Fixtures 01.ppt

FIXTURES

1. Definition2. Introduction3. Purpose/Advantages4. Classification5. Design considerations6. Tooling cost7. Tooling operation8. Tooling details9. Materials

A fixture is a work-holding or support device used in the manufacturing industry. What makes a fixture unique is that each one is built to fit a particular part or shape. The main purpose of a fixture is to locate and in some cases hold a work piece during either a machining operation or some other industrial process.

2. INTRODUCTIONA jig differs from a fixture in that it guides the tool to its

correct position in addition to locating and supporting the work piece.

A fixture is a means through which a part is securely fastened to the machine tool table to accurately locate, support and hold the part during the machining operation.

In addition to the function of holding the work piece, the fixtures also provide for setting the cutting tool for the actual machining operation. Generally a fixture is supposed to be securely fastened to the machine tool table.

1. DEFINITION

http://en.wikipedia.org/wiki/Manufacturing

http://en.wikipedia.org/wiki/Machining

http://en.wikipedia.org/wiki/Jig_(tool)

Fixtures are widely used in large batch production to ensure the easy setup and achieving the desired accuracy. It can be used in a variety of machine tools such as Lathe, milling, grinding, etc. though the milling fixtures are the most widely used in view of the complex requirements for the milling operation.

2. INTRODUCTION

3. PURPOSE/ADVANTAGESThe primary purposes of jigs and fixtures is to: Reduce the cost of production Maintain consistent quality Maximize efficiency Enable a variety of parts to be made to correct specifications Reduce operator errors

CLASSIFICATION OF FIXTURES1.Based on standardisation2. Based on process3. Based on construction4. Based on production application

4. CLASSIFICATION

CLASSIFICATION - STANDARDISATIONA. General PurposeB. Special purpose

1. General PurposeThey are usually relatively inexpensive and can be used to hold a variety and range of sizes of work pieces (examples: Vises, chucks, split collets – Standard Fixtures).

In many instances, the shape of the part and the machining to be performed allow for the use of a general-purpose work holder such as a vise, collet, or chuck. These work holders are adaptable to different machines and many different parts. Since they are not part-specific, their versatility allows for repeated use on a variety of different or limited-production runs. The cost of these work holders would usually be averaged over years and might not even be a factor in job-cost calculations. The general-purpose nature of these work holders necessitates a higher level of operator care and attention to maintain consistency and accuracy. For these reasons, general-purpose work holders are not preferred for lengthy production runs.

A. General Purpose

B. Special Purpose They are designed and built to hold a particular work piece for a specific operation on a specific machine or process. Fixtures vary in design from relatively simple tools to expensive, complicated devices. Fixtures also help to simplify metalworking operations performed on special equipment. Fixtures have a much-wider scope of application than jigs. These work holders are designed for applications where the cutting tools cannot be guided as easily as a drill. A fixture can be used in almost any operation that requires a precise relationship in the position of a tool to a work piece.

CLASSIFICATION - STANDARDISATION

CLASSIFICATION - PROCESSFixtures are classified either by the machine they are used on, or by the process they perform on a particular machine tool.

More-common classifications of fixtures used for manufacturing:EXTERNAL-MACHINING APPLICATIONS: Flat-Surface Machining • Milling fixtures • Surface-grinding fixtures • Planing fixtures • Shaping fixtures Cylindrical-Surface Machining • Lathe fixtures • Cylindrical-grinding fixtures Irregular-Surface Machining • Band-sawing fixtures • External-broaching fixtures

CLASSIFICATION - PROCESS

Milling Fixtures:A Milling fixture is a work holding device which is firmly clamped to the table of the milling machine. It holds the work piece in correct position as the table movement carries it past the cutter or cutters.

Essentials of Milling Fixtures:A. Base:A heavy base is the most important element of a milling fixture. It is a plate with a flat and smooth under face. The complete fixture is built up from this plate. Keys are provided on the under face of the plate which are used for easy and accurate aligning of the fixture on the milling machine table. By inserting them into one the T slot in the table. These keys are usually set in keyways on the under face of the plate and are held in place by a socket head cap screw for end key. The fixture is fastened to the machine table with the help of two T bolts engaging in T slots of the work table.



