Rolling to shape: low cost, high potential Essay

Rolling to shape: Low cost, high potential



The IBM Corp, Lexington, KY, buys typewriter filter shafts
manufactured by cold rolling their shape into AISI 1018 steel bar. This
precision component meets a flatness requirement that mandates a
no-twist tolerance of 0.002 max/12 ft.



General Electric, Schenectady, NY, as well as Allis-Chalmers and
others, purchase miniature cold-rolled I beams to serve as spacers. Made
from 1008 and stainless-steel wire, they allow coolants to circulate
between laminations in large electric motors and generators.



MacLean-Fogg, Mundelein, IL, purchases self-piercing nut stock for
the auto industry, produced from 1015 steel bar that’s cold rolled
to a hardness of from 90 R(B) to 95 R(B).



These examples indicate a trend toward increased cold-roll shaping
of steel bar and wire, and there are several reasons for it: If a part
can be cold rolled (i.e., embodies close to one continuous profile over
its full length), then the technique can be the lowest cost of all metal
parts-making processes; cold rollers claim their process ups tensile and
yield strengths as much as 20 percent over the starting material;
surface finishes down to 8 rms are feasible, particularly in alloy
grades; and standard tolerances are 0.0015 in thickness and 0.002 in
width, all of which reduce material and secondary operation costs.



According to William F Schreiner, manager at Charter Wire,
Milwaukee, WI, a leading cold roller, “Increasingly, designers and
buyers of small parts and components are learning about cold rolling to
net or near-net shape. Now our industry realizes an annual volume
approaching $100 million and cold rolls some 100-million lb of steel
yearly. Potential cost reduction is, obviously, a primary stimulant;
however, improved properties and reduced component inventories through
frequent and scheduled deliveries are other reasons.’


Actually, the sole impediment to cold rolling’s growth is the
difficulty of discovering all the applications for which it’s
suited. Already, it’s a components-manufacturing method for
industries ranging from agricultural equipment, auto and electrical to
filtering, office equipment and railroad.



The process is applicable to any metal that can be shaped at room
temperature. Most often, the starting material is hot rolled, coiled,
rod or bar. All low- and medium-carbon steels can be rolled; even high
carbons with special techniques. Also used are the resulphurized 1100
series, rephosphorized 1200 series, the 1300 manganese steel series, and
leaded grades where finish machining will complete a component.



In alloy steel bar, such compositions as 4037, 4130, 4140, 8617,
and 8620 are commonly chosen. In rod (from which wire is drawn), carbon
steel grades from AISI 1006 to 1065 are employed. Other metals are also
cold-rolled–aluminum, copper, silicon bronze, and stainless steels, for
example.



What the process does is put bar and wire into any number of
cross-sectional shapes–symmetrical and irregular, with grooves, beads,
curves, and all manner of indentations and protrusions. Since these
special forms are mostly custom shapes for the OEM market, custom-made
rolls are required, each precision-ground with its stage of the
progressive profile. For some standard shapes (i.e., ovals,
half-rounds, hexagons, squares, flats etc), however, stock rolls are
available.



Design considerations



As for part design, symmetrical units with rounded corners are the
easiest and most economical to produce. Moderate asymmetry can be
accommodated by special tool design or rolling variations. Extremely
complex, asymmetrical sections, D shapes, and hexagons are often
produced by a combination of cold rolling and drawing.



A rule of thumb is don’t exceed a 10:1 ratio of part width to
thickness (height). Other caveats include no tight tolerances unless
they serve a functional purpose, no groove deeper than it is wide, and
no thin projections extending from a mass of material. Also, in
general, the larger a radius, the better it can be rolled.



Where finishing is required, cold-rolled parts can be bent,
blanked, cut, drawn, drilled, ground, pierced, punched, riveted,
soldered, stamped, tapped, threaded and welded. Moreover, rolls can be
ground to impart embossing, notches and teeth into the part.



Photo: Cold rolling is a progressive forming technigue. At left,
the process starts with 3/4 round bar, which proceeds through sizing,
flattening, roughing, and finishing passes to produce the component at
right.



Photo: This circle of shapes shows cold-rolled cross sections with
one profile the full length of a bar or Wire. Uniform parts are sliced
from the length, with finishing either unnecessary or minimal. If cold
rolling can be used to produce a part, it’s the least costly of all
metal parts-making methods.