60 Degree V's vs 45 Degree V's and Centerless Grinding

[JoeLee]

A friend of mine recently bought a small centerless grinder. The problem that he's running into is that he can't get anything to come out perfectly round unless the stock is perfectly round to start off with like a hardened and ground dowel pin. Anything else that he tries to grind actually come out what I believe you would call trilobular. If you mic it it will measure perfectly round because the lobes are spaced 120 degrees apart so you measuring from the high spot of a lobe to between the low spot of the other two lobes, but if you put it on a surface plate and roll it, it will roll back an forth, or if you spin it and indicate it with a dial it will show the highs and lows so it's not perfectly round.
I don't have any experience with centerless grinding but I remember hearing a long time ago that you need to use a 60 degree V in order to get the part to come out perfectly round. Perhaps some one can enlighten me.
I always thought that if a part is lobed it will just repeat in a centerless grinder, I would think it would need to be turned or ground on centers to be trued up and then centerless ground.

JL.................

[easymike29]

https://www.gagesite.com/documents/Metrology%20Toolbox/Geometry.PDF

http://www.mmsonline.com/articles/centerless-grinding-not-magic

Gene

[Rustybolt]

The blade,( rest), should be slightly above center. If the blade puts the work on a center plane with the wheels it will grind lobes. Cinncinati Milicron has a booklet you can get that explains it all.
You don't use a V.

[Paul Alciatore]

Mike,

Thanks for those references. They are excellent! Although I have never been within 100 yards of a centerless grinder, I had long wondered how centerless grinding could possibly work. This was from viewing the rough sketches of the process which invariably showed the workblade on the center and the work at the centerline between the two wheels. Just as I had wondered, this is a formula for a lobed workpiece, not a round one. My suspicions were correct. The second reference shows how the workblade must be positioned to prevent this and how to produce true round shapes. It is all clear now.

And the first one is an excellent discussion of the geometry.

I have saved both pages for future reference.

Thanks!

[Rustybolt]

Who's grinder does he have?

[oldtiffie]

Imagine if you "mike" a cylinder that has say two ground precision same-size cylinders on each of them with one produced by a centre-grinder with the job accurately located each end on a centre and a centre-drilled/bored 60 degree hole and the other on a centre-less grinder supported on a wedge and driven only by two grinding wheels or one "rubber" and the other a grinding wheel.

If you use a micrometer to measure them in say three places - 120 degrees apart they will both be the same.

The cylinder produced on the centre grinder will still be round (no movement on a good dial indicator resting on the cylinder which is supported in a good vee-block) and there will be no movement on the dial indicator when the job is rotated in the vee-block. That is to say that there are no "lobes" at 120 degree spacing.

Repeat with the cylinder produced on the centre-less grinder and the cylinder ground on the centre-less grinder will or may cause the indicator to fluctuate (rise and fall) at 120 degree regular spacing/intervals).

Many times it is no consequence but it often is.

Many rods - such as O1 (silver steel) - are ground continuously with one rod following another (each other) with the feed being generated by the centre-less grinder.

Further, bent rods fed into a centre-less grinder will still "grind to size" (with or without "Lobe-ing") as normal but the ground rod when it exits the grinder will be as bent as it was when it was fed into the centre-less-grinder.

"Lobe-ing" does not always happen but it sometime does often for no apparent reason.

http://en.wikipedia.org/wiki/Silver_steel

[Rustybolt]

long bars are generally ground with the blade below center.
Ive seen an infeed operation where they ground a round spigot on the end of a square bar.
Bowling balls used to be centerless ground.