Pattern of a pattern

[will_bentley]

Hello,

We've recently started looking at tolerancing our drawings using GTOL's rather than +-.

Part of the design is a pattern of a pattern. 2x2 hole pattern acting as a mounting surface for a roller block. 6 of these blocks along the track. Currently dimensioned as shown in the 1st image (without any datums, I've put them in as notes temporarily for a reference)

I'm getting in a mess trying to figure out how to do this using symmetrical and position tolerances. For a single hole I understand the principles, but how to properly define the other holes relative to the 1st while maintaining the symmetry in is what's throwing me.

2. I've seen in a few places don't put a datum tag directly on to a centreline because it is imaginary. I've defined ours as the centre plane between 2 plates, currently assuming this is correct practice (2nd picture)

[Kelly_Bramble]

I'm going to assume that your in the USA - therefore your GD&T standard is ASME Y14.5-2009.

Ok, first you cannot use a datum with flatness and the CZ (Common Zone) is not used correctly..

Your drawing is confusing however in general you would identify a single feature in the hole pattern a datum (let's call your master datum) then define a position tolerance of the related hole features back to that datum (master datum). Additionally to control rotation you will an additional datum probably a feature external to the pattern. You would indicated 6X in a detail view.

If your'e a Engineers Edge Premium member see Chapter 7, page 7.29 of my book Geometric Boundaries II for an example.

http://www.engineersedge.com/geometr...ies-2-book.htm