Cold Forming...When CNC Just Doesn't Cut It...
Deringer-Ney Inc. offers decades of experience and over two centuries of high value material management to assist in design and fabrication of your most challenging parts. When a product engineer requires a small metal component with precise tolerances, the first metal forming process that usually comes to mind is computer numerical control (CNC) machining. Although machining certainly has its place, the product engineer should seriously consider a cold formed part, especially when specifying difficult-to-machine materials or expensive alloys containing precious metals.
Cold forming is a process similar to forging, but is performed on metallic materials at or near room temperatures. Using this process, a metal blank is cut from wire stock, then formed at high speed and pressure using tool steel or carbide dies. There are many advantages of this forming method over CNC screw machining: high throughput, increased strength and hardness, and minimal scrap. For metals that are very difficult to machine due to their poor chip-breaking tendencies, as well as those that tend to form built-up edges on machine tools, cold forming is often the best approach. With this forming technique, tiny, complex parts with tolerances of 0.0005” (13um) may be produced.
Productivity and Throughput
Cold forming machines run at lower cycle times than Swiss screw machines, potentially resulting in large savings at high volumes. A cold forming machine works through a main crank shaft, producing a part with every one or two revolutions. Depending on the material, geometry, and part size, run rates can be from 30 to 250 pieces per minute. In contrast, Swiss screw machines will typically produce from 2 to 10 parts per minute, with other types of machining centers even slower.
The raw material for a cold formed part is in the form of a coil or spool of material. Compare this with Swiss screw machine raw material, which is required to be straight, single pieces of bar stock, typically 10’ (3m) in length. Material handling, transport, and storage of coils is simple compared with the handling of bar stock. This results in additional savings and material flexibility that enables lower pricing on high volume part purchases. Deringer-Ney will not only manufacture custom-designed cold formed parts, but will also assist the product engineer in the optimim design for a cold forming operation.
Some materials are notoriously difficult to machine. This especially holds true for pure, non-alloyed metals that tend to be “gummy”. Such materials may leave long, stringy metal chips that interfere with the tool path or even cold weld back onto the surface of the workpiece. These materials include nickel (Ni), silver (Ag), gold (Au), palladium (Pd), copper (Cu), niobium (Nb), and tantalum (Ta). For example, commercially pure grades of nickel (i.e. Ni 200) are known to adhere to machine tools, resulting in material build-up, welding, or even burning of the cutting tool bit. This leads to poor surface finish and rapid tool wear. In contrast, nickel is readily cold formed. Nickel can also gall and adhere to cold forming tools and dies, but with proper die material selection, lubrication, and tool maintenance, nickel can be made to cold form. Other examples are significantly easier to cold form than machine; pure gold, silver, palladium, and copper.
Many materials can be cold formed including pure metals and metal alloys. The material must be sufficiently ductile, malleable, and usually requires an annealed or low cold work (i.e.quarter hard) temper. It must be soft enough to survive the spreading, bending, and/or extrusion during the forming operation without crumbling, cracking, or rupturing. Microstructural aspects including grain size and texture are important to material behavior in the cold forming operation; selection of alloy, temper, and vendor may require consultation with one of DNI’s R&D metallurgists. Common physical properties of the wire stock are elongation above 20% and ultimate tensile strength below 90ksi. One strategy that may be used to achieve optimum part properties is to select an age hardenable metal alloy, cold form components in an annealed state, and then perform an age hardening heat treatment after the fact.
Lead (Pb), sulfur (S), selenium (Se), copper (Cu), and tellurium (Te) are alloying elements added to screw machine bar stock materials. These additives are specifically chosen to improve overall machinability, but can lead to potential problems in meeting certain environmental requirements. These elements are used to help improve cutting tool life, surface quality, and chip breaking characteristics. Additional alloying elements are not required in materials used for cold forming. Designing for the use of a cold formed part could help meet future goals toward Restriction of Hazardous Substances (RoHS) directive.
