Composite VS Multiple Single Segment Feature Control Frames

Composite VS Multiple single-segment feature control frames

A composite feature control frame (FCF) contains two tolerance zone frameworks. One of the characteristics of composite tolerance is there is one geometric tolerance across two frameworks.

Top frame - Pattern Locating Tolerance zone framework (PLTZF)- Behaves like a normal position tolerance.

Bottom frame - Feature Relating Tolerance Zone Framework(FRTZF)- Refines orientation of the pattern.

Composite Rules:

1. Position symbol spans across the two frames.
2. Bottom frame refines the location and orientation of holes only to other holes.
3. If datums are called in the bottom frame, refine only orientation to the specified datums.
4. The top frame must match the bottom frame

Ex: Top frame has the location and orientation of the pattern with respect to datum A, B and C. The bottom frame has an orientation of the pattern with respect to datum A and rotation of the pattern with respect to datum A and B. You can see an alignment matching between the top and bottom frames. What if the pattern didn't need to lock to datum B, but locking to datum C is required? By the rules, datum C could not be placed under datum B. To call out datum C in the FCF, datum B should be called out, Even though datum B would not be used.

Multiple Single segments (MSS)

A multiple single segment feature control frame contains two independent tolerance zone frameworks. The position symbol is shown on each frame, rather than having a shared symbol in composite FCF.

In composite and Multiple single segments FCF, the top framework conveys the same meaning. The difference comes into play in the bottom framework, which refines location and orientation. In composite FCF, it refines the orientation of the pattern to any referenced datums and the location to itself.

Another difference is that the bottom frame of the MSS feature control frame can call out any datum in any order unless it is an exact copy of the top datum. Because the top and bottom frames are completely independent, an exact match of data on the bottom frame will no longer be needed or useful. In this case, the bottom face will eliminate the top frame's requirement altogether because the bottom frame has a tighter tolerance.

Multiple segment rules:

1. Both feature control frames are completely independent.
2. The bottom frame now refines the location and orientation of the top frame.
3. The bottom frame can have any data in any order but does not apply to the top frame.

Let us see how tolerance will be affected.

In composite FCF, the @0.1 tolerance zones in the bottom frame are locked to each other with basic dimensions but can shift within the @5 zones(they are allowed to shift in any direction). The lower frame keeps the pattern from rotating and orientated to datum A.

In multiple single segment FCF, the @0.1 tolerance zone is locked to the basic dimension of 40.This results in a slotted tolerance zone. Tolerance can move from left to right from the basic dimension but cannot shift in any other direction.

You can see that the tolerance value and reference datum are the same between the composite and multiple single-segment FCF, which results in a tighter tolerance. This is because the MSS frames are independent, and the bottom frame refines the location and orientation of the top frame, while in composite, FCF only refines the location of the holes in the pattern and orientation to any datum referenced.