Groving Thriding

8/20/2019 Groving Thriding

1/9

Cutting Tool

Applications

C u t t i n g T o o l A p p l i c a t i o n s

By George Schnei

der, Jr. CMf gE

http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/


2/9

2 Tooling & Production/Chapter 6 www.toolingandproduction.com

6.1 IntroductionGrooving and threading are both single point machining operations performed on lathes,automatic lathes, or machining centers. Outside diameter (OD) and face grooving oper-ations are shown in Figures 6.1a and 6.1b respectively. Internal threading or tapping willbe discussed in a later chapter.

6.2 Grooving or Recessing OperationsGrooving or recessing operations, sometimes also called necking operations, are oftendone on workpiece shoulders to ensure the correct fit for mating parts (Fig. 6.2a) Whena thread is required to run the full length of the part to a shoulder, a groove is usuallymachined to allow full travel of the nut. (Fig. 6.2b) Grooving the workpiece prior to cylin-drical grinding operations allows the grinding wheel to completely grind the workpiecewithout touching the shoulder.(Fig. 6.2c)

Chapter 6

Grooving& Threading

Upcoming Chapters

Metal RemovalCutting-Tool Materials

Metal Removal Methods

Machinability of Metals

Single Point MachiningTurning Tools and Operations

Turning Methods and MachinesGrooving and Threading

Shaping and Planing

Hole Making ProcessesDrills and Drilling Operations

Drilling Methods and Machines

Boring Operations and Machines

Reaming and Tapping

Multi Point MachiningMilling Cutters and Operations

Milling Methods and Machines

Broaches and Broaching

Saws and Sawing

Abrasive ProcessesGrinding Wheels and Operations

Grinding Methods and Machines

Lapping and Honing

George Schneider, Jr.CMfgEProfessor Emeritus

Engineering Technology

Lawrence Technological UniversityFormer Chairman

Detroit Chapter ONESociety of Manufacturing Engineers

Former PresidentInternational Excutive BoardSociety of Carbide & Tool Engineers

Lawrence Tech. Univ.: http://www.ltu.edu

Prentice Hall: http://www.prenhall.com

FIGURE 6.1b: Face Grooving operation(Courtesy: Valenite Inc.)

FIGURE 6.1a: Outside diameter (OD) grooving operation (Courtesy: Valenite Inc.)

http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/


3/9

6.2.1 Face GroovingWith face grooving operations the toolis fed axially rather than radiallytowards the end surface of the work-piece. The tool must be adapted to theradial curve of the groove and the bladeis therefore curved. When the machinespindle rotates in a counter-clockwisedirection, a right-hand version of thetool is used and a left-hand version isused when the machine spindle rotatesclockwise. A face grooving operation isshown in Figure 6.1b.

So that both insert and toolholder fitinto the groove, both the outer and innerdiameters of the groove must be consid-ered. The diameter measured to the out-side of the blade determines the limit forthe smallest possible diameter which

www.toolingandproduction.com Chapter 6/Tooling & Production 3

can be machined, and the diameter mea-sured to the inside of the blade deter-mines the limit for the largest possiblegroove diameter.

6.2.2 Internal GroovingThe main problem with internal groov-ing is chip evacuation. There is a veryhigh risk of chip jamming which canresult in tool breakage, especially whenmachining small diameters. The chipshave to be removed from the groovethen change direction 90 degrees and

pass the side of the toolholder to finallybe removed from the hole. Introducingintermittent feed into the program is thebest way to obtain short chips. An inter-nal grooving holder with insert is shownin Figure 6.3.Vibration is another common problemassociated with internal grooving.Stability is related to the overhang, orhow far into the workpiece the groove isto be machined. The risk of vibration isreduced by using the largest toolholderpossible. The overhang should notexceed 2 - 2.5 × the diameter. Internalgrooving is a critical operation and it isimportant to choose a tool which opti-mizes chip evacuation with vibration-free machining.

Grooving tools are usually ground tothe dimensions and shape required for aparticular job. Most grooving tools aresimilar in appearance to the cutoff tool,except that the corners are carefullyrounded because they reduce the possi-bility of cracks in the part, especially if

the part is to be heat treated.

6.3 Parting or Cut Off OperationsIn parting operations the workpiecerotates while the tool carries out a radi-al feed movement. As with face turn-ing, the tool is fed from the periphery of the workpiece towards the center and

the cutting speed is reduced to zero - buthere the similarities end. A typical part-ing operation is shown in Figure 6.4.

