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    Design Tools Practical Design Hints

  • Part A

  • Part B

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Part A: Machining

The following represents a collection of practical hints for saving time, labor, material, and costs. These involve designs for minimum tooling, efficient clamping and holding, simplifying subassemblies and efficient turning, boring, drilling, milling, broaching, grinding and tapping. In the figures, design (a) generally represents the incorrect design, while constructions (b, c, d, etc.) illustrate improvements. The symbols ∇ , ∇∇ , ∇∇∇ denote rough-machined, finish-machined and ground surface finishes, respectively.

(a) Minimizing Tooling Costs

Avoid round shapes in screw-machine work which require special forming tools. In design (b) straight cutting tools can feed practically simultaneously at right angles and at 45° to the screw-machine axis.

If round shapes are necessary, avoid radii equal to half part width (a). Such radii are sensitive to machining accuracy and tool and work alignment. Radii greater than one half part width (b) avoid the need for such accuracy.

The initial cost and maintenance of a milling cutter with curved teeth (a) is greater than that of a cutter with straight teeth. The latter are more suited to high production rates and can also be made with a combination of two standard cutters, if desired (b).

A milled or ground slot with square ends (b) requires a less expensive milling cutter than a slot with rounded ends (a), which necessitates a cutter with curved teeth.

The need for two face cutters of different size, as in (a), may be avoided if the locating surfaces in the housing can be simplified to a few appropriately spaced ribs (b, c).

To accommodate a shouldered shaft with a small taper (a), a special combination countersinking and counterboring tool is required for the housing. Redesign (b) requires only a standard countersinking tool and locates the shaft relative to the housing equally well.

(b) Minimizing Machining

To minimize machining of the casting shown in partial cross-section in (a), relieve base, leaving only a peripheral bearing area; and design upper surface so that machining is limited to circular ring sections around holes (b). Finish machining of the ring section involves simple counterbores, rather than milling or grinding.

The amount of drilling of a long hole (a) for guiding a reciprocating rod can be reduced by the cutout shown in (b), relieving the central portion of the bar without impairing its function.

To reduce the machining required in design (a), the heights of the three projections, which are to be machined, should be equal, if possible. They should also be lined up horizontally in order to minimize the width of the machining operation.

If the surfaces to be machined cannot lie in the same plane (a), at least they should be parallel (b).

When machining two shaft diameters, the square shoulder requires a special setup or a separate squaring tool (a). A tapered shoulder (b) can be produced by the same tool used for turning.

In the design of a clamp, an eccentric hole (b) requires less metal removal (from the original bar stock) than a concentric hole (a).

(c) Allowing Adequate Clearances For Cutting Tools

At the junction of a reduced-diameter shaft to the main body, the shoulder should not interfere with the cutting tool, as it does in (a). A relief groove or neck is desirable to free the tool after cutting (b). NOTE: Short screws in which the threads are formed by thread-rolling or thread-milling do not always require relief. The latter saves machining and increases part strength.

To avoid interference with machining of upper surface, U, surface A should be moved outward and the upper surface, U, should be redesigned to lie in one plane, if possible, to allow single-pass machining.

For internal boring or drilling (a), add a relief groove as in (b, c) for tool clearance.

The same applies to internal grinding (a). A clearance hole should be added (b) so that the grinding wheel will run freely after completion of its tasks.

Always allow free drill exits, especially with hidden inclined ribs.

Similarly, allow clearances for shaping and planning tools. Ditto for milling cutters and grinding wheels.

Instead of a simple relief groove, as in (a), a change of part shape (b) is more effective in planning, shaping, milling, and grinding.

In order to accommodate a large milling cutter or grinding wheel (a), the relief groove may need to be quite long (b).

To provide relief for an internal, blind keyway (a), the design may include a hole (b), or an internally bored groove (c). The latter is expensive but may be desirable when there are several circumferentially disposed keyways (as in a spline), or when drilled holes are not feasible.

Tapered surfaces require ample tool clearance. For this reason, the shoulder in (a) is best avoided as in (b).

Another illustration: for the tapered surface in (a), grinding-wheel clearance is increased by providing a neck in the attached shaft (b).

(d) Effective Clamping in Machining & Assembly

When it is impossible to grip or hold a part for machining (a), extra metal can be added (b) and removed (machined off) after completion of machining.

