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Cold Forming VS. Screw Machine

This is a repost from our original website!

I get it!  With the cost of personal injury lawsuits and class action lawsuits driving not only risk up but costs up it is incumbent upon the design engineer to ensure the safety of the product through design.  I believe the disconnect happens in the engineering schools where not enough is taught about the benefits of forming vs cutting and the limitations of both.  For instance, there is a distinct advantage to screw machining when volumes are low (depending on the size of the product, “low” could mean 5,000 pieces per year or 50,000 pieces per year.  This is primarily because there is little or no tooling cost involved with screw machining.  On the flip side of that screw machining is a much slower process which involves much higher amounts of scrap (waste or “mudda” in lean terminology) and results in “apples to apples comparison with cold heading” a weaker product.  Let’s break our comparison down to two separate discussions; piece price and design limitations.  For simplicity we can assume we are discussing cylindrical parts.

Piece price

-raw material – It is obvious that in order to create a part by screw machine your raw material must be at least as large as the largest diameter.  Different diameters are created by removing material through cutting.  The removed material, while still a part of the cost of the part, is scrapped.

          - to cold head a similar part you are moving material into cavities by the use of pressure and displacement.  Subsequently, the only scrap produced is created at set-up or when bad parts are made.  Bottom line, unless the product is a straight pin with no chamfers and no diameter changes, cold heading material costs will always be less expensive.

-set-up-  I must first admit that I have no experience with screw machines.  My experience is with NC (numerically controlled) secondary operation lathes.  For the most part with todays technology, set-up time is almost an insignificant cost to the screw machine process.  Often times the set-up is a program that can be initially created while the machine is running another part and tweaked at initial set-up then locked in for future runs.

            -  cold heading set-up is definitely a significant cost issue.  Machine manufactures have been designing servo-motor driven machine adjustments in order to reduce set-up time.  These advancements have definitely reduced the impact of set-up time  however, the increased cost of these features on the more basic machines results in increase charge out rates for both set-up and run times offsetting much of the reduction benefits from the machine design.  Bottom line, the higher cost of set-up for cold heading vs. screw machine is one of the factors that makes screw machining more economical for shorter runs.

-tooling-  Tools for screw machining include the holding device (a collet or chuck) which is not necessarily dedicated to the part but can be used across a series of parts the same diameter.  The cutters are typically holders with inserts that once again are not necessarily dedicated to one particular part.  Tools for screw machining, once again are usually an insignificant part of the cost of the product.

             -cold heading tools conversely can cost thousands of dollars and are often used only for the part they were designed for.  Tool costs will vary from a simple rivet at $1000 to a complex 5 die part at $30,000.  This is initial cost tooling.  Perishable tools add another chunk to the piece price as more complex parts involving more pressure invariably result in more perishable tools and more costs.  Bottom line, the higher cost of tooling for cold heading vs. screw machining is another factor the makes screw machining more economical for shorter runs. 

 

Runtime charge out-  let’s make an assumption here for simplicity.  We will assume that a cold heading shop and a screw machine shop have similar overhead costs and similar labor costs per hour.  Here is where it gets interesting!  A screw machine (depending on part complexity) will run a simple part at a speedy 20 parts per minute.  A cold header will run the same part at  400 parts per minute.  Even if you put 20 machines in a plant and were able to run them with one operator, you can’t compete on high volume as the shear cost of floor space would kill you!  If on the other hand you only need 2000 pieces. Now you run them in 5 minutes on the cold header but have to charge in $200 for set-up and $1000 for tooling for a piece cost of maybe 70 cents each.  The screw machine costs are $300 for material and $200 for set-up and run and it takes less than two hours at a piece cost of 25 cents each.

Design Limitations Cold Heading vs. Screw Machining

Tolerances- Truth is, screw machining can most likely hold tighter tolerances than most cold heading process.  This is particularly true with Suisse type screw machines.  The types of things that require these tighter tolerances typically are lower volume parts. From a cost perspective screw machining is the most cost effective way to produce these parts anyway.  Most engineers with little knowledge of cold forming are surprised to find out just how accurate and repeatable cold heading can be.

Typical cold heading/forming tolerances

Contained Diameter  +/- .04mm (.0015”)

Free form diameter  +/- .25mm (.010”)  typical used for large heads and washers.

Length       +/-.10mm (.004”)  up to 25mm ( 1”) long

                   +/-.20mm (.008”)  up to 50mm (2”) long

 

Geometry- This is where the debate can get complicated.  Let’s try and keep it simple by first assuming normal cold heading tolerances are acceptable for the application.  If this is the case, and the annual requirements are say 100,000 pcs or more it is wise as an engineer to design the part for cold forming. 

Undercuts-   these are typically created to allow strength at diameter changes without a radius restricting clearances.  When turning operations are  used these undercuts are usually located on the smaller diameter at the transition from small to large.  This is not feasible for cold forming.  In fact cold forming typically requires that diameters go from largest to smallest in either direction but not from large to small and then back to large again.  Without expensive collapsible tooling, this is virtually impossible.  If a “0” radius condition is required and the strength of a reasonable radius is also required the undercut can be designed into the surface above the smaller diameter at the transition from small to large.  This does not restrict the “release” of the material from the heading die.

In conclusion, as a design engineer, you need to consider the subtle differences between screw machining and cold forming and the subsequent cost implications of each when estimating eventual annual usage.  

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Tim Brennan
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December 11, 2017
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