As the cutting tool progressestowards the center, another factor takeseffect. As the diameter of the workpieceis reduced, the radial cutting force willcause the material to break before theinsert has cut through it. This results ina pip or burr being formed in the centerof the workpiece. This pip will alwaysbe there after parting, but its size can bereduced by choosing the correct insertgeometry, feed rate, and support for the

sagging workpiece.In a parting operation, there is mater-

ial on both sides of the insert. Thismeans that the tools used are narrowand that the length of the toolholderincreases with an increased diameter.Therefore, stability becomes a criticalfactor.

Since the size of the tool and tool-holder must be optimized to meetrequirements, only a small surface ispresent for drawing off heat, and there-

Groove for mating parts

(a)

Groove to allowfull travel of nut

b)

Nut

Thread

Groove for grinding relief

c)

Grindingwheel

FIGURE 6.2: Grooving operations to provide clearance for a) mating parts; b) full travel of nut; c) grinding relief.

FIGURE 6.4: Parting or cut-off operation(Courtesy: Iscar Metals, Inc.)

FIGURE 6.3: Internal grooving holderwith insert (Courtesy: Valenite Inc.)

Chap. 6: Grooving & Threading

http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/


4/9



fore cutting fluid becomes important.Unfortunately, because of the spacerestrictions, the supply of cutting fluidis obstructed by the chips. Since chipevacuation is difficult and there is noth-

ing against which to break the chips, theside surfaces can easily be damagedduring the operation.

6.3.1 Insert GeometryAt the beginning of a cut the insert willwork at a relatively high cuttingspeed,and must be able to resist plasticdeformation. The speed reduces as thetool approaches the center, at whichpoint it becomes zero.

Modern machines can be pro-grammed so that the spindle speed isautomatically increased towards the

center, so that the cutting speed is keptconstant. But the maximum spindlespeed of the machine will be reachedbefore the tool reaches the center, andthis could result in insert edge build-up.Therefore a tough tool material will beneeded to resist edge build-up as thetool gets closer to the center.

Advanced insert geometries are nec-essary for performing parting andgrooving operations in a satisfactoryway. A positive rake insert gives lowercutting forces and thus lower pressureon the workpiece, and this will reducethe size of the pip. However, a largepositive rake angle means a weaker cut-ting edge.

The insert can have various leadangles. On straight or neutral inserts thelead angle is zero. This design providesa stronger cutting edge and a better sur-face finish, while maintaining closertolerances with respect to perpendicularalignment. With an increased leadangle the axial cutting force increases,

causing theinsert todeflect.

With largelead angles thedeflection canbe so strongthat a rounding

of the end sur-faces occursand results in aconvex or aconcave endsurface. Areduced leadangle produceslarger radial

cutting forces but this can cause vibra-tion problems especially when smalldiameters are machined. In groovingoperations a radial displacement of theinsert results in an inaccurate groove

depth.

6.3.2 Chip ControlWith parting and grooving operations,the insert has machined surfaces on bothsides of the feed direction. Thereforethe chips must be formed in such a waythat they are narrower than the groove,otherwise the surfaces can be damaged.In addition, the chips must be formed insuch a way that they can be evacuatedfrom the groove without disrupting themachining with long, unwieldy chipcoils. Therefore the chips are formed in

two directions: bent across their widthand rolled together longitudinally toform a spiral spring-shaped chip.Figure 6.5 shows three chip controlinserts.

In order to produce this ideal chipshape the insert is usually provided witha chip former as shown in Figure 6.5,which takes into account both themachining conditions and the work-piece material. It is shaped in such away as to form a bank which the chipscan climb against during machining.After a number of revolutions the chipswill break automatically. The diameterof the spiral spring chips is influencedby the width of the insert, the height of the bank, the feed, and the workpiecematerial.

6.3.3 Tool PositioningAs with conventional turning, it isimportant that the cutting edge be posi-tioned on the same level as the centerline. In order to achieve satisfactory

results, a maximum deviation in posi-tioning of only ±.004 inch from the cen-ter line is acceptable.

As the cutting edge deviates from thecenter line, the rake angle and the clear-ance angle will be changed. Thischange is due to the radius of the work-piece. A clearance angle that is too

small may cause the cutting edge to rubagainst the workpiece. If the cuttingedge is positioned too low, the tool willleave material in the center and a pipwill be formed.

