design hints

7/28/2019 Design Hints

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ART A: MACHININGhe following represents a collection of practical hints for saving time, labor, material and coshese involve designs for minimum tooling, efficient clamping and holding, simplifyingbassemblies and efficient turning, boring, drilling, milling, broaching, grinding and tapping.e figures, design (a) generally represents the incorrect design, while constructions (b, c, d ustrate improvements. The symbols V, VV ,VVV denote rough-machined,finish-machined anound surface finishes, respectively.

(a) Minimizing Tooling Costs

Avoid round shapes in screw-machine work,which require special forming tools. In desig(b) straight cutting tools can feed practicallysimultaneously at right angles and at 45º toscrew-machine axis.

If round shapes are necessary, avoid radii eqto half part width (a). Such radii are sensitivmachining accuracy and tool and workalignment. Radii greater than one half partwidth (b) avoid the need for such accuracy.

The initial cost and maintenance of a millingcutter with curved teeth (a) is greater than tof a cutter with straight teeth. The latter aremore suited to high production rates and canalso be made with a combination of twostandard cutters, if desired (b).

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

The need for two face cutters of different sizas in (a), may be avoided it the locatingsurfaces in the housing can be simplified to few appropriately spaced ribs (b, c).

Catalog D220

Reprinted by permission of Mr. Frederico Strasser



To accommodate a shouldered shaft with asmall taper (a), a special combinationcountersinking and counterboring tool isrequired for the housing. Redesign (b) requionly a standard countersinking tool and locathe shaft relative to the housing equally welt

(b) Minimizing Machining

To minimize machining of the casting shownpartial cross-section in (a), relieve base, leaonly a peripheral bearing area; and designupper surface so that machining is limited tocircular ring sections around holes (b). Finishmachining of the ring section involves simplecounterbores, rather than milling or grinding

The amount of drilling of a long hole (a) forguiding a reciprocating rod can be reduced bthe cutout shown in (b), relieving the centraportion of the bar without impairing its funct

To reduce the machining required in design the heights of the three projections, which ato be machined, should be equal, if possibleThey should also be lined up horizontally inorder to minimize the width of the machiningoperation.

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

When machining two shaft diameters, thesquare shoulder requires a special setup or aseparate squaring tool (a). A tapered should(b) can be produced by the same tool used fturning.

o accommodate a shouldered shaft with a small taper



In the design of a clamp, an eccentric hole (requires less metal removal (from the

originbar stock) than a concentric hole (a).

(C) Allowing Adequate Clearances ForCutting Tools

At the junction of a reduced-diameter shaft the main body, the shoulder should not interwith the cutting tool, as it does in (a). A reliegroove or neck is desirable to free the tool acutting (b). Note: Short screws in which thethreads are formed by thread-rolling or

thread-milling do not always require relief . latter saves machining and increases partstrength.

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

For internal boring or drilling (a), add a reliegroove as in (b, C) for tool clearance.

The same applies to internal grinding (a). Aclearance hole should be added (b) so that tgrinding wheel will run freely after completioits task.

Always allow free drill exits, especially withhidden inclined ribs.

Similarly, allow clearances for shaping andplaning tools. Ditto for milling cutters andgrinding wheels.

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

in order to accommodate a large milling cuttor grinding wheel (a), the relief groove mayneed to be quite long (b).

To provide relief for an internal, blind keywa(a), the design may include a hole (b), or aninternally bored groove (c). The latter Isexpensive, but may be desirable when there

several circumferentially disposed keyways (in a spline), or when drilled holes are notfeasible.

Tapered surfaces require ample tool clearancFor this reason the shoulder in (a) Is bestavoided as in (b).

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

(d) Effective Clamping in Machining andAssembly

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

Sometimes clamping is achieved by permane

metal deformation--in this case from a tapersurface (a) to a cylindrical surface (b).

nstead of a simple relief groove



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

Avoid the need for shims, plates etc. in clam(a), by arranging bearing surfaces to becoplanar (b). The latter also leads to a saferfaster clamping operation.

