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.
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.
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|Publication:||Tooling & Production|
|Date:||Oct 1, 1984|
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