SCREW THREADS.pdf

4.83

CHAPTER 4.8

FIGURE 4.8.1 Unified national thread.

FIGURE 4.8.2 Square thread.

SCREW THREADS

Engineering StaffTeledyne Landis Machine

Waynesboro, Pennsylvania

THREAD SYSTEMS

Figures 4.8.1 through 4.8.6 illustrate common thread forms.The unified national screw thread (see Fig. 4.8.1) was adopted in 1948 as the pre-

ferred system for fasteners in the United States, Great Britain, and Canada. It is verysimilar to the earlier American standard system. Common designations are UNC(coarse), UNF (fine), UNEF (extra fine), and UNS (special).

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Source: DESIGN FOR MANUFACTURABILITY HANDBOOK

4.84 MACHINED COMPONENTS

FIGURE 4.8.3 General-purpose Acme thread.

FIGURE 4.8.4 National buttress thread.

FIGURE 4.8.5 NPT pipe thread.

FIGURE 4.8.6 ISO metric thread.



SCREW THREADS

SCREW THREADS 4.85

There are three common thread classes in the unified system. Class 1 has the loos-est fit and the broadest dimensional tolerances, Class 2 is the most common class forfasteners with closer fits and tolerances, and Class 3 is for more precise or criticalapplications. The letter A designates external threads and the letter B internal threads.

The standard method for designating a screw thread is to specify in sequence thenominal size, number of threads per inch, thread-series symbol, and thread-class sym-bol, supplemented optionally by pitch diameter and its tolerance. An example of anexternal thread designation and what it means is1⁄4–20–UNC–3A

� thread-class designation� thread-series designation�number of threads per inch (pitch)�

nominal size (in)

The square thread form (see Fig. 4.8.2) is the most efficient form for the transmis-sion of power. However, it is more expensive to produce than other forms and has beenlargely superseded by the Acme thread form.

Acme threads (see Fig. 4.8.3) are also used for power transmission and are easier tomanufacture than square threads, but their power-transmission capabilities are slightlylower. Some valve stems and many lead screws use this thread form.

Buttress threads (see Fig. 4.8.4) transmit power in one direction with virtually thefull efficiency of a square thread but are relatively easily produced because of thetapered backside of the tooth form. They are used in military applications and whentubular members are screwed together.

American standard taper pipe thread (NPT), with the form shown in Fig. 4.8.5, isthe standard thread for piping in the United States. Straight (nontapered) pipe threadsand dry-seal pipe threads have similar forms.

The ISO (International Organization for Standardization) metric screw thread (seeFig. 4.8.6) is the prime metric screw thread for fasteners.

Standard nomenclature for thread forms is illustrated by Fig. 4.8.7. Coarse threadsare suitable for general use, particularly in machines and other fastener applications inwhich quick and easy assembly is important. Fine threads are used when the designrequires increased strength or reduced weight.

FIGURE 4.8.7 Standard nomenclature for screw-thread elements.



SCREW THREADS

In addition to fasteners—bolts, machine screws, setscrews, cap screws, and studs—and power applications (vises, clamps, fixtures, and screw jacks), screw threads areused to control position accurately as in machine lead screws and vehicle-steeringmechanisms, to feed materials, and to change rotary to linear motion.

The size range of commercial screw threads is vast. Screw threads as small as 0.3mm (0.012 in) in diameter and 140 threads per centimeter (360 per inch) are used inwatches. At the other extreme, 600-mm (14-in) pipe is threaded with 8.5-mm-pitch(two threads per inch) pipe thread.

Self-tapping screws are used for wood, sheet metal, fiberboard, and other softermaterials. The thread form differs from that used in machine screws and the shank isnormally tapered. Figure 7.1.31 illustrates some typical self-tapping screws.

THREAD-MAKING PROCESSES: THEIRAPPLICATIONS AND ECONOMICS

Hand Dies

An acorn or button die for external threads must be employed by hand; it is the leastdesirable of the methods that can be used to cut external threads. (See Fig. 4.8.8.)However, such dies can be used to advantage when a limited number of small-to medi-um-sized threads are to be cut and when accuracy of the thread lead in relation to thethread axis is not essential. Compared with other thread-making tooling, they are rela-

tively inexpensive and easy to use.