For facilitating the machining of a crank. shaend lugs have been added (one at each end)Each lug has two holes in order to be able tohold and turn crankshaft and crank. Aftercompletion of machining the lugs are remove

(e) Simplifying Machining With Compos

AssembliesThe one-piece machined shaft (a) cansometimes be replaced by cold-drawn rod antubing (b); in the case of low loads the tubinmay be replaced by split retaining rings (c).Constructions (b) and (c) eliminate or reducboth machining and metal removal.

in the case of a shaft (a) with a hub, machinintegrally from stock involves removing arelatively large amount of metal, or resorting

an expensive forging. Alternative designsinvolve a cold-drawn rod or bar to which aseparate ring has been added and fastened the shaft by a set screw (b), press fit (c), orwelds (d, e).

Sometimes a ground shaft with a groundshoulder (a) can be replaced by a one-diamecenterless-ground shaft and a split retainingring.

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Similarly (a case history), a three-piece assemblywhich was originally assembled with a stepped shring-shaped washer and dowel pin (a), wasredesigned with a one-diameter shaft and two do

pins (b).

Another case history Involved a shaft in which alocating ring and set screw (a) were replaced by aexternal, split retaining ring (b)

For retention of a spring a special screw and tapphole (a) has been replaced by an internal retaininring (b).

Another method of replacing a stepped shaft withseveral diameters (a)--in this case with a single s

threaded at one end to receive the matchingbushing. Design (b) involves less metal removal aless machining, except for the added thread.

The advantage of a one-diameter construction iseven more pronounced in the case of internal boror drilling. Instead of boring from two sides (twosetups, requiring close alignment) as in (a), aone-diameter bore with an insert is preferable. Thinsert can be in the form of a bushing (b), or twosplit retaining rings (C).

A specific illustration of the preceding principle: tball-bearing housing design (a) is simplifiedconsiderably by means of the composite construcconsisting of a one-diameter bore and split retainrings (b).

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

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A blind bore with a deep recess (a) is expensIf feasible, change to a one-diameter bore apress-fitted bushing (b).

(f) Effective Turning And Boring

Starting out with a shaft which is too big isuneconomical (a shaft with an O.D. as large in (a) requires too much machining). In thiscase the middle portion could have been leftrough. Starting with drawn rod of the right s(b), only the two ends need to be machinedand metal removal is greatly reduced.

Square ends (a) represent a hazard (injury tfingers), are easily, damaged and more diffic

to assemble. Although rounding off is better a small chamfer (C) is safest and leastexpensive.

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

Long small diameter bores (a) should beavoided, due to high machining costs andpossibility of tool breakage.

Long bores for guiding shafts (a) should beenlarged in the middle section (rough boringsuffices), in order to facilitate guiding the shand reduce finish machining.

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

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A blind bore with a deep recess



Machining shaft (a) between centers is hindeby the internal thread. A design change to aexternal thread (b), if feasible, would permitsuch machining and eliminate the need forspecial tooling.

In the case of screw-machine work on roundstock whose OD (d) does not need machinin(a), keep the amount of metal removal to aminimum by maximizing dimension d1 andminimizing dimension L1, as in (b).

For efficient screw-machine work let the cutttools feed longitudinally and, if possible,simultaneously. In order to facilitate this, pa(a) should be redesigned so that the right-ediameter is the smallest. In addition, the

grooves should be narrow in order to avoidspecial tooling (b).

In the multiple-step machining of a part inscrew-machine work, machining time isgoverned by the deepest step (e.g. in the caof a single radially moving cutter havingmultiple cutting edges). Hence, part (a) shobe redesigned so that all steps have the samdepth (b).

(g) Effective Drilling

If only a short portion of a bore is used in apress-fit assembly, stich as in (a), it is notalways necessary to ream the entire hole, buis necessary to leave sufficient clearance forreaming (b).

To start a drill, the surface to be drilled shoube perpendicular to the drill axis in order toprevent the drill from skidding, wear andbreaking. Adding a flat as in (b) is desirable.

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Machining shaft



The same principle applies when drilling an inclinedsurface.

