Jis and Fixture Notes

JIGS AND FIXTURES DESIGN

JIGA Jig may be defined as a device which holds and locates a work piece, guides and controls one or more cutting tools.

FIXTUREA Fixture may be defined as a device which holds and locates a work piece during an inspection or for a manufacturing operation. The Fixture does not guide the tool.

DEFINITIONS

The following are the advantages of employing Jigs and Fixtures in mass production work.

(1) It eliminates the marking out, measuring, and other setting methods before machining.

(2) It increases the machining accuracy, because the work piece is automatically located and the tool is guided without making any manual adjustment.

(3) It enables production of identical parts which are interchangeable. This facilitates the assembly operation.

ADVANTAGES OF EMPLOYING JIGS AND FIXTURES

(4) It increases the production capacity by enabling a number of work pieces to be machined in the single set up

(5) It reduces the operator's labour and consequent fatigue as the handling operations are minimised and simplified.

(6) It enables semi-skilled operator to perform the operations as the setting operation of the tool and the works are mechanised. This saves labour cost.

(7) It reduces the expenditure on the quality control of the finished products.

(8) It reduces the overall cost of machining by fully or partly automatising the process.

The following are the fundamental differences between a Fixture with a Jig

(1) A Fixture holds and position the work but does not guide the cutting tool, whereas a Jig holds, locates and as well as guides the tool.

(2) The Fixtures are generally heavier in construction and are bolted rigidly on the machine table, whereas the Jigs are made lighter for quicker handling, and clamping with the table is often unnecessary.

(3) The fixtures are employed for holding work in Milling, Grinding, Planning, or Turning operations, whereas the Jigs are used for holding the work and guiding the tool particularly in Drilling, Reaming or Tapping operations.

DIFFERENCE BETWEEN FIXTURE WITH JIG

PRINCIPLES OF JIGS AND FIXTURES DESIGN

The following are the essential factors which must be considered in designing a Jig or a Fixture Study of the

(1) Study of the component/work piece.(2) Type and Capacity of the machine.(3) Locating elements.(4) Loading and unloading arrangement.(5) Clamping arrangement.(6) Power devices for operating the clamping elements.(7) Clearance required between the Jig and the component.(8) Indexing devices.(9) Tool guiding and cutter setting elements.(10 Fool-proofing arrangement.(11) Ejecting devices. (12) Swarf removal arrangement.(13) Rigidity and vibration problem.(14) Table fixing arrangement.(15) Safety devices.(16) Methods of manufacture of the jig base, body or frame.

Generally, all the jigs and fixtures consist of

LOCATING ELEMENTS: These position the work piece accurately with

respect to the tool guiding or setting elements in the fixture.

CLAMPING ELEMENTS: These hold the work piece securely in the

located position during the operation.

TOOL GUIDING AND SETTING ELEMENTS: These aids are used in

guiding or setting of the tools in correct position with respect to the work

pieces, e.g. drill bushes guide the drills accurately onto the work piece.

ELEMENTS OF JIGS AND FIXTURES

Generally, all Jigs / fixtures consist of the following elements

LOCATORS

A locator is usually a fixed component of a fixture. It is used to establish

and maintain the position of a part in the fixture by constraining the

movement of the part. For work pieces of greater variability in shapes

and surface conditions, a locator can also be adjustable.

CLAMPS

A clamp is a force-actuating mechanism of a fixture. The forces exerted

by the clamps hold a part securely in the fixture against all other external

forces.

ELEMENTS OF JIGS AND FIXTURES

SUPPORTS

A support is a fixed or adjustable element of a fixture. When severe part

displacement/deflection is expected under the action of imposed clamping

and processing forces, supports are added and placed below the work

piece so as to prevent or constrain deformation. Supports in excess of

what is required for the determination of the location of the part should be

compatible with the locators and clamps.

FIXTURE / BASE BODY

Fixture body, or tool body, is the major structural element of a fixture. It

maintains the spatial relationship between the fixturing elements

mentioned above, viz., locators, clamps, supports, and the machine tool

on which the part is to be processed.

