Why can't I indicate my part?

[Pat of TN]

Fellas, I am in dire need of assistance. I've run into a problem that is pretty close to driving me off the deep end.Most of the time, when I try to indicate a piece in on my lathe (Atlas 618) in a three-jaw chuck, for some reason it is impossible.

What happens is rather simple.

Normally, on a larger beefier lathe I guess, when you indicate your part and tap on the high side, in brings everything a little closer in. The high side is lower, the lower side is higher. If you have a runout of .010, and you tap the high side down to .008, the runout becomes .006 - .002 is taken off each side.

But for some reason, on my lathe, when I tap the high side, it doesn't do that. It just makes the lower side lower.

For example - I zero the indicator on the low side. The high side reads .010. I adjust the high side to .007. I spin the piece, and then the low side reads -.003... the same runout.

This is driving me crazy. I have never had this problem on the larger lathes we have at school.

For reference - again, the lathe is an Atlas 6x18. Chuck is a three-jaw, 4". Indicator is a Fowler DTI, graduated in .0005.

Thanks, guys.

 

[xalky]

A 4 jaw independent jaw would make it far simpler to indicate it in perfectly, or close. With a 3 jaw scroll your limited by the accuracy of the jaws and the scroll on the chuck. It will only ever be as good as the chuck. You can do some pretty accurate work in a 3 jaw provided that you never unchuck the piece. Once it's unchucked from a 3 jaw, the odds of getting it back in to that zero point are slim to none.

 

[JimDawson]

I suggest that the indicator is moving when you tap the part, and the part in the chuck is not moving. Maybe the chuck is loose on the spindle, or the backing plate is loose?

 

[DMS]

Scroll chucks (which every 3 jaw I have seen) are not considered to be repeatable. Basically, once you pull the part out, don't plan on ever getting it back in the same way again. The best thing to do is to plan out your operations so that you can do everything without removing the part. If you can't do that, then you should consider using something that will give you the reapeatability you are looking for, like a 4 jaw independent chuck, turning between centers, or a collet system.

Another option is to use a 3 jaw with the "set tru" feature. this basically allows you to adjust the chuck's alignment to the lathe spindle. Keep in mind that because of the way scroll chucks work, this will only ever be accurate for the diameter of stock that you have when you set it.

The general rule I have heard for 3 jaw chucks is that you should not expect repeatability any better than about 0.003". That's a general rule of course; my 3 jaw (an older import) is pretty consistent at 0.0015 TIR, which is good, but not good enough for high precision work. If I need something to be dead on, I go for the 4 jaw. With that, I can get things within 0.0005" without too much teeth gnashing, and I can get to about half that with a little effort.

- - - Updated - - -

One thing to add, if you don't have an independent chuck, and you can't use one of the other options, you can shim use shims to center yourself, it's a bit time consuming (aluminum foil makes nice shim material).

 

[rock_breaker]

Until this morning I wasn't aware you could indicate a 3 jaw chuck. I go with XALKY and DMS on this.
That being said I did inherit an adjustable 3 jaw "Buck" chuck. It has provisions for loosening a clamping plate then do your indexing up to about .030 then re-tighten. I indexed a truely round piece as close to 0.000 run-out as I could and that is where it has been for several years.

Have a good day.

Ray

 

[Bill C.]

I suggest that the indicator is moving when you tap the part, and the part in the chuck is not moving. Maybe the chuck is loose on the spindle, or the backing plate is loose?

I agree re-tighten the indicator clamps, sounds like its bouncing on him.

 

[Tony Wells]

Just an observation, but in my experience, it's never a good idea to tap on the material while the indicator contact is touching it. Most are not build as shock resistant. This applies all the more to DTI's.

I have, on occasion, tapped on the outside of the high jaw itself (instead of the material), sometimes pretty hard, to get a worn 3 jaw a little closer. But then, you should re-tighten it because the ultimate result could easily be the hardened jaws imprinting on a softer material and lessening the chuck pressure.

 

[Pat of TN]

Hmm... so, the main problems most likely result from having a 60-year old three-jaw, heh... I'm going to be buying a four-jaw pretty soon, so after that my worries will be over. I know three-jaws are not repeatable, especially ones three times older than I am, but I've always put an indicator on the part to adjust the runout. Ahh, I guess it's just time to retire the poor ol' thing. Thanks for all the advice, fellas.

 

[Grumpy Gator]

Pat,
Keep in mind that all round bar ain't nescesarily round.Sometimes if you loosen the chuck and rotate the part to a new spot you can get a better runout reading.
My uncle/teacher taught me his method:
Put the piece in the chuck with just enough hanging out to face off and drill a center.
Take it out and flip it over and do the other end and leave enuff sticking out to take a light pass on the od to clean up.{It don't have to be perfect} Then flip it arround and grab by it by the cleaned up end and run a dead center on the tailstock side.
You can save a lot of time if you get in the habit of faceing and center drilling the stock left in the lathe after you part off the piece you need.
**************Just Saying****************G**************

 

[Chucketn]

For centering round stock in a 3 jaw scroll chuck, I made a nudger tool. It's 2 skate board bearings, one mounted on each end of a length of keystock. Bearings are mounted at 90° to each other. One end for axial allignment, one for face allignment. I put the key stock in a QC tool holder made just for it and bring the bearing to bear on the surface of the stock. The stock is lightly clamped in the chuck, the lathe is run slowly, and when running true, I shut off the lathe and tighten the chuck.

This process centers the stock very well.



Chuck