Home made lapping plate - 2 vs 3 plates

[compact8]

I have made a set of two grey cast iron lapping plates and rubbing them on dyed granite reference surface indicates that they are sufficiently flat. I expect that they will need be re-conditioned to restore the flatness after some use and my question is whether it is really necessary to use the third plate in the process. For what it's worth I use garnet abrasives only.

From what I have read on the subject, rubbing two plates against each other will result in the top plate becoming concave and the bottom plate convex so the third plate is needed to correct that. My question is can the correction be done by simply swapping the positions of the two plates so that the top one becomes bottom and vice versa ?

 

[Moderatemixed]

I’m kind of interested in seeing where this thread goes as I’m looking for as much info as I can find on lapping. My understanding is (in your case) that the surface plate that you used to flatten the two becomes the third surface. If you now use the tow plates to flatten each other you in theory will have achieved flatness. Again as I understand it to be the case, so long as three separate surfaces are used, flatness can be achieved. To answer your question though, and correct me if I’ve misunderstood, if you simply call the bottom plate the top and the top the bottom it doesn’t change how the two are flattening the other, but that a third surface is required.

Good luck.

Derek


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[compact8]

If you now use the tow plates to flatten each other you in theory will have achieved flatness.

According to what I have read, you will get perfect mating between the two surfaces, ie, they contact each other at all points but the surface of the top plate will be concave and the bottom one convex. I am yet to see it by myself but I am convinced that if I keep on rubbing the two plates against each other this will happen eventually.

With the bottom plate held stationary and the top plate is moved back and forth, there will be an unwanted torque that tends to tilt the top plate. As the result, the inner area of the top plate and the edge areas of the bottom plate will be cut more. For the same reason lapping metal on a piece of sand paper laid on a flat surface will result in the surface of the metal having the edge areas cut down more.

 

[Moderatemixed]

What I believe you are misunderstanding is that yes, the two plates will be perfect mirror images of one another, but they are not necessarily flat. Tear a piece of paper and put both pieces back together, they will fit perfectly but the tear is not straight. The long and the short is that if you want flat you need a “tie breaker”.


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[benmychree]

The surface plate does not constitute the 3rd plate, and none of the lapping plates will ever be better than the reference surface (the surface plate), that is why 3 plates ground or scraped in sequence are necessary for a truly flat surface. Rubbing two plates together will never result in a truly flat surface.

 

[DavidR8]

I think Tom Lipton does an excellent job of explaining the process.

 

[gradient]

Three surfaces are needed to produce a flat. Look at how telescope mirrors are made. There two pieces of "flat" glass are rubbed together with a grinding medium producing a concave part (the mirror) and a convex part (not used unless you need a convex surface). As noted by compact8, the edges of one rub against the center of the other to produce the two curved surfaces.

 

[benmychree]

Another significant point is that after the two plates are charged with abrasive, they could not be used in contact with the surface plate without causing wear and inaccuracy to it.

 

[rabler]

I have made a set of two grey cast iron lapping plates and rubbing them on dyed granite reference surface indicates that they are sufficiently flat. I expect that they will need be re-conditioned to restore the flatness after some use and my question is whether it is really necessary to use the third plate in the process. For what it's worth I use garnet abrasives only.

From what I have read on the subject, rubbing two plates against each other will result in the top plate becoming concave and the bottom plate convex so the third plate is needed to correct that. My question is can the correction be done by simply swapping the positions of the two plates so that the top one becomes bottom and vice versa ?

A lot comes down to how flat is flat enough.

At an atomic level, I don't think things are ever perfectly "flat" given atomic structure, so there is an ultimate limit to what you can achieve. Like most things in machining, what level of precision do you need?

 

[compact8]

Look at how telescope mirrors are made. There two pieces of "flat" glass are rubbed together with a grinding medium producing a concave part (the mirror) and a convex part (not used unless you need a convex surface).

Yes, that's what I have read. As the rubbing action always causes the top plate to become concave and the bottom one convex, my imagination is putting a convex plate on the top and a concave one at the bottom MAY make both plates flat again. No ?