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Specifying requirements for 8 RMS lapped finish?

I have various flat, circular (1.5" - 2" dia) disks made of aluminum, steel, and titanium. I need to specify an 8 RMS lapped finish on the flat surfaces. This should be machine lapped for an orbital (?) pattern. The surfaces that are to be lapped will have been lathe turned prior to lapping.

For the turned surfaces, what finish and flatness should I specify prior to lapping so that the lapping will go quickly and smoothly?

After lapping, what flatness can I reasonably expect?

How exactly do I call out the lapping requirements on the drawing (symbols + notes)? (Any links to example drawings would be helpful.)

Thanks,

Donald R. Culp
- Baltimore, Maryland
2005

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First of two simultaneous responses --

Before the advent of modern high precision CNC machining, lapping was considered a third operation after grinding. Flatness and roughness requirements are easily met by a grinder (within 0.001" per foot and 32 microinches RMS or better). If your turning operation can meet this, lapping can proceed quickly, if not you'll have to find a better turn or grind the discs. But it's better to talk to the person responsible for the lapping, he should know.

Guillermo Marrufo
Monterrey, NL, Mexico
2005

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Second of two simultaneous responses --

First of all I would get either the ANSI Y14.5 Standard or a good book on technical drawing....such as Technical Drawing by Frederick E. Giesecke or even the Machineries Handbook, these should be available from amazon.com. These texts will tell you how to call out surface finishes. I believe by turning the best surface finish that you will achieve would be 63 RMS. Then you would have to have it grind to about 16-32 RMS. And then you should be able to achieve close to an 8 RMS surface finish with lapping. The idea is to step down in surface finishes using different operations because each operation will only be able to achieve a certain RMS value, and therefore it would not be economical to just turn the part and then lap it. I am assuming that you will be having a job shop fabricate these parts for you, any reasonable job shop will be able to at least tell you where you could have this done if they cannot perform it in house. Have fun....

Bryan Parker
- New Mexico
2005