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Speaking of Precision

Speaking of Precision is a knowledge preservation and thought leadership blog covering the precision machining industry, its materials and services. With over 36 years of hands on experience in steelmaking, manufacturing, quality, and management, Miles Free (Milo) Director of Industry Research and Technology at PMPA helps answer "How?" "With what?" and occasionally "Really?"

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Five Reasons Why Cold Drawing Is Essentially Unlubricated

Posted December 22, 2011 8:30 AM by Milo

Yes we flood oil and sometimes put a film of lime or borax on our steel hot roll, but the fact is that our cold drawing process is essentially "unlubricated."

The film between the die and the rod is probably less than three or four molecules thick...

Here are five attributes of Dry Friction and how they apply to cold drawing:

  1. Frictional resistance is nearly proportional to pressure. Check. In cold drawing, friction is not only proportional to pressure, it is also proportional to total area.
  2. Friction is nearly independent of speed at low pressures. While we would be challenged to identify "low pressure" s in cold drawing, the fact that we can start drawing by "pointing" the bar at low speed makes this point. Check.
  3. Friction is not greatly affected by temperature. The first draw bar of the day with equipment at ambient and those drawn mid shift when the dies are reading in the 300-400 degrees Fahrenheit range do not vary in "pull" required. Watch the ammeter. Check.
  4. Friction depends on the nature of the surfaces. If you don't believe that this is true, just rough up the die and start drawing. This is why die maintenance is so important. Check.
  5. Friction of rest is slightly greater than the friction of motion. Again watch the ammeter. It takes just a small amount more of power to start a pull than it does to sustain the pull. Check.

Bottom line: Only a fool would try to cold draw steel without lubrication. But the fact of the matter is that the cold drawing is essentially an unlubricated process, if one thinks about the attributes of dry friction given above, as applied to our process.

I tend to think of the "lubricants" that we apply as being inert pressure agents that merely separate the surfaces of the die and the work with a few molecules of material to physically prevent the two materials from welding under the extreme pressure. The steel never touches the die- the die just provides a backup for the lube which is really doing the deformation of the steel by hydrodynamic pressure.

Die Graphic

Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which originally appeared here.

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#1

Re: Five Reasons Why Cold Drawing Is Essentially Unlubricated

12/23/2011 4:46 AM

Interesting and informative as usual.
Seasons greeting to ya Milo.
Del

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Re: Five Reasons Why Cold Drawing Is Essentially Unlubricated

12/23/2011 8:26 AM

Good morning and thank you Del. All the best to you and the Missus. Hope your arrows fly straight an true. Milo

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Re: Five Reasons Why Cold Drawing Is Essentially Unlubricated

12/23/2011 8:31 AM

Lubricants are funny that way. The lubricant film in an automobile engine is a few angstroms thick, but try operating the engine without it and you will learn how important it is. Similar results with ultrasonic welding. Truly good lubricants will inhibit welding. Contaminates that don't provide lubrication, have little affect.

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Re: Five Reasons Why Cold Drawing Is Essentially Unlubricated

12/23/2011 12:44 PM

This kind of blog info just "shows to go me" that I don't know near as much as I think I do in the metal fab world, even after nearly 50 years of putzing around with it.

Thanks, Milo. Merry Christmas <-- substitute personal beliefs accordingly, if necessary

Hooker

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Re: Five Reasons Why Cold Drawing Is Essentially Unlubricated

12/24/2011 9:39 AM

Fifty years is a great start. I am humbled when I think of the stuff my mentors never got around to explaining... Thanks for your thoughtful comment on this thread and your contributions to our CR4 community. Milo

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Re: Five Reasons Why Cold Drawing Is Essentially Unlubricated

12/24/2011 3:01 AM

I've done quite a bit of hand drawing of silver from a cast cylindrical bar and eventually into wire through stages. (wife is a jeweler).

Only lube used is candle wax.

Inbuilt ammeter in my arms confirms all of Milo's friction observations.

Annealing between stages is critical. Final draw is left unannnealed.

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Re: Five Reasons Why Cold Drawing Is Essentially Unlubricated

12/24/2011 9:42 AM

"Ammeters in arms"- nothing like experience! Thanks for sharing your experience! Milo

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