Computer Integrated Manufacturing - CAD/CAM

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ACKNOWLEDGEMENT

The objectives of this course, ME 340-2, would not be possibly met without long hours andenthusiasm from our ME staffs and teaching assistants. Specifically, we would like to thank Prof.

Bruce Ankenman, Prof. James Conley, Milos Coric, Richard Dojutrek, Jesse Havens, AdrishMajumdar, Neil Krishnan and Shawn Cheng for their contributions to this course packet.

Michael BeltranJian Cao

Kornel F. EhmannHenry Stoll


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TABLE OF CONTENTS

COURSE OBJECTIVES. i

SECTION 1: PLASTIC INJECTION MOLDING... 1

Process and Tooling.... 3

Design for Plastic Injection Molding..... 11

SECTION 2: GEOMETRIC MODELING... 25

Introduction to Geometric Modeling ..... 27

Introduction to 2D & 3D Data Transformation ..... 41Introduction to Curve Representation ..... 47

SECTION 3: NC PROGRAMMING.... 65

Introduction to Machine Programming... 67

SECTION 4: GEOMETRIC DIMENSIONING AND TOLERANCING 83

SECTION 5: STATISTICAL TOLERANCING. 107

SECTION 6: METROLOGY ... 125

Measurement System Analysis... 127

SECTION 7: GROUP TECHNOLOGY... 137


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ME 340-2 Course Objectives

By the end of this class, you should be able to:

1. Design a part appropriate for injection molding and mass production, with clearlabeled detail.

2. Use the NX Modeling environment to model a part, and create a correspondingmold for that part.

3. Use the NX Manufacturing environment to create a manufacturing program withefficient use of individual operations to manufacture a mold.

4. Understand the basics of polymers and polymer processes.

5. Observe the cost, quality, flexibility, and rate of bulk manufacturing processes.

6. Use Geometric Dimensioning and Tolerancing when needed to complete a partand mold design.

7. Understand the fundamentals of CAD systems and the methods used torepresent complex forms and data transformation.

8. Comprehend the use of statistical quality control in mass production, and theconnection to GD&T.

9. Use a measurement system to statistically quantify an injection moldingproduction run.

10. Comprehend the vast number of mass production manufacturing systems usedin industry.

You will create a part in your head to your hand.


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Page 1

SECTION 1

PLASTIC INJECTION MOLDING


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PLASTIC INJECTION MOLDING SECTION 1

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SECTION 1 PLASTIC INJECTION MOLDING

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PROCESS AND TOOLING

Injection molding is a relatively new manufacturing process, in comparison to older, moretraditional processes such as die casting, machining, metal stamping and the compression moldingof plastics. Small, commercially available injection molding machines began to appear in the

1930's. By the late 1940's, the industry was well established and starting to become sophisticated.Since becoming well established, the injection molding industry and its suppliers have been veryresourceful in developing and refining the plastics materials, machinery and procedures used toinjection mold parts. This continuing improvement has allowed the industry to make inroads intovirtually every major type of product. Today, more than 25 percent of all the plastic materialsbeing used pass through injection molding machines. Injection molding is obviously an importantplastic-processing technique that product designers and manufacturing engineers must understand.

INJECTION MOLDING PROCESS

In plastic injection molding, plastic material is heated to form a "melt" which is then forced

under pressure into the mold cavity to form the part. The essential features of an injection moldingmachine are shown in Fig. 1. Plastic material in granular form is fed into the hopper. From thehopper, the material passes onto the flights of the rotating feed screw which conveys the materialforward, bringing it into contact with the electrically heated barrel. Contact with the heated barreland the heat developed by friction and shear melts (or plasticizes) the plastic. The buildup ofmaterial which accumulates in front of the screw pushes the screw back against a preset pressure.When the predetermined amount of material has accumulated in front of the screw, the screwmotor stops turning. The hydraulic piston, using the screw as a ram, moves forward and pushesthe melt into the mold cavity. The melt is held in the cavity under pressure until the gate solidifies.The pressure in the mold cavity is usually in the range of 35-140 MPa (5-20 ksi). The molded partremains in the cavity until it cools sufficiently to retain its shape. The mold then opens, the part is

ejected, and the process can be repeated.

Because the material is molten when injected into the mold, complex shapes and gooddimensional accuracy can be achieved. Molds with moving cores and unscrewing mandrels allowthe molding of parts with undercuts and internal and external threads. Injection molding is a high-rate production process, with good dimensional control. Typical cycle times range from 5 to 60seconds, but can be several minutes for thermosetting materials. The molds, generally made of toolsteels or beryllium copper, may have multiple cavities so that more than one part can be made inone cycle. Proper mold design and control of material flow in the mold cavities are importantfactors in the quality of the product. Other factors affecting quality are injection pressure,temperature, and condition of the resin.

Injection molded parts are generally molded to final desired dimensions, and no subsequentfinishing operations are required. However, the plastic in the channels (sprue and runners) thatconnect the mold cavity to the end of the barrel must be removed. More expensive molds haveheated sprues and runners, which eliminate the need for trimming. When thermoplastic is molded,the sprues and runners can be chopped and recycled.


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Figure 1Reciprocating screw injection molding machine [1].

Injection-molding machines are usually horizontal and are rated according to the capacity ofthe mold and the clamping force on the mold. Typical machine clamping force range from 0.9 MNto 2.2 MN (100 tons to 250 tons). Mold clamping force determines the size of part or number ofmold cavities that are allowed. For example, suppose a part having a projected area (area normal

to the mold parting plane) of 14 in2is to be injection molded at a molding pressure of 10 ksi by a200-ton injection molding machine. Clamping force to hold the mold halves together would then

be (14 in2)(10,000 psi) = 140,000 lb. The capacity of the machine is 200 tons, so (200)(2000) =400,000 lb of clamping force is available. Therefore, the mold could accommodate two cavitiesallowing two parts to be molded per cycle.

MOLD DESIGN

Figure 2 shows a two-plate mold the term "two-plate" refers to the fact that the mold has a singleparting line. The parting line of a mold can best be defined as that surface where a mold separatesto permit ejection of the molded plastic (molded parts, gates, and runners). The part being made inthe mold shown in Fig. 2 is a shallow dish, edge-gated. The temperature control channels are inboth the cavity and core which is the most preferred arrangement. The support pillars prevent thesupport plate from buckling under the pressure of the injection material. The knockout (KO) baris attached to the injection molding machine and is the actuator for the knockout plates. After theplastic has been injected and cooled, plate B moves away from plate A to open the mold. As plateB moves away from plate A, the molded part and runner separate from plate A and remain in plateB. Also, the "Z" type sprue puller pulls the sprue from the sprue bushing and keeps it with plateB. Upon further travel of plate B, the KO bar stops the KO plates from further travel which in turnejects the part, runner, and sprue from the mold cavity in plate B.


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Figure 2Schematic of a "two-plate" mold [2].

Many variations of the "two-plate" mold are possible. For example, three plates can be usedinstead of two. In this case, there are two parting lines, one for the part and a second for the sprueand runners. By timing the opening of the mold, the part can be separated from the runner andejected separately from the sprue and runners. This adds cost to the mold but avoids the need toremove the gates and runners from the part in a subsequent operation. In a hot runner mold, theneed to eject and regrind the sprue and runners is eliminated altogether by heating the sprue andrunner passages to maintain them in the liquid state. Many trade-offs exist between the variousapproaches and the actual choices made depend on a variety of factors such as the number of moldcavities, nature of the part, resin used, and production volumes involved.

Runners

Runners conduct the melt from the sprue to the mold cavity. Two types of runners are used:full round and trapezoidal in cross section. The full round runner has the advantage of having thesmallest periphery for a given diameter or cross-sectional area, and hence the least chilling effecton the plastic melt. It is important that the runner not freeze before the mold cavity is completelyfull, so the full round runner cross section is greatly preferred. The only time a trapezoidal crosssection runner would be employed is when it is impossible to use a full round runner. Trapezoidalrunners have a taper of about 50per side and standard cutters are available for milling them in the


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mold half. Half round runner cross sections should never be used, as a trapezoidal shape is muchsuperior.