ELEMENTS OF JIGS AND FIXTURES

FIXTURE / JIG DESIGNThe following outputs are included in the jig / fixture plan Fixture / Jig type and Complexity Number of work pieces per fixture Orientation of work piece within Jig / fixture Locating datum faces Clamping surfaces Support surfaces, if any

The following outputs are included in the fixture / Jig layout Positions of locators Positions of clamps Positions of supports, if any Type of locators Type of clamps Type of supports Clamping forces and sequence

Detailed design of locators Detailed design of clamps Detailed design of supports, if any

FIXTURE DESIGN

JIGS AND FIXTURE DESIGN FLOW CHART

DEGREES OF FREEDOM OF A WORK PIECE

Three rectilinear displacements along the mutually orthogonal

co-ordinate axes

Three angular displacements with respect to the same axes

3-2-1 PRINCIPLE OF LOCATION

Three supports restricting five degrees of freedom.

As shown in figure, three locators, or supports, are placed under the

work piece. The three locators are usually positioned on the primary

locating surface. This restricts axial movement downward, along the z -

axis (+) and radially about the x (+ and -) and y axes (+ and -).

3-2-1 PRINCIPLE OF LOCATION

Z +

Rotation Y + -

Rotation X + -

X

Y

Two supports restricting three degrees of freedom.

The next two locators are normally placed on the secondary locating

surface, They restrict an additional three degrees of freedom by arresting

the axial movement along the y-axis (+) and the radial movement about

the z-axis (+ and -).

3-2-1 PRINCIPLE OF LOCATION

X

Y

Y + Rotation Z + -

Total Six locators restricts nine degrees of freedom,

completing 3-2-1 locations.

The final locator, is positioned at the end of the part. It restricts the axial

movement in one direction along the x-axis (+). Together, these six

locators restrict a total of nine degrees of freedom. The remaining three

degrees of freedom (-x, -y, and -z) will be restricted by the clamps.

3-2-1 PRINCIPLE OF LOCATION

X

Y

X +

Final One support restricting one degree of freedom.

Based on the surface from which the work piece is located, the locators are classified in to three groups

CYLINDRICAL SURFACE Conical locator. Cylindrical locator. Female locator. Diamond locator. Mandrel locator. Collet locator. Vee locator - Screw adjusted V locator.

Cam operated V locator.

Quick action V locator.

PLANE SURFACE Adjustable locator. Spring loaded pad. Equalizer rocker locator.

PROFILE SURFACE Six point locator. Profile location by pins. Eccentric locator. Nest & cavity locator.

TYPES OF LOCATORS

ADJUSTABLE LOCATORS

Adjustable locators require less precision to mount on the tool body. For a typical work holder, they are mounted at approximately the correct position and then adjusted to the exact location.

In the design of an adjustable locator, four factors should be considered :

1) the stability and precision of the location;

2) the ability to compensate for variance in work piece sizes and locating areas

3) the ability to compensate for wear on the locating element

4) the ability to maintain the fixed location points.

The main disadvantage of adjustable locators is that the operator has the most responsibility of ensuring accuracy and precision, not the tool builder.

SPRING-LOADED LOCATORS

Spring-loaded locating help ensure the correct by pushing the work piece against the fixed locating

SIX POINT LOCATOR

Equalizing supports are a form of adjustable support used when a

compensating support is required. Although these supports can be fixed in

position, in most cases equalizing supports float to accommodate work piece

variations. As one side of the equalizing support is depressed, the other side

raises the same amount to maintain part contact. In most cases adjustable

and equalizing supports are used along with solid supports.

EQUALIZING SUPPORTS

PROFILE LOCATION BY PINS

For regular or complex shapes, a nesting locator can also be cast from plastic compounds or low-melting-point alloys.

Nesting locator completely locates the outside surface of a work piece. it can be machined if the part shape is simple.

NESTING

CONICAL LOCATORS

The main problem with using this type of locator is in the position of the work piece relative to the work holder. The locating hole can contact the locator at any point on its conical surface. So slight differences in the hole diameter affect the work piece height, as illustrated in the figure This vertical height variation must be considered in the design of the work holder.