Sometimes clamping is achieved by permanent metal deformation – in this case from a tapered surface (a) to a cylindrical surface.

Clamping of a rib is facilitated by increasing the width of a thin rib (a) to more substantial proportions (b).

Avoid the need for shims, plates, etc. in clamping (a), by arranging bearing surfaces to be coplanar (b). the latter also leads to a safer and faster clamping operation.

For facilitating the machining of a crankshaft, end lugs have been added (one at each end). Each lug has two holes in order to be able to hold and turn crankshaft and crank. After completion of machining the lugs are removed.

(e) Simplifying Machining with Composite Assemblies

The one-piece machined shaft (a) can sometimes be replaced by cold-drawn rod and tubing (b); in the case of low loads the tubing may be replaced by split retaining rings (c). Constructions (b) and (c) eliminate or reduce both machining and metal removal.

In the case of a shaft (a) with a hub, machining integrally from stock involves removing a relatively large amount of metal, or resorting to an expensive forging. Alternative designs involve a cold-drawn rod or bar to which a separate ring has been added and fastened to the shaft by a set screw (b), press fit (c), or welds (d, e).

Sometimes a ground shaft with a ground shoulder (a) can be replaced by a one-diameter centerless-ground shaft and a split retaining ring.

Similarly (a case history), a three-piece assembly, which was originally assembled with a stepped shaft, ring-shaped washer and dowel pin (a), was redesigned with a one-diameter shaft and two dowel pins (b).

Another case history involved a shaft in which a locating ring and set screw (a) were replaced by an external, split retaining ring (b).

For retention of a spring a special screw and tapped hole (a) has been replaced by an internal retaining ring (b).

Another method of replacing a stepped shaft with several diameters (a) – in this case with a single shaft threaded at one end to receive the matching bushing. Design (b) involves less metal removal and less machining, except for the added thread.

The advantage of a one-diameter construction is even more pronounced in the case of internal boring or drilling. Instead of boring from two sides (two setups, requiring close alignment) as in (a), a one-diameter bore with an insert is preferable. The insert can be in the form of a bushing (b), or two split retaining rings (c).

A specific illustration pf the preceding principle: the ball-bearing housing design (a) is simplified considerably by means of the composite construction consisting of a one-diameter bore and split retaining rings (b).

A one-piece construction with an internal broach (a) can be replaced by a one-diameter bore into which an internally broached bushing is press fitted (b). Machining costs are significantly reduced in this way.

A blind bore with a deep recess (a) is expensive. If feasible, change to a one-diameter bore and press fitted bushing (b).

(f) Effective Turning and Boring

Starting out with a shaft which is too big is uneconomical (a shaft with an O.D. as large as in (a) requires too much machining). In this case the middle portion could have been left rough. Starting with drawn rod of the right size (b), only the two ends need to be machined; and metal removal is greatly reduced.

Square ends (a) represent a hazard (injury to fingers), are easily damaged and more difficult to assemble. Although rounding off is better (b), a small chamfer (c) is safest and least expensive.

To protect a conical cavity from damage (a) when machining between centers, a small recess (b) suffices.

Long small diameter bores (a) should be avoided, due to high machining costs and possibility of tool breakage.

Long bores for guiding shafts (a) should be enlarged in the middle section (rough boring suffices), in order to facilitate guiding the shaft and reduce finish machining.

Whenever possible, design for minimum setup time. Design (a) requires two setups (boring from each side), while design (b) requires only one setup (boring from right side only).

Machining shaft (a) between centers is hindered by the internal thread. A design change to an external thread (b), if feasible, would permit such machining and eliminate the need for special tooling.

In the case of screw-machine work on round stock whose O.D. (d) does not need machining (a), keep the amount of metal removal to a minimum by maximizing dimension d₁ and minimizing dimension L₁, as in (b).

For efficient screw-machine work let the cutting tools feed longitudinally and, if possible, simultaneously. In order to facilitate this, part (a) should be redesigned so that the right-end diameter is the smallest. In addition, the grooves should be narrow in order to avoid special tooling (b).

In the multiple-step machining of a part in screw-machine work, machining time is governed by the deepest step (e.g. in the case of a single radially moving cutter having multiple cutting edges). Hence, part (a) should be redesigned so that all steps have the same depth (b).