Gates

The gate is the connection between the runner system and the molded part. It must permit

enough material to flow into the mold to fill out the part plus such additional material as is requiredto overcome the normal thermal shrinkage of the part. The location of the gate, its type and size,strongly affect the molding process and the physical properties of the molded part. In general, thedesign of the gate must be "balanced". A large gate minimizes pressure drop and assures an amplesupply of melt to fill the cavity. But the gate must be small enough to freeze first and preventbackflow of melt out of the cavity. The gate should also be sized and shaped to make degatingeasy. Fig. 3 shows several gating designs. Typically, gate geometries are selected to be compatiblewith the particular part being molded. For example, in gating hollow tubes, flow considerationssuggest the use of a "diaphragm gate".

The most common gate is the "edge gate" where the part is gated on some point on the edge.

The edge gate is an easy method from the stand point of mold construction and for many parts, isthe only practical way to gate. A "submarine gate" is one that goes through the steel of the cavity.When the mold opens the part sticks to the core and shears the gate at the piece thereby automatingthe degating process. The submarine gate works because of the flexibility of the runner and thejudicious placement of the knockout pins.

If possible, gates should be located on the thickest portion of the part so that the plastic flow isfrom thick to thin. Avoid thin to thick flows since pressure drop results in random flow which inturn results in lower strength and appearance defects. Typically, the dimensions of the gate aresized based on experience or on analysis using computer programs such as "mold flow". Once themold has been fabricated, its performance is tested and gate geometries are refined by "trial and

error" if necessary.

Vent ing

When the plastic fills the mold, it displaces air. Unless the air is removed quickly, severalthings may happen. It may ignite, causing a characteristic burn. It may compress preventingcomplete filling of a feature. It may also lead to premature cooling or to air entrapment andsubsequent weak spots. Location of the vents and gates are directly related. As the plastic flowsinto the mold cavity, air must be able to escape from the cavity. Computer modeling can be usedto help decide on vent location. Also, sample parts may be molded prior to installing the vents andthe results analyzed for underfills and burn marks to help decide where the vents should be located.

Size of the vent depends on the viscosity of the plastic being molded. The vent should be largeenough to allow hot air to escape but small enough to prevent the plastic melt from flashing.Clearance between knockout pins and their holes also provide venting.


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Figure 3Gating designs [2].

Part Ejection

After a part has been molded, it must be removed from the mold. Typically, the part ismechanically ejected by KO (knockout) pins, KO sleeves, stripper plates, stripper rings, or air,either singly or in combination. Ejector pin location can be critical and should be carefully chosento avoid stress concentration, poor appearance (ejector pins can leave unsightly marks on the part),and distortion of the part. The geometry of the part and the plastic material are the two most

important parameters in ejection. It is desirable to have a minimum draft of 1 0per side, thoughmany parts will work with less draft. The quality of polishing is also important. Often, polishingand stoning only in the direction of ejection will solve a difficult problem. A common cause forsticking in a mold is related to the elasticity of steel and is called "packing". When the injectionpressure is applied to the melt, the steel deforms. When the pressure is removed, the steel returnsto its original shape, acting as a clamp on the plastic part. Because of the complexities imposed bydraft, surface finish, packing, etc., there is no mathematical way to determine the amount of


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ejection force needed. Hence, designing the mold for proper part ejection is entirely a matter ofexperience. A mold must eject reliably to be economical in production.

Part ing Line

When a mold closes, the core and cavity, or two cavities meet, producing an air space into

which the plastic is injected. From the inside, the mating junction between the mold halves appearsas a line. This line also appears on the part and is called the "parting line". A part may have severalparting lines. The selection of the parting line is largely influenced by the shape of the part, tapers,method of ejection, type of mold, esthetic considerations, post-molding operations, inserts,venting, wall thickness, number of cavities, and the location and type of gating. Hence, selectionof the parting line is a very important decision in the design of a plastic injection molded part. Bysimply considering a part having a general rectangular shape, it is seen that the parting linedetermines the detail shape of the part and the surfaces which must have draft; the surfaces onwhich the ejector pins can act; where and how the part is gated and the mold vented; the tolerancesthat can be maintained; and the projection area of the part which in turn determines the clampingforce required during the molding process.

Undercuts

Side action (camming) is used to overcome the effect of an undercut, which is an interferenceby the mold that prevents mechanical ejection of the part. There are two types of cam action: onemoves the cams independent of the machine action (Fig. 4), and the other moves the cam by themolding machine action (Fig. 5). Molding of internal threads and some types of external threadsrequires unscrewing devices. Camming and unscrewing devices add considerable complexity andcost to the mold and should be avoided whenever possible. One trade-off is to avoid the cost ofcamming by utilizing secondary operations such as drilling or machining. This can be undesirablebecause it replaces a one time tooling cost with an on-going production cost. A more preferableapproach is to eliminate camming by designing the part such that functional and appearance

requirements are satisfied without undercuts.

Figure 4Externally operated cam for molding hole in plastic [2].


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Figure 5Mechanical cam action using cam pins [2].

Mold Cool ing

The mold is essentially the means through which heat is removed from the plastic to solidifyit to the point of ejection. Cycle time of the molding process depends on the amount of cooling.Hence, proper cooling of the mold is essential for its economical operation. Mold temperaturecontrol consists of regulating the temperature and flow rate of the cooling fluid. The functioningof the mold and the quality of the part are also highly dependent on the location of the coolingchannels. Factors to consider in designing the mold for cooling include number of coolingchannels, location of the channels, size and shape of the channels, and thermal properties of themold material. Ultimately, the excellence of the cooling system depends on the ingenuity andability of the mold designer and the mold maker.

REFERENCES

1. Glenn L. Beall, "Plastic Part Design for Economical Injection Molding", Glenn BeallEngineering, Inc., Gurnee, IL.

2. Plastic Engineering Handbook, Van Nostrand Reinhold Company, 1976.


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DESIGN FOR PLASTIC INJECTION MOLDING

INTRODUCTIONPlastic injection molding is a near net-shape manufacturing process that is widely used to

produce large quantities of production parts. In plastic injection molding, plastic material is heatedto form a "melt" which is then forced under pressure into the mold cavity to form the part. Becausethe material is molten when injected into the mold, complex shapes and good dimensional accuracycan be achieved. Molds with moving cores and unscrewing mandrels allow the molding of parts

with undercuts and internal and external threads. The molds may have multiple cavities so thatmore than one part can be made in one cycle. Proper mold design and control of material flow inthe mold cavities are important factors in the quality of the product. Other factors affecting qualityare injection pressure, temperature, and condition of the resin.

PROCESS LIMITATIONS AND REQUIREMENTSThere are three major product/process interactions that must be considered in the design of a

plastic injection molded part: material shrinkage, gating location, and parting line selection.Shrinkage occurs as the molten plastic cools in the mold to form the solid part. Since thick sectionscool more slowly than thin sections, most shrinkage defects occur when different regions of thepart have different section thickness (Fig. 1). In such cases, warpage and shape distortion can occur

as the part cools because thinner sections cool more quickly and shrink less than thicker sections.Also, because cooler regions are more rigid than hotter regions, stress can develop due to

constraints that develop as the part contracts onto the mold cores (male parts of the mold). Often,these stresses are frozen-in as the part solidifies resulting in undesirable residual stress thatdegrades the parts strength and functionality.


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Figure 1Shrinkage occurs as the molten plastic cools in the mold to form the solid part.

Figure 2Shrinkage defects are avoided by nominal wall design.

(a) Effects of Shrinkage

(b) Dimensional Instability

Radiusing inside corners eliminates stress and improvesmelt flow

Radiusing outside corners improves melt flow and eliminatesair traps

Coring out thick sections to give uniform wall thicknesshelps minimize nonuniform mold shrinkage


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Shrinkage defects are eliminated or avoided by employing a uniform wall thicknesseverywhere in the part (Fig. 2). The uniform wall thickness is often referred to as the nominal wallsince it is the target value when no other wall thickness is specified. By using a nominal wall,thermal mass, and therefore shrinkage, is the same everywhere in the part. Avoiding sharp cornersalso reduces shrinkage problems (Fig. 2). Sharp corners, especially inside corners, cause severe

molded-in stresses as the material shrinks onto the core. Sharp corners also cause poor flowpatterns, reduced mechanical properties, increased tool wear, and stress concentration (Muccio,1991).