Single-Point Threading


FIGURE 4.8.8 Button die for external threads.(Courtesy Cleveland Twist Drill Co., subsidiary ofAcme-Cleveland Corp.)



SCREW THREADS

SCREW THREADS 4.87

With this method, a single-point tool having a profile corresponding to the profile ofthe thread is used as a means of generating the thread. Internal or external threads canbe produced by this method. A lathe is used, and its carriage is moved longitudinallyalong the part by a lead screw that is gear-driven from the spindle. The lead screwmoves the carriage and hence the tool at a rate exactly equal to the lead or pitch of thethread being produced. Generally, the thread is produced by making successive multi-ple passes. (See Fig. 4.8.9.)

Single-point threading is used more often when the workpiece is too large in diam-eter, the pitch too coarse, the material too difficult to machine, or the quantity toosmall to warrant using a die head. Holes as small as 8 mm (5⁄16 in) can be threaded bythis method.

Thread-Cutting Die Heads

Die heads (not to be confused with thread-rolling heads) are an efficient and popularmeans of threading. They are versatile, have relatively wide ranges, and are made in avariety of models and sizes for application to many types of machines, including lathes, chuckers, multiple-spindle screw and threading machines, drill presses, andother types.

Die heads have four or five insert form cutters. When the head is fed axially fromthe end of the work, the threads are cut. Once engaged, the head is self-feeding at therate of the thread lead. Cutter inserts can be removed for resharpening. Figure 4.8.10illustrates a stationary, self-opening die head used with production lathes, chuckers,and screw machines.

Die heads can be used economically from low to moderate to high production lev-els depending on the circumstances. Compared with rolling, the blank need not haveits diameter controlled as accurately because a certain amount of oversize can betrimmed away by the throat section of the chaser. Die-head chasers cost less than

FIGURE 4.8.9 Single-point screw-thread cutting.(Courtesy Teledyne Landis Machine.)



SCREW THREADS

thread rollers and usually can be salvaged if partially damaged. Setup for die-head cut-ting is also usually faster than for thread rolling.

Pipe-thread cutting is a common application for die heads.

Thread Milling

This process involves the use of a form-milling cutter that machines the thread form as


FIGURE 4.8.10 Stationary self-opening diehead. (Courtesy Teledyne Landis Machine.)

FIGURE 4.8.11 Thread milling. (CourtesyTeledyne Landis Machine.)



SCREW THREADS

the workpiece revolves. The most common type is the multiple-rib or multiple-formtype, as shown in Fig. 4.8.11. Single-rib cutters are also used. With these, the work-piece must make as many revolutions as there are threads on the work.

Thread milling can be applied internally and externally and can be used to producemost thread forms regardless of whether they are straight or tapered. Minimum inter-nal thread size is determined by the diameter of the cutter. Since interference is morepronounced because the cutter does not clear itself, the cutter normally should notexceed one-third of the hole diameter.

Thread forms that have flanks approaching 90° (to axis) are impossible to millbecause the cutter cannot enter the cut without shaving the flank.

Some very coarse threads that are to be ground are rough-milled and then finishedby grinding, possibly after a heat treatment.

Although thread milling is slower than die cutting, it is often necessary that athread be milled because of coarse pitch, large or odd-shaped parts, a high helix angle,extremely long thread lengths, workpiece geometry, poor machinability of the work-

piece material, or other considerations.As such, a single part or 10,000 piecesmight be an economical productionquantity.

Tapping

This process involves the use of a cylin-drical form cutter, a tap, that has multi-ple cutting edges. The tap rotates and isfed axially into the work to produceinternal threads. Both solid (Fig. 4.8.12)and collapsible (Fig. 4.8.13) taps areused. The operation can be carried outby hand or with drill presses, lathes,automatic screw machines, or specialtapping machines.

Solid taps are used mainly to threaddiameters ranging from 1.2 mm (0.047in) to 150 mm (6 in). While the collapsi-

ble tap is limited by design factors on the low side to around 32-mm (11⁄4-in) diame-ters, it is supplied for diameters as large as 600 mm (24 in). Solid taps are most eco-nomical in the 1.5-mm (1⁄16-in) to 50-mm (2-in) range. Although it is necessary toreverse the tap to back it out, in many cases the reversing operation can be done atmuch higher speed to reduce backout time.