Round pins can be used for both internal and external work piece location. For internal location, the diameter of the pin must match the size of the locating hole. These locators come in many standard sizes, and are readily available,

For external location, the size of the locating pin is not as critical as internal. Here, a standard pin size strong enough to resist machining forces is the best choice.

These pins have a shank larger than the head. The purpose of the shoulder is to prevent the pin from being pushed into the tooling plate

Unlike the plain pins, shoulder-type pins are made in two styles Press-fit type and lock screw type,

ROUND LOCATING PINS

Vee locators are a specialized form of locating element. They are used mainly for round or cylindrical work pieces. The two basic styles of Vee locators are the Pad and the Vee Block as shown in the figure.

Vee locators can be used to locate a variety of part shapes

Vee pads are well suited for corner mounting a square or rectangular work piece.

VEE LOCATORS

DIAMOND LOCATING PINS

Another style of locating pin frequently seen in Jig-and-Fixture design is the diamond, or relieved locating pin. Like round locating pins, diamond pins are available in either the plain or shoulder-type, as shown in figure. These locating pins are the most-common form of relieved locating pin in work holders. To limit the pin's contact area, the diamond locating pin is made with four machined flats. The exact width of the contact area varies with the size of the pin, and is usually equal to one-third of the diameter on each side.

Diamond pins are generally used as shown in figure. Here the diamond pin acts as a radial locator to restrict movement of the work piece around the concentric locator, shown by the round-pin locator. The diamond pin is positioned to restrict the radial movement of the part. Since a diamond pin locates in only one axis, the contact areas of the pin must be positioned as shown in the figure. Positioning the pin any other way would allow the part to move about the concentric locator.

DIAMOND PIN CALCULLATION

G

B

H

F -0.008

Another condition to avoid in work holder design is redundant or duplicate location. Redundant locators restrict the same degree of freedom more than once.

The work pieces in figure, show several examples. The part at (a) shows how a flat surface can be redundantly located. The part should be located on only one not both side surfaces. Since the sizes of parts can vary, within their tolerances, the likelihood of all parts resting simultaneously on both surfaces is remote. The example at (b) points out the same problem with concentric diameters. Either diameter can locate the part, but not both.

REDUNDANT LOCATION

Fool proofing prevents improper loading of a work piece. The problem is most prevalent with parts that are symmetrical or located concentrically.

The simplest way to foolproof a work holder is to position one or two pins in a location that ensures correct orientation. With some work pieces, however, more-creative approaches to fool proofing must be taken as shown in figure

Fool proofing the locating prevents improper work piece loading

FOOL PROOFING

CLAMPING

BASIC PRINCIPLE OF CLAMPING

Once a work piece is located, it is necessary to press it against the locating surfaces and hold it there against the forces acting upon it. The tool designer refers to this action as Clamping and the mechanisms used for this action are known as Clamps.

However, simple or complex, all clamps must fulfill four essential requirements:

1) The work piece must be held rigidly while the cutting tools are in operation.

2) The time required for loading and unloading the tool must be as short as possible, which means the clamping device must be quick-acting.

3) when subjected to vibration, chatter, or heavy pressure, the clamping must be positive.

4) The clamp must not damage the work piece.

TYPES OF CLAMPS:

STRAP CLAMPS

Eccentric cams should usually not be used for clamping because they do not provide positive locking.

CAM CLAMPS :

Cam clamps are also based on the principle of the inclined plane. The most common forms of Cam Clamps are the Eccentric Cam and Spiral Cam. Depending on their arrangement, these cams can be used as Direct pressure Clamps or Indirect pressure Clamps.

Spiral cams have a continious rise to lock positively at any point in the clamping range

A toggle action locks positively because one of the pivots moves past

the center line of two other pivots, against a stop.

hold-down

push/pull,

latch

squeeze action.

The four basic Toggle actions are as shown below

LATCH CLAMPS :

The Latch Clamp is employed to clamp a job by a Latch. A pivoted latch and leaf is used for this. The leaf is closed on the job and is kept in position by the Latch. The work is gripped by the spring fitted to the leaf. To unload the work piece the laid end of the latch is pushed by the hand to cause the leaf to swing open as shown in the Figure.

ADVANTAGE :

The main advantage is the ease and speed of manipulation .