A well-designed plastic injection molding can typically be broken down into three basicelements: the nominal wall, projections off that wall, and depressions in that wall (Fig. 3). Oncethe basic configuration and functional features of the part have been decided, nominal wall designis implemented by isolating each element and designing it to have a uniform wall thickness (Fig.4). By decomposing the overall part into basic elements, it is relatively easy to correctly design thepart as a plastic injection molding, no matter how complex it might be.

The gate is the connection between the runner system and the molded part. In general, thelocation of the gate as well as its type and size, strongly affect the molding process and the physical

properties of the molded part. Often, the location and design of the gate must be considered duringthe design of the part. For example, the gate location can be critical if defects and/or possible partfailure due to the weakening effect of weld lines is to be avoided (Fig. 5). Similarly, the gatelocation is critical when the orientation of glass fibers or other filler is important (Fig. 6 a). Also,the gate location can be important from an appearance or assembly standpoint (Fig. 6b). Typically,the location and dimensions of the gate are based on experience or on analysis using commerciallyavailable computer programs (Fig. 7). Once the mold has been fabricated, its performance is testedand, if necessary, the gate geometry is refined by trial and error.

When a mold closes, the core and cavity, or two cavities meet, producing an air space intowhich the plastic is injected. From the inside, the mating junction between the mold halves appearsas a line. This line also appears on the part and is called theparting line(Fig. 8). A part may haveseveral parting lines. The selection of the parting line is largely influenced by the shape of the part,tapers, method of ejection, type of mold, esthetic considerations, post-molding operations, inserts,venting, wall thickness, number of cavities, and the location and type of gating. Selection of theparting line is a very important decision because it impacts many aspects of the part. Consider forexample a part having a general rectangular shape. It is seen that the parting line determines:

The detail shape of the part. The surfaces which must have draft. The number of undercuts. The surfaces on which the ejector pins can act. Where and how the part is gated and the mold vented. The tolerances that can be maintained. The projection area of the part, which in turn determines the clamping force required during

the molding process.


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Figure 3Basic elements of a plastic part.

Figure 4Isolate each element in the design and consider it individually (adapted from Beall,1985).

Maintain nominal wall

Radius all cornerswhere practical

Maximize draft Avoid undercuts


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Figure 5Weld lines form when the plastic flow divides into two or more paths. (a) This gatelocation causes a weld line to form in a critical region of the part. (b) This gate location causes

the weld line to form in a less critical region.

Figure 6Gate location can be critical. (a) Fiber orientation is determined by gate location. (b) A

gate might result in an unpredictable inconsistency that might interfere with assembly or partfeeding.

Figure 7Typical gating systems. (a) The Gate connects the runner to the mold cavity and isdesigned to control the flow of plastic into the mold. (b) There are many different gating systems

that can be used depending on the needs of the particular part involved.

Weld

Line

Load

Weld

Line

Load

Gate

Gate

(a) Poor (b) Better

(a)

Gate

(b)

(a) (b)


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Figure 8Selection of the parting line is a very important decision because it influences manyaspects of the part design and tool cost.

To facilitate proper release of the injection molded part from the tool, part surfaces that areperpendicular to the parting line must be tapered in the direction of mold movement. This taper,which is commonly referred to as draft, allows the molded part to break free by creating a clearanceas soon as the mold starts to open (Fig. 9). Draft is necessary to offset the effect of shrinkage,which causes the plastic part to grip cores making part ejection from the mold difficult. Althoughthere are exceptions, a draft of 1/20per side is considered a minimum, with 1 1/20to 30per sidefrequently recommended. In general, the larger the draft, the easier the part will be to eject. Thisin turn will allow faster cycle times since the part does not need to cool as much in the mold beforeit is strong enough to be ejected.

Undercutsare geometrical features that prevent mold opening and mechanical ejection of thepart. Side action (camming) is used to overcome the effect of an undercut (Fig. 10). Molding ofinternal threads and some types of external threads require unscrewing devices. Camming andunscrewing devices add considerable complexity and cost to the mold and should be avoidedwhenever possible. One trade-off is to avoid the cost of camming by utilizing secondary operationssuch as drilling or machining. This can be undesirable however because it replaces a one timetooling cost with a recurrent production cost. A more preferable approach is to eliminate cammingby designing the part and selecting the parting line such that functional and appearancerequirements are satisfied without undercuts.

DESIGN RELATED COST INTERACTIONSThe cost of a plastic injection molding is determined by its size, configuration, and detail

geometry. By understanding how geometry decisions affect the cost, it is possible to makegeometry decisions that reduce cost. Developing this understanding is a three-step process:

1. Develop a cost model based on the major sources of recurring and non-recurring costassociated with the part.


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Figure 9Draft is essential for reliable ejection.

Figure 10Holes, depressions and projections not perpendicular to the parting line require coresthat move to allow part ejection.

2. Identify the key design related cost drivers.3. Understand the relationship between the design related cost drivers and the cost sources.

Develop insight into the effect of each cost driver on cost and how part cost can be reduced.

Step 1 is implemented by first identifying the major cost sources and then expressing part costin terms of these cost sources. For plastic injection molding, the major sources of cost include toolcost, material cost, and production cost. Unit part cost is calculated as follows,

Y

tCCV

N

CPartCost

cycleH

MT

/ (1)

whereT

C = total tooling cost ($)

MC = material cost ($/in3)

HC = machine cost including labor ($/hr)N = lifetime number of parts made using the toolV = part volume (in3)

cyclet = cycle time to mold one part (hr)

Y = process yield (usable parts/N)Equation (1) is very clear as to what should be done to reduce cost:


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Design to minimize tooling cost. Design to minimize material cost. Design to minimize cycle time. Design to maximize process yield.

In step 2, we use our understanding of the manufacturing process to identify the design decisionsthat drive these costs. For plastic injection molding, the major design related cost drivers includethe parting line, the number and complexity of undercuts, the nominal wall thickness, the amountof draft, surface finish specifications, and dimensional tolerance specifications.

Once the key design related cost drivers have been identified, the relationship between eachdriver and part cost is delineated in step 3. For plastic injection molding, these relationships aresummarized as follows:

Nominal Wall Thickness:Of all the issues in plastic injection molding design, selecting theproper nominal wall thickness is probably the most important design decision that is made. Justabout every aspect of the parts appearance, functionality, and manufacturability relates to, affects

or will be influenced by the wall thickness. From a cost standpoint, it is safe to say that the nominalwall thickness will impact all of the cost sources in one way or another. The most obvious ismaterial cost since wall thickness directly affects the part volume with a thinner wall being moredesirable. Also, a thinner wall will facilitate faster cooling and as a result, potentially shorter cycletimes. At the same time, a thinner wall requires more injection pressure and time to fill the mold.If the wall is to thin, yield may be adversely affected because flow is restricted and the part willnot fill out properly. Also, high injection pressure can result in flash or highly stressed parts. Inaddition, thinner walls can require more mold detail and closer core/cavity alignment and fit whichwill increase tooling cost. Thick-walled parts, on the other hand, cool slower, shrink more, andhave more risk for sink marks and voids. Thick walls also create more random plastic flow patterns,which reduce the dimensional stability and predictability of the part. Hence, in addition toincreasing material cost, thicker walls can also decrease yield and increase cycle time.Recommended nominal and minimum wall thicknesses for common thermoplastics when injectionmolded are given in Table 1. Nominal wall design guidelines are given in Figures 1113.