Thread Grinding

Center-type grinding and centerless cylindrical grinding as described in Chaps. 4.13and 4.14 are used in the production of some screw threads. Single- or multiple-rib-form wheels are employed with center-type grinding, while multiple-rib wheels areemployed with centerless grinding. There is axial motion between the work and thewheel as the work rotates. Figures 4.8.14 and 4.8.15 illustrate the processes.

With center-type grinding, regardless of whether a single- or a multiple-rib wheel

SCREW THREADS 4.89

FIGURE 4.8.12 Solid tap for cutting internalthreads. (Courtesy Teledyne Landis Machine.)



SCREW THREADS


FIGURE 4.8.13 Collapsible tap. (Courtesy Teledyne LandisMachine.)

FIGURE 4.8.14 Center-type thread grinding. (Courtesy Teledyne Landis Machine.)



SCREW THREADS

SCREW THREADS 4.91

FIGURE 4.8.15 Centerless thread grinding. (Courtesy Teledyne Landis Machine.)

is used, the material specifications and the form, length, and quality of thread willdetermine the number of passes required to complete it. The number of passes canvary from one to five or six. With centerless grinding, the part is normally finished inone pass through the machine. As the work moves across the wheel, first it is sized tothe correct diameter and then the threads are formed.

Threaded parts that are ground include those which are too hard to cut, mill, or roll,when a fine finish is required or when precision form, lead, and pitch requirementsmust be held before and, most particularly, after hardening. Forms that are producedinclude API, NPT, and other taper pipe threads, 60° unified and metric, 55°Whitworth, 29° and 40° worm, 47°30″ British Association, 53°8″ Lowenhertz,Buttress, and others.

Centerless-ground threaded parts include continuous threaded parts such assetscrews, studs, threaded bushings, threaded size-adjusting bushings for boring heads,thread gauges, worm gears, powdered-iron screws, and self-threading insert bushings.

Materials that can be thread-ground include hardened and annealed screw stock,the alloyed high-speed tool and stainless steels, and sintered iron. The last-named isused widely for continuously threaded screws.

Center-type thread grinders are applicable to short as well as long production runs.Single-rib grinding wheels are more applicable to low production quantities and multi-rib wheels to mass production.

Setup times for hand operation range from 1⁄2 to 1 h and, for automatic operation,from 11⁄2 to 2 h.

Centerless thread grinding is used for high production quantities. On diameters 10mm (3⁄8 in) and larger, moderate quantities of 10 to 15,000 make for economical setupsand reasonable runs. When grinding diameters smaller than 10 mm, particularly whendiameter and length are the same or the length is up to 11⁄2 times the diameter, thequantity should exceed 15,000. On the smaller diameters, the setup is proportionallyharder and takes longer. However, the production rate is higher than on the largerdiameters. It is possible to run a 1⁄4-in, 20-pitch, 11⁄4-mm-long setscrew at 7500 pieces



SCREW THREADS

per hour.

Thread Rolling

Thread and form rolling is accomplished by having hardened-steel dies penetrateround blanks. The exertion of adequate force displaces the material into the voids andproduces a form the reverse of that on the die. Figure 4.8.16 shows the processschematically when flat dies are used. The main advantages of rolled threads overthreads produced by other manufacturing processes are that they have improved physi-c a l

characteristics, greater accuracy, and a high degree of surface finish. Another advan-tage is that there is no waste, since no material is removed in the formation of thethread.

With regard to the physical characteristics of a rolled thread, there is a substantialincrease in the tensile and shear strengths and resistance to fatigue. When a thread isproduced by other manufacturing processes, the grain fibers of the metal are severedin the formation of the thread. However, when a thread is rolled, the grain fibers aremade to flow in continuous unbroken lines following the contour of the thread. This isshown in Fig. 4.8.17.

Thread rolling is accomplished with reciprocating flat-die rolling machines,machines of the cylindrical-die type, thread-rolling heads, thread-rolling attachments,single-bump rolling equipment, and planetary rolling machines. Because the work-piece diameter before thread rolling should be controlled accurately, centerless grind-ing sometimes precedes the operation.

In addition to straight and taper threads, such forms as oil grooves and worm andgear forms are produced routinely by cold forming on thread-rolling machines.Sometimes, however, the geometry of the part is not conducive to thread-rolling appli-cations. Figure 4.8.18 presents illustrations of typical parts that have had threads andother forms produced by the rolling process.

