Introduction

In manufacturing, threading is the process of creating a screw thread. More screw threads are produced each year than any other machine element. There are many methods of generating threads, including subtractive methods (many kinds of thread cutting and grinding, as detailed below); deformative or transformative methods (rolling and forming; molding and casting); additive methods (such as 3D printing); or combinations thereof.

1. Casting:

                        In casting threads are formed by the geometry of mould cavity in the mould or die. When material freezes in mould, it retains the shape after mould is removed. Generally low melting point alloy threads are made by this process; hence they are soft and not durable.      

                These type of cast threads are used in vending machines, sewing machine, type writer parts, Plastic moulding is employed for plastics only. This material may have threads cast in place, but most of the plastics are so easily worked that tapping will be more economical.

                Cast threads in metal parts may be finished by machining, but it increases cost. Therefore, machining is not used in parts where extra precision and accuracy is not needed in order to achieve lower cost.

 

                         

 

                                                Fig.1 Thread Manufacturing by Casting.

2. Rolling:

                  Rolling is the most economical and fastest method of making threads. It is a cold working process whereby a plastic deformation takes place. No metal is removed and no chips are produced. Cold rolling strengthens the thread in tension, shear and fatigue because the fibres are not severed as in other thread making methods, but are elongated and reformed in continuous lines which increase the strength.

              The tools used are generally expensive, but die life is long and production of accurate threads can be maintained at high speeds over a long period. Thread rolling can be accomplished using either flat dies or circular dies.

 

                             

                                               Fig.2 Thread manufacturing by Rolling

 

              The majority of external threads produced are made by rolling. Such items as electric light bulb threads, wood screws, machines and cap screws, gimlet pointed stamped screws, sheet metal screws, hooks and eyes, as well as many parts are threaded and knurled by this process.

              Flat dies are used in conjunction with special machines. The top and bottom dies are fixed over top and bottom slides, which are driven in opposite directions by hydraulic cylinders. The thread is formed complete within one pass of the blank between the dies. Each die is grooved with the thread profile, the thread grooves being inclined as the thread helix angle to avoid interference during rolling. The die length is such that the blank rotates 4-5 times during one pass. Depending upon the material properties, the rolling speeds are between 0.50 to 1.25 m/sec.

                In the case of circular dies, these are contained in a die head. Three numbers highly polished thread rolls are mounted on large, friction free spindles. The thread grooves in the rolls are not annular, but are true helical grooves.

 

3. Chasing:

                       Thread chasing is the process of cutting a thread on a lathe with a chasing tool which in effect comprises of several single point tools blanked together in a single tool called a chaser. Fig shows a tangential type chaser for cutting external thread and circular chaser to cut internal threads. This is relatively slow method of making a thread.

                   Tight custom fits can be made on a lathe, as well as multiple threads, threads on tapers, threads on diameters not practical to thread with a die, threads that are not standard or those which are so seldom cut that buying a tap or die would be impracticable, or threads with a quick lead; all these are well suited for chasing.

 

 

                          

                                               Fig.3 Thread manufacturing by Chasing.

                        

                   This is only method of producing square threads, as other methods develop interference on the helix. When a work piece is to be machined in a lathe and also requires threads, internal or external but concentric with the turning operation, it is more economical to do the threading in the lathe either by chasing or with a tap or die.

                 Chasing lends itself better to non-ferrous materials rather than ferrous. Multi-start threads can be chased without any indexing of work piece. Taper threads can be generated by chasing, if chasing attachment is used in conjunction with taper attachment.

 

4. Die-Cutting:

                This method is the most widely used method of producing external threads. Dies are relatively rapid producers and thus are economical. The quality and accuracy of such thread is only moderate but is acceptable for most mass-produced articles. For a small shop, chasing may be less expensive than stocking a complete set of taps and dies.

Threading dies are of two general types:

(i) Bottom or Round Split Dies and

(ii) Adjustable dies.

(i) Bottom or Round Split Dies:

These are primarily intended for hand use but may be used in machines also. These dies are round with a radial cut closed by a screw which allows them to be adjusted within a narrow range for a tight or loose fit. These are held and operated by means of a die stock.

                                         

                                                   Fig.4 Split-die

(ii) Adjustable Dies:

These consist of two pieces which are held in a collet or mounted directly in a die stock. By a taper arrangement these may be moved towards or away from the centre so as to provide various degrees of fit. They are primarily intended for hand use.

5. Self-Opening Die Heads:

                  Die boxes are used for high production of external threads on capstan and turret lathes. Die heads contain dies or chasers and each is suitable for a given range of sizes.

Three types of die heads commonly used are:

(i) Radial dies, (ii) Tangential dies, and (iii) Circular dies as shown in Fig

                        

                                                        Fig.5 Die Heads

                    The dies open automatically when the required length of thread is cut. When the turret slide movement is arrested by a stop, the front part of the head continues to move forward by a small amount until the dies spring outwards, away from the work under the action of a scroll or cam.

                   Provision is made for taking roughing or finishing cuts by moving a detent pin to the appropriate position. Since the dies trip open after screwing operation, the work spindle need not be reversed in order to screw the die head off again. The dies can be closed by the operator after each screwing operation, by pushing a handle which partially rotates the front portion of the head.

                  Die heads for internal threads are collapsible taps. These withdraw or collapse inwards when a hardened steel ring around the tap strikes the end face of the work.

Threading dies are used for cutting or sizing external screw threads in a single pass. Dies are most widely used for cutting threads up to 52 mm in diameter.