B. Setting Blocks:After the fixture has been securely clamped to the machine table , the work piece which is correctly located in the fixture , has to be set in correct relationship to the cutters. This is achieved by the use of setting blocks and feeler gauges. The setting blocks is fixed to the fixture. Feeler gauges are placed between the cutter and reference planes on the setting block so that the correct depth of the cut and correct lateral setting is obtained. The block is made of hardened steel and with the reference planes(feeler surfaces) grooved. In it correct setting , the cutter should clear the feeler surfaces by at least 0.08cm to avoid any damage to the block when the machine table is moved back to unload the fixture. The thickness of the feeler gauge to be used should be stamped on the fixture base near the setting block.


FixturesMilling fixturesFixture componentsFixture design considerationsSequence in laying out a fixture

Fixture components

Fixture baseFixture components and the work piece are usually located on a base, which is securely fastened to the milling machine table.



3: Locating and Clamping Elements: The same design principles of location and clamping apply for milling fixtures have been discussed above.

Some Design Principles for Milling Fixtures:1: Pressure of cut should always be against the solid part of the fixture(fig a).2: Clamps should always operate from the front of the fixture (fig B)3: The work piece should be supported as near the tool thrust as possible(fig c)

2: Lathe Fixtures(Turning fixtures)The standard work holding devices or fixtures for lathe are: Three and four jaw chucks Collets Face plate Mandrels Milling viceIf the job can be held easily and quickly in the above mentioned standard devices, then there is no need for special work holding devices. However many jobs particularly casting and forging, because of their shapes, cannot be conveniently held by any of the standard devices. It then becomes necessary to build a special work holding device for the job. Such a device is called lathe fixture.

A lathe fixture consists of a base , location and clamping devices. A lathe fixture can be fixed to the lathe either by holding in the chuck jaws or fixing to a face plate.

Basic Design Principles for Turning or Lathe Fixtures:1. To avoid vibration while revolving , the fixture should be accurately balanced.2. There should be no projections of the fixture which may cause injury to the operator.3. The fixture should be rigid and overhang should be kept minimum possible so that there is no bending action.4. Clamps used to fix the fixture to the lathe should be designed properly so that they don’t get loosed by centrifugal force.5. The fixture should be as light weight as possible since it is rotating.6. The fixture must be small enough so that it can be mounted and revolved without hitting the bed of the lathe.

3:Grinding Fixtures:The work holding devices for grinding operations will depend upon the type of the grinding operation and the machine used.(A): Fixture for External Grinding:A mandrel is the most common fixture used for grinding external surface of the work piece, a mandrel is hardened and is held between centers of a machine. The mandrel is used for internal chucking or round work piece with bores. The work piece is located and held on the mandrel with the help of the bore so that the external surface may be machined truly concentric to the bore. The various types of mandrel are:

Taper Mandrel: In this type of mandrel, the outer chucking surface is given a slender taper of about 0.5mm per meter

Straight Mandrel: It differs from the taper mandrel in that it has straight or un tapered chucking surface.Combination taper and straight:In this type of mandrel , a portion of the outer diameter of the mandrel is straight and the rest of the is tapered.

C: Fixtures for Surface Grinding:The work piece can be held for machining on a surface grinder in the following waysIt may be clamped directly to the machine table or to an angle plate and so onIt may be held in a viceThe work piece may be held by means of a magnetic chuck or a vacuum chuck. Here the work piece is held without any mechanical clampingThe work piece may be held in a special fixture

B: Fixtures for Internal Grinding:For grinding internal surfaces of simple circular work piece, the chuck may be used as a standard work holding device. It required special jaws can be provided for the chuck. However , for many components special fixtures may have to be made which are designed on same lines, as the lathe fixtures.