The principle applies also at the exit side of the drilhole, even though it is not as critical as the entranc

side. While (b) Is preferable to (a), both entrance aexit need to be "perpendicular design's", as in (c) a(d).

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

Blind holes with square ends are expensive. Best anleast expensive design is to bottom with a tapercorresponding to the 118º angle of a standard drill If a square end cannot be avoided, include as much

the taper as possible (c), or better still, add asmall-diameter bore (d). If a dowel pin is to fit in thmain bore, design (d) is necessary in order to be ato remove the pin, if needed, and to avoid trapped which may impede assembly.

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

When drilled holes intersect, the drill deflects whensecond hole is drilled. The distance, "1", between t

axes of the holes should be sufficient, as in (b), forfollowing procedure: first, hole "d is finish drilled; tthe second hole is predrilled with diameter d1 finalldiameter d1 is in. creased to d2 using a counterborwith a guided pilot of diameter d1. If the distance `too short, as In (a), hole "d" would have to be filledwith a tight plug (of the same material as theworkpiece), which is removed after completion of tdrilling.

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he same principle applies when drilling an inclined surface



When the radial position of two parts, such as a shaand housing, must be coordinated with a "Dutchmanpin" (a), the two parts should have the same matericharacteristics (e.g. hardness, ultimate strength,chip-forming strength etc.) in order to prevent drilldeflection towards the softer material, breakage andinaccurate machining.

In the case of a piloted counterbore or stepped reamthe ratio of the hole diameters (d1:d2) should confoto the established ratio furnished by the toolmanufacturer.

(h) Efficient Milling

To minimize machining costs avoid blind keyways (aor keyways extending up to a square shoulder (whic

would Involve needlessly expensive machining). Whdesign (C) represents an improvement, it is better sto add a curved exit, which permits use of a moresturdy milling cutter (d). The best design is a Woodrkey (e).

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

The flat-bottomed slot in the fork-like work-piece (arequires a large travel on the part of the milling cutt

By curving the bottom so as to match the radius of tcutter, as in (b), the tool travel (and hence machinintime) are significantly reduced.

The milling of a flat surface with a projection (a)requires cuts in two directions and careful machiningavoid a slight mismatch between the milled surfacespossible, raise one surface intentionally, as in (b), inorder to reduce the precision needed for the millingoperation.

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When the radial position of two parts



When milling evenly spaced circumferentiallydisposed slots, an-even number of slots (a)

requires twice as many passes as slots. An oddnumber of slots requires only as many passes slots (b). Hence, everything else being equal, odd number of slots is preferable.

If the reduced-diameter portion of a shaft (a) be milled, a small shoulder should be providedfeasible, in order to prevent damage by the mcutter to the main shoulder.

(I) Effective Broaching

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

If two slots are needed in a tapered bore. desi(a) involves two setups and two separatemachining operations. If. on the other hand, tbroached slots are de signed parallel to the boaxis (b), they an be produced in one setup and

one operation.Broaching slots of different widths at requiresbroaching cutters of different sizes. Slots of thsame width, but different depths, can bemachined with the same broach (b) and thus son tooling.

Unsymmetrical part design leads to tool deflecand extra load. A symmetrical design (b) ispreferable.

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When milling evenly spaced circumferentially disposed slots



Burrs are difficult to remove from broachedinternal splines. If slots are produced afterbroaching, burrs are created internally. Hencdesign (a) should be modified so as to avoidteeth adjacent to the slot (b).

(J) Effective Grinding

When several radii are to be ground (a), it isdesirable to keep the radii equal, If possible In this way a single grinding wheel sufficeswithout redressing.

An analogous principle applies to grindingtapered shafts (a). if the tapers are equal(b)single setup suffices for both tapers.

When only a portion of a shaft needs to be f

ground, this fact should be specified on thedrawing, so as to avoid unnecessary machinA diametral difference of about 0.040 in.between the two portions of the shaft wouldensure recognition of the distinction and invominimum turning.

When grinding between centers, as In (a), tdriving dog would be held by means of a grosurface [left end in (a)). An additional ungrosurface, as in (b), for holding the driving dogpreferable and also permits one-setup

machining. The added shaft portion is removafter grinding.

