Page 349 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

< Various Rolling Processes and Mills 2

~ tt.
Section 13.5

Edging
Idler
Rounding Main roll
(driven)
roll
roll
roll
;$.
i

Workpiece
(H) (D) (C) (Ol)

FIGURE I3.|5 (a) Schematic illustration of a ring-rolling operation. Thickness reduction
results in an increase in the part diameter. (b) through ld) Examples of cross sections that can
be formed by ring rolling.

Typical applications of ring rolling are large rings for rockets and turbines, jet-
engine cases, gearwheel rims, ball-bearing and roller-bearing races, flanges, and
reinforcing rings for pipes. The process can be carried out at room temperature or at
an elevated temperature, depending on the size (which can be up to 3 m, in diame-
ter), strength, and ductility of the workpiece material. Compared with other manu-
facturing processes capable of producing the same part, the advantages of ring
rolling are short production times, material savings, close dimensional tolerances,
and favorable grain flow in the product, thus enhancing its strength in the desired
direction. The design of the profile rolls requires considerable experience. Analytical
techniques are being developed to rely less on established practice and help minimize
defects in rolled products.

Thread Rolling. 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. 13.16a). In another
method, threads are formed with rotary dies (Fig. 13.16c), 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. 13.17a)-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. 1317).
Spur and helical gears can be produced by cold-rolling processes similar to
thread rolling (see also Section 24.7). The process may be carried out on solid cylin-
drical 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.