(g) Effective Drilling

If only a short portion of a bore is used in a press-fit assembly, such as in (a), it is not always necessary to ream the entire hole, but it is necessary to leave sufficient clearance for reaming (b).

To start a drill, the surface to be drilled should be perpendicular to the drill axis in order to prevent the drill from skidding, wear and breaking. Adding a flat as in (b) is desirable.

The same principle applies when drilling an inclined surface.

The principle applies also at the exit side of the drilled hole, even though it is not as critical as the entrance side. While (b) is preferable to (a), both entrance and exit need to be “perpendicular designs”, as in (c) and (d).

To avoid interference with the drill exit, such as in (a), arrange holes at a safe distance from other portions of the workpiece (b).

Blind holes with square ends are expensive. Best and least expensive design is to bottom with a taper corresponding to the 118° angle of a standard drill (b). If a square end cannot be avoided, include as much of the taper as possible (c), or better still, add a small diameter bore (d). If a dowel pin is to fit in the main bore, design (d) is necessary in order to be able to remove the pin, if needed, and to avoid trapped air, which may impede assembly.

Tapered reamers should not work against shoulders (a), as this requires special tooling and more maintenance. Change to design (b), if possible.

When drilled holes intersect, the drill deflects when the second hole is drilled. The distance, “1”, between the axis of the holes should be sufficient, as in (b) for the following procedure: first, hole “d” is finish drilled; then the second hole is predrilled with diameter d₁; finally, diameter d₁ is increased to d₂ using a counterbore with a guided pilot of diameter d₁. If the distance “1” is too short, as in (a), hole “d” would have to be filled with a tight plug (of the same material as the workpiece), which is removed after completion of the drilling.

When the radial position of two parts, such as a shaft and housing, must be coordinated with a “Dutchman’s pin” (a), the two parts should have the same material characteristics (e.g., hardness, ultimate strength, chip formatting strength etc.) in order to prevent drill deflection towards the softer material, breakage and/or inaccurate machining.

In case of a piloted counterbore or stepped reamer, the ratio of the hole diameters (d₁: d₂) should conform to the established ratio furnished by the tool manufacturer.

(h) Efficient Milling

To minimize machining costs avoid blind keyways (a), or keyways extending up to a square shoulder (which would involve needlessly expensive machining). While design (c) represents an improvement, it is better still to add a curved exit, which permits use of a sturdier milling cutter (d). The best design is a Woodruff key (e).

To reduce setup time, a keyway on a tapered surface (a) should be parallel to the shaft axis, if possible, as in (b).

The flat-bottomed slot in the fork-like workpiece (a), requires a large travel on the part of the milling cutter. By curving the bottom to match the radius of the cutter, as in (b), the tool travel (and hence machining time) is significantly reduced.

The milling of a flat surface with a projection (a) requires cuts in two directions and careful machining to avoid a slight mismatch between the milled surfaces. If possible, raise one surface intentionally, as in (b), in order to reduce the precision needed for the milling operation.

When milling evenly spaced circumferentially disposed slots, an even number of slots (a) requires twice as many passes as slots. An odd number of slots requires only as many passes as slots (b). Hence, everything else being equal, an odd number of slots is preferable.

If the reduced-diameter portion of a shaft (a) is a milled, a small shoulder should be provided, if feasible, in order to prevent damage by the milling cutter to the main shoulder.

(i) Effective Broaching

As in drilling, both entrance and exit surfaces of the workpiece should be perpendicular to the workpiece, the exit side being more critical in the case of broaching. Hence, change design (a) to (c), if possible, with (b) intermediate between the two.

If two slots are needed in a tapered bore, design (a) involves two setups and two separate machining operations. If, on the other hand, the broached slots are designed parallel to the bore axis (b), they can be produced in one setup and one operation.

Broaching slots of different widths (a) requires broaching cutters of different sizes. Slots of the same width, but different depths, can be machined with the same broach (b) and thus save on tooling.

Unsymmetrical part design leads to tool deflection and extra load. A symmetrical design (b) is preferable.

Burrs are difficult to remove from broached internal splines. If slots are produced after broaching, burrs are created internally. Hence, design (a) should be modified to avoid teeth adjacent to the slot (b).

(j) Effective Grinding

When several radii are to be ground (a), it is desirable to keep radii equal, if possible (b). In this way a single grinding wheel suffices without redressing.

