Unit 4B Thread Manufacturing

Thread Manufacturing methods covering thread casting, thread rolling, thread cutting on lathe, thread grinding, thread whirling, thread milling, thread rolling etc

1. Machining & Machining Tools Unit-4B 1 Thread Manufacturing

2. Methods of Thread Production

3. Casting & Molding Threads are directly formed by the geometry of the mold cavity in the mold or die. When the material freezes in the mold, it retains the shape after the mold is removed. The accuracy and finish depend upon the casting process used.  Cast threads in metal parts may be finished by machining if needed for extra precision and surface finish with additional cost Threads made by sand casting are rough and are not used much, except occasionally in  non-precision machinery. Drawbacks of sandcasting can be overcome by shell molding Lost wax method can produce highly accurate threads of good finish. But the method is costly and difficult.

4. Casting & Molding Threads made by die casting and permanent mold casting are very accurate and of high surface finish. Die casting applicable only to low melting point non ferrous metals and therefor are not fit for repeated use, because of their low strength and less durability. Sewing machine vending machines, typewriter parts and toys may have their threads cast in place by die casting and permanent mould casting. Such parts are rarely taken apart, so, the method is very satisfactory. Major application of plastic molded threads in plastic bottles and die-cast threads in  cable glands  (connectors/fittings ).

5. Thread Rolling Threads are formed into blank by pressing threading die against the blank . As the thread shaped ridges on the dies penetrate the blank material, material is displaced from the bottom of the thread and forces radially out to form the thread crests. Cold working process in which work material is shifted by plastic deformation with the help of a pair of dies having same threads which are desired. There are four types of the thread rolling machine: Reciprocating , flat die machines Cylindrical die machines. Rotary planetary machine Sector die machines

6. Reciprocating Flat Die Machines Flat dies: one remain fixed ( bottom die) and the other moving parallel ( reciprocating die) , are used in three configurations: Horizontal : most convenient and common Vertical : occupies less space and facilitates cleaning and lubrication under gravity Inclined : derives benefit of both horizontal and vertical features All the flat dies are made of hardened cold die steel and provided with linear parallel threads like grooves of geometry as that of the desired thread. Blank is placed on stationary die and then moving die slides over the blank. Due to such action blank roll between two dies and threads are formed .

7. Cylindrical Thread Rolling Machines 2 Roll Machine: Two identical circular dies with parallel axis are rotated in the same direction and speed. One stays fixed in a position the other is moved radially desirably depending upon the thread depth 3 Roll machine: one die is move radially outward for let the work blank enter between die and then closes and rotate to roll threads. All three dies are moved equally in to penetrate into the work placed at the centre

8. Rotary Planetary Thread Rolling Machine In rotary planetary machine, the job is rolled between a central die that rotates continuously about a fixed axis, and one or more concave-shaped die segment located adjacent to the periphery of the rotating die. This being a continuous process is the fastest method of thread rolling.

9. Sector Circular Die Thread Rolling Simplest and fastest due to auto-feed of the blanks.

10. Thread Rolling Advantages Advantages: The grain fibers remain continuous and follow the contours of the threaded surface. Due to his, the threads are less easily sheared off than machined threads . During thread rolling, the material is strained plastically and is work-hardened , and is, therefore, stronger against both tension and fatigue , especially the latter . The surface of rolled thread is harder than a cut thread, so wear resistance increases . Surface finish is better as controlled by the rolls. Dimensional accuracy is better, as very little wear occurs on the rolls as it would on a cutting tool . faster production, no skilled labor needed, chip less no wastage, economical, requires simpler machines and tools

11. Thread Rolling Limitations Applicable for threads of smaller diameter, shorter length and finer pitch Cannot provide that high accuracy Is applicable for relatively softer metals Is used mostly for making external screw threads Needs separate dies for different threads

12. Thread Cutting on Lathe Machine By various cutting tools made of HSS or often cemented carbide tools. Centre lathes to single spindle automats are used Special purpose lathes and CNC lathes including turning centers used Threads are produced in centre lathes by various methods By single point cutting tool By thread chasing (covered later) By attachments of die threading and tapping (covered later)

13. Cutting with Single Point Tool on Lathe A single-point cutting tool is used to produce a thread form on a cylinder or cone. The tool moves linearly while the precise rotation of the workpiece determines the lead of the thread. The process can be done to create external or internal threads (male or female). In external thread cutting, the piece can either be held in a chuck or mounted between two centers. With internal thread cutting, the piece is held in a chuck. The tool moves across the piece linearly, taking chips off the workpiece with each pass.

