Jig and fixtures

OBJECTIVES

After completing this unit, the student should beable to:

• Identify the classes of jigs and fixtures.• Identify the types of jigs and fixtures.• Choose a class and type of jig and fixture for

selected operations on sample parts.

JIGS AND FIXTURES

Jigs and fixtures are production-workholding devicesused to manufacture duplicate parts accurately. Thecorrect relationship and alignment between the cutter,or other tool, and the workpiece must be maintained.To do this, a jig or fixture is designed and built to hold,support, and locate every part to ensure that each isdrilled or machined within the specified limits.

Jigs and fixtures are so closely related that theterms are sometimes confused or used interchangeably.The difference is in the way the tool is guided to theworkpiece.

A jig is a special device that holds, supports, or isplaced on a part to be machined. It is a productiontool made so that it not only locates and holds theworkpiece but also guides the cutting tool as the oper-

ation is performed. Jigs are usually fitted with hard-ened steel bushings for guiding drills or other cuttingtools (Figure 2–1A).

As a rule, small jigs are not fastened to the drillpress table. If, however, holes above .25 inch in diam-eter are to be drilled, it is usually necessary to fastenthe jig to the table securely.

A fixture is a production tool that locates, holds,and supports the work securely so the requiredmachining operations can be performed. Set blocksand feeler or thickness gauges are used with fixturesto reference the cutter to the workpiece (Figure 2–1B).A fixture should be securely fastened to the table ofthe machine upon which the work is done. Thoughlargely used on milling machines, fixtures are alsodesigned to hold work for various operations on mostof the standard machine tools.

Fixtures vary in design from relatively simpletools to expensive, complicated devices. Fixtures alsohelp to simplify metalworking operations performedon special equipment.

CLASSES OF JIGS

Jigs may be divided into two general classes: boringjigs and drill jigs. Boring jigs are used to bore holesthat either are too large to drill or must be made anodd size (Figure 2–2). Drill jigs are used to drill,

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UNIT 2

Types and Functions of Jigs and Fixtures

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ream, tap, chamfer, counterbore, countersink, reversespotface, or reverse countersink (Figure 2–3). Thebasic jig is almost the same for either machining oper-ation. The only difference is in the size of the bushingsused.

TYPES OF JIGS

Drill jigs may be divided into two general types, openand closed. Open jigs are for simple operations wherework is done on only one side of the part. Closed, or

box, jigs are used for parts that must be machined onmore than one side. The names used to identify thesejigs refer to how the tool is built.

Template jigs are normally used for accuracy ratherthan speed. This type of jig fits over, on, or into the workand is not usually clamped (Figure 2–4). Templates arethe least expensive and simplest type of jig to use. Theymay or may not have bushings. When bushings are notused, the whole jig plate is normally hardened.

UNIT 2 Types and Functions of Jigs and Fixtures 9

Figure 2–1 Referencing the tool to the work.

Figure 2–2 Boring jig.


10 SECTION I Basic Types and Functions of Jigs and Fixtures

Figure 2–3 Operations common to a drill jig.

Figure 2–4 Template jigs.


Plate jigs are similar to templates (Figure 2–5).The only difference is that plate jigs have built-inclamps to hold the work. These jigs can also be madewith or without bushings, depending on the numberof parts to be made. Plate jigs are sometimes madewith legs to raise the jig off the table for large work.This style is called a table jig (Figure 2–6).

Sandwich jigs are a form of plate jig with a backplate (Figure 2–7). This type of jig is ideal for thin orsoft parts that could bend or warp in another style ofjig. Here again, the use of bushings is determined bythe number of parts to be made.

Angle-plate jigs are used to hold parts that aremachined at right angles to their mounting locators(Figure 2–8). Pulleys, collars, and gears are some ofthe parts that use this type of jig. A variation is themodified angle-plate jig, which is used for machiningangles other than 90 degrees (Figure 2–9). Both ofthese examples have clearance problems with the cut-ting tool. As the drill exits the product being drilled,it has little or no room for the drill point to clear theproduct completely, produce a round hole all the waythrough the part wall, and avoid drilling the part loca-tor. This is most noticeable in Figure 2–9, where anangled hole requires additional clearance to therelieved portion of the part locator. Additional clear-ance here would allow the drill to complete the holeand avoid drilling the relieved portion of the locator.The part locator will most likely be hardened and the


Figure 2–5 Plate jig.