An analogous principal applies to grinding tapered shafts (a). If the tapers are equal (b), a single setup suffices for both tapers.

When only a portion of a shaft needs to be finish ground, this fact should be specified on the drawing, so to avoid unnecessary machining. A diametral difference of about 0.040 in. between the two portions of the shaft would ensure recognition of the distinction and involve minimum turning.

When grinding between centers, as in (a), the driving dog would be held by means of a ground surface [left end in (a)]. An additional unground surface, as in (b), for holding the driving dog, is preferable and permits one-setup machining. The added shaft portion is removed after grinding.

Levers which are integral with shafts (a) should be designed so as not to interfere with the grinding wheel, for example, by necking the shaft adjacent to the lever (b).

In order to avoid complicated setups or special grinding wheels in the case of surfaces which are not readily accessible (a), modify the design by increasing the cutout area as in (b), so to provide easier access for the grinding wheel.

The grinding of the stepped shaft (a), requires that the ends of the part be center-drilled for grinding (in two setups). A split bushing added to the central portion in design (b) would be equivalent and allow centerless grinding. The most advantageous design, if permissible, is the one-diameter shaft (c).

For centerless grinding, replacing the two-diameter shaft (a) with a one-diameter shaft (b) with provision for the addition of a split retaining ring is advantageous, when possible (this permits one-setup centerless grinding).

In the case of a cylindrical shaft with a slot, the shaft needs to be machined (centerless ground) before the milling of the slot or keyway.

The grinding between centers of the cylindrical shaft with a thin-wall section (arising from the two-diameter bore) is difficult. Thin-walled tubing, on the other hand, can be centerless ground even with the two internal diameters.

Grinding journal seats on a shaft with two shoulders (a) is expensive. One shoulder, as in (b), is preferable. Of course, clearance at both ends is even more desirable.

A shaft with a sizable bore at one end (a) requires an artificial center in order to permit grinding. One remedy is to turn down a short portion of the workpiece, so that an artificial center can be mounted on it (b).

When only one of the two parallel surfaces needs to be ground, try to arrange for the non-machined surface to lie below the other, in order to provide tool clearance.

(k) Effective Tapping

The first suggestion regarding tapping is: Don’t! Its easier, safer and less expensive to use a nut and bolt, when feasible.

Blind tapped holes should not be threaded all the way (a). A less expensive design, which also reduces the probability of tool breakage, is to leave an untapped portion, the length of which should not be less than the O.D. of the thread (b).

In the case of a partially tapped bore, the diameter of the smooth portion should, if possible, be equal to the internal diameter of the threads (c). Design (a) requires either two setups, or a tap, the unsupported length of which is larger than necessary. Design (b) could involve one setup (drilling from the bottom followed by tapping), but the unsupported length of the tap is larger than in (c), in which the machining proceeds from the top. Since tapping is a more critical operation than boring, the tapping requires the most favorable circumstances possible.

In tapping from a square shoulder (a), there is more chance of material breakage at entry than with a slight countersunk surface as in (b). The countersink also provides better bearing conditions for the nut or bolt, which is added during assembly.

(l) Miscellaneous

Inspection: checking the center distance between two blind holes (a) is difficult. If feasible, one of the holes should be a through hole (b).

Avoiding stress concentration: In order to avoid the buildup of stresses, move hole “h” [see (a)] away from the sudden change of shaft cross-section (at the shoulder). Relocate closer to the upper end of the shaft, as in (b).

Part B: Assembly

This section contains suggestions for designing parts for ease of assembly and disassembly. Specific topics include the following: avoiding dimensional overspecification of mating parts, minimizing the need for close tolerances, allowing for thermal expansion and wear, design for ease of assembly, accessibility and ease of disassembly. As in Part A, design (a) generally represents the incorrect design, while other constructions (b, c, d, etc.) represent improvements.

(a) Avoiding Overspecification of dimensions of Mating Parts

The depth of the bore for the stepped shaft, (a), should not need to match the length of the small-diameter portion of the shaft. This requires unnecessarily close tolerances, so that the shoulder seats square against the wall. In design (b) the hole is slightly elongated, thereby avoiding the need for close tolerances while insuring a square seat for the shoulder.

The axial position of the busing relative to the housing is overdetermined in (a). A one-diameter shaft, (b), eliminates this drawback.

