To get the greatest benefit from jigs and fixtures, a basic understanding of their construction is necessary. Jigs and fixtures are identified one of two ways: either by the machine with which they are identified or by their basic construction. A jig, for instance, may be referred to as a “drill jig.” But if it is made from a flat plate, it may also be called a “plate jig.” Likewise, a mill fixture made from an angle plate may also be called an “angle-plate fixture” The best place to begin a discussion of jig and fixture construction is with the base element of all workholders, the tool body.
The tool body provides the mounting area for all the locators
, and other devices that position and hold the workpiece. The specific design and construction of a tool body are normally determined by the workpiece, the operations to be performed, and the production volume. Economy is also a key element in good design.
The three general categories of tool bodies are cast, welded, and built up, Figure 4-1. Each type of construction can be used for any workpiece, but one is often a better choice than the others. The first step toward an economic design is to know and weigh the strengths and weaknesses of each.
Figure 4-1. The three basic forms of tool-body construction are cast, welded, and built up.
Cast tool bodies are made in a variety of styles and types. The most common casting materials for tool bodies include cast iron, cast aluminum, and cast magnesium. Cast materials occasionally found in specialized elements for tool bodies, rather than in complete tool bodies, are low-melting-point alloys and epoxy resins.
Cast tool bodies can have complex and detailed shapes. Such shapes require fewer secondary machining operations. Cast materials dampen vibration. They are most often found in relatively permanent workholders; workholders not subject to drastic changes. Cast tool bodies have three major drawbacks: (1) they are not easily modified for part changes; (2) their fabrication cost is high; (3) they require a lengthy lead time between design and finished tool body.
Welded tool bodies are also made from a wide variety of materials. The most common welded tool bodies are made of steel or aluminum. Welded tool bodies are inexpensive to build and they are usually easy to modify.
They require minimal lead time. Welded tool bodies are also quite durable and rigid. They provide an excellent strength-to-weight ratio.
Heat distortion is the major problem with welded tool bodies. For best results, and to ensure stability of the tool body, welded tool bodies should be stress relieved before final machining and use. Be aware, however, this will add to the preparation lead time and cost, as well. Another problem with welded tool bodies is in the use of dissimilar materials. When a steel block, for example, is added to an aluminum tool body, it should usually be attached with threaded fasteners rather than by welding to the body.
Built up tool bodies are the most common tool body today. These tool bodies are very easy to build, and usually require the least amount of lead time between design and finished tool. The built up tool body is also easy to modify for changes in the part design. Like the welded body, built up tools are durable and rigid, and have a good strength-to-weight ratio. Depending on the complexity of the design, the built up tool body may be the least expensive to construct.
Built up tool bodies are usually made of individual elements, assembled with screws and dowel pins. The built up tool body is often used for precision machining operations, inspection tools, and some assembly tooling.
Preformed materials can often reduce the cost of machining tool bodies. These preformed materials include precision tooling plates, tooling blocks, risers, cast sections, and angle brackets. Other materials include ground flat stock, drill rod, or drill blanks, and also structural sections such as steel angles, channels, or beam.
The major advantage to using preformed and standard parts is the reduced labor cost in fabricating the workholder.
Tooling plates are standard, commercially available base elements used to construct a variety of different workholders. Like other fixturing elements, these plates come in several variations to meet most fixturing requirements.
Rectangular Tooling Plates: Of all standard tooling plates, the rectangular ones, Figure 4-2, are the most popular. Their rectangular form works well for a wide variety of workholders. The plates come in a wide range of sizes, from 12” x 16” to 24” x 32”. Rectangular tooling plates are made of ASTM Class 40 gray cast iron, machined flat and parallel.
Figure 4-2. Rectangular tooling plates are available in a full range of standard sizes, for vertical milling machines.
Round Tooling Plates: Another tooling-plate variation is the round tooling plate, Figure 4-3. Round tooling plates work well on rotary or indexing tables. These tooling plates are available in 400mm, 500mm, and 600mm diameters. They are made of ASTM Class 40 gray cast iron, and have a series of mounting holes.
Figure 4-3. Round tooling plates are ideal for rotary or indexing tables.
Square Pallet Tooling Plates: Square pallet tooling plates, Figure 4-4, are another form of tooling plate.