DISADVANTAGE :

1.Construction is limited to relatively light work.

2.Difficult to secure rigid clamping.

SCREW CLAMPS :

Screw clamps are among the simplest and least-expensive. Screw clamps offer the designer more clamping options than many other clamps. For clamping effectiveness and size/force ratio, screw clamps make excellent work holders as shown in the figure-23a. But they also have a drawback; their inherently slow clamping speed limit their use in high-production Jigs and Fixtures.

QUICK ACTING SCREWS AND KNOBS :

Quick-acting knobs are another variation of quick-acting threaded fastener.

TOOL GUIDING ELEMENTS

The cutter is set relative to the work in a fixture by adjusting the machine or by using the cutter Setting Block. The tools are guided in Jigs by Drill Bushes which are fitted on the jig plates. There are three different types of Jig bushes;

Slip Bush, (Fixed and Renewable) and Liner Bush

FIXED BUSH :

Fixed bushes are most common and least expensive, permanent bushes.These are press fitted and are designed for one step operation such as Drilling or Reaming. The bushings are pressed directly into the jig plate. They are held in plates by the force of press fit as shown in the figure

FIXED RENEWABLE BUSH :

Fixed renewable bushings are typically employed in long production runs where bushing changes are needed. The fixed renewable bush is intended for single step applications such as Drilling or Reaming. These bushings are changed only when the bushings wear. Fixed renewable bushings are held in space with a lock screw which prevents any movement of the bush during machining cycle as shown in the figure.

SLIP RENEWABLE BUSH:

These are convenient for applications when multiple operations are performed in same hole. One example is Drilling and Reaming the same hole.The first Slip Renewable bush is installed and the hole is drilled. The drill bush is removed. Then the reaming bush is installed and the hole reamed to size. The Slip Renewable side allows rapid change over. The bush is rotated clockwise to lock it in place and rotated counterclockwise for removal as shown in figure

DRILL JIGS

1.TEMPLATE JIG :

Template Jigs are normally used for accuracy rather than speed. This type of Jig fits over, on, or into the work and is not usually clamped. Templates are the least expensive and simplest type of Jig to use. They may or may not have bushings, when bushings are not used, the whole jig plate is normally hardened.

2. PLATE JIG :

Plate Jigs are similar to Templates. The only difference is that Plate Jigs have built-in-drive clamps to hold the work. These Jigs can also be made with or without bushings, depending on the number of parts to be made. Plate Jigs are sometimes made with legs to raise the jig off the table for large work. This style is called a Table Jig.

3. SANDWICH JIGS :

Sandwich Jigs are a form of plate jig with a back plate. This type of jig is ideal for thin or soft parts which could bend or warp, in other type of Jig. Here again, the use of bushings is determined by the number of parts to be made.

4. ANGLE-PLATE JIGS :

Angle-Plate Jigs are used to hold parts which are machined at right angles to their mounting locators. Pulleys, collars and gears are some of the parts which use this type of Jig as shown in the figure.

A variation of this jig is the modified Angle-Plate Jig. This jig is used for machining angles other than 90 degrees as shown in the figure.

05. BOX JIG or TUMBLE JIGS:

 These usually totally surround the part. This style of jig allows the part to be completely machined on every surface without repositioning the work in the Jig.

06. CHANNEL JIGS :

Channel Jigs are the simplest form of Box Jig. The work is held between two sides and machined from the third. In some cases, where jig feet are used, the work can be machined on three sides.

07. LEAF JIGS :

Leaf Jigs are small box jigs with a hinged leaf to allow for easier loading and unloading. The main differences between Leaf and Box Jigs are size and part location. Leaf Jigs are normally smaller than Box Jigs. Sometimes they are made so that they do not completely surround the part. Leaf Jigs are usually equipped with a handle for easier movement.

01.Work

03.Drill bush

10.Button

11.Hinge pin

12.Set screw

13.Leaf.

14.Leaf clamping screw.

08. INDEXING JIGS :

Indexing Jigs are used for circular hole patterns in which the part is indexed successively to the different positions under a single bushing. The location of the holes may be taken from the first hole drilled, or from other holes in the part. Sometimes an indexing plate or device is incorporated in the Jig.