This case is analogous to the preceding. In this case both the radial and axial position of the bushing is overdetermined (a). To avoid the need for matching four dimensions between bushing and housing, add an axial and radial clearance as shown in (b). This reduces the number of matching dimensions to two.

The same principle applies in the assembly of a hollow shaft and bushing. In (a) the mating shaft lengths must match, and this requires close tolerances. In design (b) the length of the reduced-diameter section of the male shaft has been slightly reduced, thereby eliminating the need for close tolerances.

Another example: the shaft-and-collar assembly (a). The need to match two radii (a) is reduced to one in the design (b).

Similarly, for the assembly of threaded tubing (a), arrange for clearance for the small-diameter section of the male part, as in (b), to avoid the need for matching the 0.4-inch dimensions.

For the clamping of a housing with a nut and bolt, the thread on the reduced-diameter portion of the bolt should extend into the housing as in (b), so that the width of the housing does not have to match the length of the unthreaded portion of the bolt, as in (a). Design (b) avoids the possibility of the nut tightening against the bolt, rather than the housing.

Another illustration of the same principle. In order to avoid play in the axial position of the ball bearing (a), allow for a small gap between the outer radial portions of the cover plate and housing as in (b). The axial constraint on the ball bearing then does not require matching dimensions of cover plate and housing.

The top of a filister-head screw (or a hexagonal socket-head screw) should not be flush with the top surface of the part, as in (a). This would require unnecessarily close tolerances for the head of a crew and the counterbore. A small increase of the depth of the counterbore (b) eliminates the need for the close tolerances.

Another similar situation: in the case of a two-diameter shaft (a), avoid the need to match both shaft diameters by added a clearance, if possible, for one of them as in (b).

The design of the hinge (a) involves several interdependent dimensions with fairly close tolerances. In design (b) the lower hinge is axially floating and in design (c) the width of middle (rolled) portion of the right-hand member has been eliminated. In designs (b) and (c) the operation of the hinge remains reasonably efficient, but the need for close tolerances has been reduced.

In assembly of a plunger, (a), which is to exert pressure on the bottom of a hole and which, at the same time, must be capable of rotation, make the matching surfaces at the bottom of the hole flat (b) to simplify machining and leave a large clearance between the pins and groove in the plunger [as in (b)], so that the pins are not under pressure.

If the distance, “n”, [see (a)], must be maintained within close tolerances, it is desirable to add a separate bushing (b) and shortened the length of the intermediate portion of the shaft to allow for a clearance between the hubs. The bushing is easier to adjust than the hub of a heavy part, such as a flywheel or large gear. The redimensioning of the shaft eliminates the need for matching the length of the middle portion of the shaft to the width of the corresponding hub.

The pin in design (a) serves both to transmit torque and to locate the ring axially on the shaft. Avoid the overdetermination of the axial position of the pin by the shaft shoulder by considering the one-diameter shaft design (b).

The matching of fillets in assembly (a) is expensive and usually not necessary. A simple countersink (b) eliminates the difficulty.

It is difficult to maintain close tolerances on distances “n” in the cast cover (a). A one-diameter bushing held in place by a set screw (b) simplifies the construction and permits some reduction in the length (and hence also the weight) of the cover (b).

Similarly, avoid matching recesses and shoulders [as is the case in (a)], if possible. A screw fastening (b) is more economical and in most case equally efficient.

The same principle applies to the assembly of flat surfaces.

If two flat interchangeable parts need to be aligned by dowel pins, the center distance between them must be held within close tolerances (a). When possible, use a pin-and-slot construction for one of the pins (b), which circumvents this difficulty.

Another illustration of a means for avoiding close tolerances in matching component dimensions (A). The assembly (a) requires matching the length of the pin to the combined width of the links. In (b) the addition of a shim and bushing (insert) eliminate the need for the matching of part dimensions.

The use of shims for tapered gears (a) or eyebolts (b) represents one solution for the problem of controlling the dimension “n” within definite limits, including the case in which the angular orientation of the eye (b) is specified.

(b) Design Changes Which Avoid the Need for Close Tolerances

When possible, avoid using countersunk screws for fastening (a). Any lateral (e.g., center-distance) or angular misalignment stresses the screw and changes its height relative to the part surface. Square-shouldered heads located in oversize counterbores (b) are preferable.

Tolerances on assemblies involving dowl pins can be loosened substantially using roll pins (b) or groove pins.