Figure 4-4. Square pallet tooling plates are available for all horizontal machining centers with standard square pallets.
The square shape is ideal for palletized arrangements where a square tooling base is necessary. Square tooling plates come in five sizes to fit standard machining-center pallets 320mm, 400mm, 500mm, 630mm, and 800mm square. Plates are made of ASTM Class 40 gray cast iron.
Rectangular Pallet Tooling Plates. Similar to square pallet tooling plates, except made for rectangular machining-center pallets 320 x 400mm, 400 x 500mm, 500 x 630mm, and 630 x 800mm. Figure 4-5 shows this type, and how it can also be mounted on a square pallet by adding a spacer.
Figure 4-5. Rectangular pallet tooling plates are for machining centers with rectangular pallets, or square pallets requiring a larger mounting surface.
Angle Tooling Plates. The angle tooling plates, Figure 4-7, are another useful tooling plate. These vertical plates allow mounting a large part approximately on the pallet’s center-line. These plates are made to fit machining-center pallets 400mm, 500mm, 630mm, and 800mm square. Angle tooling plates are made from ASTM Class 45 cast iron.
Platform Tooling Plates. Platform tooling plates are a variation of the square tooling plate. These plates are specifically designed for a mounting surface that must be elevated off the machine-tool table. As shown in Figure 4-6, the raised mounting surface permits easier access to the workpiece with horizontal machining centers. The added height provides the necessary clearance for the machine-tool spindle. The design also eliminates the dead space between the machine-tool table and the minimum operating height of the spindle.
Added height is also beneficial when machining short parts on a vertical machining center to avoid Z-axis limit errors. Platform tooling plates come in three sizes for 500mm, 630mm, and 800mm pallets. Platform tooling plates are made of ASTM Class 45 cast iron.
Figure 4-6. Platform tooling plates provide a raised horizontal mounting surface for easier workpiece access on horizontal machining centers.
Angle Tooling Plates. The angle tooling plates, Figure 4-7, are another useful tooling plate. These vertical plates allow mounting a large part approximately on the pallet’s centerline. These plates are made to fit machining-center pallets 400mm, 500mm, 630mm, and 800mm square. Angle tooling plates are made from ASTM Class 45 cast iron.
Figure 4-7. Angle tooling plates provide a vertical mounting surface on horizontal machining centers, ideal for extremely large parts.
Tooling blocks are often used on horizontal machining centers. The most-common tooling blocks are the two-sided and four-sided styles. These blocks work both for mounting workpieces directly, or for mounting other workholders. All working faces are accurately finish mac hined to tight tolerances, and qualified to the base.
Dual mounting capability, Figure 4-8, allows both JIS mounting (locating from two reference edges) and DIN mounting (locating from center and radial holes).
Figure 4-8. Tooling blocks can be located either using two reference edges (JIS standard) or using center and radial holes (DIN standard).
Two-Sided Tooling Blocks. The two-sided tooling block, Figure 4-9, is for mounting workpieces or workholders on two opposite sides. Two-sided tooling blocks work well for fixturing two large workpieces.
These tooling blocks come in five different pallet sizes: 320mm, 400mm, 500mm, 630mm, and 800mm.
Figure 4-9. Two-sided tooling blocks have two identical wide mounting surfaces for fixturing large parts.
Four-Sided Tooling Blocks. The four-sided tooling block, Figure 4-10, mounts workpieces or workholders on four identical sides. Four-sided tooling blocks, with their four working surfaces, are typically chosen to maximize production. These tooling blocks are available in five different pallet sizes: 320mm, 400mm, 500mm, 630mm, and 800mm.
Figure 4-10. Four-sided tooling blocks have four identical mounting surfaces for fixturing medium-size parts.
PRECISION CAST SECTIONS
Precision Cast Sections
Precision cast sections come in a variety of shapes and sizes. Most cast sections are available in standard lengths of 25.00”, and all sizes and styles are available in precut 6” lengths, with squareness and parallelism within .005”/foot on all working surfaces. The sections can also be ordered cut to any specified length. The two common cast-section materials are cast iron and cast aluminum. The cast iron sections are made of ASTM
Class 40 cast iron with a tensile strength of 40,000 to 45,000 psi. The aluminum sections are 319 aluminum with a tensile strength of 30,000 psi. Cast elements are used mainly for major structural elements of jigs and fixtures rather than as accessory items. Depending on the workholder design, it is possible to build a complete workholder by simply combining different sections.
