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.
TOOL BODIES
The tool body provides the mounting area for all the locators, clamps, supports, 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.
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
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.
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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.
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.
Figure 4-4. Square pallet tooling plates are available for all horizontal machining centers with standard square pallets.
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.
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.
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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
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 machined 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).
JIS STANDARD DIN STANDARD
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.
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Figure 4-10. Four-sided tooling blocks have four identical mounting surfaces for fixturing medium-size parts.
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.
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.
EQUAL T SECTION OFFSET T-SECTION
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.
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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 4-15 shows a fixturing application with the L section.
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.
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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.
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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”.
ANGLE BRACKET ANGLE BRACKET WITH LOCATING HOLES
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 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. 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”.
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Figure 4-28. Adjustable angle brackets have a close-tolerance hinge for accurate location.
!= !
! !"#$
!" ! =70°,! = 4000
4 !"#70° =1064 !"#
!" ! =15°,! = 4000
4 !"#15° =3864 !"#
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
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.
1. 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:
F = Force on each hoist ring W = Total weight = 4000 lbs.
N = Number of hoist rings = 4 A = Lifting Angle
2. 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.
3. 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.
4. Do not allow hoist rings to bind. Use a spreader bar, Figure 4-30, if necessary, to avoid binding.
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POOR GOOD
Figure 4-30. Use a spreader bar to avoid binding hoist rings.
5. Do not use spacers between the hoist ring and the mounting surface.
6. 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.
7. 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.
8. Never lift with a hook or other device that could deform the lifting ring. Use only cable designed for lifting.
9. Do not apply shock loads. Always lift gradually. Repeat magnaflux testing if shock loading ever occurs.
10. 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.
Conventional eye bolt Swivel hoist ring under under heavy side load. same load.
Figure 4-33. Hoist rings should be used in place of eye bolts for all heavy lifting applications.