In comparison with a cutting tool that is less expensive, tends to wear quickly, butcan be easily reground, the roll set is more expensive, may or may not be regrindable,and must produce more parts to justify its cost. Therefore, the rolling process general-ly must be matched to longer runs or to cases in which extra tool cost can be amor-tized. In some cases, when the necessary rolling die is available and can be substitutedfor a thread-cutting head, rolling may be economical for moderate-sized lots. Thread-rolling dies have a long life (from tens of thousands to millions of pieces), andresharpening during the life of the die is not necessary.


FIGURE 4.8.16 Thread rolling with flat dies. (Courtesy TeledyneLandis Machine.)



SCREW THREADS

SCREW THREADS 4.93

FIGURE 4.8.17 Grain structure of cut threads (a) contrasted with the stronger,plastically deformed structure of rolled threads (b). (Courtesy Teledyne LandisMachine.)

Cold-Form Tapping

Forming taps produce internal screw threads by plastic flow of material near the holewalls rather than by metal removal, as with conventional cutting taps. Figure 4.8.19illustrates a typical forming tap.

The method has the advantages that no chips are formed, that the threads arestrong, and that tapping speeds are higher. However, only a limited range of soft, duc-tile materials is suitable for cold-form tapping, and the percentage of thread is bestheld to 65 percent or less to avoid overfilling at the minor diameter. Torque require-ments for tapping are also higher than for cutting taps. The characteristic thread formhas a small groove at the crest of the thread, the width of which decreases with higherpercentages of thread.



SCREW THREADS


FIGURE 4.8.18 Typical parts with rolled threads. (Courtesy TeledyneLandis Machine.)

FIGURE 4.8.19 Cold-forming tap. (From American Machinist.)

SUITABLE SCREW-THREAD MATERIALS

Cut Threads

Often, the end use of the workpiece or considerations other than the threading opera-tion dictate the selection of the material. However, when a choice is possible, selecting



SCREW THREADS

SCREW THREADS 4.95

one of the free-cutting grades of material will give a more accurate thread of smootherfinish. Compared with threading non-free-machining grades, producing a thread onfree-cutting material will result in higher production at lower machining and toolcosts. Soft, non-free-machining metals are especially difficult to thread, for they pro-duce stringy chips that weld to the cutting edge.

In many cases metals selected on the basis of cost are more expensive in the end. Aproportionately greater amount of time is spent obtaining a satisfactory thread. Also, high-er tool cost is involved, tool life is poorer, and more downtime is required for tool changes.

Materials suitable for threading follow generally those suitable for most machiningoperations. Brasses and bronzes cut better and at higher speed than steels, free-machining steels cut better than unleaded or non-free-machining grades, and as carboncontent increases and/or additives such as chromium or molybdenum are introduced,machinability drops quite rapidly. Aluminum, in bar stock, is generally quite good, butcast aluminum can be quite abrasive and cause excessive tool wear. Cast iron is brittleand presents a problem of maintaining a good form on the crest of the thread. Low-carbon steels, such as the 1010 and 1020 grades, while soft enough for easy machin-ing, tend to tear, and it is difficult to obtain a good finish.

In steels, it is difficult to cut good threads when the Brinell hardness is below 160.This is due mainly to the difficulty in breaking the chip in such soft steel. In hardermaterials, the chip can be broken more easily. Easier breaking causes less interferenceat the cutting face of the tool and allows freer cutting and a smoother finish. Materialsabove Rc 34 are usually not suitable for die chasers and taps, which, generally speak-ing, are manufactured from high-speed steels. The single-point process using carbideis better suited for materials above Rc 34.

Difficult-to-machine materials sometimes can be more advantageously threaded bythread milling. In setting speeds and feeds, consideration must be given to the work-piece hardness and cutter material, taking into account the fact that 60° thread formsdo not make for a cutter with strong teeth.

Ground Threads

Materials generally suitable for other form-grinding operations are satisfactory alsofor ground threads. The most suitable materials are the hardened steels and any metalsthat will be heat-treated above Rc 33 before threading. Aluminum and comparable softmaterials are the most difficult to grind because they tend to load the wheel and causeburning.