Parting Line:The parting line establishes many aspects of both the part geometry and theprocess. The parting line choice affects the size of the injection molding machine required and thecomplexity and size of the mold. This in turn affects tooling cost, machine cost, and to a lesserextent cycle time. To minimize part cost, the parting line should be selected very early in the designprocess. The goal should be a parting line that keeps tool complexity to a minimum. For example,the selection of the parting line in Fig. 14 avoids the requirement of a moving core in the mold. Aplanar parting line is preferable to a non-planer parting line. Also, when possible, a parting linethat places the whole part in one mold half can greatly reduce tool cost since only one mold halfneeds to be machined and alignment and registration requirements across the parting line areavoided.


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Table 1Suggested Wall Thickness for Various Thermoplastic Materials

Material ShortSections(< 0.5in)

SmallSections(0.5 ~ 1 in)

AverageSections(1 ~ 8 in)

Large Sections

(> 8 in)

Acetal 0.6 (0.025) 0.9 (0.035) 1.9 (0.075) 3.2-4.7 (0.125-0.185)

Acrylic 0.6 (0.025) 0.9 (0.035) 3.2 (0.090) 3.2-6.3(0.125-0.250)Acrylonitrile butadienestyrene (ABS)

0.9 (0.035) 1.3 (0.050) 1.9 (0.075) 3.2-4.7 (0.125-0.185)

Cellucose acetatebutyrate

0.6 (0.025) 1.3 (0.050) 1.9 (0.075) 3.2-4.7 (0.125-0.185)

Nylon 0.3 (0.012) 0.6 (0.025) 1.5 (0.060) 2.4-3.2 (0.093-0.125)

Polycarbonate 0.4 (0.015) 0.8 (0.030) 1.8 (0.070) 2.4-3.2 (0.093-0.125)

Low-densitypolyethylene

0.9 (0.035) 1.3 (0.050) 1.6 (0.062) 2.4-3.2 (0.093-0.125)

High-densitypolyethylene

0.9 (0.035) 1.3 (0.050) 1.9 (0.075) 3.2-4.7 (0.125-0.185)

Polypropylene 0.6 (0.025) 0.9 (0.035) 1.9 (0.075) 3.2-4.7 (0.125-0.185)Polystyrene 0.8 (0.030) 1.3 (0.050) 1.6 (0.062) 3.2-6.3(0.125-0.250)

Flexible polyvinylchloride

0.6 (0.025) 1.3 (0.050) 1.9 (0.075) 3.2-4.7 (0.125-0.185)

Rigid polyvinylchloride

0.9 (0.035) 1.6 (0.062) 2.4 (0.093) 3.2-4.7 (0.125-0.185)

Undercuts:Anything that can be done to avoid undercuts or reduce their complexity willreduce tooling cost and development time. Also, since complex side actions require additionaltime, avoiding or simplifying undercuts can often reduce cycle time. The best way to avoid

undercuts is to select the parting line wisely. In addition, it is often possible to design so undercutscan be created using simple tooling without the need for expensive side actions (Fig. 15). Also, ifthe undercut is made shallow enough, the part may be strippable from the mold without the needfor a core pull. Note that strippable undercuts will also increase cycle time since the mold halvesmust be opened before the ejector pins can strip out the part and the part must also be strong enoughto endure the ejection forces. Maximum allowable strippable undercuts for some common injectionmolded thermo plastics are given in Table 2.

Draft:Generous draft is highly desirable for low part cost. Minimal draft, especially whencombined with deep draws (Fig. 16), increase tool cost because highly polished mold surfaces orspecial mold surface treatments are required for proper part ejection. Alternatively, a regular

application of mold release spray may be necessary during production. This increases cycle timeand production cost. Minimal draft also increases cycle time because the part must be stronger andtherefore cooler before it can be reliably ejected. Yield may also be adversely affected whendifficult part ejection results in damaged parts. A difficulty that is commonly encountered whenapplying draft to a part is the creation of unacceptably heavy walls. One remedy is to use paralleldraft which allows the wall sections to be kept uniform (Fig. 16).

Table 2Maximum Allowable Undercut for Common Materials


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Material

Average Maximum

Strippable Undercut, Smm (in.)

AcrylicAcrylonitrile Butadiene Styrene (ABS)NylonPolycarbonate

PolyethylenePolypropylenePolystyrenePolysulfoneVinyl, Flexible

1.5 (0.060)1.8 (0.070)1.5 (0.060)1.0 (0.040)

2.0 (0.080)1.5 (0.060)1.0 (0.040)1.0 (0.040)2.5 (0.100)

Figure 11Coordinate corner radii with wall thickness to improve flow and reduce stress.

Generous radii improve flow and reduce molded in stress and stress concentration. Sharpcorners, on the other hand, reduce flow rate, intefere with smooth flow, create the possiblility of

turbulence, and pull and tear long-chain polymeer molecules.

Less than S

Avoid sharp corners except at the parting lineAvoid sharp corners except at the parting line


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Decrease projection thickness to limit thickness in region ofunction with wall.

Projections are difficult to fill, vent, and eject. Limit height to 2.5o 3 W. In general, two or more short reinforcing ribs will be

easier to mold than one tall reinforcing rib.

Distance between adjacent projections should be greater than 2W

Figure 12Design projections to maintain nominal wall.

Figure 13Design holes and depressions to maintain the nominal wall.

Can be through holes or blindholes

Grooves and slots are justelongated holes

Either telescoping or butt

cores may be used

Note:

Figure 14.A demonstration of the impact of the parting line selectionon the complexity of the tooling. The selection shown

here eliminates the need of a moving core.


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Figure 15Creating undercuts with simple tooling.

Figure 16Generous draft enables reliable part ejection and short cycle time.

Figure 17Lips and bosses aid in alignment of assembled plastic parts.

Surface Finish:The appearance and feel of the finished part depends on the surface finish. Ingeneral, dull, matte, or textured finishes are preferred to glossy finishes, which tend to accentuatesink marks and other surface imperfections. Textured surfaces and very smooth surfaces increasetool cost because of the extra effort required to prepare the mold surface. Surface finish can alsoadversely impact cycle time and yield if it is not properly designed. To avoid problems, care must

be taken to ensure that the draft of the part is adequate to allow proper part ejection. Also, thetexture features must have positive draft.

Tolerance:An advantage of the plastic injection molding process is that it allows surprisinglygood dimensional control. However, tight dimensional and form tolerance specifications increasetooling cost and reduce cycle time and yield. Fine-tolerance molds are more costly than looser-tolerance molds. Fine tolerance also increases processing cost because closer process control ofpressure, temperature, and cycle time is required, cycle time may be increased, and shrink fixtures

Side ShutoffsMolding Holes Parallelto Parting Plane

Moldingsnap-fitflexingfingerwithoutside core

Draft

Angle

Depth of

Draw

Parallel

Draft

Plastic

Steel

Plastic

Casting


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may be required after the part has been removed from the mold. Therefore, whenever possible,component tolerances should be relaxed as much as possible. Plastic parts are more flexible thanmetals. Often, this property can be leveraged by the knowledgeable designer to reduce tolerancerequirements. For example, the plastic part can be made to deform slightly for a better fit byproviding lips and locating bosses that ensure alignment with mating parts (Fig. 17). Also, low-

shrinkage materials and glass-filled or mineral-filled materials can be molded more accurately thanhigh-shrinkage or unfilled materials. Note also that the use of a greater number of mold cavitiesreduces precision. In general, the allowable dimensional tolerance should be increased by 5% foreach cavity after the first. Suggested dimensional tolerances for common injection molded plasticsare given in Table 3.

SUMMARYThe key to designing a good plastic injection molded part is to clearly understand how part

design decisions drive cost. A qualitative depiction of the relationship between cost sources anddesign related cost drivers for plastic injection molding is illustrated graphically in Fig 18.

Figure 18Relationship between design related cost drivers and cost sources for the plasticinjection molding process.

FURTHER READINGThis book provides a broad overview of plastic part design with detail discussion and

illustrations treating many aspects of the subject.Muccio, Edward A., Plastic Part Technology, ASM International, Materials Park, Ohio44073-0002, 1901.