6.3.4 Operational StabilityWith conventional external turning thetool overhang is not affected by thelength of the workpiece. The size of thetoolholder can be chosen so that it with-stands the stresses which arise duringthe operation. However, with partingand grooving operations, consideration

must be given to the depth of insertionand the width of the groove, whichmeans that stability must often be com-promised to meet specifications.

To obtain the best possible stability,the overhang should be as small as pos-sible, so a holder f or the shortest possi-ble insertion depth should be chosen.Wider inserts can be used in order toimprove the stability, but more materialis wasted in the form of chips. This canbe expensive with large batches andwhen machining expensive materials.

Vibration can also arise as a result of

the deflection of the workpiece. Thecloser the chuck is to the parting posi-tion, the lower the effect of the stressesand the deflection of the workpiece willbe. Therefore, if a workpiece has a ten-dency to vibrate, the machining shouldbe done as close to the chuck as possi-ble.

The risk of vibration must be kept toa minimum in order to obtain acceptableresults in quality and tool life. In addi-tion to choosing the best tool and moststable set up, the cutting data must beadapted to minimize the tendency of thetool and workpiece to vibrate.

6.3.5 Toolholder and InsertSelection

Modern parting and grooving cuttingtools consist of a toolholder and anindexable insert developed specificallyfor a particular operation. The majorityof inserts produced over the last decadewere designed to work with the SELF-GRIP concept. This clamping method

FIGURE 6.5: Chip control grooving and parting inserts (Courtesy: Iscar Metals, Inc.)



5/9



incorporates no external screws orlevers to hold the insert in place asshown in Figure 6.6. Instead, it relieson the rotation of the part and tool pres-sure to keep the insert seated in awedge-style pocket. The insertsdesigned for this type of holder are usu-

ally single ended and their geometrypermits unlimited depth of cut.

With double-ended inserts, alsoknown as “dogbones”, the depth of cutis limited by the second cutting edge asshown in Figure 6.7. Dogbone insertstraditionally can only cut as deep as theoverall length of the insert. Once thedepth is reached, the trailing edge willbegin to rub inside the groove that thetool is creating. In addition, dogboneinserts usually are secured by a screw-top clamp, which also limits the depthof cut as again shown in Figure 6.7.

Coatings for grooving and partinginserts vary from supplier to supplier.But titanium carbon nitride applied bythe PVD process has practically becomethe industry standard for lower cuttingspeeds and tougher applications. And

when conditions arestable and highercutting speeds arerequired, the choicenarrows to titanium-aluminum-nitridecoatings applied bythe PVD process, or

the newer medium-temperature CVDprocess. The reasonis because TiAlNacts as a good heatbarrier for the car-bide substrate and

can handle elevated temperatures.Figure 6.8 shows a variety or groovingand parting tool holders with coatedindexable inserts..

6.4 Grooving and PartingRecommendations

The ability to efficiently cut off work-pieces and blanks in lathes has alwaysbeen important in getting the job com-pleted. Even in special purpose cutoff machines, a good parting tool is at theheart of the operation. Today’s modernindexable insert parting and groovingtools provide the same productivity lev-els as modern turning tools.

In parting operations, the objective isto separate one part of the workpiecefrom the other as efficiently and reliablyas possible. In grooving operations, theprinciple is the same, although these

operations are less sensitive because thegrooves are usually not as deep. Ingrooving, the shape, accuracy and sur-face finish arethe mainrequirements

that must be met.Some important hints for applying

grooving and parting tools:• always use plenty of cutting fluid.• set the center-height of the cuttingedge accurately• make sure toolholder/blade is accu-rately positioned at 90 degrees to the

workpiece axis.• use toolholder with the shortest possi-ble length of insertion for the operationin question.• select the largest shank/bar for the tool• adapt the cutting speed to avoid vibra-tions• reduce the feed rate for the final partwhen parting-off bar material/compo-nents.• for axial grooving, make the firstplunging cut at the largest diameter, far-thest out on the face, to minimize therisk of chip jamming

• use the smallest possible lead angle foravoiding pips/burrs in parting-off • when possible, use a toolholder with astrengthening radius between shank andblade.

6.5 Screw Threads and ThreadingThe screw thread dates back to 250 B.C,when it was invented by Archimedes.For centuries wooden screws, hand-made by skilled craftsmen, were usedfor wine presses and carpenters’ clampsthroughout Europe and Asia. Precisionin screw and thread manufacture did not

come into being until the screwcuttinglathe was invented by Henry Maudslayin 1797.

FIGURE 6.8: Various OD and ID grooving holders. (Courtesy: Iscar Metals, Inc.)

