TIR

[Mr Ron]

This may sound like a dumb question, but if I don't ask, then I won't know. If the TIR on a lathe spindle is say .003", what is the result of turning down a bar? I believe the bar will be perfectly concentric around the axis (perfectly round and not oval). If that is the case, is TIR really that much of an issue? As long as it turns out round, that's what matters. I might not be able to depend on cross slide micrometer dial readings, but that just means I will have to make many cuts and measurements (trial and error) until I get the reading I want. It may not be acceptable in an industrial setting, but for the home amateur machinist, it works. I'm sure there are many amateurs out there who have lathes with less than ideal TIR spindles and just because it's out .003 or more, we are not going to give up. I believe a lathe with less than perfect spindles, can still do precision work, even within tenths; it just takes more care and time.

[John Hasler]

Where are you measuring the TIR? Inside the taper? How are you mounting the work? Is the spindle axis parallel to the ways?

Spindle runout does not impact use of the cross-slide dial. You don't know where you are until after the first cut anyway, and after that the work is concentric regardless of where it started.

[Magicniner]

Get a back plate for your lathe and machine it in-situ for the chuck of your choice.

[earlgo]

The TIR on a chuck or lathe spindle is preferred to be 'perfect' so one can re-chuck up a part and not have to worry about not being able to cut a few thou off or cut concentric features.
Consider if you will, the use of a 4 jaw chuck. One can randomly chuck a part, either round or square or oval and proceed to turn a precision part. The precision part is then separated from the stub that was gripped in the chuck.
With a 4 jaw chuck a previously machined part can be re-chucked to the same axis with the help of a DTI and a bit of care. This is difficult to do with a universal 3-jaw unless the part and an adjacent jaw is carefully marked so that it can be reinserted in the same orientation. It may still be off a bit due to the vagaries of the scroll.
The point of this is to say that run-out should not affect anything unless the part is re-chucked.
--earlgo

[John Hasler]

earlgo writes:
> The point of this is to say that run-out should not affect anything unless the part is re-chucked.

It puts a real crimp in the use of the taper.

[earlgo]

The original post on this thread that showed up on my confuser did not mention a taper. I even read it again to be sure. My apologies if I missed it.
--earlgo

[Magicniner]

Run out will take the fun out of using a good collet chuck.

[Harold_V]

The consideration would be why there is runout. If it's due to a less than acceptable bearing in the headstock, it can manifest itself in several ways, including yielding less than a round cut. If it's just a matter of a true running spindle but a chuck with runout, it can be addressed by the use of soft jaws, or, as has been suggested, a four jaw.

Work accomplished in a chuck that does not run true, assuming good bearings in the headstock, will be round. The error in chucking will manifest itself as eccentricity between the chucked surface and the machined surfaces. It may, or may not, be a problem, depending on the intended use of the piece being machined. It also complicates a second operation, where the piece must be reversed and machined on the opposite end. The amount of runout will equate to that amount of eccentricity between the two ends. Usually not desirable.

H

[johnfreese]

No matter how bad the runout of the chuck a turned surface will be true to the spindle axis. When you need to re-chuck a part the chuck runout becomes a problem. That is where the 4 jaw comes into play. You can dial in work to rune perfectly true.

[Magicniner]

No matter how bad the runout of the chuck a turned surface will be true to the spindle axis. When you need to re-chuck a part the chuck runout becomes a problem. That is where the 4 jaw comes into play. You can dial in work to rune perfectly true.

What if the run-out is "at one end" and the affect on rotational axis results in the spindle physical axis describing a cone?

[John Hasler]

The original post on this thread that showed up on my confuser did not mention a taper.
--earlgo

This is true. I just wanted to point out that out.

[John Hasler]

No matter how bad the runout of the chuck a turned surface will be true to the spindle axis. When you need to re-chuck a part the chuck runout becomes a problem. That is where the 4 jaw comes into play. You can dial in work to rune perfectly true.

What if the run-out is "at one end" and the affect on rotational axis results in the spindle physical axis describing a cone?

Not sure what you mean by that. As long as the axis of rotation is parallel to the bed you can turn a flat faceplate that will run true and turn round cylinders of constant diameter. Even if it isn't you can turn a center that will run true.