Why does cylindrical grinding use a stationary center on the work headstock?

[dwyers]

When turning between centers on a lathe, the headstock center spins with the lathe spindle and yje job driven by a dog. However when using a conventional (not centerlss) cylindrical grinder, the work headstock spins but the centrer is locked stationary and the job is spun with a drive dog.
Why is this?

 

[sfriedberg]

Better concentricity. The classic advantage of dead centers.

 

[eKretz]

If they could easily do it that way in the lathe, they would.

 

[Bakafish]

Maybe I'm misunderstanding the setup of the grinder he's discussing here (pictures clarify these things), but I thought the fact that you can repoint the lathe spindle mounted center for concentricity was an advantage. Wouldn't a cylindrical grinder that could presumably grind in the 'driving' center have a better shot at concentric accuracy than having the center fixed and depending on it being well aligned and constrained by a thin layer of high pressure grease? What am I missing?

 

[eKretz]

Dead center is always better for concentricity because it spins the part around the center. Any live center will spin in bearings and those almost without exception impart roundness, deflection and runout errors. Dead centers come with their own set of limitations though - namely that the center hole needs to be in good shape and center pressure needs to be a little more carefully monitored. Dead center lubricants are squeezed out to a very thin and highly uniform layer. The center hole "pip" acts as a small reservoir also. No worries about irregularities there.

 

[Bakafish]

Interesting. No bearings would seem better than bearings, I guess that checks out. Complicates the drive, but if it's worth it, it's worth it.

 

[Halcohead]

It’s worth adding part rotation rates are slower on cylindrical grinders than turning, so spinning on a dead center will generate less heat and wear.

Rolling element bearings will struggle to have error motion smaller than 1/2um (20microinches), and no amount of in-place grinding or turning will improve this, since the errors are not synchronized to the bearing rotation (they’re caused by ball size variation, ball roundness errors, debris/contamination, etc). By contrast a dead center can be lapped to almost arbitrary roundness, as can its mating center hole. So part roundness can be quite a bit better running on dead centers.

Dead centers can also be more stiffly mounted, but this seems like a secondary consideration.

 

[fmari --MariTool-]

I think some of us are confusing concentricity for roundness.
Using a dead center will give better concentricity between the ground diameter and the center. But that is not that hard to get with a live center.

The biggest advantage is better roundness. a simple dead center and a decently round center can give you roundness never achievable with a live center. Lapping the center will give even better results giving you roundness that is most cases you cannot measure.

 

[eKretz]

I think some of us are confusing concentricity for roundness.
Using a dead center will give better concentricity between the ground diameter and the center. But that is not that hard to get with a live center.

The biggest advantage is better roundness. a simple dead center and a decently round center can give you roundness never achievable with a live center. Lapping the center will give even better results giving you roundness that is most cases you cannot measure.

I glossed over this because the main question seemed to be mostly concentricity related, but yes there are several other benefits. Also far better repeatability if the part needs to be removed and replaced in the machine.

 

[LexD]

I have a "true dead centre" grinding attachment for use when using a toolpost grinder for "between centre grinding" in the lathe.

 

[michiganbuck]

I had a cylindrical grinder job in trade school that had to be turned around because of length and dogging, and got a parting line where the two grinds met in the middle..could not even measure/find the error, but it looked like crap. After I learned the ropes, I figured the headstock center was off.
I had a respected old-timer who told me "Whatever the error in the center point figure you will get double that on the part because it takes off one side and leaves on at the other side."
With thinking that I have solved grinder machine problems and have had great success in high-precision grinding.
I look for less than half center error comparing the specs on the part
To get a tenth on a part, I want less than 50 millionths on the center...
A 2 tenths error live center I regrind, or throw it in the dumpster.
Some China centers can be ground dead true because of poor bearings, bearing not set with ascetics lined up, or the diameters running with a wobble.
We had an expensive production grinder at the big shop that they bought a new spindle for a ton of money and it still failed..When I found that out I have them bring me the center..and I ground to near zero and the machine worked just fine.. likey there was nothing wrong with the old spindle.
Note: it is just about impossible to take apart a live center to regrind the point, that is what is wrong with China centers ..the process is wrong,
A live center spindle has to be ground between centers to be right, so all diameters run near zero..then you snub off the point end center and grind the point running in its own bearings.
*I have to laugh at YouTubers showing how they take apart a live center and regrind it.