FIGURE 6.6: Self-grip tool holders use no external screw to hold the insert ( Courtesy: Iscar Metals, Inc.)

FIGURE 6.7: Dogbone insert tool holders have limited depth of cut (Courtesy: Iscar Metals, Inc.)



6/9



In the early 1800’s, Maudslay alsobegan to study the production of uni-form and accurate screw threads. Untilthen no two screws were alike; manu-facturers made as many threads per inchon bolts and nuts as suited their own

needs. For example, one manufacturermade 10 threads per inch in 1/2 -in. -diameter threaded parts, whereas anoth-er made 12 threads, and so forth.During this period the need for threadstandards became acute.

Despite many attempts at standard-ization, it was not until World War I thatthread standards were developed. Thethread profile was designated theAmerican National thread form andwas the principal type of thread manu-factured in the United States until WorldWar II.

During World War II the UnitedStates manufactured military equipmentthat used the American National threadform, which presented interchangeabili-ty problems with machinery made inCanada and Great Britain. Not untilafter World War II in 1948, did thesecountries agree upon a Unified threadform to provide interchangeability of threaded parts.The Unified thread formis essentially the same as the oldAmerican National, except that it has arounded root and either a rounded or flatcrest. The Unified thread form ismechanically interchangeable with theformer American National threads of the same diameter and pitch. Today it isthe principal thread form manufacturedand used by the United States.

6.5.1 Screw Thread NomenclatureScrew threads have many dimensions.It is important in modern manufacturingto have a working knowledge of screwthread terminology to identify and cal-

culate the dimen-sions correctly.Screw set nomen-clature is shown inFigure 6.9.

The majordiameter is thelargest diameter of

the screw thread.On an externalthread it is the out-side diameter; onan internal thread itis the diameter atthe bottom or rootof the thread.

The minor diameter is the smallestdiameter of a screw thread. On an exter-nal thread, the minor diameter is at thebottom of the thread; on an internalthread the minor diameter is the diame-ter located at the crest.

The pitch diameter is an imaginarydiameter that passes through the threadsat the point where the widths of thegroove and the thread are equal. Thepitch diameter is the most importantdimension on a screw thread; it is thebasis from which all thread measure-ments are taken.

The root is the bottom surface con-necting two sides of a thread. The crestis the top surface connecting two sidesof a thread. Pitch is the linear distancefrom corresponding points on adjacentthreads. The pitch is equal to 1 divided

by the total number of threads per inch(P=1/[no.threads/in.]). A screw having asingle lead with 16 threads per inch hasa pitch equal to 1/16 in., commonlyreferred to as a “16-pitch thread”.

The lead is the axial distance athreaded part advances in one completerotation. On a single lead threaded part,the lead is equal to the pitch.

The depth is the distance, measuredradially, between the crest and the rootof a thread. This distance is often calledthe depth of thread.

The flank is the side of the thread.Thread angle is the angle between theflanks of the thread. For example,Unified and Metric screw threads have athread angle of 60 degrees. Helix is thecurved groove formed around a cylinderor inside a hole.

A right-handed thread is a screwthread that requires right-hand or clock-wise rotation to tighten it. A left-hand-ed thread is a screw thread that requiresleft-hand or counterclockwise rotation

to tighten it. Thread fit is the range of tightness or looseness between externaland internal mating threads. Threadseries are groups of diameter and pitchcombinations that are distinguishedfrom each other by the number of threads per inch applied to a specificdiameter. The two common thread

series used in industry are the coarseand fine series. specified as UNC andUNF.

6.5.2 Unified Thread FormThe Unified screw thread has a 60degree thread angle with a rounded rootand a crest that is flat or rounded. Asmentioned earlier, this is the principalthread form used for screw thread fas-teners used in the United States. TheUnified screw thread system includessix main thread series:1. Unified Coarse (UNC)

2. Unified Fine (UNF)3. Unified Extra-Fine (UNEF)4. Unified 8-Pitch (8 UN)5. Unified 12-Pitch (12 UN)6. Unified 16-Pitch (16 UN)

The coarse-thread series (UNC) isone of the more commonly used serieson nuts, bolts, and screws. It is usedwhen lower-tensile-strength materials(aluminum, cast iron, brass, plastics,etc.) require threaded parts. Coarsethreads have a greater depth of threadand are required on these types of mate-rials to prevent stripping the internal

threads.The fine-thread series (UNF) is used

on higher-tensile-strength materialswhere coarse threads are not required.Because they have more threads perinch, they are also used where maxi-mum length of engagement between theexternal and internal threads is needed.