T Sections. T-shaped cast sections, as shown in Figure 4-11, are made in two different styles: equal T
sections and offset T sections. Equal and offset T sections come in 6.00” lengths, or 25.00” lengths. As shown in Figure 4-12, both the equal and offset T sections have basically a square cross-sectional profile where width and height are the same. The major difference between the two styles, as shown, is the position of the vertical member in relation to the horizontal portion. The vertical member of the equal T section is positioned in the middle of the bottom portion. It is moved to one side on the offset T section. Both styles are available in five different sizes ranging from 3.00” x 3.00” to 8.00” x 8.00”. The web thickness of these sections is proportional to the overall size, ranging from .63” to 1.25”. Figure 4-13 shows an application where either style T section can be used.
Figure 4-11. T sections are made in two styles: equal T sections and offset T sections.
Figure 4-12. The vertical member is positioned in the middle of an equal T section, while it is moved to one side on an offset T section.
Figure 4-13. An application where either T section may be used.
L Sections. The L-shaped cast section, Figure 4-14, has a right-angle shape and is often used for applications when the bottom portion of a T section might get in the way. As shown, both the vertical and horizontal sides are the same. L sections come in five different sizes ranging from 3.00” x 3.00” to 8.00” x 8.00”, and are available in either 6.00” or 25.00” lengths. The web thickness is proportional to the overall size.
Figure 4-14. The L section.
Figure 4-15. A typical fixturing application using an L section.
U Sections . U-shaped cast sections, Figure 4-16, are widely used for channel-type workholders. These sections have a square cross-sectional profile with identical height and width dimensions, as shown. U-sections are available in seven sizes ranging from 1.75” square to 8” square. The web thickness of these sections is, once again, proportional to the overall size. The smaller U sections are made in 19.00” lengths.
The larger sizes are available in full 25.00” lengths. All sizes are available in 6.00” lengths. Figure 4-17 shows two workholders constructed from this type of cast section.
Figure 4-16. The U section.
Figure 4-17. Examples of workholders constructed from U sections.
V Sections. V-shaped cast sections, Figure 4-18, are useful when a V-shaped element is needed for either locating or clamping. Thin portions of this material are often used as V pads. Longer lengths are frequently used as V blocks, Figure 4-19. V sections have a rectangular cross section with the width greater than the height. The V-shaped groove is machined to 90º ± 10’. V sections come in three sizes, ranging from 1.00” x 2.00” to 2.50” x 4.00” in standard 6.00” and 18.00” lengths.
Figure 4-18. The V section.
Figure 4-19. V sections are often used as V blocks for locating cylindrical parts.
Square Sections. Square cast sections, Figure 4-20, are typically used as major structural elements.
Applications include riser elements, supports, or four-sided tooling blocks, as shown in Figure 4-21. Square sections are made in four standard sizes from 3.00” square to 8.00” square. Each size is available in either 6.00” or 25.00” lengths. All external surfaces except the ends are precisely machined.
Figure 4-20. The square cast section.
Figure 4-21. A fixturing application with a square cast section used as a tooling block.
Rectangular Sections. Rectangular cast sections, Figure 4-22, like square sections, are often used as structure elements in workholders. These sections work well for base elements, riser blocks, or similar features. Rectangular sections come in three standard sizes from 4.00” x 6.0” to 8.00” x 10.00”. Here, too, all external surfaces except the ends are precisely machined. They are available in 6.00 and 25.00” lengths.
Figure 4-22. The rectangular cast section.
H Sections. H-shaped cast sections, Figure 4-23, are a unique design well suited for either complete tool bodies or structural elements. These sections are basically square and come in five sizes ranging from 3.00” x 3.00” to 8.00” x 8.00”. All H sections are made in 6.00” and 25.00” length. Figure 4-24 shows an application with the H section as a tool body.
Figure 4-23. The H section.
Figure 4-24. A typical application with an H section as a tool body.
