Gunsmithing Lathe problem, anyone have a similar issue?

[ken226]

I have an old Smithy AT-300 lathe/Mill combo that I've been using for years. I've chambered some bolt actions that hold 1/2 moa groups with ease and can cut very accurately with it, but I have a minor (to me) problem that I'm not real certain about.

My 3 jaw has about .001 runout, so I do most of my precision work with a 4 jaw but I have an issue with it. Actually, the same issue pops up when I put the indicator on something in the 3 jaw as well.

When I set up something, say for example a piece of drill rod in either the 3 or 4 jaw, then put the indicator on it in a magnetic base, then rotate the chuck by hand, there is about 4-5 degrees of rotation in the chuck before I feel the belts and pulleys in the gearbox start to turn. This slack, per-se doesn't bother me, but right at the moment the slack is taken up, the indicator needle moves up by .0005.

The needle stays there, .0005 over where it was before I started rotating the chuck by hand. As long as im applying pressure keeping the slack out of the pulleys it stays there. If I take my hand off the chuck, the needle drops .0005.

So basically, I start turning the chuck by hand and when the slack is taken up and the electric motor starts to turn with the chuck, the indicator needle goes up by .0005. when I stop turning the chuck, the needle drops back to its original position, it drops by .0005 back to its original position.

The headstock feels rock solid, I cant feel any "give" in the headstock bearings, but I cant dial in any better than .0005 with this "slack".

What ive been doing to dial it in is to rotate the chuck until the slack is out and I can feel the resistance of the electric motor, then zero the indicator while maintaining this pressure on the chuck. The I rotate it, measure the runout, adjust the chuck by half the indicated runout, and measure again, over and over until dialed in. All while, very annoyingly, maintaining a torsional pressure on the chuck to keep the .0005 of slop from jacking up my progress.

What might it be?

I'm not as articulate as id like to be, does my explanation make sense?

 

[bptactical]

Worn spindle bearing(s).
You can't feel .0005 by hand.
You may be able to adjust them, not sure on that machine though.
When you take the backlash out of the gear train by rotating the chuck it has a jacking effect on the outboard end of the spindle.

 

[Mad_Charlie]

Worn spindle bearing(s).
You can't feel .0005 by hand.
You may be able to adjust them, not sure on that machine though.
When you take the backlash out of the gear train by rotating the chuck it has a jacking effect on the outboard end of the spindle.

Yep. Bearings.

 

[longshooter]

Also, some peeps will run their machines for a period, to warm things up, before measuring and doing precision work. Under load, the bearings will have a wedge of oil they are running on.

 

[LRI]

Warm it up first.

If the spindle pulls oil from the transmission maybe try a slightly heavier oil or an oil stabilizer like STP or Lucas.

 

[ken226]

Warm it up first.

If the spindle pulls oil from the transmission maybe try a slightly heavier oil or an oil stabilizer like STP or Lucas.

Ok, ill definitely have a go at that. Totally makes sense, the bearings can't possibly be at "0 clearance" cold, or they would bind when they warmed up.

Now that im thinking about it, I've always started indicating with the machine dead cold. The bearings are likely designed to run smooth at operating temp, so the clearance will be a bit looser when cold.


I'm going to do an oil change this weekend anyway, so if you don't my me begging a little more free advice, what type of oil would you recommend? Ill throw some oil stabilizer in too!

Thanks.

 

[bohem]

I would advise only changing 1 thing at a time for now.

The oil needs to be changed, then changing it is something that you regularly do and I wouldn't consider it to be a new variable.

Running the machine and then dialing is a good idea. I would avoid the oil additives for now until you see how that works first.