 

[Bakafish]

I have a "true dead centre" grinding attachment for use when using a toolpost grinder for "between centre grinding" in the lathe.

This discussion had me thinking about how one might make exactly such an attachment :-)

 

[michiganbuck]

One suitable method I used was to set the live center point under a wheel and put on an O-ring as a belt the run the live center in its own bearings when grinding the point. Some/mant you have to load the point end with a little pressure so they are full tight to the thrust you would get using them I have used a die grinder and the drive motor. One set up I used an O Ring on the center and rugged a hand crank.
Mostly ran them on a B&S `13 grinder, but have done them on the surface grinder.
For dead machine centers, I rigged them between bearings so they just floated in the bearings.
Dead (not turning) or ground in the spindle centers eliminate the run-out problem
centers ground in the spindle, are good/true if not removed from as they were ground.
OH.. A perfect machine center knocked into an older spindle taper is likely running out, so indicate it or grind it to be less than half your part spec.

 

[michiganbuck]

I consider myself a grinder guy, but I am confused about what makes a cylindrical grinder different from an OD grinder. Usually, a universal Od grinder has a turnable wheel head and a taper-adjusting table. Is the difference that an OD grinder is often also an ID grinder?
Here is some discussion on the subject:

Cylindrical grinder - Wikipedia

en.wikipedia.org

Cylindrical Grinders: Everything You Need to Know — Rost Machining & Fabrication (RMF)

Cylindrical grinders have long been used in the manufacturing of a wide variety of creative goods. Keep reading to learn more.

rostmf.com

 

[dwyers]

Thanks guys for all your answers and comments. Your advice has given me a clear understanding.
What I gather is, as long as your center and spindle for the same center are in good shape, keep the center stationary so that it is not subject to any bearings and bearing related consequences, runouts etc. as would occur, if you lock the tab or pin and spin it within the driven main (outer) spindle.
I've heard the same thing said about lathe tailstock dead center vs live center.

 

[John Garner]

I'm sensing that the term "live center" is being used in two different ways within this thread. One sense of live center is one having a point that rotates relative to its mounting shank-flange-or-whatever; the other sense is one having a point that is stationary with respect to its mounting shank.

I understand michiganbuck's description of his point-gringing technique appears to suit point-rotates-relative-to-shank center. In contrast, Bakafish seems to be thinking in terms of a center having a point that is stationary relative to its shank.

My cylindrical-grinder experience is limited to a couple to tech-school class sessions in 1971 or '72, so I'm FAR from being a subject-matter specialist. But I do remember that each student made a DIY center (on a lathe) that we were to mount in the grinder spindle, and true to the grinder spindle bearings.

IIRC, out instructor defined a live center as having a point that rotates with the workpiece, and a dead center as one having a point that rotates relative to the workpiece.

 

[EmGo]

each student made a DIY center (on a lathe) that we were to mount in the grinder spindle, and true to the grinder spindle bearings.

Most cylindrical grinders don't have spindle bearings. Tailstock center is dead, so is headstock center. The dogs will mount to a ring that rotates around the stationary center to drive the part. A few will have live centers, i.e. centers that rotate, but those tend to be more in the "universal" or "toolroom" category.

All the production-type cylindrical grinders I can think of have stationary centers at both ends. Doubtless there's an exception here and there but in the majority ...

 

[Bakafish]

I didn't use the terms 'live' or 'dead' for a reason, as my example might have been confusing in the context of a dead center being mounted in the spindle of a lathe. I have a couple of Gepy high precision live centers, but I understand that (when set up correctly) no bearings beat bearings, no matter how good they might be. With errors being so small it can be hard to mentally model this advantage though, my intuition still tells me spinning something between a pair of dead centers is inferior, but logically I can understand that it is better.

 

[michiganbuck]

A dead center stationary (not turning at all) with only the work/part turning
And a center that has been ground in the spindle and tuning with that spindle are the most accurate centers..but the ground in the spindle one can be subject to spindle bearing defects.

A perfect center just placed in a spindle taper is subject to any imperfections of that taper..usually having some errors..

 

[johnoder]

BS thought it was the only way. Attachments from 1920. See last line of text. They did not say why. The locking pin on lower left is plain to see