14. Cutting with Single Point Tool on Lathe Operator should be fully conversant with different terms, types, starts and shape of threads. External or internal threads may be cut on lathe either with the help of a die or tap respectively or a thread cutting tool may be used . A certain relation is needed between job revolutions and revolutions of lead screw to control the linear movement of the tool, parallel to the job length when half nut is engaged with lead screw. The tool should be ground to the proper shape and profile of the thread to be cut. Many lathes are provided with quick-change gearbox in which different ratios of spindle and lead screw revolutions can be readily obtained by simply shifting the gear change lever. In other lathes, for cutting different pitches of threads, every time gears are changed. For cutting threads on a lathe, headstock spindle is connected with the lead screw through gears in such a way that positive carriage feed is obtained and the lead screw is driven at a predetermined speed in relation to the spindle speed.

15. Cutting with Single Point Tool on Lathe

16. Cutting with Single Point Tool on Lathe It includes only three gears, driver, driven and some suitable intermediate gear . The intermediate gear has no effect on the ratio. It simply acts as a connection that is used to fill up the distance gap between the driver, and the driven gear and makes the lead screw rotate in the same direction of the machine spindle . When cutting a screw thread, the tool is moved along the bed and is driven by a nut engaging with the lead screw. The lead screw is driven by a train of gears from the machine spindle. The gear train may be arranged in one of the following ways: Simple gear train.

17. Cutting with Single Point Tool on Lathe B. Compound gear train. Sometimes driver and driven gear ratio becomes so typical that the selection of gears to arrange in a simple train becomes difficult. For example Dr and Dn ratio is 4/26, and then a set of gears, which can give this ratio in the simple train, may become difficult. In such cases the solution is obtained with the use of compound wheel train,  the gear ratio become Gears supplied with lathes, generally, range from 20 to 120 teeth in steps of 5 teeth with two 40s or two 60s. The lead screw on lathes is always single-threaded and of a pitch varying from 5 to 10 mm depending on the size of the machine. For English lathes, the most common screw threads have 2, 4, or 6 tpi .

18. Example Calculate suitable gear trains for the following cases : a. 2.5 mm pitch on a 6 mm lead screw b. 11 tpi on a 4 tpi lead screw c. 7 threads in 10 mm on 6 mm lead screw d. 7/22 in. pitch, 3 start on a lathe with 2 tpi e. 2.5 mm pitch on a 4 tpi lead screw f. 12 tpi on a lathe having 6 mm pitch lead screw

19. Cutting with Single Point Tool on Lathe Thread Catching Required depth of a thread cannot be obtained in a single cut of tool and several successive cuts are needed. In the first cut, tool travels up to the job length, and then it is to be taken to the starting position for giving the second cut. Now if tool is not fed exactly in the previous groove, it may form a new groove thus wastage of the job. Modern machines are provided with facilities to reverse the rotation of job, which will bring the tool to original position without disengaging the half-nut. But few machines are not equipped with such facilities. In those machines tool is to be brought back by some other means.

20. Cutting with Single Point Tool on Lathe Thread Catching The process of setting the tool repeatedly in the previously formed groove is called thread catching or thread chasing. Few methods of thread chasing are given below ( a) At the end of each travel of the tool, take it back from the job surface and stop the machine. Disengage half-nut from lead screw and return it by hand to the starting point of the cut. By trial method, set the tool in right groove, start the machine to have the fresh cut after giving a certain feed to the tool. ( b) Do not disengage half-nut from lead screw at the end of each cut. By reversing the direction of lead screw, bring the carriage to the starting position of the cut. A mark may be given on the bed by the side of the carriage; this will help in setting the carriage immediately. This method is useful for cutting threads on short lengths or in blind holes or for threads of odd pitches, which are difficult to cut even with the help of chasing dial method .

21. Cutting with Single Point Tool on Lathe Procedure for External Threads Hold the job between the lathe centres or in chuck as required and turn it to the size of the major diameter of thread to be cut. Select suitable change gear set for cutting required pitch of thread and arrange them. At the end of job, which will be starting end for every cut while threading, turn a very small step of diameter equal to the minor diameter to be cut i.e. recessing. Set the tool properly, swivel and clamp the compound rest at an angle if required as discussed already. Adjust the spindle speed for threading. Bring the tool at the starting point, take a certain depth of cut and engage the half nut with lead screw. This will move the tool along the job length forming a thread groove. Have few successive cuts to obtain the required depth of thread. That small step turned at the job and will guide you to feed the tool up to a required depth. When threading is complete, stop the lathe and measure the depth of the threads with a thread pitch gauge.

