Thread Rolling Manufacturing- Introduction, Principle
Thread rolling is a cold-forming process by which straight or tapered threads are formed on round rods or wire. The threads are formed on the rod or wire with each stroke of a pair of flat reciprocating dies (Fig.).
In another method, threads are formed with rotary dies (Fig. ), at production rates as high as 80 pieces per second. Typical products are screws, bolts, and similar threaded parts. Depending on die design, the major diameter of a rolled thread may or may not be larger than a machined thread (Fig.)-that is, the same as the blank diameter.
The thread-rolling process is capable of generating other shapes as well, such as grooves and various gear forms, and it is used to produce almost all threaded fasteners at high production rates. The thread-rolling process has the advantages of generating threads with good strength (due to cold working) and without any loss of material (scrap). The surface finish produced is very smooth, and the process induces compressive residual stresses on the workpiece surfaces, thus improving fatigue life. Thread rolling is superior to other methods of thread manufacture, because machining the threads cuts through the grain-flow lines of the material, whereas rolling the threads results in a grain-flow pattern that improves the strength of the thread (Fig. ).
Spur and helical gears can be produced by cold-rolling processes similar to thread rolling . The process may be carried out on solid cylindrical blanks or on precut gears. Cold rolling of gears has extensive applications in automatic transmissions and in power tools. Internal thread rolling can be carried out with a fluteless forming tap. This operation is similar to external thread rolling, and it produces accurate internal threads with good strength
Lubrication is important in thread-rolling operations in order to obtain a good surface finish and surface integrity and to minimize defects. Lubrication affects the manner in which the material deforms during deformation, which is an important consideration because of the possibility of internal defects being developed . Typically made of hardened steel, rolling dies are expensive because of their complex shape. They usually cannot be reground after they are Worn. With proper die materials and preparation, however, die life may range up to millions of pieces.