iron casting surface appearance acceptance vs rms or Ra

[Tmoose]

We have the ASTM A802 comparators, and the MSS SP-55 Quality standard with photographs for tactile and visual casting evaluation.

We recently heard that one casting supplier is expecting a tolerance based on an RMS reading.

Here's a device available to estimate a casting's RMS finish. It's maximum is 900, and does not readily equate to the ASTM/MSS .

http://67.59.156.7/merchant2/merchant.mvc?Screen=PROD&Product_Code=16039

Simplified RMS 900 microinches would seem to have peak to valley depths on the order 0.002 inch or less. I'm guessing the "best" A802 comparator (A1) is worse than that. (I'm trying to get the A802 plates' RMS finish measured).

Is there a source for a more direct ASTM > RMS equivalent?

thanks,

Dan T