Position and Align
Position and Align
(OP)
I have an extruded shape - round on the outside and round except two "key-ways" 180 degrees apart on the inside. The OD is Datum A and the keyways are Datum B. I have to machine a slot in the end; Looking down at the end (the cross section if you will) I will endmill a slot central to a depth leaving two "ears". I have a geometrical callout to that slot width to be True-Position of .004 to A and B. No diametral symbol. My question is; If my slot is slightly off center say .001 (TP of .002) and there is angular misalignment how do I calculate TP total (needed for process capability reasons). Do I have TP to A then another TP to B? or do I lose angular misalignment tolerance as my centrality gets larger? Or if anyone knows where a good example is I would really appreciate it. Thanks in advance.





RE: Position and Align
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Heckler
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RE: Position and Align
Maybe use semmetry?
Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-05)
RE: Position and Align
RE: Position and Align
Not true. Not necessary. Setting orientation with the keyway (Datum B) infers direction parallel and perpendicular to that datum. The frame of reference looking down the part axis is now fully defined. Center of "A" provides position of center, orientation of "B" defines "clock position" of part from which dimension directions are drawn.
RE: Position and Align
RE: Position and Align
The keyway feature will define a center plane of the part as a datum. This plane is parallel/coincident to one plane of datum A and perpendicular to the other plane, but not mutually perpendicular to them. The minimum requirement is to have "3 mutually perpendicular planes." I'd guess that would be one of the end surfaces of the extruded shape, defined with a control related to datum A.
RE: Position and Align
You didn't actually tolerance the position of the slot width; what is toleranced is the mid-plane of the slot. Inspection of that feature (the slot) means verifying the size first, then establishing the mid-plane of the slot feature. The tolerance zone emenates from the basic (theoretically perfect) position wrt the datums. The actual Positional Error of the physical part corresponds to the total difference (one end to the other) of the displacement from Datum-B, which includes the angular error you are concerned with. I'm not sure if that's clear.
Jim Sykes, P.Eng, GDTP-S
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RE: Position and Align
RE: Position and Align
Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development