Datum Pattern Dimensioning

[JLang17]

When you have a pattern of holes as a datum, do the dimensions for other features come from the centerplane derived from the pattern, or from the center of one of the holes? I've been searching around and I keep finding it done both ways, are both ways really allowed? I believe inspection is done by establishing the centerplane, so I'm inclined to assume that dimensioning to the centerplane is the way to go.

 

[KENAT]

Given that the pattern will be located with basic dimensions, so no tolerance stack as such, then it doesn't really matter does it?

If inspection is going to establish center plane(s) then dimensioning from that might save a little math but that's about it.

It's a bit like when you locate a hole pattern from an edge. You can either go from the edge to each hole, or from the edge to the first hole, then from that hole to the next etc. The schemes have advantages & disadvantages in different stages of the design & manufacturing process.

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[JLang17]

Saving a little math was the only practical reason I could think of, but since I'm not experienced I figured I better consult with the masterminds. I also thought since the centerplane is what goes into establishing the DRF, technically this is what you should dimension from.

 

[axym]

KENAT is right. It makes no difference how the basic dimensions are laid out. The only requirement is that there must be a way to calculate the true position of the considered feature - in other words, the nominal displacements from the datum features to the considered feature. If you need to add or subtract basic dimensions in order to calculate the dimension that you need, that is allowed.

This is related to the often-discussed issue of where the "datum axis" or "datum center planes" or "origin" of a pattern of holes is. To me, it is arbitrary - the designer can pick a particular origin or let the inspector pick one. As KENAT said, different schemes have advantages and disadvantages to different users of the drawing. But functionally they are all identical.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca