What is "Work Tolerance"?

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Harold_V
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Re: What is "Work Tolerance"?

Post by Harold_V »

Don't grip the center (of the material). Grip it near the end, where you have rigidity. Dial it dead true using a DTI---then face it fully. When you start the center drill, do that by gripping it only by a short amount, so it is free to seek center. Tighten the center drill by hand only. When it makes contact with the turning material, apply only enough pressure to cause it to seek center, which it will gladly do if it's not held too rigidly. When it is at what you assume to be center, apply very slight pressure, so it can drill, but still move about if necessary. Once you assured that it has picked center, you can then apply enough pressure to drill to the desired depth. Repeat that for the opposite end.

When you have your centers, test the shaft for concentricity by mounting it between two dead centers, as I suggested previously. You'll likely discover that it isn't true, and it may not be straight. Even drill rod is rarely truly round and straight, and for good reason.

Ground material is generally run through a centerless grinder, and in some instances, a centerless sander. Both are capable of producing straight and round material, assuming they are operated properly. They aren't always. A centerless that is operated at center has the potential to yield three-sided material that measures dead round, yet it isn't. Long material that is not dead straight when ground, and not passed through the machine numerous times, allowing the machine to remove the bowing, often results in material that isn't round, often with a flat.

The reasons mentioned above are why I suggested that any test piece should be ground on the drilled centers. Only then will you achieve the needed degree of precision that will yield reliable results. Anything less is akin to taking measurements with a caliper. You get readings, but they may or may not have credibility.

H
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dml66
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Re: What is "Work Tolerance"?

Post by dml66 »

After going down the headstock alignment rabbit hole, I concluded it wasn't the problem; a virgin replacement headstock resulted in slightly worse taper :cry: .

I decided to take one last shot at fixing this lathe's taper, approximately 0.001" per inch, by trying to see if a twist in the bed might resolve the issue. I have to say it took a lot for me to mentally let go of my belief that a 10" bed could not possibly be twisted, lol.

Using a couple 14" long pieces of 2"x1/2" C-channel, I secured those to the lathe's legs through the drip tray. The C-channels sit on 14"x3"x1/4" aluminum flats. Four 3/8"x6" bolts secure the lathe to the bench top. The benchtop thickness where these bolts pass through to inside the cabinet is 4-1/2", it should be pretty rigid. Once I bolted the lathe to the bench I confirmed it was still turning the same taper.

Loosening the back tailstock-end bolt, I pried the C-channel up off the aluminum and inserted various feeler gages starting at 0.001", then re-tightened the bolt. Each successive feeler gage thickness increase resulted in less taper; once I hit the 0.003" feeler gage, the measured taper was 0.0003" over 3" :D

And yes, I did acquire a 1"-2" Mitutoyo 0.0001" micrometer in order to be more certain of my measurements.

I want to thank you all for your help with this, it means a lot to me. Also, if you see anything I've done here that may be causing a false positive, please let me know. Time to make some chips :D .

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NP317
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Re: What is "Work Tolerance"?

Post by NP317 »

Well there you go!
Another job well done.
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Bill Shields
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Re: What is "Work Tolerance"?

Post by Bill Shields »

Just do not assume that it will stay where it is.

Sn unseasoned casting is not stable for years....
Too many things going on to bother listing them.
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rmac
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Re: What is "Work Tolerance"?

Post by rmac »

Bill Shields wrote: Sn unseasoned casting is not stable for years....
Even at 4-1/2" thick, a wooden bench isn't either ...
dml66
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Re: What is "Work Tolerance"?

Post by dml66 »

Advice well taken.

This is why I tried to make it "easy" to adjust the bed as needed in the future.
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Bill Shields
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Re: What is "Work Tolerance"?

Post by Bill Shields »

Before meehanite, castings used to be seasoned for months if not years.

Today 'solution annealing' has found well deserved favor as an alternate for less expensive materials.

Do not assume either has been applied....or if it has, then the bed was not properly ground.

It is a delicate balance...

accuracy vs. affordability.

Without googling it ..anybody know who brought meehanite into the production world?
Too many things going on to bother listing them.
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ctwo
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Re: What is "Work Tolerance"?

Post by ctwo »

First I heard of it. And I looked it up before reading the second sentence of your post... :D
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dml66
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Re: What is "Work Tolerance"?

Post by dml66 »

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Harold_V
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Re: What is "Work Tolerance"?

Post by Harold_V »

It would be highly unlikely that any of the inexpensive lathes built by China use the Meehanite process. I wouldn't be the least bit surprised if they were simply pouring grey iron melted in a cupola, although they may be using induction for melting. They make that equipment there, too.

Castings have stress and will relax under multiple conditions, time being one of them. That, of course, demands a huge inventory of castings, each of which must serve time aging. That, too, is a tall expectation from Chinese items. I suspect that the castings are still warm when they are machined!

Case in point. I purchased a hand powered oil transfer pump made in China (HF). I had intentions of powering the pump (which I did, and it works exceedingly well). I was shocked at the condition of the machined parts, none of which had been aged. Nothing was flat or well machined and the fits were highly compromised. I re-machined the housings to correct errors, then made a new rotor, which solved all the problems. The amount of movement of the raw castings after machining (in China) was remarkable. Once machined, they were reasonably stable, stresses having been chiefly relieved, both by machining and time.

H
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Bill Shields
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Re: What is "Work Tolerance"?

Post by Bill Shields »

Ah...another 🦕 who recognizes meehanite.
Too many things going on to bother listing them.
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Harold_V
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Re: What is "Work Tolerance"?

Post by Harold_V »

Bill Shields wrote: Wed Jun 15, 2022 6:01 am Ah...another 🦕 who recognizes meehanite.
Chuckle!
Yeah, something that I've been aware of since my early years in the shop (late 50's). What surprises me is that many here are not aware of its existence.

I wonder---are any of you guys familiar with Graphmo? Graphair?

H
Wise people talk because they have something to say. Fools talk because they have to say something.
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