The standard way of making a flange that is grooved for an o-ring is to leave one flange flat and then cut the groove (square or rectangular) in the mating flange. There are tables for the proper groove dimensions (Roth's book noted elsewhere - hint hint buy a copy) but in lieu of that: the o-ring should crush by about 25 to 28% and should not squeeze out of the groove. (I.e. depth of the groove is about 75% of the diameter of the o-ring and cross sectional area of the groove is greater than the x-section area of the o-ring. Less crush and you will possible suffer from leaks (not enough force applied by the o-ring), more crush and the elastomer will take a set and sealing force will decline over time.
When the flanges are tightened, there will be no gap for particles, etc. to shine on the o-ring therefore you don't need the step.
On all ISO o-ring seals that use center rings, the metal part of the center ring also shields the o-ring from bombardment. The elastomer never "sees" the chamber.
When machining the flange, make sure all tooling marks go with the seal, not across. Don't use sandpaper and don't use grease with the o-ring.
Steve
Created on Thursday, March 08, 2001 12:45 PM EDT by Steve Hansen