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