Figure 2–6 Table jig.


drill will be lost as a result of any attempted drilling.Additional clearance on the relieved diameter of thepart locator may be possible. A larger clearance holein the locator could also be added if the relieveddiameter cannot be reduced. The additional designconsideration added to the locator would include thefeature to provide the correct orientation of this clear-ance hole or machined relief to line up with the bush-ing location.

Box jigs, or tumble jigs, usually totally surroundthe part (Figure 2–10). This style of jig allows thepart to be completely machined on every surfacewithout the need to reposition the work in the jig.

Channel jigs are the simplest form of box jig (Figure2–11). The work is held between two sides andmachined from the third side. In some cases, where jigfeet are used, the work can be machined on three sides.

Leaf jigs are small box jigs with a hinged leaf toallow for easier loading and unloading (Figure 2–12).The main differences between leaf jigs and box jigsare size and part location. Leaf jigs are normallysmaller than box jigs and are sometimes made so thatthey do not completely surround the part. They areusually equipped with a handle for easier movement.

Indexing jigs are used to accurately space holesor other machined areas around a part. To do this, the


Figure 2–7 Sandwich jig.

Figure 2–8 Angle-plate jig. Figure 2–9 Modified angle-plate jig.


jig uses either the part itself or a reference plate and aplunger (Figure 2–13). Larger indexing jigs are calledrotary jigs.

Trunnion jigs are a form of rotary jig for verylarge or odd-shaped parts (Figure 2–14). The part isfirst put into a box-type carrier and then loaded on thetrunnion. This jig is well suited for large, heavy partsthat must be machined with several separate plate-type jigs.

Pump jigs are commercially made jigs that must beadapted by the user (Figure 2–15). The lever-activatedplate makes this tool very fast to load and unload. Sincethe tool is already made and only needs to be modified,a great deal of time is saved by using this jig.

Multistation jigs are made in any of the formsalready discussed (Figure 2–16). The main feature ofthis jig is how it locates the work. While one part is

drilled, another can be reamed and a third counter-bored. The final station is used for unloading the fin-ished parts and loading fresh parts. This jig iscommonly used on multiple-spindle machines. Itcould also work on single-spindle models.

There are several other jigs that are combinationsof the types described. These complex jigs are often sospecialized that they cannot be classified. Regardless ofthe jig selected, it must suit the part, perform the opera-tion accurately, and be simple and safe to operate.

TYPES OF FIXTURES

The names used to describe the various types of fix-tures are determined mainly by how the tool is built.Jigs and fixtures are made basically the same way asfar as locators and positioners are concerned. Themain construction difference is mass. Because of theincreased tool forces, fixtures are built stronger andheavier than a jig would be for the same part.

Plate fixtures are the simplest form of fixture(Figure 2–17). The basic fixture is made from a flatplate that has a variety of clamps and locators to holdand locate the part. The simplicity of this fixturemakes it useful for most machining operations. Itsadaptability makes it popular.

The angle-plate fixture is a variation of the platefixture (Figure 2–18). With this tool, the part is nor-mally machined at a right angle to its locator. While


Figure 2–11 Channel jig.

Figure 2–10 Box or tumble jig.


most angle-plate fixtures are made at 90 degrees,there are times when other angles are needed. Inthese cases, a modified angle-plate fixture can beused (Figure 2–19).

Vise-jaw fixtures are used for machining smallparts (Figure 2–20). With this type of tool, the stan-dard vise jaws are replaced with jaws that are formedto fit the part. Vise-jaw fixtures are the least expen-


Figure 2–13 Indexing jig.

Figure 2–12 Leaf jig.


sive type of fixture to make. Their use is limited onlyby the sizes of the vises available.

Indexing fixtures are very similar to indexing jigs(Figure 2–21). These fixtures are used for machiningparts that must have machined details evenly spaced.

The parts shown in Figure 2–22 are examples of theuses of an indexing fixture.

