concentricity problem

[r.b.enriquez]

Hello there!

I am new to this site and sure glad I've found CNC zone.com on the
net.

I've been running an F-100 Gildemeister CNC 3-axis lathe in my
company as a trainee operator after working with Brown & Sharp
for 3 years. The Gildemeister machine is a great machine for
cutting, facing, roughing etc, etc.

My problem is the concentricity. Sometimes I could get it to
.005 to .001 to .0015 at the most after a while of playing with
the pads of the jaws 1, jaws 2 and jaws 3. Sometimes it takes
me much longer to correct the eccentricity using dial indicator.

My concern is, is there a quickest way of solving concentricity or
removing eccentricity on a workpiece clamp to the jaws.
I would really appreciate any help that you may give me.

Thanks

Ray

[BobWarfield]

It sounds like you have a 3-jaw chuck. Unfortunately they are well known problems for concentricity.

You have a few choices to improve. First, there are various ways to tune up or upgrade 3 jaws. People say a set of custom jaws for you application will help tremendously.

4-jaws offer extremely good concentricity, but you have to dial them in each time.

A collet chuck or lever type closer will be much better than your 3-jaw and is the most likely solution for production work. You can get pneumatic versions as well.

You can also turn between centers for the ultimate in concentricity. You'll want to read up on each of these possibilities and decide what will be easiest and make the most sense for your application.

Best,

BW

[lerman]

If this is the first operation on rough bar stock, just start with stock that is oversize enough that you can turn the circumference to be concentric.

If it is a second operation, use a collet to precisely center your part.

Ken

[RICHARD ZASTROW]

If the jaws are removeable, replace the hard jaws with soft jaws and bore the jaws to size. If hard / serrated surfaces are required, install hard/serrated pads in the bored soft jaws.

[r.b.enriquez]

Thanks very much Richard for the reply.
Really appreciated your advice on the concentricity
problem on my F-100 Gildemeister machine.
Yeah I think I would go for a collet chuck to
try it. Thanks again.

Ray

[Hogger]

Collet chucks can be very accurate but they can have runout too, especially after they get dirty, if your shooting for better than .002 T.I.R. then you need to bore softjaws for the 3-jaw or get a independant 4-jaw chuck.

[Zumba]

Get a "set-tru" 3 or 6 jaw chuck. Bison makes decent ones. There are better chucks out there for more $$, but anything cheaper than Bison isn't worth your time. I use an 8" 3-jaw Bison on my machine. These chucks allow you to dial in a part similar to how you'd dial in a 4-jaw. The chuck has four large set screws that push against a special backplate. You can dial a part in to zero.

The benefit is speed, as you only have to dial in a part once. Concentricity will repeat within a couple tenths for each subsequent part that is similar in size. Claimed repeatability is .0005 but IME, it's more like .0002-.0003.

Another benefit is that you can precisely dial in odd parts such as ACME/Ball screws. Dialing in a screw is typically pretty tricky due to the threads. With a set-tru chuck, you first dial in a bar that's of the same diameter, e.g. a 1" Thomson shaft, in order to get the settings right for a 1" ballscrew.

Lastly, 3-jaw chucks close down tighter than 4 and 6 jaw chucks. A 4-jaw chuck of a given size may have a 1/2" minimum capacity, whereas the 3-jaw will go all the way down to 3/16" or so.

[r.b.enriquez]

Thanks Mr. Machinist for the information you sent. I apologize
that it is only now that I responded. Anyhow I really do
appreciate your response. Really I got to use 3 jaws chuck
for the simple reason that you have stated. I running very
small parts. Thanks again for your advice.

Ray