What's new
What's new

Lathe soft jaw basic questions

Cole2534

Diamond
Joined
Sep 10, 2010
Location
Oklahoma City, OK
I have some questions about using soft jaws in the CNC lathe. We'll assume these to all be about gripping the OD of round stock. Machine is a QT15n with either a 10" chuck or 16c collet nose.

1) when boring the jaws- do you bore them on size, or a touch over?

2) what the best way to preload the jaws? monster jaws has a nice little ring, but they're darn proud of it.

3) how much grip surface do you strive for? approaching 100%, akin to a collet, or more like 25% like traditional hard jaws?

4) When do you choose AL vs steel?

5) When do you just say screw it toss on the collet chuck?

6) any other pointers?

Thanks, Cole
 
Last edited:
I use a 16c collet nose on my lathe whenever possible. its pretty quick to swap out. As for chuck jaw material if its going to be a repeating job or the bore dia. is a common fraction I'll use steel, AL for short run stuff or for plastic parts. I aim to bore on size. preload with a slug held just shy of the bore depth. try not to have the bore fall in the jaws counterbores. scribe a witness mark on the jaws to the master help setting them up next time.
 
I use a 16c collet nose on my lathe whenever possible. its pretty quick to swap out. As for chuck jaw material if its going to be a repeating job or the bore dia. is a common fraction I'll use steel, AL for short run stuff or for plastic parts. I aim to bore on size. preload with a slug held just shy of the bore depth. try not to have the bore fall in the jaws counterbores. scribe a witness mark on the jaws to the master help setting them up next time.
I do the same. Most of my lathe work is bar pulled in the 1-1.5" range, perfect for 16c. But I also love the flexibility of the chuck because the machine control is so easy to setup that my manual lathes are basically paper weights. I was hoping that good use of soft jaws would bridge that gap.

As for loading the jaws, I dont think that method will work on bars.
 
I have some questions about using soft jaws in the CNC lathe. We'll assume these to all be about gripping the OD of round stock. Machine is a QT15n with either a 10" chuck or 16c collet nose.

1) when boring the jaws- do you bore them on size, or a touch over?

2) what the best way to preload the jaws? monster jaws has a nice little ring, but they're darn proud of it.

3) how much grip surface do you strive for? approaching 100%, akin to a collet, or more like 25% like traditional hard jaws?

4) When do you choose AL vs steel?

5) When do you just say screw it toss on the collet chuck?

6) any other pointers?

Thanks, Cole
I usually bore jaws about .02" larger than the diameter I have to grip.
I use spacers to have the jaws clamp on. The adjustable boring rings are nice, too, but expensive and you have to watch the chucking pressures when clamping.
I use steel most of the time except for very high speed applications.
Collet chucks are great for bar feed work and smaller diameters.
I always relieve the back of the jaw a few degrees so the work piece doesn't push out..

I replace chuck jaw bolts every year with new, high quality, brand name bolts. I grease the chucks every other day.
 
The more I think about the boring rings...I bet a could a guy could lay out common jaw positions, and have 2 or 3 rings that covered probably 90% of his work.

How well does bar pulling work in jaws? I think it'd be fine as long as your gripper was in good shape and the spindle liner was appropriately sized.
 
The more I think about the boring rings...I bet a could a guy could lay out common jaw positions, and have 2 or 3 rings that covered probably 90% of his work.

How well does bar pulling work in jaws? I think it'd be fine as long as your gripper was in good shape and the spindle liner was appropriately sized.
It works well, provided there's sufficient clearance when the jaws are open and the pull out speeds aren't too fast.
 
I have always preferred to bore a bit undersized. That makes the jaws contact 6 places on the part rather than 3.
That makes sense. Are you boring them with the chuck closed?
It works well, provided there's sufficient clearance when the jaws are open and the pull out speeds aren't too fast.
nah, I only pull at 100ipm.
 
Last edited:
One of the big collet advatages is no loss of grip force at high rpm. If running bar in the range of the collet chuck use it. Use a good spindle liner.

Use an impact wrench to tighten soft jaws. Use the clamping disc to bore when you can, else use the adjustable thingie. We always use steel jaws and sometimes cut them down so they are lighter. For od clamping a bit smaller or on size is the way to go. If you cut oversize you only have line contact. Cut a small taper, a few thousandths, bigger at the back. Amazing how much difference that makes. Put a step at the back of the jaws, maybe 1/2 wide, so the central jaw part doesn't contact the back of the part. With sawn parts we use 1/32 TNR to bore jaws and never worry about corner relief but you must have enough corner relief for parts finished on the back if you want to hold parallel. Don't use max clamping force unless you have to and your chuck will last way longer. Grease often. Chuck jaws are not like bearings. Too much grease is better than too little. If a jaw won't take grease fix it right away or you'll be buying a new chuck.
 
I have 2 different sized Rivelica out locks for boring soft jaws they work great. As someone said when boring jaws with a boring ring turn chuck psi down MAX PSI 100
 








 
Back
Top