Levers which are Integral with shafts (a) shobe designed so as not to interfere with thegrinding wheel, for example, by necking theshaft adjacent to the lever (b).

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Burrs are difficult to remove from broached internal splines



In order to avoid complicated setups or specgrinding wheels in the case of surfaces whic

are not readily accessible (a), modify the deby increasing the cutout area as in (b), so asprovide easier access for the grinding wheel

The grinding of the stepped shaft (a), requirthat the ends of the part be centerdrilled forgrinding (In two setups). A split bushing addto the central portion in design (b) would beequivalent and allow centerless grinding. Themost advantageous design, if permissible, isone diameter shaft (c).

For centerless grinding, replacing the twodiameter shaft (a) with a one-diameter shafwith provision for the addition of a split retaring is advantageous, when possible (thispermits one-setup centerless grinding).

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

The grinding between centers of the cylindricshaft with a thin-wail section (arising from t

two-diameter bore) is difficult. Thin-walledtubing, on the other hand, can be centerlessground even with the two internal diameters

Grinding journal seats on a shaft with twoshoulders (a) is expensive. One shoulder, as(b), Is preferable. Of course, clearance at boends Is even more desirable.

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n order to avoid complicated setups or special grinding wheels in the case of surfaces which are not readily accessible



A shaft with a sizable bore at one end (a) requan artificial center in order to permit grinding. remedy is to turn down a short portion of theworkpiece, so that an artificial center can bemounted on it (b).

When only one of two parallel surfaces needs tbe ground, try to arrange for the non-machinesurface to lie below the other, in order to provtool clearance.

(k) Effective Tapping

The first suggestion with regard to tapping is:

Don't! It is easier, safer and less expensive to a nut and bolt, when feasible.

Blind tapped holes should not be threaded all tway (a). A less expensive design, which alsoreduces the probability of tool breakage, is toleave an untapped portion, the length of whichshould not be less than the O.D. of the thread

In the case of a partially tapped bore, thediameter of the smooth portion should, if possbe equal to the internal diameter of the thread

(c). Design (a) requires either two setups, or atap, the unsupported length of which is largerthan necessary. Design (b) could involve onesetup (drilling from the bottom followed bytapping), but the unsupported length of the talarger than in (c), in which the machiningproceeds from the top. Since tapping is a morecritical operation than boring, the tapping requthe most favorable circumstances possible.

In tapping from a square shoulder (a), there ismore chance of material breakage at entry tha

with a slight countersunk surface as in (b). Thcountersink also provides better bearingconditions for the nut or bolt, which is addedduring assembly.

A shaft with a sizable bore at one end



(I) Miscellaneous

Inspection: checking the center distancebetween two blind holes (a) is difficult. If feasible, one of the holes should be a throughole (b).

Avoiding stress concentration: In order to avthe buildup-of stresses, move hole "h" [see away from the sudden change of shaftcross-section (at the shoulder). Relocate cloto the upper end of the shaft, as in (b).

ART B: ASSEMBLY

his section contains suggestions for designing parts for ease of assembly and disassembly.pecific topics include the following: avoiding dimensional overspecification of mating parts,inimizing the need for close tolerances, allowing for thermal expansion and wear, design for

ase of assembly, accessibility and ease of disassembly. As in Part A, design (a) generallypresents the incorrect design, while other constructions (b, c, d etc.) represent improveme

(a) Avoiding Overspecification Of Dimensions Of Mating Parts

The depth of the bore for the stepped shaft,

should not need to match the length of thesmall-diameter portion of the shaft. Thisrequires unnecessarily close tolerances, so tthe shoulder seats square against the wall. Idesign (b) the hole is slightly elongated, theavoiding the need for close tolerances whileinsuring a square seat for the shoulder.

The axial position of the bushing relative to housing is overdetermined in (a). A onediamshaft, (b), eliminates this drawback.