Formed Threads

Different properties are required for thread forming than for cutting, and materials thatcan be cut may not be suitable for thread rolling or cold-form tapping. Factors thatpromote thread formability are low hardness, a low yield point, elongation of 12 per-cent or more, a fine-grained microstructure, and freedom from work hardening.

Leaded and sulfurized steel and leaded brasses do not work out well for threadrolling and should be considered only for cut threads. The use of thread rolling is alsogenerally not recommended for materials that exceed Rc 32 hardness. With materialsharder than this, die life is substantially reduced. Table 4.8.1 indicates the rollabilityand expectable die life for commonly thread-rolled metals.



SCREW THREADS

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SCREW THREADS

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SCREW THREADS

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SCREW THREADS

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SCREW THREADS

Cold-form tapping requires even greater cold workability than external-threadrolling. Cold-workable grades of brass, copper, and aluminum and low-carbon steelare the most commonly used materials.

DESIGN RECOMMENDATIONS FOR SCREWTHREADS

External threads made by all processes should not terminate too close to a shoulder orother larger diameter. Space must be provided for the thread-cutting tool. In fact, thereshould be an area of thread relief or undercut where the diameter of the workpiece isless than the minor thread diameter. (See Fig. 4.8.20.) This allows room for the throat

angle of the thread cutter, which would otherwise produce an incomplete thread at theend. It also reduces the chance of tool breakage. The width of this relief depends onthe size of the part, coarseness of the thread, and throat angle of the threading tool.From 1.5 mm (1⁄16 in) to 19 mm (3⁄4 in) or more should be allowed. When possible, thewidth of the relief should be increased to allow use of chasers having the maximumlength of the throat or chamfer. This will provide maximum efficiency of the operationand maximum tool life.

Internal threads should have a similar relief or undercut even though, for blindholes, it necessitates an added recessing operation before threading. Blind holes, evenif not provided with an undercut, require some unthreaded length at the bottom forchip clearance. Best and most economical of all is the through hole, which providesboth chip clearance and relief if the threads extend to the opposite surface. Figure4.8.21 illustrates these alternatives.

In many applications no more than 60 or 65 percent of the thread height is requiredfor adequate thread strength. Threads in this range machine more easily, requiringonly 75 percent of the torque needed for conventional threads. If high strength is notrequired, consider the use of a reduced-height thread form. (See Fig. 4.8.22.)

Similarly, the length threaded should be kept as short as possible consistent withthe functional requirements of the part. Shorter threads machine more quickly andprovide longer tool life. For internal threads, where tap breakage may be a problem,limit the depth of the threaded portion to two diameters.


FIGURE 4.8.20 Allow thread relief at the end of the threaded length.



SCREW THREADS

4.101

FIGURE 4.8.21 Allow chip clearance with internal threads.

FIGURE 4.8.22 A reduced-height-thread form will machine moreeasily than a full thread and has adequate strength for most applications.



SCREW THREADS


The design of threaded products should include a chamfer at the ends of the exter-nal threads and a countersink at the ends of the internal threads. These inclined sur-faces prevent the formation of finlike threads at the ends, help to minimize burrs, andassist the threading tool in starting to cut or form the threads. (See Fig. 4.8.23.)

Aside from chamfers and countersinks, the surface at the starting end of the screwthread should be flat and square with the thread’s center axis. Otherwise, proper start-ing of the thread-making tool may be difficult.

Slots, cross holes, and flats should not be placed where they intersect screwthreads. Most thread-making processes are adversely affected by surface interruptions,and burrs are almost inevitable where the surfaces intersect. Burrs on thread surfacesare especially costly to remove. When cross holes are unavoidable, they should becountersunk.

Standard thread forms and sizes with off-the-shelf threading tools are always moreeconomical than threads made with special tools.

Tubular parts must have a wall heavy enough to withstand the pressure of the cut-ting or forming action. This stricture applies to both internal and external threads.Castings and forgings of odd shapes should not have thin sections at a portion of thethread’s circumference. Otherwise, out-of-roundness will occur.

Tolerances closer than required for the given function should not be specified.Class 2 threads are usually satisfactory for most work.

Threads to be ground should not be specified to have sharp corners at the root.Normally, a radius of 0.08 mm (0.003 in) is the very minimum that can be expected,and much larger radii on the order of 0.25 mm (0.010 in) are preferable. (See Fig.4.8.24.)