22. Cutting with Single Point Tool on Lathe Procedure for Internal Threads Hold the job in a chuck. Make a hole equal to the minor diameter of the thread. A common practice is to make the hole slightly larger than minor diameter of thread for providing a clearance between the mating parts. Turn a small groove at the end of the thread length equal to the major diameter of the thread to guide the operator for maintaining the correct depth of thread. Rest of the procedure is the same as for cutting external threads. Special care is needed while threading a blind hole By moving carriage, bring the tool to the job front and let it go inside the hole till it reaches the point at, which the last thread will terminate. At this position, make a line on the bed to guide the operator that he is not to allow the carriage to come towards the left ahead of this mark

23. Cutting Right hand & Left Hand Threads Cutting Right Hand Threads Direction of rotation of job and lead screw should be same i.e. towards the operator (counter clock wise). For threading on job, start the cut from tail stock side. After engaging half nut, see that the tool moves from the tail stock side to headstock side. Cutting Left Hand Threads Reverse the direction of rotation of lead screw with the direction reversing mechanism lever. Start the cut on job from headstock side so that when half nut is engaged tool should move from headstock to tail stock side.

24. Cutting Multistart Threads Cutting procedure of multi start threads is similar to that of cutting single start threads. In multi start thread, circumference of the job should be divided in to as many parts as the starts of the threads and thus every part of the divisions of the circumference of job becomes the starting point for the new start. Several methods are given below to divide the job circumference to take cuts for different starts ( a) Faceplate of lathe may be divided and marked in as many parts as the starts are to be had on the job. Every mark will give the starting position for new start of the thread. ( b) Indexing faceplate of typical construction for cutting multi start threads are usually available. On the plate, means are provided to rotate the job through whatever fractional part of the job circumference is needed .

25. Cutting Multistart Threads ( c) Compound rest is also used for adjusting the tool to have the correct spacing while cutting multi start threads. Set the compound rest parallel to work axis. When one start of the thread is cut, then the tool is moved a distance equal to the pitch to be cut. This method is mostly used for cutting internal threads. Make sure that there is no backlash in the compound rest screw. The rest of the procedure is to get the lines of different starts inscribed on the job surface along the length with a thread cutting tool, cut rough profiles of different starts. For finishing the threads, finish first start completely and then one after the other.

26. Tapered Threads The only precaution used for cutting threads on a tapered surface is that the threading tool is set perpendicular to the lathe axis and not to the tapered surface. Rests of the procedure is same. Only taper turning attachment is employed for taper turning and thread cutting . All these methods are covered in ppts of Lathe & related operations Unit-3

27. Threads Cutting on Drilling Machine using tapping attachment Internal threads of relatively smaller diameter, length and pitch are produced by using tapping attachment with its taper shank fitted axially in the spindle bore. Just at the point of start of return, the lower part of the attachments momentarily gets delinked from the upper part and is then up and rotated respectively by the spring and the clutch to move at per with the upper part fitted into the spindle. This is necessary for the safe return of the tap without damaging the through or blind hole. The tapping attachment is pushed slowly inside the drilled hole at low speed for cutting threads and at the end of this stroke, it is withdrawn slowly by rotating in reversed direction.

28. Die Threading External threads cutting on cylindrical or taper surface by means of Solid and self opening dies is called die threading. Dies are look like nuts but it is made of Hardened Steel or HSS with sharp cutting edges . These hand operated dies are coaxially rotated around the premachined rod like blank with the help of handle or die stock. Types of die Solid die Self opening die

29. Die Threading by Solid dies Solid or button die: used for making threads of usually small pitch and diameter in one pass. Spring die: the die ring is provided with a slit, the width of which is adjustable by a screw to enable elastically slight reduction in the bore and thus cut the thread in number of passes with lesser force on hands.