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This case is analogous to the preceding. In tcase both the radial and axial position of thebushing is overdetermined (a). To avoid theneed for matching four dimensions betweenbushing and housing, add an axial and radiaclearance, as shown in (b). This reduces thenumber of matching dimensions to two.

The same principle applies in the assembly ohollow shaft and bushing. In (a) the matingshaft lengths have to match and this require

close tolerances. In design (b) the length of reduced-diameter section of the male shaft hbeen slightly reduced, thereby eliminating thneed for close tolerances.

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

Similarly, for the assembly of threaded tubin(a), arrange for clearance for the smalldiamesection of the male part, as in (b), to avoid t

need for matching the O.4ùinch dimensions.For the clamping of a housing with a nut andbolt, the thread on the reduced-diameter poof the bolt should extend into the housing as(b), so that the width of the housing does nohave to match the length of the unthreadedportion of the bolt, as an (a). Design (b) avothe possibility of the nut tightening against tbolt, rather than the housing.

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Another illustration of the same principle. In ordto avoid play in the axial position of the ballbearing (a), allow for a small gap between theouter radial portions of the cover plate and houas in (b). The axial constraint on the ball bearin

then does not require matching dimensions of cover plate and housing.

The top of a filister-head screw (or a hexagonasocket-head screw) should not be flush with thetop surface of the part, as In (a). This wouldrequire unnecessarily close tolerances for the hof a screw and the counterbore. A small increasthe depth of the counterbore (b) eliminates theneed for the close tolerances.

Another similar situation: in the case of a

two-diameter shaft (a), avoid the need to matcboth shaft diameters by adding a clearance, if possible, for one of them, as In (b).

The design of the hinge (a) involves severalinterdependent dimensions with fairly closetolerances. In design (b) the lower hinge is axiafloating and in design (c) the width of the midd(rolled) portion of the righthand member has beeliminated. In designs (b) and (C) the operatiothe hinge remains reasonably efficient, but theneed for close tolerances has been reduced.

In the assembly of a plunger, (a), which is to epressure on the bottom of a hole and which, atsame time, must be capable of rotation, make tmatching surfaces at the bottom of the hole flato simplify machining and leave a large clearanbetween the pins and groove in the plunger [as(b)], so that the pins are not under pressure.

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If the distance, "n',, [see (a)], has to be maintainedwithin close tolerances, it is desirable to add a separa

bushing (b) and shorten the length of the intermediaportion of the shaft to allow for a clearance between hubs. The bushing is easier to adjust than the hub ofheavy part, such as a flywheel or large gear. Theredimensioning of the shaft eliminates the need formatching the length of the middle portion of the shafthe width of the corresponding hub.

The pin in design (a) serves both to transmit torque ato locate the ring axially on the shaft. Avoid the

overdetermination of the axial position of the pin by tshaft shoulder by considering the one-diameter shaftdesign (b).

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

It is difficult to maintain close tolerances on distance in the cast cover (a). A one-diameter bushing held inplace by a set screw (b) simplifies the construction apermits some reduction in the length (and hence alsoweight) of the cover (b).

Similarly, avoid matching recesses and shoulders [asthe case in (a)], if possible. A screw fastening (b) ismore economical and in most cases equally efficient

The same principle applies to the assembly of flatsurfaces.



If two flat interchangeable parts need to be alignby dowel pins, the center distance between themto be held within close tolerances (a). When poss

use a pin. and-slot construction for one of the pin(b), which circumvents this difficulty.

Another illustration of a means for avoiding closetolerances in matching component dimensions (AThe assembly (a) requires matching the length ofpin to the combined width of the links. In (b) theaddition of a shim and bushing (insert) eliminate need for the matching of part dimensions.

The use of shims for tapered gears (a) or eyebolt(b) represents one solution for the problem of

controlling the dimension "n" within definite limitsincluding the case in which the angular orientatiothe eye (b) is specified.

(b) Design Changes Which Avoid The Need FTolerances

When possible, avoid using countersunk screws fofastening (a). Any lateral (e.g. center-distance) oangular misalignment stresses the screw andchanges Its height relative to the part surface.Square-shouldered heads located in oversize

counterbores (b) are preferable.Tolerances on assemblies involving dowel pins caloosened substantially by the use of roll pins (b) groove pins.