09. TRUNION JIGS :

Trunion Jigs are a form of rotary Jig for very large or odd-shaped parts. The part is first put into a box-type carrier and then loaded on precision bearing mounts called Trunnions. This Jig is well suited for large, heavy parts which must be machined with several separate Plate-Type Jigs.Trunnion jigs allow easily turning a large part to work on all sides.

10. PUMP JIGS:

Pump Jigs are commercially made Jigs which must be adopted by the user. The lever activated plate makes this tool very fast to load and unload. Since the tool is already made and only needs to be modified, a great deal of time is saved by using this Jig.

11. MULTISTATION JIGS :

Multistation Jigs, are for repetitive simultaneous operations on several identical parts. In most cases, almost any Jig may be used with a multistation arrangement. The unique feature of a Multistation Jig is the way the Jigs are mounted and arranged with respect to the machining stations. In this example, the jig has four stations: #1 is the load/unload station; #2 is for Drilling; #3 is the Reaming station; #4 is where the work piece is counter bored. An indexing arrangement is also included with this Jig to accurately position the jigs at each station.

FIXTURE

PLATE FIXTURE :

The Plate Fixture is the most-basic and most-common fixture. The Plate Fixture is built with a mill fixture base, cast flat section, tooling plate, or similar plate material. All locators, supports and clamps are mounted directly to the plate. A complete plate fixture can be built using only standard, off-shelf components.

Plate fixtures usually hold a work piece parallel to the machine table

ANGLE-PLATE FIXTURES : 

Angle-Plate Fixtures are a variation of the basic Plate Fixture. They are useful when the locating surface is at an angle to the machine table. The two main variations of Angle-Plate Fixtures are the right angle and modified-angle plate fixtures. Right-angle Plate Fixtures, are constructed at 90 degrees to the base. The right-angle plate fixtures can be built with tooling blocks, T-cast sections, L-cast sections, angle brackets, or any comparable material. Adjustable angles or sine plates may be used to build the modified-angle plate fixtures.

TURNING FIXTURE:

 Commonly used work holding devices as fixtures in Turning are Soft Jaws, Chucks,(as Three Jaw and Four Jaw Chuck), Face Plate etc.

Turning Fixtures are used for complicated work pieces and are in effect simplification of the technique of butting the work piece to a Face Plate.

The Fixture body is located on the machine spindle and bolted in position. It carries the work piece location and clamping system. Sometimes the work piece is located and clamped to a separate shaft that projects from the fixture body.

When the weight of the work piece and the fixture are imbalanced, the balancing weight may be incorporated.

A setting face machined relative to the location system and a typical hardened setting piece is used to set the depth of the tool.

A pilot bush may be used to guide the boring bar.

MILLING FIXTURE :

 A Milling Fixture is located accurately on the machine table and then bolted in position; the table is positioned relative to the cutter or cutters with the aid of the Setting Block. The work piece is located on the Fixture base and then clamped in position. The cutter is not guided during cutting.

The location and clamping systems are similar to those used for drill jigs, but as the cutting forces are high, interrupted, and tend to lift the work piece, the clamping forces must be big; hexagonal nuts are usually used to clamp the work piece rather than hand nuts. The details that are peculiar to Milling Fixtures are the Setting Block and the Location Tenons.

Setting Block :

The setting block as shown in the figure, is located relative to the fixture location system and retained in position by screws; it has two hardened setting faces, so that the table can be positioned both horizontally and vertically (note that the table is located from one side of the cutter only). The table setting is done with a 0.25 mm feeler between the cutter and the setting face so that the block will not be damaged by the cutter during machining. The setting block is positioned so that the cutter is between the vertical face of the block and the operator during setting; this arrangement gives maximum convenience for the operator during setting.

Tenons :

The two Tenons (as shown in the figure) are made from case hardened steel and are located on the underside of the fixture base; these two tenons sit in one of the tee slots that run along the length of the machine table so that the fixture is located relative to the table feed; the two Tenons should be as far apart as possible, to produce maximum accuracy. The Fixture is bolted to the table by two or four tee bolts that are placed in the tee slots (these bolts are not called up as part of the fixture).

Example - 1