Flat Sections. Flat cast sections, Figure 4-25, are the simplest and most-basic type of cast section. These sections are used where a cast iron material is preferred over steel flat stock. Flat sections are available in five width and thickness combinations. The sizes range from .63” x 3.00” to 1.25” x 8.00”, and 6.00” and 25.00”
lengths. Flat sections work well as base elements for smaller jigs or fixtures, or as structural elements for larger workholders.
Figure 4-25. The flat cast section.
PRECISION ANGLE BRACKETS
Angle brackets are often used when a right-angle alignment or reference is required. Although angle brackets are commonly thought of as 90º elements, there are also adjustable-angle styles of angle brackets and plates.
Plain Angle Brackets. The plain angle bracket, Figure 4-26, is available with or without locating holes.
Angle brackets are often used when a fixed 90º angle is required. The right angle of these plates is closely controlled and is accurate to 90º ± .08º. These brackets are made in ASTM A36 steel or 6061-T6 aluminum.
Angle brackets come in 10 different sizes ranging from 2.00” x 2.50” to 6.00” x 6.00” with both equal and unequal leg lengths. The web thickness of these sections is also proportional to the overall size, ranging from .22” to .44”.
Figure 4-26. Angle brackets are machined flat and parallel to close tolerances. They are also available with precision locating holes for 3-axis accuracy.
The gusseted angle bracket, Figure 4-27, is a variation of the standard angle bracket. This angle bracket is made with a gusset between the horizontal and vertical legs. The gusset stiffens the angle bracket and reduces any distortion when heavy loads are applied. These angle brackets also have a right angle accurate to 90º ± .08º. These brackets are made in ASTM A36 steel or 6061-T6 aluminum.
Gusseted angle brackets are available in 10 different sizes ranging from 2.00” x 3.00” to 6.00” x 6.00”.
Figure 4-27. Gusseted angle brackets have a gusset for added rigidity and strength.
Adjustable angle brackets, Figure 4-28, are another variation of the plain angle bracket. These brackets are made with a close-tolerance hinge between the horizontal and vertical legs.
The hinge permits the bracket to pivot so it may be set at any desired angle. The most-basic adjustable angle bracket is the plain type, shown at (a). This type has a bolt and nut arrangement for the hinge. The gusseted adjustable angle bracket, shown at (b), also has a bolt and nut hinge, but it also has a gusset mount on both legs. The mount permits the two legs to be connected with a gusset that is either bolted or welded to the mounts. For applications where the angle bracket must be disassembled, the removable-pin-type adjustable angle can be used. This angle bracket, shown at (c), uses an L pin to attach the horizontal and vertical legs.
These brackets are made of 1018 steel. The plain and gusseted adjustable angle brackets come in three different sizes ranging from 3.00” x 3.00” to 4.00” x 4.00”, with equal or unequal leg lengths. The removable pin type is made in two sizes, 4.00” x 4.00” and 6.00” x 6.00”.
Figure 4-28. Adjustable angle brackets have a close-tolerance hinge for accurate location.
Figure 4-29. The load on a hoist ring is not simply total weight divided by the number of hoist rings. Shallow lift angles can cause very-large resultant forces.
Hoist rings should, for safety reasons, be added to any tool weighing over 30 pounds. The following are design considerations when selecting hoist rings.
Hoist-Ring Safety Precautions. Simply following a few basic safety precautions makes working with hoist rings both safer and more efficient.
- The load on each hoist ring is not simply total weight divided by the number of hoist rings. The resultant forces can be significantly greater at shallow lift angles and with unevenly distributed loads. In the example shown in Figure 4-30:
- Despite the 5:1 safety factor on hoist rings, never exceed the rated load capacity. This safety margin is needed in case of misuse, which could drastically lower load capacity.
- Tensile strength of fixture-plate material should be above 80,000 psi to achieve full load rating. For weaker material, consider through-hole mounting with a nut and washer on the other side.
- Do not allow hoist rings to bind. Use a spreader bar, Figure 4-30, if necessary, to avoid binding.
- Do not use spacers between the hoist ring and the mounting surface.
- Do not apply shock loads. Always lift gradually. Repeat magnaflux testing if shock loading ever occurs.
- Never lift with a hook or other device that could deform the lifting ring. Use only cable designed for lifting.
- Tighten mounting screws to the torque recommended. Because screws can loosen in extended service, periodically check torque. For hoist rings not furnished with screws, mount only with high-quality socket-head cap screws.