Detail design for plastic injection molding guidelines are provided in Chapter 6.2 of this book.Handbook of Product Design for Manufacturing, James G. Bralla (Ed.), McGraw-Hill,

1986.These excellent books provide design guidelines as well as methods for estimating cycle time andcost of plastic injection moldings.

Boothroyd, G., Dewhurst, P., and Knight, W., Product Design for Manufacture andAssembly, Marcel Dekker, 1994.Dixon, J. and Poli, C., Engineering Design and Design for Manufacturing, Field StonePublishers, 331 Field Hill Road, Conway, Massachusetts 01341, 1995.

Cost

Drivers

Cost Sources

Parting

Line

Under-

cuts

Nominal

Wall

Draft Surface

Finish

Toler-

ance

Tooling Cost

Material Cost

Cycle Time

Yield

Major Influence Moderate Influence Little Influence


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Table 3Dimensional Tolerances for Injection Molded Thermoplastic Parts*


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SECTION 2

GEOMETRIC MODEL ING


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GEOMETRIC MODELING SECTION 2

Page 26



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SECTION 2 GEOMETRIC MODELING

Page 27

Northwestern University

INTRODUCTION TO GEOMETRICMODELING

ME 340-2

Computer Integrated Manufacturing

- CAD/CAM -


Third Angle Projection


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Standard Views in Third Angle Projection


Wireframe Models


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Wire Frame Ambiguity


Surface Models

Polygon Mesh for a Funnel A Polygon Mesh


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Geometric Modeling Approaches

- Example -


Geometric Modeling Approaches- Multi-view of a Part -


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- Boundary Representation (BREP) -


Euler - Poincare Formula

v - e + f- h = 2 (s - p)

v - number of vertices

e - number of edges

f - number of faces or peripheral loops

h - number of hole loops

s - number of shells

p - number of passages (through holes)

16 - 24 + 10 - 2 = 2 (1 -1)


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- Constructive Solid Geometry (CSG) -


Typical CSG Primitives


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Solid Construction Methods

Translational Sweeping

Rotational Sweeping

Skinning


Sweeping


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CSG Method


Volume Listing (Decomposition Model)

Voxel Representation

Octree Representation


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Feature-based Modeling

Slot and Hole Features

Chamfer, Hole

Pocket, Fillet


Basic Architecture of a CAD System


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Data Bases for Computer Graphics

Example Object



Example of a Relational Database


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Example of Hierarchical Database



Example of a Network Database


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Basic Data Structures - Linked Lists

Double Linked List




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Double Linked List


Basic Data Structures Tree Structure


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Tree Structure for a Quadtree


Graphic Standards


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INTRODUCTION TO 2D & 3D DATA TRANSFORMATION1

INTRODUCTION

The two basic tasks required to display an image of an object on a graphics device are: (1)specifying the location of all the points on the object in space, and (2) determining which locationson the display monitor are to be occupied by those points. In this section, we will show themathematic behind CAD on how to determine the point location and how to transfer points inspace.

Projection

In perspective projection, all the points on the object of interest are connected to the center ofprojection, usually located along the line from the viewsite to the viewpoint, and the intersectionpoints between these lines and the screen comprise the projected image. The screen is locatedbetween the viewpoint and the viewsite. In parallel projection, the parallel lines are cast from allthe points on the object in the viewing direction defined by the viewsite and the viewpoint asbefore, and the intersection points between these lines and the screen comprise the image.Similar to perspective projection, the screen is oriented perpendicular to the projection directionfor orthogonal projection.

Let the subscript, v, denote coordinates associated with a viewing coordinate system, and let thesubscript,s, denotes the distances associated with the projection on the screen. See Figures 2.1

and 2.2 for the graphical representation. Applying the similarity rule, we can obtain

1Taken from Principles of CAD/CAM/CAE Systems by Kunwoo Lee, Addison Wesley, 1999.


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Fig. 2.1 Viewpoint and viewsite Fig.2.2 Calculation of projection point in theperspective view.

v

v

s XZL

SX

and vv

s YZL

SY

(Eq.1)

Translation

When an object is translated by a, b, cin thex, yand z directions, respectively, from its initialposition at which its model coordinate system coincides with the world coordinate system, theworld coordinates of a point on the object at the new position, (Xw, Yw, Zw) are obtained as follows:

cZZ

bYY

aXX

mw

mw

mw

(Eq. 2)

which can be rewritten in the matrix format,

1

),,(

11000

100

010

001

1

m

m

m

m

m

m

w

w

w

Z

Y

X

cbaTransZ

Y

X

c

b

a

Z

Y

X

(Eq.3)

The graphical representation can be found in Fig. 2.3.


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Scaling

To scale an object up or down sx times in x, sy times in y, and sz times in the z direction, thefollowing transformation matrix is applied:

1

),,(

11000

000

000

000

1

m

m

m

m

m

m

z

y

x

w

w

w

Z

Y

X

zyxSZ

Y

X

s

s

s

Z

Y

X

(Eq. 6)

Fig. 2.3 Translation of an object.


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Rotation

Suppose that an object is rotated by about thexaxis of the world coordinate system together withits model coordinate system, i.e., from Q(Ym, Zm)toQ(Ym, Zm)(Fig. 2.4). The relationship betweenQand Qcan be represented as

1

),(

11000

0cossin0

0sincos0

0001

1

m

m

m

m

m

m

w

w

w

Z

Y

X

xRotZ

Y

X

Z

Y

X

(Eq. 4)

Note that here, follows the right-hand rule, i.e., in the case represented in Fig. 2.4., a positive indicates a rotation in the counter-clockwise direction.

Similarly, the transformation matrix for the rotation about theyorz axis can be expressed as

1000

0100

00cossin

00sincos

),(

1000

0cos0sin0010

0sin0cos

),(

zRot

yRot

(Eq. 5)

One important fact about rotation transformation is that order matters, which means that the finallocation of a point will be most likely different if the rotation order is changed, for example, from(x, y and z) to (z, x and y).

Fig.2.4 Rotation about thexaxis.


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Example 1:

Example 2:


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INTRODUCTION TO CURVE REPRESENTAION

INTRODUCTION

In automotive design, a designer will hand draw an outline, this drawing then be inputted into aCAD system, the system is required to reproduce that hand-drawn curve as accurate as possibleand use minimum of data storage. In this section, we will outline key characteristics of variouscurve representation methods.


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Hermite curve

Bezier curve

B-spline curve

Bspline demo


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SECTION 3

NC PROGRAMMING


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SECTION 3 NC PROGRAMMING

Page 67

INTRODUCTION TO MACHINE PROGRAMMING

General Information

GENERAL PROGRAM ORGANIZATION

The NC Part Program consists of a series of commands to the ACRAMATIC 850 MC controlto give instructions for machining the workpiece. The control interprets these commands anddirects the machine motions. All functions of the machine are controlled by the programmedcommands during automatic execution. This includes position of the machine slides; slide feedrate selection; selection of the direction, speed starting and stopping of the spindle; and the controlof auxiliary equipment.

The program consists of a series of blocks. Generally, each block contains the commandsrequired to perform a single step in the machining operation such as feeding the tool at the specifiedfeed rate from one point to another. The workpiece is machined by executing one block afteranother in sequence until the entire workpiece is complete.

A variable block length format is used. Only the commands pertaining to new functions or

changes need to be programmed in the block.To minimize programming instructions, a block may cause execution of a series of events.

Automatic cycles and sub-routines may be called into action to perform repetitive operations.These may contain the instructions to perform a series of events such as making a series of similarholes or performing all of the passes required to machine a pocket, frame, etc.

Each block contains a number of words. The word begins with a letter, called an address, andis followed by numerical information. Each address is used to define a single function. An X word,for example, is used to designate the coordinate position to which the X-axis is to move. The lengthof the words may vary with their function. The format of each word used by the control is describedin detail later in this manual. A plus or minus sign may be programmed for words allowing apositive or negative value.

BLOCK FORMAT

Blocks use a word address, tab ignore, variable block length format. These blocks contain theconventional, step-by-step, programming commands.