Fixtures for Milling: Vise

Vise fixed to a milling table, holding rectangular part

V-slot vise jaws hold cylindrical parts horizontally/vertically

Vise fixed to a milling table, holding rectangular part

V-slot vise jaws hold cylindrical parts horizontally/vertically

Vise on sine-bar to hold part at an anglerelative to the spindle

Universal angle vise can index parts along any direction

Vise on sine-bar to hold part at an anglerelative to the spindle

Universal angle vise can index parts along any direction


Strap clamp

Clamp support(clamp and support have teeth)

Parallel bars raise the partabove table surface – allowmaking through holes

Bolt (bolt-head is inserted into T-slot in table)

Workpiece

Strap clamp

Clamp support(clamp and support have teeth)

Parallel bars raise the partabove table surface – allowmaking through holes

Bolt (bolt-head is inserted into T-slot in table)

Workpiece

Fixtures for Milling: ClampsCLASSIFICATION - PROCESS

3: Locating and Clamping Elements: The same design principles of location and clamping apply for milling fixtures have been discussed above

ClampsClamps counteract forces from the feed of the table and rotation of the cutter.

Set blocksCutter set blocks are mounted on the fixture to properly position the milling cutter in relation to the work piece.


INTERNAL-MACHINING APPLICATIONS: Cylindrical- and Irregular-Hole Machining • Drill jigs • Boring jigs • Electrical-discharge-machining fixtures • Punching fixtures • Internal-broaching fixturesNON-MACHINING APPLICATIONS: Assembly • Welding fixtures • Mechanical-assembly fixtures (Riveting, stapling, stitching, pinning, etc.)


• Soldering fixtures

NON-MACHINING APPLICATIONS: Inspection • Mechanical-inspection fixtures • Optical-inspection fixtures • Electronic-inspection fixturesFinishing • Painting fixtures • Plating fixtures • Polishing fixtures • Lapping fixtures • Honing fixtures


Miscellaneous • Layout templates • Testing fixtures • Heat-treating fixtures

CLASSIFICATION – CONSTRUCTION

Fixtures also may be identified by their basic construction features. For example, a lathe fixture made to turn radii is classified as a lathe radius turning fixture. But if this same fixture were a simple plate with a variety of locators and clamps mounted on a faceplate, it is also a plate fixture. Like jigs fixtures are made in a variety of different forms. While many fixtures use a combination of different features, almost all can be divided into five distinct groups. These include plate fixtures, angle plate fixtures, vise jaw fixtures, indexing fixtures, and multi part, or multi station

Plate fixtures, as their name implies, are constructed from a plate with a variety of locators, supports, and clamps (Figure 5-8). Plate fixtures are the most common type of fixture. Their versatility makes them adaptable for a wide range of different machine tools. Plate fixtures may be made from any number of different materials, depending on the application of the fixture.


The angle plate fixture (Figure 5-9) is a modified form of plate fixture. Here rather than having a reference surface parallel to the mounting surface, the angle plate fixture has a reference surface perpendicular to its mounting surface. This construction is very useful for those machining operations which are performed perpendicular to the primary reference surface of the fixture.


Vise jaw fixtures are basically modified vise jaw inserts which are machined to suit a particular work piece. In use, these modified vise jaws are installed in place of the Vise jaw fixtures are basically modified vise jaw inserts standard, hardened jaws normally furnished with milling machine vises. Vise jaw fixtures are the least expensive type of fixture to produce, and since there are so few parts involved, they are also the simplest to modify. Figure 5-1) shows several examples of parts which could easily be fixtured with this type of work holder. Indexing fixture, like indexing jigs, are used to reference work pieces which must have machine details located at prescribed spacings.


CLASSIFICATION – PRODUCTION APPLICATION Permanent and temporary work holders PERMANENT AND TEMPORARY WORKHOLDERSJigs and fixtures are most often found where parts are produced in large quantities, or produced to complex specifications for a moderate quantity. With the same design principles and logic, work holding devices can be adapted for limited-production applications. The major difference between permanent and temporary work holders is the cost/benefit relationship between the work holder and the process. Some applications require jigs and fixtures solely for speed; others require less speed and higher precision. The requirements of the application have a direct impact on the type of jig or fixture built and, consequently, the cost.