Centerless-ground threads should have a length-to-diameter ratio of at least 1:1, butpreferably the length should be longer than the diameter. Parts to be centerless-thread-ground also should not have large burrs, be flattened or egg-shaped from shearing, orbe bent or crooked. Taper and flatness also should be avoided because they will not beremoved by the thread-grinding operation.

FIGURE 4.8.23 Specify chamfers and countersinks at the endsof threaded sections.



SCREW THREADS

SCREW THREADS 4.103

Parts for thread rolling have similar requirements of roundness, straightness, andfreedom from taper and burrs. Uniformity of hardness is also important for threadrolling, as is accurate blank diameter.

Except for the largest sizes, coarse threads are slightly more economical to producethan fine threads and should be specified in preference to fine threads if the part’sfunction permits. Coarse threads also assemble more rapidly.

DIMENSIONAL FACTORS AND TOLERANCES

The same factors induce dimensional variations in screw threads as affect the dimen-sions of other types of surfaces produced by thread-making equipment. The accuracyand conditions of tooling and equipment are key factors for all thread-making process-es. So are the skill of the worker, the suitability of the material, and the feed rate of thethreading tool.

When conditions are optimal and extra care and extra time (sometimes consider-able) are taken, Class 4 and 5 threads can be produced by all the methods covered bythis chapter. Suitably accurate measuring equipment is also required to guide and con-trol the accuracy of the final results.

Hand dies are normally not capable of the highest precision and are primarilyapplicable to Class 1 and 2 threads only. Other thread-cutting methods can be used forClasses 1 through 5, with costs increasing sharply at the higher precision levels. Bestlead accuracy occurs when the advance of the thread-cutting tool is controlled by leadscrew rather than by the tap or die. Surface finishes smoother than 1.6 �m (63 �in) arenot normally attainable by thread cutting.

With thread milling, by using a careful setup and moderate feed, the outside, pitch,and root diameters can be held to �0.025 mm (0.001 in). Lead depends on the accura-cy of the lead screw of the machine and can be held as close as 0.001 mm/cm (0.0001in/in). Surface finish can be held to 1.6 �m (63 �in), with finer finishes sometimespossible with finer feed. Thread milling is a suitable method for accurate classes ofthread, especially if the workpiece material has some machinability limitations.

At one time, thread grinding was essential to achieve Class 4 and 5 threads. Thiscircumstance has changed, and Class 5 threads are now both rolled and cut. Grinding

FIGURE 4.8.24 Ground threads require a generous root radius.



SCREW THREADS


is used most frequently when the hardness of workpiece precludes other methods. Iffinishes and accuracy greater than those specified for Class 3 threads are required,reduced production rates should be expected. Center-type grinders can hold flankangles of thread forms to �1⁄4° and lead accuracy to within �0.002 mm/cm (0.0002in/in) and the cumulative error to not more than 0.06 mm in 300 mm (0.0024 in/ft).

All classes of thread can be rolled. The piece-to-piece accuracy of rolled threadsdepends on various factors, particularly the consistency of the blank diameter and theuniformity of material and structure from piece to piece. Tolerances cannot be met ifthere are variations in these factors. Centerless grinding is a common preliminaryoperation to thread rolling to ensure an accurate blank diameter. The surface finish ofrolled threads is superior to that of cut threads or about as smooth as the surface of therolling dies. Generally, a limit of 0.8 �m (32 �in) can be specified if a smooth surfaceis required.

Tables 4.8.2 and 4.8.3 provide information on the approximate dimensions and tol-erances of standard screw threads. Additional data on screw-thread dimensions andtolerances can be found in the publication Screw Thread Standards for FederalServices, Handbook H28, published by the U.S. Department of Commerce. A similarpublication is Unified Inch Screw Threads (ANSI B1.1-1974), published by theAmerican National Standards Institute.



SCREW THREADS

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SCREW THREADS

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the

nom

inal

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er.



SCREW THREADS

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ref

eren

ce.

Sou

rce:

Fro

m A

mer

ican

Nat

iona

l Sta

ndar

d A

NS

I B

1.13

-197

9,pu

blis

hed

by th

e A

mer

ican

Soc

iety

of

Mec

hani

cal E

ngin

eers

.



SCREW THREADS



SCREW THREADS

Documents

SCREW THREADS.pdf