Reading the sparks

[tornitore45]

I was checking my live centers and found both around 0.004 off on the cone but dead centered on the cylindrical part of the point.
So I thought: If I can get them centered in the spindle I can grind them true.
The fixture to immobilize the live center turned out to be unnecessary, the center turned with the spindle even when grinding.

I rigged up my poor man tool post grinder and proceeded grinding 0.001 per pass advancing slowly.
I expected that the cascade of sparks would fade to nothing at some point but it always gave a few sparks.

Is that normal?

I have heard the term "sparking the wheel" in surface grinding contest but have no idea of what it is or how it relates.

Or we need to go into thermal motion of atoms, quantum mechanic fluctuations or minute vibrations?

[spro]

I don't think it is that. There is something else going on with the live center. Its use is under pressure, taking up the bearings play. They do wear but are supposed to be concentric under pressure. It is something to duplicate that with a TPG.

[Harold_V]

Is that normal?

When grinding on a lathe, especially with a less than acceptable grinding spindle, yes, it's normal. The resulting finish, when compared to a finish achieved by a cylindrical grinder, will help you understand why.

Edit. In reading spro's post, that, too, is a consideration. A center, loaded, may display differently than one in the free state. However, in either instance, sparking out will almost endlessly produce sparks, although diminishing slowly. That's due to the less than optimum condition of the wheel, to say nothing of the less than high precision bearings in use.

H

[tornitore45]

I have no hopes to do a proper operation in my home shop, but for argument sake, how would a manufacture of high quality live center achieve the final assembled grinding of a center assuring a load while grinding to the very tip?
Is using a wide wheel working the entire cone produce a sufficient axial component.

The resulting finish, when compared to a finish achieved by a cylindrical grinder, will help you understand why.

Right, the finish was nice but far from what usually seen on ground shafts.

[Harold_V]

I am not familiar with the setup that might be used to grind such an object, but I suspect it isn't done in one operation. It could be (by centerless, plunge grinding), but I suspect that isn't the case.

Bottom line on an object such as a center would be that it is likely ground perfectly concentric before assembly, relying on the bearings of the center assembly to maintain proper concentricity (in use of the live center). I can see how the point might be ground last, having been used as a male center while the shank was ground. Once ground, the shank likely fit a socket, which then allowed the point to be final ground. Just speculating. I have no idea if that's how they are ground.

While plunge grinding isn't unheard of, given the opportunity, a ground surface is best accomplished by feeding across the part. That tends to yield the best possible surface finish, removing any irregularities of the generated surface. That's very much a part of "sparking out". A centerless grinder works that way when through feeding. The bulk of material is removed from the part by the corner of the wheel, with the balance of the wheel (which is generally quite wide) doing the sparking. A cylindrical grinder is often operated in the same manner, and for the same reason.

H

[tornitore45]

Thank you Harold.