problem about my first turning

[yang_cnc]

Hello:

I turned a 2" aluminum cylinder.

Starting from diameter of 0.625", I turned it down to 0.53". Just some random turn, from time to time, I swapped two ends and turning the other ends.

The result, I am not so surprised, is that I got a cylinder which is no longer concentric.

Just after I cut off the cylinder from its stock, I use a test indicator, I can observe about 0.002" wobble, which I think is OK.

But now the wobble is as large as 0.01", I can see it wobbling if the RPM of the chuck is high, I almost start to suspect my chuck is not concentric.

I believe my turning skill, if any, is so low As my first turn, my happiness is more than frustration

But can someone enlighten me: what is essential to guarantee concentricity? Thank you very much!

[HuFlungDung]

For precision double end turning, you'll need to turn between head and tail centers, use an adjustable chuck, or use a precision collet holding system.

I'm not a big fan of turning between centers, because of the nuisance of doing it, that, and I don't like lathe dogs swinging around at high speeds.

So I would recommend using an adjustable 4 jaw chuck and a dial indicator. Through patient experimentation, you can learn how to set something up as close as you want, to be concentric to an existing surface.

If you've got a really good chuck with good smooth jaws, you can always hope that the part runs concentric anywhere along its length as you dial it in. But often, there may be some small amount of tilt, which means it will dial concentric on only one position, and further along, it will not dial in at zero. This requires using a soft hammer to tap the part in such a manner that the wobbling end wobbles less than it did. Doing this will also disturb the area that you originally dialed in, so then you will go back and forth, dialing, tapping, dialing, tapping, until you're ready to call it good

The easiest method to set something up in the 4 jaw is if it has a cylindrical surface, and also a squared off face, square to the axis of rotation. The trick is that this good face has to be accessible when you flip the part, or you have to be able to seat that face against the face of the chuck or the face of the jaws, to guarantee that the piece does not wobble. Then, you can dial in on the concentric reference surface, and it should be good in every way.

[Rally]

4 Jaw as mentioned! Only way to go.
3 or 6 jaw self centering will work best with soft jaws that are bored for that job.

[chrbau]

Taking the piece out of the chuck and turning it the other way is a no-no.
If you just want "cylinders" you don't need much more than you got. If you chuck it, turn it and then cut it, you will have cylinders. Unless your ways are not true, in wich case you will have a cone.
If you have a DTI you can set it up and test your chuck. Try with a piece of silversteel (ground axel steel) or better still, a parallel test bar.

[chrbau]

Forgot: when you have measured with the DTI you can try and adjust the chuck you got.

[yang_cnc]

Originally Posted by chrbau Taking the piece out of the chuck and turning it the other way is a no-no. If you just want "cylinders" you don't need much more than you got. If you chuck it, turn it and then cut it, you will have cylinders. Unless your ways are not true, in wich case you will have a cone. If you have a DTI you can set it up and test your chuck. Try with a piece of silversteel (ground axel steel) or better still, a parallel test bar.

sure, it is a no-no, now i know.

I just need to keep working on it until it is done with the stock, and then cut it off. This way, I may waster a bit of material, but I will not chuck it, release it, rechuck it.... I think this procedure is highly problematic since I keep losing my start point (the previous chuck position)

Also, thank all guys for your advices! cheer!

[jpelosi2002]

When concentricity is an issue, it's best to bore first, then mount in an arbour and turn the OD.