In the case of a flywheel-and-shaft assembly in which two bushings constrain the axial position of the flywheel, a groove pin (b) eliminates the need for who a two-diameter shaft, which would be needed to facilitate assembly in case (a).

A key in a tapered shaft should never touch the bottom of the corresponding seat, as might occur in (a). A broached, straight keyway (b) represents an improved design.

(c) Allowing for Thermal Expansion

To allow for the thermal expansion of a rotating shaft and avoid possible binding (a), it is preferable to mount the ball bearings in lapped holes (b) with one bearing held in position by a split retaining ring and the other bearing allowed to float axially in the smooth machined bore.

In the case of collars which control the axial positions of a shaft (a), it is preferable to install the collars at one end of the shaft only, thereby allowing for thermal expansion (b).

(d) Allowing for Wear in Design and Assembly

In order to loosen the tolerances on the link lengths b, c, d, of a four-bar linkage, it is sometimes desirable to provide for an adjustable floating link. The length of the floating link can be adjusted, for example, by means of a three-piece threaded connection (b), consisting of a nut and one left-and one right-handed threaded link.

In the case of a rotating shaft with a precision assembly (a), a split bushing tapered towards the exit side (b) permits takeup of wear and thus prolongs the service life of the assembly.

Another illustration of wear compensation by means of an extra tapered key (b), which also eases the tolerances on the dovetail.

In the case of repeated assembly/disassembly of a shaft, which is press-fitted into a plate (a), an integral clamp with a screw-and-nut fastener (b) is more expensive, but more efficient.

A similar case involving a press-fitted dovetail insert (a). For looser tolerances and ease of repeated assembly/disassembly it is preferable initially to have a running fit between dovetail and slide and add a slot and oversized pin (b).

(e) Design for Ease of Assembly and Disassembly

Sometimes an alignment pin had to be inserted in a blind hole e.g., when one part is too long, and the aligning hole is too far away from the side of the workpiece to permit a transverse expulsion hole. In such a case an air-vent hole (a) or a flat on one side of the dowel pin (b) is recommended in order to prevent air compression during assembly.

Relying on threads for alignment is not reliable (a). For threaded plungers, alignment should be provided by adding a small, smooth, cylindrical shaft at the foot of the plunger (b). In the case of long plungers, two aligning diameters are desirable (c). These will ensure squareness in assembly.

In assembling a pin, plunger or shaft with the mating hole, avoid a square-ended shaft entering a square shoulder (a). Countersinking the bore and the entering portion of the shaft facilities smooth entry without nicking corners.

Similarly, in the case of screws and bolts (a), addition of a tapered, pointed end and a corresponding countersink of the tapped hole (b) facilitates assembly.

Long shafts and bushings are easier to assemble with a two-diameter construction. In such cases avoid having to match counterbore depth with the length of the reduced-diameter portion of the shaft (a). The depth of the counterbore should be slightly less than the length of the reduced-diameter portion of the shaft.

In order to secure the axial position of a threaded cap, the use of a slot and matching strip (a) implies that locking can occur only after assembly of the cap. The circumferential knurl in (b) permits clamping in practically any position of the cap.

(f) Design for Accessibility

In an assembly in which the bolt head is not readily accessible (a), an access hole for a wrench is provided in design (b) or – better still – the bolt can be replaced by readily assembled threaded pins (c).

Similarly a nut and bolt (a) should be located for easy accessibility as in (b) for tubular or Allen wrenches (the latter for a hexagonal socket head).

(g) Design for Ease of Disassembly

When using dowel pins for alignment, avoid blind holes (a). The addition of a small diameter exit hole (b) permits introducing a punch for extracting the dowel pin. Better still, if possible, use a one-diameter bore (c); and if one part is too long for easy extraction, increase the diameter of the clearance hole (d), which does not require close tolerances.

When planning for disassembly the threads of screws used for safety, locking or locating purposes, such as in (a), should be arranged to avoid widening (mushrooming) of the thread ends, which would make it difficult to remove the screw. Adding a reduced diameter section (b) remedies this difficult.

In separating parts with a light press fit, separating bolts and extractions are useful. A simple bolt design is shown in (a) and a bolt-and-conical-wedge assembly is shown in (b). The conical-wedge assembly is more expensive, but limits the need for exerting a high force at the beginning of the extraction operation.

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