In the case of a flywheel-and-shaft assembly inwhich two bushings constrain the axial position oflywheel, a groove pin (b) eliminates the need fortwo-diameter shaft, which would be needed tofacilitate assembly in case (a).

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A key in a tapered shaft should never touch the botof the corresponding seat, as might occur in (a). Abroached, straight keyway (b) represents an impro

design.

(C) Allowing For Thermal Expansion

To allow for the thermal expansion of a rotating shaand avoid possible binding (a), it is preferable tomount the ball bearings In lapped holes (b) with onbearing held In postion by a split retaining ring andother bearing allowed to float axially in the smoothmachined bore.

In the case of collars which control the axial positio

of a shaft (a), it Is preferable to install the collars aone end of the shaft only, thereby allowing for therexpansion (b).

(d) Allowing For Wear In Design And Assembl

In order to loosen the tolerances on the link lengthc, d, of a four-bar linkage, It is sometimes desirablprovide for an adjustable floating link. The length othe floating link can be adjusted, for example, bymeans of a three piece threaded connection (b),consisting of a nut and one left- and one right-hand

threaded link.In the case of a rotating shaft with a precisionassembly (a), a split bushing tapered towards the eside (b) permits takeup of wear and thus prolongs tservice life of the assembly.

Another Illustration of wear compensation by meanan extra tapered key (b), which also eases thetolerances on the dovetail.

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

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

(e) Design For Ease Of Assembly AndDisassembly

Sometimes an alignment pin has to be insertin a blind hole e.g. when one part is too longand the aligning hole is too far away from th

side of the workpiece to permit a transverseexpulsion hole. In such a case an air-vent ho(a) or a flat on one side of the dowel pin (b)recommended In order to prevent aircompression during assembly.

Relying on threads for alignment is not reliab(a). For threaded plungers alignment shouldprovided by adding a small, smooth, cylindrishaft at the foot of the plunger (b). In the caof long plungers, two aligning diameters aredesirable (c). These will ensure squareness i

assembly.

In assembling a pin, plunger or shaft with thmating hole, avoid a square-ended shaftentering a square shoulder (a). Countersinkithe bore and the entering portion of the shaffacilitates smooth entry without nicking corn(b).

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n the case of repeated assembly



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

Long shafts and bushings are easier to assemwith a two-diameter construction. In such caavoid having to match counterbore depth withe length of the reduced-diameter portion othe shaft (a). The depth of the counterboreshould be slightly less than the length of thereduced-diameter portion of the shaft.

In order to secure the axial position of athreaded cap, the use of a slot and matchingstrip (a) implies that locking can occur only assembly of the cap. The circumferential knu(b) permits clamping iii practically any positiof the cap.

(f) Design For Accessibility

In an assembly in which the bolt head is notreadily accessible (a), an access hole for awrench Is provided in design (b) or-betterstill-the bolt can be replaced by readilyassembled threaded pins (c).

Similarly a nut and bolt (a) should be locateeasy accessibility as in (b) for tubular or Alle

wrenches (the latter for a hexagonal sockethead).

(g) Design For Ease Of Disassembly.

When using dowel pins for alignment, avoidblind holes (a). The addition of a small-diamexit hole (b) permits Introducing a punch forextracting the dowel pin. Better still, if possiuse a one-diameter bore (c); and if one parttoo long for easy extraction, increase thediameter of the clearance hole (d), which do

not require close tolerances.

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When planning for disassembly the threads oscrews used for safety, locking or locatingpurposes, such as in (a), should be arrangedas to avoid widening (mushrooming) of thethread ends, which would make it difficult toremove the screw. Adding a reduced-diametsection (b) remedies this difficulty.

In separating parts with a light press fit,separating bolts and extractors are useful. Asimple bolt design is shown in (a) and abolt-and-conical-wedge assembly is shown i(b). The conical-wedge assembly is moreexpensive, but limits the need for exerting ahigh force at the beginning of the extractionoperation.

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design hints

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