Rolling out complex metal parts

By Glenn W. Eberly
Rathbone Precision Metals Inc.
A Subsidiary of Carpenter Technology Corp.
Palmer, Mass.
www.rathboneshapes.com

Edited by Jean M. Hoffman

It's a common misconception that cold forming is more expensive and time consuming than machining. But it's not, at least for difficult, precise shapes in quantity. In these cases, precision cold forming boasts better finishes, tighter tolerances, and more consistent dimensional repeatability than their machined counterparts.

Cold-formed components require minimal secondary operations such as milling and polishing. They exhibit predictable dimensional uniformity. This "near-net-shape" capability reduces raw material waste and translates in significant bottom-line benefits for machining components from high-cost alloys.

COLD, HARD FACTS
Although cold forming has been refined significantly over the years to meet demands of modern OEMs and their subcontractors, the basic principles of cold drawing and cold rolling haven't changed much since the Industrial Revolution.

Cold rolling compresses and spreads metal bar or coil stock between a series of preshaped rollers. This improves the metal's mechanical properties. Of the two processes, cold rolling imparts better surface finishes and is more suited for simpler profiles with less-demanding tolerances.

Cold drawing, on the other hand, uses extreme pressure to pull metals, such as copper and brass, and stainless or carbon steel through a controlled series of progressively smaller dies at ambient temperature. It gradually establishes part contours while cold working the metal, increasing its tensile strength and hardness. A skillfully crafted series of carbide dies creates complex shapes with consistent corners/fillets and outside radii to ±0.005 in. Other profiles include dovetails and undercuts. Finished shapes can be cut to length or supplied as coils or straight lengths.

Cold drawing is often confused with hot extrusion, which pushes hot metal billets through extrusion dies. Steel and stainless hot extrusions have surface finishes of 250 R_a and greater, linear and radial tolerances of ±0.02 and ±0.062 in., respectively, and forms profiles up to 6in. diameter.

In contrast, cold drawing produces standard surface finishes of 63 R_aor better while holding standard linear and radial tolerances at ±0.002 in. and ±0.005 in., respectively. Additionally, well-equipped and experienced cold-drawing shops hold linear tolerances to ±0.0005 in. and maintain radial tolerances of ±0.002 in.

Size limitations for cold-formed parts depend on part configurations. Cold-rolled components typically measure less than 1.5 in. diameter, while cold drawn generally run from 0.032 to 2.5 in. diameter. This comparatively small work envelope is the biggest limit of cold forming. Usually, it is neither cost effective nor practical to draw stock much larger than 2.5 in., where casting, hot extrusions, and machining are better suited.

ART AND SCIENCE
Designers trying their first precision cold-drawn or cold-rolled part will be familiar with the make-to-order ritual of inquiry-quotation-order, although the manufacturing process may seem foreign when compared to machining.

Industry veterans often say that there's an element of art in what is essentially a metalworking science practiced by cold formers. Therefore, it is to the designer's advantage to work closely with an experienced cold drawer when designing cold-formed parts for the first time. Their experience in tooling and process design is central to quality cold forming.

Experienced cold formers are well versed in the idiosyncrasies of cold forming such as the metallurgical reactions of various alloys to high pressure, as well as the most efficient methods for achieving specified tolerances.

This experience can make the difference between high-performance parts and parts that don't measure up. That means "measure," literally. Cold-drawn and cold-rolled parts should follow the same manufacturing standards as any other fabricated component. The major difference in the quality of finished cold-formed parts is almost exclusively a function of how good the progressive tooling is. This contrasts to machined parts that are largely a reflection of the proficiency of the machinist controlling the CNC.

Preproduction samples are generally sent to customers for inspection, a comparatively rare practice with contract machine shapes. Cold-worked samples come from the same tooling and processing as the production material, so they give designers a good idea of future production quality.

For repeat projects, cold formers typically help establish both just-in-time and stocking programs of finished profiles or initial raw materials. Blanket orders with scheduled release dates are yet another commonly used approach to further reduce turnaround times. Additionally, cold-forming manufacturers will often supply limited sample materials for assisting machine shops with initial fixture/tooling setup and production parts approval program (PPAP) requirements.

WEIGHING THE CHOICES
Customer-specific tooling —specifically the progressive carbide dies crafted by cold-forming houses — is the most significant upfront cost in new orders. This one-time expense ranges from $300 to $6,000, depending on complexity and size. Development, which includes tooling production, also depends on profile complexity, mill workload, and raw material. It may take from three to 10 weeks before a sample part is in the designer's hand for approval.

When weighing the pros and cons of cold drawing and cold rolling, designers need to consider a number of factors.

Production volume is the top consideration. Up-front costs and turn-around times don't lend themselves to prototypes or one-time, low-volume runs.

Material availability is another factor that influences turnaround time when selecting cold forming. Forming houses generally stock a number of common grades and sizes, but for certain highly engineered parts, lead-times may be at the mercy of the mill producing the raw stock.

Material availability in rod or coil form can also increase lead times. It is often more efficient for a cold drawer to internally process raw material from coils than to process the same shape from bar stock.

As a rule of thumb, cold drawing is the best option for repeatedly ordered components. These components are usually processed through long-wearing carbide dies maintained and replaced at the expense of the cold drawer. Cold-drawn profiles aren't subject to faster tool wear, machine temperature variations, and variability in operator experience as often as their machined counterparts. Cold drawing also minimizes lot-to-lot and part-to-part discrepancies helping to lower reject rates.