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Screw Machine vs. Cold Heading

When examining the difference between screw machined parts and cold-formed parts, cold-formed parts rise above in many areas such as raw material savings, elimination of secondary operations and large head-to-shank ratios. One of the most important differences is the strength.

Cold-formed parts are stronger than screw machined parts for the following reasons:

• Working hardening – As metal is moved around during cold forming, it is work hardening each time it is struck. If a softer material such as low carbon steel, aluminum, brass, etc. is used, it will get stronger as it goes through each dye. Some grades of stainless steel do not respond to any form of post heat treatment and only get harder if they are cold headed. Therefore heading a stainless steel part will make it much stronger than screw machining it.

• Better material – Material used in cold heading is specially designed for upsetting and extrusion. It is usually classified as a better grade of material than screw machine bar stock.

• Uninterrupted grain flow – Grain lines are compacted to follow the geometry of the part, resulting in a stronger part.

• Less stress risers – Cold forming moves metal with dyes, so dyes and punches need to be designed for increased tool life. This usually entails larger under head radius, reducing the stress under head.

There is a lot of competition from screw machine sources, but cold heading uses cold heading technology to form parts to net or near net shape and provide cost savings to our customers. Cold heading is very successful converting current screw machine parts into cold heading. One reason is screw machining generates a lot of scrap.

However, not all parts are better suited for cold forming. Some parts such as the ones listed below have characteristics that make them a better candidate for screw machining:
Small or no head to shank ratio
No unique shapes
No recesses
Short parts
Few different diameters
Low cost material used
Low volume

Cold heading is recognized to provide cost savings. Heading scrap losses an average 1-3 percent, while screw machining can produce scrap losses as high as 75 percent. Cold forming also offers better very smooth surface finishes while machining leaves a rough finish. Surface smoothness is called Micro Inch Finish. A typical cold-headed part has a Micro Inch Finish of 63 or better where a machined part will be 63 -125.

When converting a part is successful, the savings can be hundreds of thousands of dollars. Usually the savings comes from reduced material costs or elimination of secondary operations such as recesses and smaller shank diameters.

Other areas where opportunities can be found are when multiple piece assemblies can be converted to a single cold-formed part. Customers may use two screw machined parts and press them together or weld two or more components together. This can be an opportunity to provide a stronger part and considerable cost savings.

Understanding the advantages of cold heading is very important in order to recognize a good cold headed part, and an opportunity for cost savings.

To learn more about the differences between Cold Heading and Screw Machines, contact us at ProvenProductivity@bossard.com.

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July 10, 2014

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