- The mounting surface must be flat and smooth for full contact under the hoist ring. Tapped mounting holes must be perpendicular to the mounting surface.
After installation, check that rings rotate and pivot freely in all directions
Standard Hoist Rings. Standard hoist rings, Figure 4-31, have a low profile and are attached directly to the mounting surface with socket-head cap screws. This is the most economical type of hoist ring. The solid forged lifting ring pivots 180º but does not rotate. These hoist rings are available for loads to 20,000 lbs.
Figure 4-31. Standard hoist rings have a forged ring that pivots 180º.
Figure 4-32. Swivel hoist rings pivot 180º and rotate 360º simultaneously to allow lifting from any direction.
Swivel Hoist Rings. Swivel hoist rings, Figure 4-32, are a form of hoist ring with a 180º pivot and 360º rotation. These hoist rings, available in the two variations shown, are mounted with a single screw. As shown in Figure 4-33, these hoist rings are always preferred over conventional eye bolts or forged lifting eyes when side loads are expected. The pivot-and-swivel combination permits the hoist ring to accommodate lifting angles that can cause a standard eye bolt or forged lifting eye to break. An inherent advantage of the swivel hoist ring when compared to the eyebolt or lifting eye is the ability of the swivel hoist ring to swivel, rather than being fixed in one orientation. Forged lifting eyes and eyebolts have their maximum lifting strength when the axis of the eye is perpendicular to the lifting angle, and when the lifting eye is screwed all the way to the shoulder. It is very difficult to achieve both of these conditions simultaneously. The swivel hoist ring solves those problems easily. These swivel hoist rings are available for loads up to 10,000 lbs. They are available in a wide variety of sizes with either black oxide or electro-less nickel plate finish, and many are available in stainless steel.
A useful hoist-ring accessory is the hoist-ring clip, shown in Figure 4-34. These clips keep the swivel hoist rings stationery and out of the way when they are not being used for lifting.
Figure 4-33. Hoist Rings should be used in place of eye bolts for all heavy lifting applications.
Figure 4-34. Hoist-ring clips keep swivel hoist rings stationary while not in use for lifting.
Other styles of hoist rings are also commercially available. These styles including Heavy-Duty Swivel Hoist Rings, Heavy-Duty Weld Mount Swivel Hoist Rings, and Shackle Hoist Rings. Figure 4-35.
Figure 4-35. Three additional types of swivel hoist rings.
Heavy-Duty Swivel Hoist Rings have a forged steel ring, which pivots 180 degrees and swivels 360 degrees simultaneously to allow lifting from any direction. They have a maximum lifting capacity of 30,000 pounds.
Heavy-Duty Weld Mount Swivel Hoist Rings are, as the name implies, attached by welding, and are available in capacities up to 24,000 pounds. Shackle Hoist Rings are designed for permanent mounting on tooling and equipment. It can be bolted or welded in place. Each of these hoist rings can pivot 180 degrees and swivel 360 degrees simultaneously to allow lifting from any direction. All have a safety factor of 5:1.
Lifting Pins. Lifting pins, Figure 4-36, are a modified form of hoist ring. Sizes are available for loads up to 3,400 lbs. These pins have a positive-locking four-ball arrangement to hold them in place during lifting. A release button at the opposite end of the pin allows quick installation and removal. Figure 4-37 shows two typical applications. These pins are made of 17-4Ph stainless steel.
Figure 4-36. Lifting pins are medium-duty hoist rings that can easily be completely removed
Figure 4-37. Typical applications for lifting pins.
Threaded inserts are widely used for construction and repair of workholders. The most common use for the inserts is to reinforce threads in new workholders or to repair threads in existing workholders. Threaded inserts provide a way to strengthen threaded holes in new workholders where repeated use may cause excessive wear, such as with aluminum or other soft fixture plates. With existing jigs and fixtures, threaded inserts are a quick way to fix stripped, damaged, or worn threads. Two primary forms of threaded inserts are Keenserts and self-tapping threaded inserts.
Keenserts, Figure 4-38, are threaded inserts for both repairing and reinforcing applications. The Keensert design uses a standard tap size for installing the insert. This feature eliminates the cost of special taps for threading the mounting holes. As shown, the inserts have unique locking keys that both securely hold the inserts in place and prevent rotational movement. The method of installing these inserts is shown in Figure 4-39.