Each word within a block has a word address followed by numerical information. A total of70 characters and 25 words are recognized by the control. Other address characters are not allowedand are detected as errors when programmed. The format of each word in the block is fixed andmay vary with the active inch/metric state and the use of the word.

1. The following sequence is used in the blocks.

Sequence number (N or :)Q wordsG wordsX, Y, Z, A or B or C, I, J, K, E, L, F, P, R, UV, W, O ,=, S, T, D, H, M

2. The sequence number must precede all other words, when used, with the exception of the blockdelete code.

3. The label word (Q) may be preceded only by sequence number and block delete code.


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4. The block delete character, sequence number and label word are the only words that may beprogrammed before the G word.

5. A block may contain a maximum of 255 characters.6. Only the address of G and M may appear more than one time within a block. Multiple G and

M codes are allowed. Up to nine G codes and four M codes may be programmed in the same

block. Conflicting command M and G codes, however, are not allowed.7. A space code is allowed but is subject to special rules. During program entry from the operatorstation, a space code is allowed at any place. During any Load, Save or Resequence

operation, however, space characters that do not occur between an open parenthesis and thenext End of Block are automatically erased from the active program.

8. The block ends with a line feed character

SEQUENCE NUMBERS

Sequence numbers may be programmed in each block. It is an unsigned six digit numberpreceded by the letter N or a Reference Rewind Stop character (colon :).

Format: N10:10

The ACRAMATIC 850 MC control does not require sequence words nor does it requiresequence word values to be unique, contain the same number of digits, or to be in order. Increasingword values, however, simplify searching. Observing a fixed length of sequence number allowssearching without getting false matches inside longer numbers. It is therefore recommended thatsequence numbers be used and have the same number of digits. It is also recommended that thevalue of the sequence number be programmed in ascending order in increments of 10 (N0020,N0030, N0040, etc.).

A sequence number having an N word address has no effect on the execution of the program

and is used only for reference and operator searching of the program. A sequence number havinga Reference Rewind Stop code address causes initialization, but does not otherwise affect programexecution.

When a sequence number is programmed, it must conform to the following rules:

1. The sequence number is the first word of the block and may be preceded only by theblock delete character.

2. Only one sequence number may be programmed in a block.3. The address must be either N or the Reference Rewind Stop character. Both cannot be

programmed in the same block.

INITIALIZATION

A Reference Rewind Stop code addressed sequence number (a colon instead of a N in front ofthe sequence number) must be programmed in the first block of the program. It may beprogrammed at any additional point where it is desired to establish a potential restarting point. Itcannot be programmed within a subroutine. When this code is executed all modal preparatoryfunctions are automatically reset to their initialized (Data Reset) states unless specificallyprogrammed otherwise. The following are automatically selected:


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G01 Linear interpolationG17 XY plane selectionG40 Cutter compensation offG60 Positioning modeG90 Absolute

G94 Feed per minuteG70 Inch selected by commissioning data

SPECIFYING POLAR COORDINATES

Polar coordinates are programmed in a block containing an E word and a departure command.The E word designates the angle from the positive direction of X axis at which the command pointis located. The presence of an E word designates that polar coordinates are used. Rectangularcoordinates are assumed when an E word does not appear within the block.

The E word has a format of E33 and may range in values from 0 to 359.999 degrees.

NOTE:The angle of polar coordinates that is used when programming the NC control is

expressed in degrees. Fractional parts of the angle are expressed as decimal fractionsof degrees and are not expressed in seconds and minutes. Decimal portions ofdegrees are used when calculating with most small calculators. Most trigonometrictables, however, use degrees, minutes and seconds, and must be converted todecimal parts.

Figure B1 shows an example polar coordinate movement and is programmed as follows:

GO X8 Y5 E60 L3

Here E and L are the angular and linear offsets of point A (the target) from the XY command point.

In this case slide movement is made directly to point A from the current position.If X and/or Y are not programmed then the polar offset is taken from the current position ofthe X and/or Y axis.

NOTE: If the modal L word is not present or has a value of zero, then the block becomesequivalent to rectangular coordinates.


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+Y

+X

8

5

60 o

3

A

Figure B1:Polar coordinates.

SHIFTING THE PROGRAM COORDINATE SYSTEM

The ACRAMATIC 850 MC control has the ability to shift the machine coordinate system tomatch the coordinates of the NC program. The coordinate shift is accomplished by a programmedPosition Set operation.

One important use of this feature is to allow writing the NC program without considering the

physical location of the workpiece on the machine. The zero point for the NC program is thenpositioned by the operator during workpiece setup.

SPINDLE CONTROL

Spindle speed commands are programmed in terms of spindle revolutions per minute. SpindleRPM is specified by a four digit S word whose format is

S1000

FEEDRATE CONTROL

The traverse rate of the axes is specified using an F word, such as:

F25

where the feedrate is expressed in terms of in/min (if using inch units) or mm/min (if using metricunits).


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MISCELLANEOUS FUNCTIONS (M Words)

Miscellaneous Functions are programmed using an M word in the range 0 to 99. These codesare used for a variety of machine related functions including coolant and Spindle Control, ToolChange initiation, Program Control, etc. (See Table B1).

TABLE B1: Quick Reference Chart - Preparatory and Miscellaneous Functions

G00 Linear Interpolation - Rapid RateG01 Linear Interpolation - Feed RateG02 Circular Interpolation - CWG03 Circular Interpolation - CCWG17 X-Y Plane SelectionG18 X-Z Plane SelectionG19 Y-Z Plane SelectionG23 Pocket Milling (Rect)G24 Inside Frame Mill (Rect)

G25 Outside Frame Mill (Circ)G26 Pocket Milling (Circ)G27 Inside Frame Mill (Circ)G28 Outside Frame Mill (Circ)G37 Hole Pattern CancelG38 Hole Pattern (Rect)G60 PositioningG61 ContouringG70 Inch ModeG71 Metric ModeG80 Fixed Cycle - Cancel

G81 Fixed Cycle - DrillG84 Fixed Cycle - TAPG90 Absolute ModeG91 Incremental ModeG92 Position SetG94 Inch (Millimeter) Per Minute Feed RateG98 Machine Coordinate

M02 End of ProgramM03 Spindle clockwise, coolant offM04 Spindle counter clockwise, coolant offM05 Spindle off, coolant off

M06 Tool ChangeM13 Spindle clockwise and coolant onM14 Spindle counter clockwise and coolant on

M02 - End Of Program

The M02 code is used at the end of the program. The following steps take place after allinterpolation has been completed for the block in which the M02 is programmed.


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1. The Z slide will rapid retract to the full retract position, as the X and Y axes travel toeither the Manual or Auto Tool Change position, simultaneously stopping the spindle.The coolant, if ON, will be turned OFF.

2. The tool in the spindle is automatically unloaded and returned to the tool storage matrix.

M06 - Tool Ch ange

The M06 Tool Change code is used to:

command the loading of the first tool into the spindle

command all intermediate tool changes

command the loading of the last tool into the spindle but not to unload the last tool.When an M06 is processed the following axis motion occurs:

1. The spindle will stop and orient, and the coolant will be turned off whilst the Z slideretracts to a clearance level beyond the tool change position.

2. The XY axes (and B, if applicable) advance to their respective tool change positions,if specified.

3. If there is no tool in the spindle, the tool storage matrix will rotate to the requested tool

location then swing radially beneath the spindle nose. The spindle advances onto theexposed tool adapter, using the power draw bar to restrain the tool. The tool storagematrix swings rapidly away from the spindle to its home position. If there is a tool inthe spindle, the mechanism will collect the tool from the spindle prior to searching forthe desired tool. Searching for the desired tool is initiated once the spindle is retractedto the clearance level beyond the Tool Change position.

After tool change the spindle will remain in tool change position which is at a considerable distanceabove the workpiece. The following movement commands should reflect the next tool position inall three axes to make sure that the tool will be positioned properly.

PREPARA TORY FUNCTIONS (G Words)See Table B1 above for reference.