PERMANENT JIGS AND FIXTURES

Work holders for high-volume production are usually permanent tools. These permanent jigs and fixtures are most often intended for a single operation on one particular part. The increased complexity of permanent work holders yields benefits in improved productivity and reduced operator decision-making, which result in the tool having a lower average cost per unit or per run. Therefore, more time and money can be justified for these work holders. In the case of hydraulic or pneumatic fixtures, inherent design advantages can dramatically improve productivity and, hence, reduce per-unit costs even further, even though the initial cost to construct these fixtures is the most expensive of all fixture alternatives. In some cases, where machine-loading considerations are paramount, such as a pallet-changing machining center, even duplicate permanent fixtures may be justified.

CLASSIFICATION – PRODUCTION APPLICATION

Permanent jigs and fixtures are typically constructed from standard tooling components and custom-made parts. Figure 1-3 shows a typical permanent work holder for a drilling operation.

CLASSIFICATION – PRODUCTION APPLICATION

Figure 1-3. A permanent work holder used for a drilling operation.

5. DESIGN CONSIDERATIONSFixtures vary in design from relatively simple tools to expensive, complicated devices. Fixtures also help to simplify metalworking operations performed on special equipment.Fixtures have a much-wider scope of application than jigs. These work holders are designed for applications where the cutting tools cannot be guided as easily as a drill. With fixtures, an edge finder, center finder, or gage blocks position the cutter. The principal considerations when choosing among work holder varieties fall into three general categories: tooling cost, tooling details, and tooling operation. Although each of these categories is separated here, in practice they are interdependent. The following are some design differences and considerations for permanent, general-purpose, and modular work holders.

Work holders are sometimes designed to serve multiple functions. For example, it is possible to have a work holder that acts both as a drill jig and a milling fixture. These tools are called combination tools or multiple-function work holders. Figure 1-6 shows a typical temporary work holder for drilling and milling operations on the same part. In this example, since the work holder has provisions for both milling and drilling, it is classified as both a drill jig and milling fixture.

Figure 1-6. A combination drill jig/milling fixture used for both types of operations on the same part.

5. DESIGN CONSIDERATIONS

With temporary work holders, the design drawings are often sent to the tool room as simple freehand sketches. Permanent tools are normally designed for long-term use. This being the case, the drawings and engineering data for the permanent jig or fixture then become a permanent record. With modular work holders, the designer may either construct drawings or specify building the work holder directly around the part. Here only a parts list and photographs or video tape are kept as a permanent record. Certain work holding applications require special fixture characteristics. For example, a particularly corrosive environment may require stainless steel components and clamps to deliver a satisfactory life cycle. In other cases, variable work piece dimensions, as in a casting, necessitate clamping devices which can compensate for these variations. Appearance of a finished part might require the use of nylon, plastic, or rubber contact points to protect the part. Similarly, the selection of tooling details can enhance the productivity of some permanent tools.


They also typically reduce machine setup time, machine cycle time, and the level of operator skill required to produce satisfactory quality output. Over a long production run, or a series of runs in the life of a tool, the average cost of the tool per piece produced can be quite low.

General-purpose work holders are more expensive than temporary tools in most cases, but their utility and flexibility often allow these work holders to be regarded as a capital cost to be amortized over a period of time without regard to actual usage. Similarly, modular fixturing is typically a capital investment to be amortized over a set lifespan, with an average cost assigned to usage for each anticipated job. Another cost to be considered is work holder disposition. Permanent fixtures require storage and maintenance to keep them available for their next use.


6. TOOLING COSTS

6. TOOLING COSTS

General-purpose tools are reused extensively, but still incur some costs for maintenance and storage. Similarly, modular fixtures will be disassembled, and the components maintained, stored, and reused frequently. The total cost of any jig or fixture is frequently the major area of consideration in many work holder designs. Although initial cost is a major element, it should not be the basis for accepting or rejecting any tooling option. A more-proper economic evaluation of the work holder design takes into consideration many other factors. Permanent fixtures have distinct advantages in the production of high-volume and high-precision parts.

7. TOOLING DETAILS

Tooling details are the overall construction characteristics and special features incorporated into the jig or fixture. Permanent work holders are designed and built to last longer than temporary work holders. So, permanent jigs and fixtures usually contain more-elaborate parts and features than temporary work holders.