Figure 4-38. Keensert threaded inserts are frequently used to reinforce threads in soft tooling plates.
Figure 4-39. Installation steps for Keensert threaded inserts.
- Drill out the old thread, if repairing an existing thread, or drill a new hole to the specified tap drill diameter (slightly larger than the normal tap drill for that thread size).
- Countersink the entry end of the hole to the specified diameter.
- Tap the new threads to the correct size with a standard tap.
- Screw in the insert until the body is slightly (.010” to .030”) below the surface. The locking keys act as a depth stop.
- Drive the keys down with several light hammer taps on the installation tool (or directly on the keys).
Figure 4-40. Standard Keensert assortments contain many different insert sizes and a tool for each size.
Keenserts are made in a variety of forms for almost any application. The inserts are available in a heavy-duty carbon-steel style in standard inch sizes from #10 through 1 ½, and in metric sizes from M5 to M20. Stainless steel, heavy-duty, and thinwall inserts are also available in either plain or internal-thread-locking styles. A variety of standard Keensert assortments are also available for UNC, UNF and metric threads, Figure 4-40.
One other style of Keensert, ideal for repair work, is the solid Keensert, Figure 4-41. As shown, these stainless steel inserts are typically used for relocating tapped or drilled holes. They come with external-threaded diameters from 5/16” to 1 3/8”.
Figure 4-41. Solid Keenserts are handy plugs for relocating misplaced holes.
An accurate relationship between the workholder and the machine tool must be established. Fixture keys not only establish location initially, they also help hold the fixture in place during machining.
The two basic styles of fixture keys are the slot-mounted and hole-mounted types. Slot-mounted fixture keys are made in two variations, the plain fixture key and the step fixture key, Figure 4-42. The plain fixture key, shown at (a), is the simplest and least expensive of the slot-mounted keys. As shown, these keys are mounted in a slot cut to a depth equal to half the thickness of the key. The key is held in place with a socket-head cap screw.
The second style of slot-mounted fixture key is the step key, shown at (b). These fixture keys are a variation of the standard fixture key. This key’s step design allows a fixture with one slot width to work on a machine table with a different slot width. Like the plain-style key, this key is held in place with a socket-head cap screw.
Fixture 4-42. Plain fixture keys and step fixture keys.
Fixture 4-43. Sure-Lock™ fixture keys are ideal for small and medium fixtures, while removable locating keys are best for large, heavy fixtures. Subplate locating keys allow mounting the same fixtures on a subplate with 3-axis location.
Hole-mounted fixture keys are also made in several variations. The most popular are the Sure-Lock™ fixture key and the removable locating key, Figure 4-43. Hole-mounted keys eliminate the need to slot fixtures. The Sure-Lock™ fixture key, shown at (a), is the most popular of all fixture keys. Keys for any machine-table slot mount in a .6250” reamed hole for interchangeability. This design has a unique locking arrangement to precisely align and lock the fixture key in the hole. As shown, Sure-Lock™ fixture keys can be secured from either the top or bottom of the fixture. These keys are made in many sizes, for all standard USA table slots from 3/8” to 13/16”, and metric table slots from 12mm to 22mm.
Locating keys, shown at (b), are the standard removable-type fixture key for large, heavy fixtures. These keys can be inserted from above after placing the fixture on the machine table, and then removed again if desired after the fixture is fastened. This design keeps the fixture’s bottom side free of obstructions. Locating keys all mount in a 1.1875” reamed hole. They are available in many sizes, for all standard USA table slots from 9/16” to 7/8”, and metric table slots from 14mm to 22mm.
Subplate locating keys, shown at (c), are designed for mounting quick-change fixture plates on a subplate. A round key and a diamond (relieved) key are used together for precise location without binding. Two standard diameters, .6250” and 1.1875”, match standard fixture-key holes. This allows mounting these same fixture plates either on a subplate (3-axis location) or directly on a machine table (2-axis location).
Pallet Fixture Keys. Fixture keys for mounting tooling plates and blocks on standard DIN pallets. Each pallet requires one center key and one radial key (for orientation). If desired, the radial key can be removed after fastening the pallet to the machine table. A tapped hole in the top of each pin makes insertion and removal easier. Center keys are available for 25, 30, and 50mm holes. Radial keys are available for 20 and 25mm holes. See Figure 4-44.