Multistation fixtures are used primarily for high-speed, high-volume production runs, where themachining cycle must be continuous. Duplex fixturesare the simplest form of multistation fixture, usingonly two stations (Figure 2–23). This form allows theloading and unloading operations to be performedwhile the machining operation is in progress. Forexample, once the machining operation is complete atstation 1, the tool is revolved and the cycle is repeatedat station 2. At the same time, the part is unloaded atstation 1 and a fresh part is loaded.

Profiling fixtures are used to guide tools formachining contours that the machine cannot normallyfollow. These contours can be either internal or exter-nal. Since the fixture continuously contacts the tool,an incorrectly cut shape is almost impossible. Theoperation in Figure 2–24 shows how the cam is accu-rately cut by maintaining contact between the fixtureand the bearing on the milling cutter. This bearing isan important part of the tool and must always beused.


Figure 2–15 Pump jig.

Figure 2–14 Trunnion jig.



Figure 2–17 Plate fixture.

Figure 2–16 Multistation jig.


CLASSIFICATION OF FIXTURES

Fixtures are normally classified by the type ofmachine on which they are used. Fixtures can also beidentified by a subclassification. For example, if afixture is designed to be used on a milling machine, itis called a milling fixture. If the task it is intended toperform is straddle milling, it is called a straddle-milling fixture. The same principle applies to a lathefixture that is designed to machine radii. It is called alathe-radius fixture.


Figure 2–18 Angle-plate fixture.

Figure 2–19 Modified angle-plate fixture.

Figure 2–20 Vise-jaw fixture.



Figure 2–22 Parts machined with an indexing fixture.

Figure 2–23 Duplex fixture.

Figure 2–21 Indexing fixture.


The following is a partial list of production oper-ations that use fixtures:

Assembling LappingBoring MillingBroaching PlaningDrilling SawingForming ShapingGauging StampingGrinding TappingHeat treating TestingHoning TurningInspecting Welding

SUMMARY

The following important concepts were presented inthis unit:

• Jigs and fixtures are production workholdingdevices designed to hold, support, and locate aworkpiece.– A jig guides the cutting tool with a drill bushing.– A fixture references the cutting tool with a set

block and feeler, or thickness gauges.• Jigs are divided into two general classes: drill jigs

and boring jigs.

• The type of jig is determined by how it is built.The two types of jigs are open and closed.– Template, plate, table, sandwich, and angle-

plate jigs are all open jigs.– Box, channel, and leaf jigs are all closed jigs.

• Other variations, such as indexing, rotary, trun-nion, pump, and multistation jigs, are made aseither open or closed jigs.

• Fixture types are determined by the way they arebuilt. The most common types are plate, angle-plate, vise-jaw, indexing, and multistation fixtures.

• Fixture classes are determined by the machinetools on which they are used and sometimes by theoperations performed. A fixture used for a straddle-milling operation is classed as a mill fixture, but itmay also be classed as a straddle-milling fixture.

REVIEW

1. What is the difference between a jig and a fixture?2. How are jigs and fixtures normally identified?3. What are set blocks used for?4. What class of jig would normally be used to tap

holes?5. A gang-milling fixture is actually what class of tool?6. Analyze the following part drawings and opera-

tions to be performed and select the best jig orfixture for each.A. Figure 2–25. Operation: Mill a slot .250 inch

by .250 inch.1. Box fixture2. Duplex fixture3. Vise-jaw fixture


Figure 2–25

Figure 2–24 Profiling fixture.


D. Figure 2–28. Operation: Drill four holes, .50inch in diameter.1. Box jig2. Angle-plate jig3. Template jig


C. Figure 2–27. Operation: Drill four holes (two.62-inch and two .25-inch).1. Channel jig2. Plate jig3. Box jig

Figure 2–28

Figure 2–29

Figure 2–27

B. Figure 2–26. Operation: Drill four .500-inch-diameter holes.1. Plate jig2. Angle-plate jig3. Channel jig

Figure 2–26

E. Figure 2–29. Operation: Mill a shoulder .75inch by .75 inch by .38 inch.1. Plate fixture2. Angle-plate fixture3. Indexing fixture


Engineering

Jig and fixtures