The extra-fine thread series(UNEF) is used when even greaterlengths of engagement are required inthinner materials. Eight, 12 and 16-pitch threads are used on larger-diame-ter threads for special applications. The8-pitch is generally regarded as a coarsethread for larger diameters, 12 pitch isthe fine series, and 16 is the extra-finethread used on the larger-diameterthreads.

The relationship between the pitchdiameter or major diameter deter-mines the helix angle of that thread. Forexample, a 12-pitch (12 UN) threadwith a 1.250-in. major diameter willhave a greater helix angle than a 12-

FIGURE 6.9: Screw Thread Nomenclature.

Threadangle Crest

Root

Flanks

PitchHelix angle

Single Depth

MajorDiameter

PitchDiameter

MinorDiameter

http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/


7/9



pitch thread with a 2.0-in. major diame-ter. Generally speaking, the lower thehelix angle, the greater the tensile stress

applied to the bolt for a given torqueapplied to the nut. The fastener with alower helix angle will also resist vibra-tion and loosening more effectively.

A grooving and threading holder isshown in Figure 6.10 and variousgrooving and threading inserts areshown in Figure 6.11

6.5.3 Acme Screw ThreadsAcme screw threads are manufacturedfor assemblies that require the carryingof heavy loads. They are used for trans-mitting motion in all types of machine

tools, jacks, large C-clamps, and vises.The Acme thread form has a 29 degreethread angle and a large flat at the crestand root (see Fig. 6.12).

Acme screw threads were designed toreplace the Square thread, which is dif-ficult to manufacture.

There are three classes of Acmethreads (2G, 3G, and 4G), each having

clearance on all diameters toprovide for free movement.Class 2G threads are used on

most assemblies. Classes 3G and 4Gare used when less backlash or loose-ness is permissible, such as on the lead

screw of a lathe or the table screw of amilling machine.

6.5.4 Tapered Pipe ThreadsPipe threads, usually designated

NPT (National Pipe Taper) are taperedthreads used for sealing threaded jointssuch as water and air pipes. Most pipethreads have a slight taper (3/4-in./ft)and are cut using special pipe taps anddies. Pipe threads can also be machinedusing the taper attachment on an enginelathe.

6.6 Thread TurningDevelopment of threading tools hascome a long way since the days of high speed tool-bits and tips ground toshape, which were then slowly fedalong by the lathe lead screw. Most of today’s threading is performed byindexable insert tools as part of a veryrapid CNC process. What used to be a

relatively difficult and time-consuming part of machining isnow standard procedure as withany other operation. A typicalpart that requires a thread isroutinely machined with fixedcycles of numerical control anda variety of other machinemechanisms and using toolswhich have the right threadshape. An ID and OD threadingoperation with coated indexableinserts is shown in Figure 6.13.

The principle of single pointthread cutting is the feed move-ment of the tool in relation to

the workpiece rotation. The point gen-erates the typical spiral groove thatmakes up the screw thread with a cer-tain pitch. Basically, threading is awell-coordinated turning operation witha form-tool. During the feed passes, the

tool is moved longitudinally along theworkpiece and then withdrawn andmoved back to the starting position forthe next pass along the same threadgroove.

The feed rate is a key factor that hasto coincide with the pitch of the thread.The coordination is obtained by variousmeans, depending on the type of machine; lead screw, cam or numericalcontrol (usually handled as a sub-rou-tine in CNC). The shape of the grooveproduced is determined by the shape of

FIGURE 6.11: Various grooving and threading inserts (Courtesy: Valenite Inc.)

FIGURE 6.10: Grooving and threading tool holders(Courtesy: Valenite Inc.)

FIGURE 6.12: General purpose Acme screw thread

FIGURE 6.13: OD and ID threadingOperation (Courtesy: Sandvik CoromantCorp.)

Flat(root)

Depth29°

Pitch

Flat(crest)

http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/http://www.engarena.com/


8/9



the insert point, and the feed rate is con-siderably higher than for ordinary turn-ing operations.

The relatively small 60 percent pointangle of the tool makes the cutting edgesusceptible to the forces and stresses of

metal cutting. To counter this, a longestablished method has been to use thethread depth to determine the cuttingdepth, and to avoid machining in onepass. Instead, the depth is machined inseveral passes. The cutting tool opensup the thread groove by cutting deeperand deeper, usually by making 5 to 16passes, depending on the thread pitch.As each pass is made, more and morematerial is removed per cut as a largerpart of the edge is engaged. For thisreason, the depth of cut is reduced suc-cessively as the passes are made.

