Lathe chuck slippage

kmanuele

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I was trying to thread a piece of 3/8 12L14 in a ER40 collet chuck with a 3/8-16 die, but the piece kept slipping (turning) in the chuck.

Installed a 3-jaw (6 "), but it still slipped no matter how tight I could get the chuck, using the normal chuck key.

I was reluctant to use a breaker bar on the chuck key.

Was able to finish with the piece in a vice.

3/8-16 is pretty aggressive and takes some effort. Maybe that's the limit of lathe chucks?

Kevin
 
you should be able to hold it in an ER chuck, but alot of times both ER and regular scroll chucks take an awful lot more tightening than you think. I wouldn't use a breaker bar but I would use some effort with the regular wrench. Another option is to use an adjustable die and crank it open for a first pass, then close it up again for a second pass for the correct fit. Or single point the bulk of the thread and finish it off with the die. In many ways that gets you the best of both worlds.
 
A few things to consider, first is the bar OD may run a bit larger than 3/8" as there is a bit of variance in the stock, if slightly over it will significantly increase the cutting force required. I would turn it down a few thousandth under 0.375 and I also put a chamfer at the start. You need to make sure the collet is oil free, any oil will cause it to slip. Improper torquing is another big problem in particular with the larger ER collets, adding a power nut or ball bearing nut can significantly increase the collet clamping pressure, I recommend the Techniks power nut or similar. Most of the ball bearing ER40 nuts are junk, the only one that works (based on the ones I have used and tossed) is from Rego Fix.
 
Re: single point. I thought of that (and like the process), but really hate doing the gear changes.

Had a second piece to do so I checked, and the die is adjustable, but the adjustment was loose. I was able to get enough spread to prevent slippage in the collet -- with some extra tightening there too.

So thanks for the suggestions.

Kevin
 
A few things to consider, first is the bar OD may run a bit larger than 3/8" as there is a bit of variance in the stock, if slightly over it will significantly increase the cutting force required. I would turn it down a few thousandth under 0.375 and I also put a chamfer at the start. You need to make sure the collet is oil free, any oil will cause it to slip. Improper torquing is another big problem in particular with the larger ER collets, adding a power nut or ball bearing nut can significantly increase the collet clamping pressure, I recommend the Techniks power nut or similar. Most of the ball bearing ER40 nuts are junk, the only one that works (based on the ones I have used and tossed) is from Rego Fix.
Stock was right at 0.3750, and maybe 1/2 thou under at one end. With die tolerance, this explains the cutting force? I could get just a bit of spread with the adjustment (it's a tiny screw), but cutting forces still seemed high to me. Noticeable drag when spinning off the die.

Kevin
 
Here's my trick. You won't see it anywhere else, it's proprietary. You sweep up a little bit of carbide dust from around your grinder, mix that with a dab of silicone grease, and smear a little of the resulting dope on your slipping work with a finger. Place back in collet, tighten, and it will never slip again. My dad came up with that trick, and the company he worked for had carbide dust as a waste product, so they patented it and put it in tubes. Also works on stripped cross-tip screws and hex sockets too.
 
Didn’t old timers wrap the work in a cigarette paper!
 
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