G00 - Linear Interpo lation at Rapid Rate

When the G00 Preparatory Function is programmed, the axes move from the current positionto the command position along a straight vector. The rate of travel when measured along the vectoris set by commissioning data.

The G00 Preparatory Function is subject to the following programming rules and conventions:1. The G00 code is functional in both the absolute and incremental modes.2. The command position may be expressed in rectangular or polar coordinates.3. The G00 code is modal and remains in effect until replaced by another G code, or the

control is initialized. A G01 code is assumed at initialization.

G01 - Linear Interpolation at Feed Rate

The G01 linear interpolation Preparatory Function commands the slides to move from thecurrent position along a straight line vector. The rate of traverse is measured along the vector andis equal to the programmed feed rate.

The G01 Preparatory Function is subject to the following programming rules and conventions:1. The G01 mode is assumed when the control is initialized.


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2. The G01 code is functional in both the absolute and incremental modes.3. The command position may be expressed in rectangular or polar coordinates.4. After the G01 there has to be an F word for the desired feedrate.

G02/G03 - Circular Interpolatio n

Circular interpolation provides the capability of moving the tool in a circular arc along a pathgenerated by the control system. The rate of travel is uniform around the arc with tangential vectorfeed rate equal to the programmed feed rate. A circular path may be generated in any of the planesXY, XZ or YZ by programming the appropriate plane select code, G17, G18 or G19 respectively.Circular interpolation may be programmed in two ways. They are:

1. Programming G02 or G03 Preparatory Functions together with I, J, K words to definethe center point of the arc;

2. Programming G02 or G03 Preparatory Functions together with an end point and a Pword to define the radius of the arc.

Preparatory Function codes G02 and G03 (per EIA standards) are used for programmingCircular Interpolation. These codes determine the direction of the circular path as viewed from the

positive end of the axis that is perpendicular to the plane of interpolation: the G02 code causes the tool to proceed in a clockwise path

the G03 code causes the tool to proceed in a counterclockwise path.These codes are programmed in the block where Circular Interpolation becomes effective, andremain effective, until a new Preparatory Function code is programmed.

Starting Point

The starting point, an X, Y and/or Z coordinate, is the result of a previous block of information,either the end point of a previous arc (Circular Interpolation) or the end point of a line (LinearInterpolation). The starting point is always described by X, Y and/or Z words.

Center PointThe center point, and X, Y and/or Z coordinate, is the center of the circular arc. The centerpoint is described by I, J and K words. The I word describes the X coordinate value, the J worddescribes the Y coordinate value and the K word describes the Z coordinate value. The I, J and Kwords are absolute values when programmed in absolute mode. These words define the signedincremental vector distance from the present X, Y and Z position to the arc center point in theincremental mode.

The I, J and K values are modal in Absolute Mode, but must be programmed in each block inIncremental Mode.

End Point

The end point, and X, Y and/or Z coordinate, is the final point where the centerline of the cutterpath completes the circular arc. The end point is always described by X, Y and/or Z words, andmust be programmed in every block using Circular Interpolation.

G90/G91 - Absolu te and Incremental Dimension Input

The algebraic sign (+ or -) specifies the direction of travel. Incremental movements may beprogrammed using either rectangular or polar coordinates.


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Figure B2 illustrates the difference between the two types of commands. It is desired to movethe X axis from point A to point B. The X axis is currently located at the absolute coordinate of X= 5 inches, a distance of 1.5 inches from point B. The programmed axis position words for themovement are:

Absolute mode X +3.5

Incremental Mode X -1.5Note that the algebraic sign of the incremental X word is negative. This specifies that the axis isto move in the negative direction, the direction towards the origin. The sign of the absolutecoordinate, however, remains positive since point B lies in the positive direction from the programzero reference point.

The current position of the axes are displayed on the CRT in absolute rectangular coordinatesregardless of whether the incremental or absolute mode is active.

Point B Point A

Program Zero

Movement Commands:

Absolute: X3.5

Incremental: X-1.5

3.5

5

+Y

+X

Figure B2:Absolute and incremental dimensions.

The absolute/incremental feature does not apply to the following cases:1. The R word used with fixed cycles is always absolute.2. The X, Y and Z axis words in a G92 - Position Set block are always absolute.3. The X, Y and Z axes words using the G98 mode for programming in machine

coordinates are always absolute.4. The L word used when specifying polar coordinates is always the incremental

displacement from the command position.

G92 - Posit io n Set

The G92 - Position Set feature enables resetting the absolute program coordinates of the X andY axes by MDI or NC program command. The current position of the designated axis (or axes) isassigned a new coordinate value. No movement of slides results.

When the G92 code is executed, the current position of the programmed axis is assigned thecoordinate value programmed in the block. Any combination of axes may be given new coordinatevalues. The values are always absolute regardless of whether the incremental or absolute mode isactive. The following block, for example, assigns the current position of the axes coordinate values


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of X = 2.000 inches and Y = 4.0000 inches. These values are displayed on the CRT as the currentaxes position after the block is executed.

G92 X2 Y4

FIXED CYCLESThe control provides a wide selection of fixed cycles which can be used to program multiple

machining movements with a minimum of input data.Each cycle has characteristics suited to its own application, but they all share some common

features and the same basic sequence of operation steps.1. Rapid the X, Y and contouring rotary axes to their commanded position,

simultaneously positioning the indexing axis if one is present and if it is commandedto move.

2. Rapid the Z axis to the clearance plane.3. Feed to depth in Z.

4. Perform cycle-specific dwell and spindle operations.5. Feed or rapid Z back to the clearance plane and the additional W distance ifprogrammed.

6. Perform cycle-specific dwell and spindle operations.

Figure B3:Hole depth.

As shown graphically in Figure B3, the fixed cycle reference plane ('R plane') represents thenominal surface of the workpiece. Since it is unsafe to position in X and Y at the R plane, fixedcycles add a clearance called 'gauge height' to the R plane, and use the resulting Z coordinate asthe 'clearance plane', at which to do rapid moves and from which to begin machining. 'Hole depth'is the true depth of the hole, measured from the R plane. Machining actually begins at the clearanceplane, so the length of the machining move is hole depth plus gauge height.


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The R plane coordinate is coded in the R word of any fixed cycle. The value is always anabsolute program coordinate, and is independent of the absolute/incremental mode setting or thenon-modal machine coordinates function G98. In drilling cycles (G81, G82, G83 and G88), the Zfeed depth is increased by the drill point length, such that the programmed Z word becomes the"full diameter" drilling depth. The drill point length is a function of drill diameter and drill point

angle, both of which must be entered into the Tool Data Table (done by the operators). Failure toprovide the system with this data will result in a Tool Data Alert which places the machine in feedhold, not in cycle.

Hole depth is coded in the Z word of any fixed cycle. The value is always an incrementaldeparture from the R plane, and is independent of the absolute/incremental mode setting or thenon-modal machine coordinates function G98.

G80 - Cancel Fixed Cycle

This cycle is used to inhibit any Z axis feed motion. The Z word specifying hole depth is notused but remains in effect for subsequent G80 series cycles. Specific actions at the G80 cycle are:

1. Simultaneously rapid the non-spindle axes to their commanded positions.

2. Rapid the Z axis to the clearance plane.

G81 - Dril l Cyc le

Specific actions of the G81 cycle are:1. Simultaneously rapid the non-spindle axes to their commanded positions.2. Rapid the Z axis to the clearance plane.3. Feed to depth in Z plus the drill point length.4. Rapid Z back to the clearance plane and the additional W distance if programmed.

G84 - Tap

Specific actions of the G84 cycle are:

1. Simultaneously rapid the non-spindle axes to their commanded positions.2. Rapid the Z axis to the clearance plane.3. Feed to depth in Z.4. Reverse the spindle.5. Feed Z back to the clearance plane and Dwell for 0.5 sec. and then rapid the additional

W distance if programmed.6. Reverse the spindle.