30. Die Threading by solid dies Split die: the die is made in two pieces, one fixed and one movable (adjustable) within the cavity of the handle or wrench to enable cut relatively larger threads or fine threads on harder blanks easily in number of passes, the die pieces can be replaced by another pair for cutting different threads within small range of variation in size and pitch. Pipe die: pipe threads of large diameter but smaller pitch are cut by manually rotating the large wrench (stock) in which the die is fitted through a guide bush

31. Die Threading by self opening dies The major drawback of the solid type dies is that they must be unscrewed from the work piece by reversing the machine spindle, to disengage the die from the work. Due to this, these dies are not suitable for use on high speed production machines. For use on high speed production machines like Turret lathes and automatics, this drawback is overcome by using self-opening die heads. In it when the required length of thread is cut, the die open automatically. At the end of the turret slide travel, the front portion of the die head continuous to move forward by a small amount until the chasers in the die head move outward in the body, under the action of a scroll or a cam. This action clears the chasers from the cut thread and enables the die head to be withdrawn without reversing the machine spindle. The die head, while cutting threads may advance its own guidance once it screws itself along the work, until the die trip opens. However, for better accuracy, there is increasing use of lead screw guides. Depending upon the type of chaser, there are three types of die heads 1.      Radial 2.      Tangent 3.      Circular

32. Die Threading by self opening dies Radial chasers: Rigid, difficult to resharpen and their life is short. Tangential chasers : long life, easy to regrinds on the cutting face. Due to this they are very suitable for heavy duty work and large batch production. Circular chasers : long working life since these can be resharpened a number of times. All the die-heads can either be stationary or revolving. When used on automatics, the feed motion of the die-head is controlled by the cam rise, which can be designed accordingly. At the end of the return stroke, the dies are closed automatically when the closing handle strikes a rod.

33. Thread Tapping Taps are the tools for cutting internal threads. A tap look and behave like a screw but made of tool steel or HSS and have sharp cutting edges (teeth formed on it by cutting flutes parallel to its axis )produced by axial grooving over the threads . The flutes perform three functions : Provide cutting edges. Conduct the cutting fluid to the cutting region, and Act as channels to carry away the chips formed by the cutting action . The flutes can be straight, spiral, helical or spiral pointed. Taps with straight flutes are most commonly used, since it is easier to cut and sharpen these flutes. Tapping can be dome manually or on drilling machines, tapping machines, turret lathes and automatics. A hole of diameter slightly larger than the minor diameter of the thread to be cut must already exist, for thread tapping. Drilling can make the hole, boring or casting. The two main types of taps are: solid taps and collapsing taps .

34. Thread Tapping-Solid Taps Solid taps are of one-piece construction. These taps are usually worked manually but can also be used on machine tools, such as lathes, drill presses and special tapping machines. Taps are made of high carbon or high-speed steel. The shank of the taps is kept plain and the end is squared. To operate the tap by hand (Hand taps), it is held at the squared end with the help of a “tap wrench”, which is used to screw the tap into the hole. To cut any particular size, hand taps are available in sets of three: taper, plug and bottoming. The three taps are identical in size and length, but differ in the amount of chamfer at the bottom end. The taper plug has about 8 to 10 threads chamber at the bottom end, the plug tap has 2 to 3 threads chamfered, whereas, a bottoming tap has no taper threads at its bottom end. The tapered are cut to the full depth gradually, so less effort is required. If a hole is open at both ends, then, after the taper tap, plug is used for finishing the treads as deep into the hole as its shape will permit. Lastly, the bottoming tap is used to finish the entire thread portion. So, the three taps should be used in the order mentioned above. The bottoming tap is the only tap, which would nearly reach the bottom of a blind hole. These taps namely, taper tap, plug tap and bottoming tap are used consecutively after drilling a tap size hole through which the taps are axially pushed helically with the help of a handle or wrench.   The taper tap allows the tap to be started straight in the hole easily so as to produce uniform and complete threads.

35. Machining: Hand Operated Tools: Internal Threads Threads are often tapped by manually rotating and feeding the taps through the drilled hole in the blank held in lathe spindle The quality of such external and internal threads will depend upon the perfection of the taps or dies and skill of the operator. During machining features like fast retraction of the tool, automatic loading or unloading, use of multiple spindles; machining rate will considerably increased.

36. Collapsible Taps Threads of large diameter are not generally cut with solid tap but with collapsible tap. This consists of a body with slots for carrying the blades. Inside the body there is a plunger with a taper on the end of which supports the blades in their operating position. A trip ring, which may be set at a suitable longitudinal position, is connected to inner plunger and the action of this ring arrests the inner plunger whilst the blade and body moves forward. This removes the taper support from the blade bases allowing them to collapse so that the tap may be withdrawn without stopping or reversing the machine. It is used when the diameter exceeds 1”.  It is tool which collapse inwards automatic when thread is completed . It permits return without unscrewing . So speed of operation is increased. Due to machining surface finish is good . The taps are provide with trip plate which set accordingly the length of thread to be cut.

