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baldysm
12-18-2010, 07:46 PM
I have a CNC lathe with 5C collets. I'm doing 100 brass parts and I use a Dunham bar puller to advance the bar stock.

The problem I am having is that the bar puller pulls the bar even after the collet is closed on the stock. The collet isn't gripping the bar stock tight enough to prevent slippage when the puller backs off.

I have seen someplace that advertised 5c collets with a friction coating or something on the inside of the collet to provide better grip. I can't remember where I saw the ad. Google wasn't helpful.

Does anyone know where I can get friction 5c collets? Or have an idea that would solve the problem without buying one?

Thanks

KEJR
12-18-2010, 07:58 PM
I have to admit that I am not entirely familiar with your bar feeder. Is it just a spring or air cylinder providing constant force, and then your CNC machine provides a "stop" block tool to stop the stock before letting go of the collet? If so I would suggest the following:

- Release your force on the puller after the collet closes, but before the hard stop moves out of place. Easy to do for air pressure, not as simple for spring force (although there may be a way).

- Could you serrate the collet? Obviously this is kind of a quick and dirty fix, but you could use a carbide grooving bar to add little notches into your collet to provide ring like teeth that dig into the stock with more pressure. This is only acceptable if you can endure the minute marks it will impart on the stock. (You'd want to debur and maybe polish the serrations a bit to minimize the marks).

- Plate (just the inside) of your collets with something really sticky like Gold. You might check the coefficient of friction of brass to various materials to find the right one.

- Finally make sure your stock is fairly clean entering the collets. You may have a problem with debris or excess oil that is causing you collet problems. That said, you might not be able to prevent this in a CNC machine!

just my $0.02

KEJR

ken
12-18-2010, 08:11 PM
You can get serrated collets from Hardinge ken

baldysm
12-18-2010, 08:37 PM
I have to admit that I am not entirely familiar with your bar feeder. Is it just a spring or air cylinder providing constant force, and then your CNC machine provides a "stop" block tool to stop the stock before letting go of the collet? If so I would suggest the following:

KEJR

The Dunham bar puller is very simple. No moving parts. It simply is shoved onto the end of the bar (while the collet is closed), and "fingers" clamp onto the bar with spring pressure. You release the collet pressure, and move the bar puller, clamp the collet closed, and pull the bar puller off.


http://www.dunhamtool.com/cnc_lathe_bar_puller.html

I'll look into the serrated collets from Hardinge

Mark Hockett
12-18-2010, 09:08 PM
What closes your collet? Hydraulic, air, lever? My collet is closed with hydraulic power which is adjustable for grip pressure. I am using this style puller on one of my CNC lathes, its running parts with the puller right now as I type this,

http://i24.photobucket.com/albums/c10/mahockett/CompactCNC1.jpg

It pulls off the part perpendicular so the part wont slide out.

DR
12-18-2010, 10:51 PM
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Does anyone know where I can get friction 5c collets? Or have an idea that would solve the problem without buying one?

Thanks


Friction collets??? I don't know them by that name. But there are collets with an abrasive type texture internally that have a substantially better grip, Zagar is a company that makes them. There are also the Hardinge serrated collets.

You may have a problem those won;t solve though. Is your stock the correct size for the collet, or more to the point is your collet the correct size for the stock?

I have the Dunham bar puller setup also. I can't imagine it gripping the bar so hard that the bar is slipping. That tells me you have a problem with the closing mechanism (or the wrong size collet).


Regarding the "friction" and serrated collets, my experience has not been good. They're purchased when you have a holding problem, in my case it was extruded aluminum alloy bar stock with varying diameter down the bar. When the diameter was at a small area, the bar would slip and gall up the abrasive or serrations and become no better than a standard smooth bore collet. Same situation with certain stainless also. Once those expensive collets become galled they're impossible to clean and basically almost worthless.

DaHui
12-19-2010, 12:45 AM
Seems like the collet is not the right size or does not have enough force applied. Shouldn't it take a heck of a lot of force to pull the stock out of the collet? Also seems like if the collet is that loose, that's not enough holding force for machining either.