The combination of programmed spindle speed and feed rate must match the lead of the tap:

Feed rate = spindle rpm lead


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G23 - Rectangu lar Pocket Mil l ing

WORDS USED IN A G23 BLOCK:

X : Coordinates of center point of the pocketY : (defaults of current axis position)

U : Incremental dimensions of the pocket

R Reference plane representing the unmachined surface of the workpieceZ The total depth of cut measured from the work surfaceI Corner Radius (must be greater than Cutter Radius)E Cycle typeK Cut depthP Width of cut, expressed as a percentage of the cutter diameter (defaults to 50%)

NOTE: Tool number, Feed rate, Spindle speed and M codes to be used for roughing areinherited from previous blocks.

CYCLE MOVEMENTS FOR RECTANGULAR POCKET MILLING G23:1. Simultaneously rapid the non-spindle axes to their calculated start positions.2. Rapid the Z axis to the clearance plane.3. Feed Z axis to cut depth (K), at 15% of inherited feed rate.4. Feed parallel to the longest side to produce a slot, of length such that remaining stock

is equal in both directions, at P % of inherited feed rate.

5. Feed towards shortest side by amount of cutter diameter P, or to the final boundary,whichever is smallest, at P% of inherited feed rate.

6. Corner Radii, if programmed, using the I word, will be generated so as to maintainequal stock conditions.

7. Repeat from step 5 until roughing at this cut depth is completed.8. If pocket not yet at full depth, retract Z axis and parallel with the longest side by gauge

height and XY clearance value, to establish a new clearance plane.9. Repeat from step 1 until full depth is reached.10.If cycle type is roughing only (E12) then cutter is retracted to programmed R plane or

W dimension, and the X, Y axes are positioned to the X, Y coordinates programmed inthe G23 block.

11.At completion, Z axis is retracted to the R plane and the X and Y axes are positionedto the programmed X and Y coordinates.


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Figure B4:Rectangular pocket milling.

Legend for Figure B4 - Rectangular pocket milling1. Calculated start point2. End point of initial slot3. Start and end point of 1st pass

4. Intermediate points during 1st pass CW or CCW movements depends upon SpindleDirection and climb or conventional Milling Requirement

G27 - Circular Frame Mil l ing Inside

WORDS USED IN A G27 BLOCK:

X Coordinates of center point of the frameY (Defaults of current axis position)U Diameter of PocketR Reference Plane representing the unmachined surface of the workpieceZ The total depth of cut measured from the work surface

J Stock to be removedE Cycle TypeK Cut depth of one cutP Width of cut, expressed as a percentage of the cutter diameter (Defaults to 50%)

NOTE: Tool number, Feed rate, Spindle speed and Misc. Functions used for roughing areinherited from previous blocks.


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CYCLE MOVEMENTS FOR INSIDE CIRCULAR FRAME MILLING G27:1. Simultaneously rapid the non-spindle axes to their calculated start positions.2. Rapid the Z axis to the clearance plane.3. Feed Z axis to cut depth.

4. Feed towards boundary by amount of twice the cutter diameter P %, or to final

boundary, whichever is the smallest. This is a semi-circular move at P% of inheritedfeed rate.5. Feed once around the frame, direction of cut is dependent upon Climb/Conventional

Milling Selection.

6. Feed out of cut by P cutter diameter, along a semi-circular path.7. Repeat from step 4 until roughing at this cut depth is completed.8. If frame is not yet to full depth then retract Z axis XY axes by gauge height and XY

clearance value, to establish a new clearance plane.9. Repeat from step 1 until full depth is reached.10.If cycle type is roughing only (E12) then cutter is retracted to the programmed R plane,

or W dimension, and the XY axes are positioned to the Y coordinates programmed in

the G27 block.11.At completion, Z axis is retracted to the R plane or W dimension if programmed, andthe X and Y axes are positioned to the programmed X and Y coordinates.


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Figure B5:Circular frame milling.

Legend for Figure B5 - Circular frame milling

1. Calculated Start Point2. Start/End Point for 1st pass

Overlap = P% of cutter diameterDirection of cutter path is dependent upon climb/conventional milling selection


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G37 - Cancel Pattern

When a G37 is programmed all pattern data is deleted. Subsequent G80 series blocks willproduce only a single hole in accordance with their specification.

G38 - Hole Pattern

Hole patterns, establish geometric patterns of holes and allow the G80 series of fixed cycles tobe performed on the active pattern. A pattern is modal until canceled by a cancel pattern code, datareset end of program or a (:) and this enables multiple G80 series operations to be carried out onthe active pattern without the need to reprogram the geometry.

A rectangular pattern is not referenced to any absolute dimensions although the incrementaldimensions between holes and between lines of holes are signed to establish the reference cornerof the pattern. The G80 series blocks, which do have absolute X Y dimensions, should use thesame reference corner as the start of the pattern. This enables the rectangular pattern to be repeatedat different locations on the work without the need to reprogram.

Words Used in the G38 block:

I Number of holes in line (max. 999)U Distance between holes in lineJ Number of lines (max. 999) (defaults to 1)

V Distance between linesO Angle of line

Use of Pattern Making Block:

1. Specify pattern by using above words. This information is stored by the control forfuture use with G80 series fixed cycles and is canceled by G37.

2. The signs of the U and V words in the G38 block determine the pattern reference corner.3. The X and Y coordinates programmed in a G80 series block SHOULD define the

position of the same reference corner, which will be the first hole in the pattern.4. Subsequent G80 series blocks, whilst the pattern is still active, SHOULD use the same

reference corner.5. At completion of each hole in the pattern, the Z axis will retract to the Reference (R)

Plane. At completion of the last hole in the pattern, the Z axis will retract to theReference (R) Plane.


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SECTION 4

GEOMETRIC DIMENSIONING AND TOLERANCING


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SECTION 4 GEOMETRIC DIMENSIONING AND TOLERANCING

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GEOMETRIC DIMENSIONING AND TOLERANCING1

INTRODUCTION

Geometric dimensioning and tolerancing (GD&T) is a means of specifying engineering designand drawing requirements with respect to actual "function" and "relationships" of part features.Furthermore, it is a technique which, properly applied, ensures the most economical and effectiveproduction of these features. Thus geometric dimensioning and tolerancing can be considered bothan engineering design drawing language and a functional production and inspection technique.Uniform understanding and interpretation among design, production and inspection groups are themajor objectives of the system.

The authorative document governing the use of geometric dimensioning and tolerancing in theUnited States is ANSI/ASME Y14.5M, "Dimensioning and Tolerancing". This standard is aconsolidation of several earlier standards and has the prime objective to provide a single standard

for practices in the United States and harmonize as much as possible USA practices withInternational Standards Organization (ISO) standards.

When should geometric dimensioning and tolerancing be used?

When part features are critical to interchangeability; When functional gaging techniques are desirable; When datum references are desirable to ensure consistency between design,

manufacturing, and verification operations; When CAD/CAM is to be used; When standard interpretation or tolerance is not already implied.

1 This teaching note is adapted from Lowell W. Foster, Geo-Metrics III, Addison-Wesley, 1994.


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GEOMETRIC DIMENSIONING AND TOLERANCING SECTION 4

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DEFINITIONS

Characteristics: Geometric characteristics, symbols, and terms used in GD&T are summarized inFigure 1. There are five types of geometric characteristics:

1. FORM tolerance - a form tolerance states how far an actual surface or feature is permittedto vary from the desired form implied by the drawing.

2. ORIENTATION tolerance - an orientation tolerance states how far an actual surface orfeature is permitted to vary relative to a datum or datums.

3. PROFILE tolerance - a profile tolerance states how far an actual surface or feature ispermitted to vary from the desired form on the drawing and/or vary relative to a datum ordatums.

4. RUNOUT tolerance - a runout tolerance states how far an actual surface or feature ispermitted to vary from the desired form implied by the drawing during full (3600) rotationof the part on a datum axis.

5. LOCATION tolerance - a location tolerance states how far an actual size feature is

permitted to vary from the perfect location implied by the drawing as related to a datum,or datums, or other features.

Kinds of Features:The geometric characteristics are also divisible into three "kinds" of featuresto which a particular characteristic is applicable:

1. INDIVIDUAL feature - a single surface, element, or size f

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