37. Thread Chasing Thread chasing is the process of cutting a thread on a lathe with a chasing tool that comprises several single-point tools banked together in a single tool called a chaser . Thread chasers are shown in Figure 4.7. Figures a and b show a tangential-type chaser for cutting external threads and Figure c shows a circular chaser for cutting internal threads.

38. Thread Chasing Chasing is used for the production of threads that are too large in diameter for a die head . It can be used for internal threads greater than 25 mm in diameter. During internal and external chasing , the chaser moves from the headstock. The chaser is moved radially into the WP for each cut by means of the cross slide screw. Thread chasing reduces the threading time by 50% compared to single-point threading.

39. Thread Chasing However , thread chasing is a relatively slow method of cutting a thread, as a small depth of cut is used per pass. Depending on the size of the thread, 20–50 passes may be required to complete a thread. Multiple threads, square threads, threads on tapers, threads on diameters not practical to thread with a die, threads that are not standard or those that are so seldom cut that buying a tap or die would be impracticable, or threads with a quick lead are all cut by chasing. Chasing lends itself better to nonferrous materials rather than ferrous ones. Multistart threads can be chased without any indexing of WP. Taper threads can be generated by chasing, if the chasing attachment is used in conjunction with taper attachment. For HSS cutters, a cutting speed of the order of 40 m/min and upward should be used. Feed varies from 5 to 7.5 cm/min for coarse threads in tough materials to 20–25 cm/min under more favorable conditions.

40. Thread Milling Thread milling is a machining process used for cutting screw threads with a single-form or multiple form milling cutter . When milling suitable: For producing internal thread which are not suitable to cut with tap. Work on large diameter beyond the capacity of die Thread milling makes smoother and more accurate threads than a tap or a die. It is more efficient than using a single-cutting-point tool in a lathe. Thread milling is the most practical method for thread cutting near shoulders or other interfaces.

41. Various Thread Milled Parts

42. Thread Milling Single-form and multiple form milling cutter thread milling operations: (a) disc cutter and (b) multiple-thread cutter

43. Thread milling with a disk milling cutter Used for cutting long and coarse threads with trapezoidal profiles. Sometimes used to machine triangular threads, Not used for cutting square threads. During threading, the cutter rotates and provides a longitudinal feed by the pitch of the thread. The axis of the cutter arbor is set at the thread helix angle to the WP axis. When cutting multiple-start threads, the wp should be turned by 1/n of a revolution (n is the number of starts) and the feed rate should be made equal to the lead of the thread.

44. Thread milling with multiple-thread milling cutter This method is used to produce short threads of 15–75 mm length and 3–6 mm pitch. The cutter should be 2–3 pitches longer than the thread being cut. S traight threads

45. Thread milling with multiple-thread milling cutter External tapered threads can be cut using multiple-thread milling cutters having threads perpendicular to the axis of the cutter For internal tapered threads , the cutter angle should be equal to the angle of the taper of the cut thread . Generally, the direction of feed is parallel to the generator of the thread surface. Tapered threads

46. Thread Milling Machines Universal thread mills . These machines have a lead screw and cut internal and external threads ( with the exception of square threads). Change gears permit milling of threads with leads of 0.8–1520 mm. Pick-off gears in the cutter drive provide a wide range of speeds. The cutter head on the cross slide can be set at the proper angle for right-hand or left-hand thread helix angles. A single-form cutter must be set at such an angle and then allowed to traverse the full length of the thread .

47. Thread Milling Machines Planetary thread mills . These machines are used to thread odd-shaped parts that are difficult to be held in a chuck. Consequently , the WP is held in a special fixture that does not rotate during thread cutting. The milling cutter rotates around its axis and revolves around the work . Double heads can be used to cut both ends of the part, and external and internal threads can also be machined at the same time. NC machines . These machines are used for thread milling together with other operations in a single WP operation. Long cutter life and high-quality threads are some of the advantages of these machines.

48. Thread Broaching Thread broaching is a newly developed thread cutting process that has been employed in the automotive field . Typical parts include internal threads on steering-gear ball nuts and ball-race nuts for various circulating ball-type assemblies. The WPs are given one or two passes (rough and finishing cuts ), heat-treated, and then finish ground on an internal thread grinder. The broaches used for the application have a special form and are guided by lead screws. Threads are cut by drawing the part and fixture against the revolving tool. Threading broaches are available in sizes up to 50 mm diameter and 750 mm length.

