Bidirectional tol. vs 2 single segment feature control frames

[jszymkowski]

Can anyone explain the difference between controlling the position of an FOS with a bi-directional tolerance vs. a 2 single segment feature control frame (non-diametric)? Both indicate direction and magnitude relative to specific datums. What are the benefits of one method over the other?

 

[dingy2]

On slot bi-directional are beneficial since there is usually more tolerance on the length of the slot than the width. If you have a positional tolerance in MMC, one must state "BOUNDARY" below each feature control frame.

One will have a FCF reflecting the length and one reflecting the width.

A single segment FCF is altogether different. The top section or 1st level reflects the position of the pattern relative to the datums such as A, B and C. The secondary or lower level FCF reflects inside the pattern. The lower section may only reflect datum A and B. Datum A usually relates to orientation (perpendicularity) while datum B is dimenioned and oriented to that datum.

Hope this helps.

Dave D.

 

[ringman]

I believe the benefit would be that the feature control frame in Y14.5 is covered in the standard as far as the interpretation of the origin and precedence of datums applied. The bi-directional method, is not clearly defined by the standard in this regard. Therefore, there is more room for mis-interpretation using bi directional.

I hope that I understood your question. I think I did.

 

[MechNorth]

If I am understanding your question correctly, then the answer would be that:
(1) a bi-directional tolerance provides an equal tolerance zone in two directions (wrt the datums) from the basic position of the FOS
(2) two single-segment positional tolerance FCFs specify a different tolerance in each of the two directions (wrt the datums) from the basic position of the FOS

Here's a couple of scenarios;
(1) a tapped hole in a plate must align with a square or circular clearance hole in another plate so that the fastener can move equally in any direction from the basic position; equal bi-directional tolerance would be suitable here
(2) a tapped hole in a plate mates up with a slotted hole in another plate so that the two plates can only move in one direction wrt each other; you would want a looser tolerance in the direction of the slot, but tighter perpendicular to the direction of the slot

Jim Sykes, P.Eng, GDTP-S
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