Today’s Machining World Archives January/February 2011 Volume 7 Issue 1
Dear Shop Doc,
On our CNC lathes we occasionally have trouble with push back when using collets on bar jobs. Our collets have smooth bores and I am wondering if a serrated collet would help or if it will just create more problems.
Chuck Force
Dear Chuck Force,
Serrated collets will probably help, but first let’s consider all of the variables.
- Bar whip—Bar whip can cause the bar to act as a lever against the collet, prying it open. You should always use a spindle liner and/or a properly sized liner set in your bar feeder to minimize bar whip.
- Collet bore—Most collet systems have some gripping range, but the bore of the collet can only be machined to one given nominal diameter, and that diameter fits the bar the best. Avoid using a collet that’s “close enough.”
- Chucking pressure—The hydraulic pressure to the rotary actuator can be adjusted. Follow the manufacturer’s recommendation for the operating range and adjust accordingly. In general, you need higher pressure for larger diameter bar and less pressure for small diameters.
- Maintenance—Make sure that the sliding components of your collet chuck are clean, lubricated and slide easily. Make sure your hydraulic oil is in good condition, the level is adequate, and the system is operating in the proper temperature range.
Serrated collets work by reducing the surface area of the collet bore, thereby increasing the pressure that the contact area of the collet exerts against the work. You can calculate the surface area of the collet bore using the formula: 2 π r2 + 2 π r h. Ignoring the area removed by the slots in the collet, a 1.0” diameter collet with a 1-1/4” land has 5.5 in² of gripping surface.
If the collet closes with 1,000 pounds of force, that force is distributed over the 5.5 in² surface area of the bore, resulting in a contact pressure of 181.8 psi. If you decrease the surface area of the collet bore by machining in serrations, you increase the contact pressure by a corresponding amount. This doesn’t multiply the holding force in any way; you are still applying the same 1,000 pounds of force to the task of holding the work. By applying the force to a smaller area with greater pressure, the collet can dig into (deform) the work. Whether or not the collet permanently marks the work (plastic deformation), or the work bounces back (elastic deformation) depends on the force applied.
Another option is to have the collet coated with a textured carbide alloy coating like Carbinite (go to www.carbinite.com for more info). The principle is the same as serrations, but instead of grooves cut into the collet bore, the bore is coated with a crystalline like carbide alloy. The coating has a texture similar to sandpaper, which provides tremendous grip.
Dan Murphy
REM Sales LLC
Dan Murphy is a regional sales manager for REM Sales LLC., a U.S. Tsugami importer. He can be reached at dmurphy@remsales.com.
2 Comments
Dear Mr. Murphy,
In your article on “Push Back Trouble Using Collets” I don’t see why would you use the forrmula
2 π r2 + 2 π r h for the surface area of the collet bore and not just 2 π r h.
The area of a cylinder surface (where the pressure is applied) is just 2 π r h and you don,t have to include two surfaces of circle with the dia. of 1″, hence 2 π r h=2 x π x .5 x 1.25 = 3.9269 in².
Best regards,
Mark Dmytrow
Dear Mr. Dmytrow,
You are correct! I made a mistake. I originally used 2 π r h and while proof reading, I decided to look up the formula to be sure it was correct and came up with the formula 2 π r2 + 2 π r h. It didn’t seem right, but I found it in three sources, so I changed my original math.
Now with no deadline looming, I realize that 2 π r2 + 2 π r h gives the surface area of the top, bottom, and periphery of a cylinder, which of course is not the case in a collet bore where the only surface in contact with the work is the periphery.
I apologize for the oversight.
Regards,
Dan Murphy