Surface finish callout.....need explanation

Can anyone explain what the following surface finish callout means?

-0.8/Ra2 3.0
-0.8 Rz1max2 25

Surface finish - Wikipedia, the free encyclopedia

This will be a start on figuring that out.

I've never seen a negative surface finish call-out on a print. The wiki link didn't give me any clues. If someone could elaborate that'd be appreciated. I'm curious myself. I'm use to seeing something like '32Ra'. I always assumed 0Ra would be perfect and unattainable.

This link gives more information: Surface roughness - Wikipedia, the free encyclopedia

Given that the definition of Ra is the arithmetic average of absolute values, I can't see how it could ever be a negative number, since an "absolute value" is by definition always positive, thus the average of a bunch of them would also have to be positive...

--Hawk

RebHawk said:

Given that the definition of Ra is the arithmetic average of absolute values, I can't see how it could ever be a negative number, since an "absolute value" is by definition always positive, thus the average of a bunch of them would also have to be positive...

Click to expand...

True.

You never know what engineers will come up with in the computer age. They probably don't know what a sign is.
I doubt any of them has ever used a ruler or other real measuring tool. There's an app for that.

- Leigh

I suspect that the dash (or hyphen) shown by the OP is really only an itemizing mark, not a negative sign. As noted, Ra can't be a negative value based on its absolute value derivation. I provided the initial Wiki link since it seemed to show some of the additional callout info structure in the best way that I could find in a few minutes. I wasn't able to look at the ASME standard as it appears that one needs to buy it for $169.00, which I think is also similar to ISO standard availability.

-0.8/Ra2 3.0 => 0.8mm cutoff/arithmetic average of roughness over 2 single measured lengths, 3.0 um max value, as I interpret what is given.

-0.8 Rz1max2 25 y'all can figure out.

Some additional info that may be more explanatory on specifics:
http://www.mitutoyo.com/pdf/1984_Surf_Roughness_PG.pdf