49. Thread Grinding In production of screw threads, grinding is employed for two purposes; Finishing the threads after machining or even rolling when High dimensional and form accuracy as well as surface finish are required, e.g., screw threads of precision machines and measuring instruments The threaded parts are essentially hardened and cannot be machined or rolled further, e.g., leadscrews of machine tools, press – screws etc. Directly originating (cutting) and simultaneously finishing threads in any hard or soft preformed blanks. This is employed generally for finer threads of small pitch on large and rigid blanks Thread grinding is used to cut threads on: taps, micrometer screws, lead screws, thread gauges and milling cutters .

50. Thread Grinding Thread grinding is the preferred method of threading when the WP hardness is greater than 36 HRC or less than 17 HRC, and when a high degree of accuracy is required. Threads are ground by the contact between a rotating WP and a rotating GW that has been shaped to the desired thread form. In addition to the rotation, there is a relative axial motion between the wheel and WP to match the pitch of the thread being ground. The process can be used to produce either external or internal threads.

51. Thread Grinding Methods

52. Center Type Thread Grinding In this operation, the WP is held between centers or in the machine chuck. The material specifications and the form, length, and quality of the thread determine the number of passes required (from one to six passes). Depending on the design of the threading wheel, the following two basic methods can be identified : Single-rib wheel traverse grinding Multirib wheel grinding 1. Single-rib wheel traverse grinding . The most versatile method for which the highest accuracy can be obtained. The single rib wheel is adaptable, by truing, to many different profile configurations (Figure a ).

53. Multirib Wheel Grinding The wheels have two or more parallel grooves or ribs around the periphery of the wheel. Each rib is trued to the form of the thread to be ground. The thread form is imparted to the wheel by diamond or crush truing. A Traverse grinding . More productive than single-rib-wheel grinding, because of the higher material removal rate per pass. However, the pitch should not exceed 1/8 of the wheel width and threading against shoulders should be completely avoided

54. Multirib Wheel Grinding B . Plunge grinding. The most productive thread grinding method; therefore used for the production of parts in substantial quantities. As shown in Figure c , the GW is advanced into the rotating WP. C. Skip-rib traverse grinding . This process uses a wheel, which has a spacing that is twice that of the thread pitch, basically used for threading accurate and fi ne pitches . Threading is accomplished in two passes. In the first pass, the wheel grinds every other thread of the WP. In the second pass, the WP is advanced by a single pitch and the untouched threads are then ground (Figure d ).

55. Multirib Wheel Grinding D. Three-rib traverse grinding . As shown in Figure below, the GW has a roughing rib ( A) that removes two-thirds of the material, and an intermediate rib (B) that takes the remainder of the material and leaves 0.13 mm for clean up by the finishing rib (C). A flattened area (D) is used to finish the crest of the thread. The process produces more accurate threads than single-rib-wheel thread grinding (a) Conventional grinding, (b) skip-rib grinding, and (c) three-rib grinding

56. Centreless Thread Grinding Centerless thread grinding is a high-production process for continuously grinding threads on cylindrical surfaces in which the workpiece is supported not by centers or chucks, but by a blade the regulating wheel rotates in the same direction as the GW. Screw threads are cut by feeding the blanks between the grinding and the regulating wheels in a continuous stream most productive method of grinding screw threads that uses either single-rib or multi rib wheels.

57. Centreless Thread Grinding Two basic methods of centerless thread grinding A. Plunge grinding . In this arrangement , the wheel is plunged into the WP to the full depth. The WP then makes one revolution, while the wheel traverses one pitch. This method gives a uniform wheel wear, but is used for short thread lengths. B. Traverse grinding. The wheel is positioned at a full thread depth, then the work is traversed past the wheel. The first thread form on the wheel removes the majority of metal and therefore is subjected to the most wear; the following threads affect the fi nishing . Traverse centerless grinding of headless screws

58. Thread Whirling Thread Whirling is a form of the Thread Milling process with the exception that the cutters are mounted on the inside of a Cutting Ring or Cutter Holder rather than the outside of a milling tool . Thread whirling uses a holder equipped with multiple inserts that is mounted into an attachment capable of helical angle adjustments and high speed concentric rotation . The workpiece, which is advanced at a slow rpm, engages the cutting head, which in turn is advanced longitudinally. The combined rotation of workpiece and cutter, and the angle of the tool head, produce the thread pitch required. Advantages of Thread Whirling: Deep Threads, Faster Setup, Increased Productivity, Increased Tool Life