It is best to have radial in-feeds whichdecrease successively as the passes areperformed. The number of in-feed pass-es must be balanced to provide the edgewith sufficient but not excessive cut intothe workpiece. Too much cutting forcewith insufficient cutting depth leads topremature tool wear.

6.6.1 Left and Right-Hand ThreadsThe difference in direction between leftand right-hand threads does not affectthe thread profile; it does, however,have some effect on the choice andcombination of tools. The method of cutting the thread depends on the work-piece design. Working towards thechuck is the most common method,even though working away from thechuck is in many cases also satisfactory.

The advantage in using right-handtools for right-hand threads and left-hand tools for left-hand threads, is thatthe holder is designed to give maximumsupport to the insert. But under normal

cutting conditions this order is not criti-cal. It is vital, however, that insertsalways be used with holders of the samehand.

6.6.2 Toolholders and Insert

SelectionCompared to conventional turning, thetool and machining parameters of threading are not so flexible. This ismainly because the feed is related to thepitch, the cutting depth is divided intopasses, and the cutting speed is limitedbecause of the pointed cutting edge.

Indexable inserts are available forexternal and internal threading. Theinserts for internal threadingare mirror images of the cor-responding external inserts.Both external and internal

inserts are available in rightand left-hand versions. Sincetolerances and cutting geome-tries differ between externaland internal inserts, it isimportant that they should notbe confused.

6.6.3 Coated ThreadingInserts

The development withinthread turning tools has beenconsiderable during the pastthirty years, since the intro-duction of the first flat insertswith a loose chip formerclamped on top of the inserts.Today’s modern inserts havedone away with most of thepossible problems that canarise with conventionalthreading inserts. This hasmade threading more closelyresemble a turning operation.

The multi-purpose PVD

coated inserts allow a wider range of cutting speeds between the area charac-terized by built-up edge formation atlower speeds and plastic deformation athigher speeds. Threading involvesmany short cutting sequences and oftenrelatively low cutting speeds throughoutmachining. Of major importance in

threading is the ability of the cuttingtool to keep the built-up edge tendencyto an absolute minimum, or prevent itentirely, depending upon the workpiecematerial. A built-up edge will causepoor surface finish and eventually leadto edge breakdown and tool failure.

6.7 Thread MillingThread milling has been an establishedmethod of manufacturing accuratescrew threads for many years. Longscrews, such as lead screws on lathesand multiple start threads, are often

manufactured by milling.Milling a screw thread is done with

either a single- or multiple-lead millingcutter. The rotating cutter is fed into thework to the required depth. The work isthen rotated and fed longitudinally at arate that will produce the proper lead onthe part (Fig. 6.14). Any class of fit orthread form can be manufactured by thethread milling process.

FIGURE 6.14: External thread milling with: a) single-row and b) multiple-row tooth cutter

(a)

Cutter Feed direction

(b)

Part

FIGURE 6.15: Shown are: a) single and b) multiple rib thread grinding wheels

Single-rib wheel

(a)

Multi-ribwheel

(b)



9/9



6.8 Thread GrindingGrinding a screw thread generally isdone when the hardness of the materialmakes cutting a thread with a die or sin-gle point tool impractical. Grindingthreads also results in greater accuracyand in superior surface finishes com-pared to what can be achieved with

other thread-cutting operations. Taps,thread chasers, thread gages, andmicrometer spindles all use groundthreads.

Ground threads are produced bythread-grinding machines. A thread-grinding machine closely resembles acylindrical grinder in appearance. Itincorporates a precision lead screw toproduce the correct pitch or lead on thethreaded part. Thread-grindingmachines also have a means of dressing

or truing the cutting periphery of thegrinding wheel so it will produce a pre-cise thread form on the part. Grindingwheels used in producing ground

threads are single-or multiple-rib. (Fig.6.15) Single-rib types are used forgrinding longer threads and feed longi-tudinally for the required length of thread. The multiple-rib type of grind-ing wheel is generally used for formingshort threads. This type of wheel is“plunged” into the workpiece to pro-

duce the thread.Internal threading or tapping will be

discussed in a later chapter as part of hole-making processes.
http://www.engarena.com/

Date post:	07-Aug-2018
Category:	Documents
Upload:	3pher
View:	216 times
Download:	0 times

Groving Thriding

Documents