# "00" or "AA" ? Explain surface plate grade codings ??



## graham-xrf (Jan 28, 2022)

Yes please!
Sometimes it's numbers, otherwise letters. I do get it that there is a numbers system where the numbers for "better" go in the opposite direction to DEFCON states. So grade "00" is fancy and expensive.

Easy to spot that some use an alphabetic system, where "A" is better than (say) "F", maybe a bit like school report cards grades.

Is there somewhere a table setting these together, along with their surface accuracy, so one can decide what is near equivalent, and by how much? Somewhere in there, it seems both schemes adopted a doubled-up expression of maxed out goodness. "00" might aspire to the same rank as "AA", but one should ask, are they so equivalent the gradings increments just mean the same thing, alphabetic or not?

This is the sort of thing one can usually find with Google, but so far, not for me this time. Just lists of vendor offerings.


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## Rex Walters (Jan 28, 2022)

What did you search for? "Surface plate grades" returns several useful resources.


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## graham-xrf (Jan 28, 2022)

Duh! It helps if one does not fumble the speling in the keywordz 
Sorry about that!


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## Rex Walters (Jan 28, 2022)

graham-xrf said:


> Duh! It helps if one does not fumble the speling in the keywordz
> Sorry about that.


No worries. There are NO dumb questions (thank heavens).

I'm not sure but I think the numeric designations are an ISO thing, most manufactures in the US at least seem to have standardized on the alphabetic lab grades.


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## graham-xrf (Jan 28, 2022)

So far as I can tell, there seem to be three categories for USA style alphabetic grading. There's B, for workshop use, and A for "inspection grade", and "AA" for laboratory calibration grade. Within the grade, there are norms for deviation depending on the size of plate. local flatness has the closer tolerance than the deviation over the whole plate.

The "other" set are relentlessly metric. There are 5 grades.
Grade "0" up to Grade "3", and a special Grade "00" as the ultimately accurate. They have deviations in microns for 13 sizes of block, from 25cm x 25cm up to 2m x 1m.

That's OK. The units don't really matter. We know how express the measures in either.


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