Figure 4-44. Two pallet fixture keys, one for center location and one for radial orientation, are used for DIN mounting on horizontal machining centers.
Jigs are made to meet the requirements and specifications of individual workpieces, resulting in an infinite number of different jigs. Even though every jig may be different, each can be grouped into one of only a few categories. The following is a description of the basic jigs and the applications where each is best.
Template jigs, Figure 4-45, are the simplest and least-expensive jigs. They are generally used for layout or light machining. They are designed for accuracy rather than speed. Template jigs do not have a self-contained clamping device. If a clamp is needed, a secondary clamp such as toggle piers may hold the jig to the workpiece. When a template jig is used for drilling multiple holes, a pin placed in the first drilled hole can reference the jig.
Figure 4-45. Template jigs are simple plates usually held in place by hand.
Plate jigs are very similar in their basic construction to template jigs. As shown in Figure 4-46, plate jigs have a self-contained clamping device. Although many different clamps can be used, a screw clamp is the most-common type with these jigs.
Figure 4-46. Plate jigs are similar to template jigs, except they include a clamp.
Table jigs are a variation of the basic template or plate jig. The table jig shown in Figure 4-47 is basically a template jig installed on legs. Table jigs are designed for applications where the surface to be machined also locates the workpiece. The location is transferred across the underside of the jig plate and down the legs to the machine table. When designing this type of jig, make sure the area to be drilled is inside the legs to prevent tipping. Although three legs will work, four are recommended with table jigs. Four legs also wobble if a chip is under one leg; three legs do not. The wobble tells the operator to clear the chips from the locating surfaces.
Figure 4-47. Table jigs are basically template or plate jigs raised up on legs.
Indexing jigs are for applications where holes must be drilled in a pattern around a center axis. This is done either with an indexing ring holding bushings, or by indexing the part itself. With a separate indexing ring, Figure 4-48, a hand-retractable plunger is frequently used. A ball-plunger can also be used for less critical positioning. Here the work piece itself serves as the indexing ring. With this design, the first hole is drilled and the part rotated to engage the ball-plunger, Figure 4-49. Once located, the part is re-clamped and a second hole is drilled. The indexing is then repeated until all the holes in the part are drilled. The angular position of the ball plunger relative to the drill determines the indexing pattern. So, for four holes, the plunger is located at 90º from the drill, 60º for six holes, 45º for eight holes, and so on.
Figure 4-48. A hand-retractable plunger can be used to positively index a ring holding drill bushing.
Figure 4-49. A ball-plunger can index using holes drilled in the workpiece.
Index plungers are heavier-duty spring-loaded locators. Precision bushings are available to fit the plungers.
They are available in a choice of mounting styles, and with either tapered or straight plungers.
Multi-station jigs, Figure 4-50, are for repetitive simultaneous operations on several identical parts. In most cases, almost any jig may be used with a multi-station arrangement. As shown, the unique feature of a multi-station 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; and #4 is where the workpiece is counterbored. An indexing arrangement is also included with this jig to accurately position the jigs at each station.
Figure 4-50. Multi-station jigs are used in a continuous, multi-step production process.
Trunnion jigs, Figure 4-51, are for large, heavy, or odd-shaped workpieces. This type of jig rotates the workpiece on precision bearing mounts called trunnions. Trunnions are made it two basic styles: standard and spherical.
Figure 4-51. Trunnion jigs allow easily turning a large part to work on all sides.
Standard trunnions, Figure 4-52, are available in either locking or revolving styles. With most trunnion jigs, trunnions are typically used in pairs: the revolving trunnion provides a rotating pivot, and the locking trunnion both rotates and locks at any rotational angle. Locking trunnions are locked in place with a friction-cone arrangement engaged by turning the locking handle. Trunnions are intended for low speed rotation, such as when repositioning a fixture to allow proper access to complete a weld.
Figure 4-52. Trunnions are usually used in pairs, one locking and one revolving.
These trunnions come in two size ranges, small, with a weight limit of 1,500 pounds at a distance of 18 inches from the face, and the larger one, with a weight limit of 2,500 pounds 18 inches out from the face. In pairs, either the weight limit, or the distance can be doubled, but not both. For larger fixtures, either longer or heavier, exceeding these limits will lead to premature failure of the trunnion(s).