Other differences between permanent and temporary work holders include the type and complexity of the individual tooling elements, the extent of secondary machining and finishing operations on the tool, the tool-design process, and the amount of detail in the work holder drawings. Since the elements for modular work holders are usually part of a complete set, or system, only rarely will additional custom components need to be made.

Permanent work holders contain different commercial tooling components based on expected tool usage. Permanent jigs intended for a high-volume drilling operation, for example, often use a renewable bushing and liner bushing together. A throwaway jig for a smaller production run often uses a simple press-fit bushing. The secondary operations normally associated with tooling include hardening, grinding, and similar operations to finish the work holder. Usually, permanent work holders are hardened and ground to assure their accuracy over a long production run. Since they are intended only for short production runs, throwaway jigs and fixtures do not require these operations. Another secondary operation frequently performed on permanent tools, but not temporary tools, is applying a protective finish, such as black oxide, chrome plating, or enamel paint. In designing a permanent work holder, the designer often makes detailed engineering drawings to show the tool room exactly what must be done to build the work holder.

7. TOOLING DETAILS

For example, utilizing small hydraulic clamps may allow loading many parts on a work holder due to the compactness of the design. This would enhance productivity by reducing load/unload time as a percentage of total cycle time. Duplicate fixtures are sometimes justified for machining centers because they allow loading of parts on one pallet during the machining cycle on the other pallet.

7. TOOLING DETAILS

8. TOOLING OPERATION The performance of any work holder is critical to the complete usefulness of the tool. If the work holder cannot perform the functions desired in the manner intended, it is completely useless, regardless of the cost or the extent of the detail. As the performance of a permanent, modular, or general-purpose work holder is considered, several factors about the machine tools must be known. These factors include the type, size, and number of machine tools needed for the intended operations.

The primary advantage of modular fixtures is that a tool with the benefits of permanent tooling (setup reduction, durability, productivity improvements, and reduced operator decision-making) can be built from a set of standard components. The fixture can be disassembled when the run is complete, to allow the reuse of the components in a different fixture. At a later time the original can be readily reconstructed from drawings, instructions, and photographic records. This reuse enables the construction of a complex, high-precision tool without requiring the corresponding dedication of the fixture components. Figure 1-5 shows how modular fixturing fits into the hierarchy of work holding options, ranking below permanent fixturing yet above general-purpose work holders. Virtually every manufacturer has good applications for each of these three options at one time or another.

Figure 1-5. The hierarchy of work holding options.

MODULAR FIXTURESModular fixtures achieve many of the advantages of a permanent tool using only a temporary setup. Depicted in Figure 1-4, these work holders combine ideas and elements of permanent and general-purpose work holding. Figure 1-4. Modular

work holders combine ideas and elements of both permanent and temporary work holding to make inexpensive-yet-durable work holders.

Other machine considerations may come into play as well. On numerically controlled machines, for example, care must be taken in fixture design to position clamps out of the cutting tool's path. Pallet machines require different fixtures than other machines. Obviously, vertical mills would be tooled differently than horizontal mills. Likewise, the way parts are loaded onto the fixture has implications for fixture design. Despite the work holder design or the size of the production run, every jig or fixture must meet certain criteria to be useful. These criteria include accuracy, durability, and safety. Accuracy, with regard to jigs and fixtures, is the ability of a work holder to produce the desired result, within the required limits and specifications, part after part, throughout the production run. To perform to this minimum level of accuracy, the work holder must also be durable. So, the jig or fixture must be designed and built to maintain the required accuracy throughout the expected part production.

If part production is continuous, year after year, the jig or fixture must be more durable than is necessary for only one production run. The final consideration, safety, is actually the most important. No matter how good the design or construction, or how well it produces the desired accuracy, if the work holder is not safe, it is useless. Safety is a primary concern in the design of any work holder.

Safety, as well as speed and reliability of part loading, can often be improved by the use of power clamping, either pneumatic or hydraulic. Once set, power clamps will repeatedly clamp with the identical force. This is not always true with manual clamps, which depend on operator diligence for the proper application of clamping force. In addition, power-clamping systems can have interlocks to the machine control which will shut the machine down if the system loses power—a clear safety advantage for both operator and machine tool.

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Fixture for gauge components

Fixtures 01.ppt

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