Lathe Re-Chucking

drawoh · Jan 27, 2016

I have a part here with an accurate shaft for pre-loaded spindle bearings, and an inside diameter on the opposite face from the shaft, for mounting a hub. The part must be machined in the lathe, then re-chucked to do the opposite feature. I am using the shaft as my datum, and I have applied a positional tolerance of Ø0.05mm on the inside diameter. It appears that our machine shop used every bit of this. This particular shop is fast and cheap.

How accurately can a shop centre a part in a lathe, when they re-check to a datum feature?

--
JHG

ornerynorsk · Jan 27, 2016

Depends on the type of chuck. Collet or between centers is preferred for greatest accuracy, 0.05mm no problem there. An ID feature on one or both ends precludes the use of centers. A good power chuck on a tight spindle with properly bored soft jaws and a conscientious operator should be able to stay inside of your tolerance, as well. Manual scroll chucks, don't count on it.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

racookpe1978 · Jan 27, 2016

0.05 mm = 0.002 ... Difficult, but doable on a good lathe with a good operator. Not extreme tolerance at all, by any means.

+/- 0.001 is routinely achieved for centerline accuracy and part diameters; 1/10 of 1/1000 can be met if you need it. Lathe accuracy (tool rigidity, bed accuracy, gear and runout problems sometimes are more limiting than initial positioning. The machinists I work with often find that a 4-clamp lathe is often more accurate (but slower) to dial in than a default 3-clamp head.

But, accurate, fast, or cheap. Pick two of the three.

JNieman · Jan 28, 2016

I would echo @ornerynorsk. His comments about workholding devices is poignant. @racookpe1978 brings up good points though I believe the numbers are on the over-cautious side compared to my experience. I work with mostly low-quantity production and R&D type manufacturing where there is only "just enough" material and no time to 'do it over' so "get it right the first time, every time" has been my career experience. The expected tolerance varies widely from shop to shop, depending on their 'culture/industry' of focus. Some shops think they are rock stars for holding +/- .005"[.1mm] on 2"[50mm] diameters whereas some shops would discipline or fire someone if they had difficulty with that.

If your machine shop is using every bit of the tolerance you have, it is quite possible that they are choosing the /fastest/ tools that will get the job done. If you tighten that tolerance, it may force them to switch to a 4-jaw chuck that requires 'dialing in' periodically, or possibly for each part, and your cost/part will increase.

Then again, are your parts working? Is there excessive fail/scrap rates? If not, why care? If so, then yea, look at how much your tolerance needs to be to still function, and open it as much as you can tolerate.

drawoh · Jan 28, 2016

JNieman said:
...

Then again, are your parts working? Is there excessive fail/scrap rates? If not, why care? If so, then yea, look at how much your tolerance needs to be to still function, and open it as much as you can tolerate.

My part is a one-off assembly fixture. The mating part will be manufactured in the hundreds (we hope). I am considering sending my piece out to a better quality shop. I am not sure why it did not got there in the first place. In addition to my specified positional error, I have a couple of assembly clearances to sort out, so this is quite a bit more than operating a lathe.

Thanks everyone.

--
JHG

desertfox · Jan 29, 2016

Hi drawoh

A standard three draw chuck should hold a component concentric to about 0.003" after it's been turned around.
Using a four jaw chuck and setting up correctly should enable you to get within about 0.001" for the same procedure.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

Tmoose · Jan 29, 2016

"pre-loaded spindle bearings"

The catalog tolerances for diameters and concentricity and shoulder abutment perpendicularity and runout for spindle bearings are WAY tighter than 0.05 mm.
How did the shaft shop do on those,and what did you ask for?

drawoh · Jan 29, 2016

Tmoose,

The spindle bearings are off-the-shelf. They are angular contact. The front faces are lapped such that when you compress them together, they preload. I installed these, not the shop. I should inspect the mounting shaft. The assembly is sitting here and I can feel no clearance. I called up an accurate tolerance. I believe most of my runout is centration between the shaft and the bore on the opposite side.

--
JHG

Tmoose · Jan 30, 2016

The catalog tolerances for shaft and housing diameters and concentricity and shoulder abutment perpendicularity and runout for spindle bearings are WAY tighter than 0.05 mm.

http://products.emersonbearing.com/Asset/barden_company_data.pdf

drawoh · Feb 1, 2016

Tmoose,

My bearing attachment is very accurate. My problem is a pocket on the opposite side of the part, that we mount stuff into. The bearing appears to be working fine.

--
JHG

MikeHalloran · Feb 1, 2016

So, final machine the pocket in the shaft bearings.

Mike Halloran
Pembroke Pines, FL, USA

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