Correlation between dots/in^2 and quantification of flatness?

Question

Is there any general correlation between bearing dots per square inch and a quantification of flatness? I ask this because once a piece goes flatter than the resolution of the dial indicator you are using (or for one's patience to jack a piece level) other methods (like optical flats, I suppose) must be used.

Could any kind of general conclusion about the flatness be assigned to the following ranges (like for example .001, .0001, 50 mils, 10 mils, respectively, or something else):

1) 5-8 spots / sq in
2) 15-20
3) 35-50
4) 100

Thanks in advance for any insights or opinions.

DoN. Nichols · Accepted Answer

There are electronic indicators -- used with a box of electronics with a big meter scale on them, which can measure will below the limits of a mechanical dial indicator. I think that they will go down to a micro-inch or maybe even better.

If those are supposed to be in order -- you should specify what kind of mils you are using. In the electronics industry, a "mil" is

0.001", so you are getting much bigger after getting smaller. Perhaps you are thinking of micro-inches instead?

I don't do scraping (at least, yet), so I don't know for sure, but I don't think that this relates directly to flatness -- just to the percentage of the material being scraped which is available as bearing surface. (And -- of course, it probably shows how far below those high spots the surface between can go -- probably as a function of the curvature of the end of the scraper.

Enjoy DoN.

Mike Henry · Answer

I believe that these are referred to as gage heads and amplifiers. Federal and B&S seem to be the brands that come up on eBay most reasonably (hsm prices anyway). I've seen analog versions with 1 grad resolutions of 1, 10, and 50 millionths of an inch.

Correlation between dots/in^2 and quantification of flatness?

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