long chuck jaws 1

[RobWard]

We currently produce plastic parts in various hardnesses.
Machining some of these parts is proving tricky.
They are soft enough that when facing off bungs 14" long 7" diameter they flick out of the lathe chuck.
We cannot use a center (we're facing off) and a steady will melt the plastic.
Milling doesn't work as the plastic crumbs or melts.
We think that chuck jaws that protrude, say, 10" from the face of the chuck would work, but can these be bought anywhere, or are we barking up the wrong tree?
(It's a Rohr three jaw chuck on a DSG Type 17 lathe).
Thanks.

 

[evelrod]

RobWard, I have done a little fab work with plastic/delrin/nylon etc. for fairly small stuff, nothing like 14 X 7. I use a 4 jaw chuck because it holds the soft stuff a bit better (a pain to work with, but it is what I have). Never thought about a steady. A thought would be to fab one up with rollers contacting the material.
That's about all I can offer, I am not a machinest by trade or avocation. I admire those who have tried over the years to teach me.


Rod

 

You can either purchase long jaws from stock, or have your shop fabricate them for you. Drill a hole in the clamping face of each jaw and insert a plug of soft (50 or 60) urethane to add grip to the part being held. You will experience the jaws "springing" when tightening down, and you may want to cut the jaws at a slight angle or do some shimming to accomodate this. Slowing down your rpm will also help retain the work piece. If you can live with a little marring on the od of the work, you can insert checkered (knurled) gripper pads in the jaws, these are available from any tooling supplier and mcmaster-carr. If you have a good machinist and allow him the time to fabricate a set of jaws, you should have no problem with these parts. It's worth the investment of time especially if this is an ongoing operation.

 

[Hush]

I used to work at a shop where they machined nylon and teflon by covering the OD with rubber tubing where they chucked. I don't know if the purpose was to add grip or to protect the work piece.

 

[sbc]

The rubber idea sounds like a quick & cheap thing
to try. Maybe a piece cut from an old inner tube?
Being a crotchety old yankee, I like to try the
easy & cheap things first.

 

[knobhead]

Try these things. I've no expreience of m/cing plastics, but if these can't hold it, nothing will. They're great, and they shouldn't mark. And no i don't work for them!

http://www.ringspann.com/uk/products/p13_16/p13zst.htm

 

[evelrod]

knobhead---OUTSTANDING!


Rod

 

[WIPLASH]

Some of these ideas are good,but they seem costly.If your not doing large production spending money on expensive clamps is not the answer.Build a slip fit bushing(7.5O.D.x7.005I.D.x1.75 long) to set on the outside Dia. of the work peice.Cross drill three (120.deg equally spaced)set screw(1/4-20)holes toward the edge of the bushing.Then you can set the steady rest up on the side that the set screws are not on.This should be easy to make ,cost effective,and should stop the walking problems.

 

I'd mill it!

Simple set-up in v blocks for side milling using an end-mill, or ing the head of a bridgeport over so you can attach your plastic cylinder pointing down over the side of the table and face mill (fly-cut)it.

Slow down (melting), climb cut (with axis brake on if necessary), and use coolant! Everyone cuts plastic dry or with an air blast like those instructions came down from the mountain on a stone tablet, but I cut them wet and never have problems of melting - even at 15,000 rpm and 3/4 endmills. (THAT is done in an enclosed cnc.)

Choose a compatible coolant, mix it up with water, pump continuiously as you cut. I use flood coolant, not mist.

You have to use sharp cornered tooling,double positive rake, like for aluminum. Negative rake tooling for steel will melt most thermoplastics every time, even with coolant.

The coolant will prolong the life of those fine sharp edges too, something that plastics typically tend to erode quickly. I won't tell you WHY the coolant will help so much just yet, but it's not it's lubricity that saves the edges, I've found it's something far less obvious, but a powerful factor non the less.