Figure 4-53. Spherical trunnions are ideal for mounting on pipe frames.
Spherical trunnions, Figure 4-53, are a locking trunnion for pipeframe mounting. The spherical bottom aids in precisely aligning the trunnions. Spherical trunnions lock the jig in position by lowering the handle.
Fixtures, like jigs, can be grouped into a few categories. These categories are most often based on the construction of the fixture. Another way to identify a fixture is by the machine it is used with.
The plate fixture is the most basic and most common fixture. The plate fixture is built with a Carr Lock® Fixture Plate, cast flat section, tooling plate, or similar plate material. All locators, supports, and clamps are mounted directly to the plate. As shown in Figure 4-54, a complete plate fixture can be built using on standard, off-the-shelf components.
Figure 4-54. Plate fixtures usually hold a workpiece parallel to the machine table.
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, Figure 4-55, are constructed at 90º to the base. The modified-angle plate fixtures have an angle other than 90º. 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.
Figure 4-55. Angle-plate fixtures usually hold a workpiece perpendicular to the machine table.
All basic workholding principles should be applied to fixtures used for welding operations. The major differences between most welding fixtures and machining fixtures are the locational tolerances and clamping methods. With welding fixtures, weight is often a problem. Many times a fixture is made up of welded sections.
The sections are usually positioned only in the areas where the parts to be joined contact the fixture. Rather than the precision locators found on most machining fixtures, small angle clips, blocs, or similar elements are used as locators.
The clamps for welding fixtures are often toggle clamps. These clamps offer the best combination of design flexibility, holding capacity, and operational speed. In addition, since most toggle clamps move completely clear of the work area when opened, loading and unloading operations are also simplified. Although the clamps may be attached with a screw, in many instances toggle clamps are welded directly to the fixture body.
A few important considerations to keep in mind when building a welding fixture are as follows:
- Always construct the fixture so that the parts to be welded can be loaded only in the correct orientation.
- Supports are needed underneath clamps to prevent distortion.
- Locators and supports must be positioned so any workpiece distortion caused by welding heat loosens, rather than tightens, the workpiece in the fixture.
- Only essential dimensions and relationships should be located and rigidly clamped.
- All areas to be welded must be easily accessible.
- When possible, welding should be done on a flat, horizontal plane.
- To minimize warping of the workpiece, provisions to dissipate excess heat must be included in the design.
- As many operations as possible should be performed before the workpiece is removed or repositioned.
- Large or heavy loads should be completely supported, and provisions for lifting the jig with a hoist must be included.
The manipulation of odd shaped, heavy, and/or bulky items for welding, inspection, or assembly can lead to operator fatigue, mistakes and injuries. Manual/hydraulic lifting columns can eliminate these conditions while simplifying the performance of these tasks. Available in lifting capacities from 225 to 1,350 pounds, these columns are easily raised or lowered by pumping a foot pedal, or releasing pressure to position the work at the optimum level for ease and accuracy. See Figure 4-56.
Figure 4-56. Lifting columns are used to easily raise and lower a fixture for convenient access.
Rotating modules are also available for these columns to allow rotation of the workpiece in either the horizontal or vertical axis. They are available as free rotating modules, or with locks operated either by hand or foot pedal. See Figure 4-57.
Figure 4-57. Rotating modules allow easily turning a large fixture for convenient access.
Inspection and gaging fixtures are subject to different requirements than machining fixtures, whether inspection is done on a CMM (Coordinate Measuring Machine) or with manual gages. The following are key differences and unique aspects of inspection fixtures:
- The workpiece must be oriented to expose all features to be inspected.
- Machined surfaces are often not used for locating, to allow them to remain exposed.
- Bridge-type CMMs require different tooling than horizontal-arm machines.
- Since most CMMs can automatically reference a part to the machine, often only approximate orientation in the fixture is required.
- Quick-acting clamps, such as toggle clamps, are widely used in inspection fixtures due to the light clamping forces usually required.
- Inspection fixtures can be either permanent or modular.
This page contains information originally published in the Jig & Fixture Handbook, 3rd Edition, Copyright 2016, Carr Lane Manufacturing Co., St. Louis, Mo.