Improved Part Strength
When machining a part, the grain structure of the final part is the same as the original stock material. For wrought hardenable alloys, the mechanical properties of the finished part is fixed by the temper of the rod stock that feeds the machining process.
On the other hand, when a material is cold formed, the part is naturally strengthened through a process called “work hardening” or “cold working”. In the process of cold working a material, the workpiece will develop hardness and strength in the areas where shaping takes place. For example, when a head shape is pressed onto wire stock to form a rivet, the surface and edge of the rivet’s head is harder than the shank.
Naturally eased edges result on cold formed parts, as opposed to screw machining where parts will have sharp edges unless intentionally chamfered. Cold formed parts have inside corners and fillets that result from the features designed into the forming dies. The metal or metal alloy flows across and around these polished die features as it work hardens in these areas. Where dimensionally acceptable, rounded / filleted corners and reduced tool marks can limit stress risers, thereby increasing the strength of a cold formed component relative to a comparable machined part.
Improved Surface Finish
As a result of cold forming, some features on the final part can be made to have a very smooth surface finish. For example, the spherical head of a rivet forms plastically against a polished tool, taking the imprint of the harder tool material and resulting in an excellent surface finish on the part. Many materials will match the finish of a polished carbide die as the part is ejected. It is important to note that if this advantage of cold forming is to be leveraged, good quality wire stock is absolutely essential...”quality in equals quality out,” when it comes to raw material.
Little to No Scrap
Material savings realized in lower per-part prices is a significant advantage of cold forming compared with CNC machining methods. Cold formed parts are produced “net material”, without any scrap inherent to the process. The forming machine feeds a fixed volume “blank” of wire from the spool, and then 100% of the blank is forged into the completed part. Nothing drops off into a scrap pan; in fact, most forming machines won’t even have one!
Conversely, screw machines are very good at producing chips. As the screw machine peels away at the raw bar stock, it eventually arrives at the final part inside. In doing so, up to 70% of the material can be cut away, resulting in substantial scrap. Reduction of scrap is an especially important consideration when designing parts with high intrinsic value, including precious metal or precious metal alloys. Reclamation of the screw machine scrap can be very expensive, time consuming, and complex; simply discarding certainly isn’t an economic option. Reducing or eliminating material reclamation results in substantial cost savings in the final part.
Designing a Cold Formed Part - 2 Factors
Cold formability depends on the part geometry including relative feature sizes, depths, lengths, radii, etc. Rules for part geometry come from our decades of cold forming experience and are broadly related to forming pressure, material yield and flow strengths, and tooling strength.
As a simple example, the volume of a part cannot exceed 3x the raw material wire diameter. The 3x diameters are “gathered”, or upset, during cold forming resulting in radial expansion that forms the part head. If the design exceeds 3x, then buckling, folding, or flash will occur.
In most cases where a new design is being considered, the overall geometry of the part can be optimized for a cold forming operation. The earlier Deringer-Ney is consulted in the design process, the easier it is to design with cold forming in mind and leverage the product improvement and savings that the technique has to offer.
Annual volume is an important factor to consider when designing a part, whether it be cold formed or machined. In the case of cold forming, a new tool can range anywhere from $300 to $10,000 depending on the part complexity and the type of material required to make the part. If the part volume is high, the tool cost is spread over a large number of parts and recovered quickly. In this way, economy of scale tends to favor the cold forming process; if annual volumes exceed a few thousand per year, cold forming should be the first method to consider.
Cold forming is a well kept secret in metals fabrication that offers a unique combination of reduced per-part cost due to low cycle time and waste relative to CNC machining, access to difficult-to-machine materials, tailored mechanical properties, and low tooling costs. Successful adoption of a cold formed component can provide the elusive dual benefit of performance improvement and cost reduction. Cold forming has different design requirements than machining, so the earlier in the design cycle a cold forming part producer is contacted, the higher the probability of successful implementation.
Learn More About Cold Heading and Cold Forming...
"Virtual Design" of Cold Formed Parts Using Finite Element Analysis
Graded Microstructures in Cold Formed Metallic Parts