                A solid threading die is in effect a hardened nut with axial openings forming cutting edges. Normally, dies have from 3 to 6 clearance holes for chip disposal.

 

                                        

                                                  Fig. 6 Threading die

 

             The die thickness is from 8 to 10 turns of the thread. The chamfer covers from 2 to 3 turns of the thread. The angle 2 j = 40° to 60° is for cutting through threads and 2j = 90° for cutting threads close to a shoulder. In standard dies the rake angle is g = 15° to 20°. The relief angle, which is α = 6° to 8°, is formed on the chamfered (cutting) section only.

 

6. Taps:

Available in many types, these are used for cutting internal threads. There are hand taps and machine taps, straight shank and bent shank taps regular pipe taps and interrupted thread pipe taps, solid taps and collapsible taps.

A tap may be compared with a screw which has teeth formed on it by cutting flutes parallel to its axis, and then has been hardened so that it will cut metals. Besides forming teeth, the flutes act as channels to carry away the chips formed by the cutting action.

Hand taps are furnished in three sets; taper, plug and bottoming. These three are identical in size, length and vital measurements, differing only in chamfer at the bottom end. The taper tap has about 10 threads chamfered at the end, the plug tap about 5 and the bottoming tap only 1. The taper tap allows the tap to be started straight in the hole easily so as to produce uniform and complete threads.

For threading purposes, these are used in the order; taper, plug and bottoming. Bottoming tap is best suited when blind holed threads are required. In cases where threads are to be made by using only one tap, the plug tap is generally used. Standard taps of sizes 5/32″ to 1/2″ are furnished with four flutes and are used for iron and steel.

These do not provide sufficient chip room for certain metals that are soft and staringly, such as copper in which case two or three-fluted taps should be used. Serial hand taps are also available and are similar to the above set except that each cut only a certain percentage of the complete thread.

They are numbered 1, 2 and 3, and are used in that order. These taps find particular use in cutting tough metals because the load is shared by the three taps, making cutting easier and producing a smoother thread.

Nut taps are made with straight shanks (short or long) and bent shanks.

The thread length l of a tap is made up of a chamfer l₁ and a sizing section l₂. The chamfer, or cutting section, has 4 turns of the thread for roughing hand taps, and 1.5 to 2 for finishing hand taps. Machine taps have from 5 to 6 turns of the thread on the same section for cutting through holes, and 2 turns for cutting blind holes. Nut taps come with chamfers having 11 to 12 turns of the thread.

The sizing section l₂ of the tap serves to finish and size the thread being cut and properly guide the tool in the hole. This portion is slightly back tapered to reduce friction.

The shank l₃ is a plain rod with the square l₄ for hand (and sometimes for machine) taps. The profile of tap flutes has an effect on the cutting process and should facilitate the removal of chips. Taps with three and five flutes have found wide use.

 

                       

 

The rake angle in taps is g = 5° to 10° for threading steels, g = 0° to 5° for cast irons, and g = 10° to 25° for non- ferrous metals and alloys. The relief angle is α = 4° to 12°. Taps are usually made with straight flutes, but for some applications resort is made to taps with helical flutes having a helix angle of ε = 8° to 15°, which improves conditions for chip removal.

7. Milling:

When threads are cut by milling, the thread is formed by a revolving milling cutter shaped to conform to the shape of thread desired. Either single or multiple cutters may be used. In the case of single cutter, all the cutting edges lie in one plane. The multiple cutters consist of several annular rows of cutting teeth.

A hob may be used for cutting threads, in which case the teeth lie along a helix. The disadvantage of the hob type cutter is that it must revolve with a fixed relation to the work, this is not true of the cutter with annular teeth.

Milled threads may be external or internal, the only limitation being the size of hole in which a cutter may be inserted. The threads cut by this method are more accurate than those cut by a die but not so accurate as those cut with a grinding wheel. A given cutter is not limited to one size of thread, as in the case with taps and dies.

This method is desirable when the pitch of the thread is too coarse to be cut with a die. This process is more efficient than threading on a lathe, especially when the work piece is long or when large number of stocks are to be removed. Parts such as lead screws are milled because of the high accuracy and fast production and because the part can usually be finished in one or two passes.

 8. Grinding:

Internal or external threads can be finish ground by means of a single or multiple edge grinding wheel. The threads are cut as grinding wheel. A vitrified bond is generally used with a fine grit of about 60. The process is carried on a special grinding machine having a master lead screw and gears and means of holding the work.

The wheel rotates at 30 m/sec and work is rotated slowly. In the case of hardening stock probably grinding is the only means of forming threads. The accuracy of grinding exceeds that of any other method and the finish is exceeded only by good thread rolling.

Pitch diameters can be ground to an accuracy of ± 0.002 mm per 2.5 cm and accuracy of lead may be maintained within 0.007 mm in 50 cm of thread length. Grinding eliminates tiny cracks due to hardening and also tearing is always present to some extent in any material removal method.

Distortions due to heat treatment may be eliminated by grinding. Parts which would be distorted by milling threads can be satisfactorily ground. The thread parts which demand high accuracies and freedom from distortion, and stress cracks are usually made by this method.

              

 

Two variations of process are:

(i) Pass over or traverse method and

(ii) Plunge method.

In first method the wheel is positioned at full thread depth and then the work is traversed past the wheel. The work table traverse is controlled by a master lead-screw and change gears are used to suit the thread pitch.

 In the case of plunge cut thread grinding, the wheel is plunged into work to full thread depth. The work piece then makes one revolution and work traverses one pitch.