[Do...] Posted February 13 Share Posted February 13 So, I have a scenario where I have a multiple single segment position callout for one hole in a part and need to understand the best practice for programming it. The top tier is pretty straight forward. I measure the hole as a cylinder and the choose the true position characteristic and select datum A (primary), datum B (secondary), then datum C (tertiary). The second tier I am not so sure about. Since it is only going back to datum A and no other datums I know that it can or should be able to rotate around A and translate along A. I have done this before for a pattern of holes but never a single hole, so I am not sure if the method or strategy is the same. I have attached a crude napkin sketch of the part and apologize for the lack of detail, but my company's policies do not allow me to share the original drawing or images of it. 20250213120434616.pdf Link to comment Share on other sites More sharing options...
[Ha...] Posted February 13 Share Posted February 13 Based on the sketch, it looks like the second tier is a refinement of the first tier. All you have to do is to enter datum A and apply MMC to the toleranced feature, no need for a best fit bore pattern. Link to comment Share on other sites More sharing options...
[Do...] Posted February 13 Author Share Posted February 13 My result for the first tier is .0248" The result of the second tier is coming in at .0256". Not sure why this would be worse as it is not as constrained as the first tier. Maybe I am looking at this wrong. I would have thought that number would have been significantly lower than the first tier. Link to comment Share on other sites More sharing options...
[Ha...] Posted February 13 Share Posted February 13 Do the evaluation settings in both position characteristics match by chance? Link to comment Share on other sites More sharing options...
[Ma...] Posted February 14 Share Posted February 14 The lower tier is only controlling perpendicularity to |A|. The result should never be greater than the upper tier result. Link to comment Share on other sites More sharing options...
[Do...] Posted February 14 Author Share Posted February 14 @Joseph Harter - Can you elaborate on what you are meaning? Link to comment Share on other sites More sharing options...
[Ma...] Posted February 14 Share Posted February 14 Please sign in to view this username. Just to clarify - do you have your datum A and B as symmetry / symmetry plane? The way you draw datums A and B incline to be used as symmetries. For second tier - just fill all three datums with datum A - watchout for (P) settings and selection based on norm you work in. Link to comment Share on other sites More sharing options...
[Do...] Posted February 14 Author Share Posted February 14 Please sign in to view this username. Datum A is set to a symmetry plane. Datum B is a symmetry point and then I utilize the distance between the two parallel lines for the FOS. Link to comment Share on other sites More sharing options...
[Ma...] Posted February 14 Share Posted February 14 I would use symmetry plane for datum B too instead of point - you have to use it's rotation - datum C will give you only position - it should not give you anything else. How long is that hole? This can help. You can also investigate how are vectors looking like for datums and cylinder ( intersections, caliper distances, ... many ways to investigate ). Link to comment Share on other sites More sharing options...
[Da...] Posted February 14 Share Posted February 14 So, you have a rectangular 'saddle', where the datum A is the center-line between the 2 'legs' of the 'saddle', datum B is the center-line of the square hole in the top, perpendicular to the datum A center-line, and datum C is the planar surface on the top, that the square is cut through. Next, you have a hole on the (if you set this up as A=x, B=y, and C=Z) which is not dimensioned, but it looks like it might be at X=0, Y is a negative number, and Z is also a negative number. And yet, A is set up as the primary datum, and C is the tertiary? Very odd. If my ascertains are correct, you will be measuring the position in the Y,Z axis for the .030, and will be just calculating the .015 as perpendicularity to the A-plane, in which an MMC callout is pointless and incorrect. Link to comment Share on other sites More sharing options...
[Do...] Posted February 17 Author Share Posted February 17 (edited) Please sign in to view this username. the hole is .440"-.446" in diameter and is only around .119" deep. Also, in a previous comment, you mentioned to watch out for (P) settings and selection based on norm you work in. What are you referring to there? Edited February 17 Link to comment Share on other sites More sharing options...
[Ma...] Posted February 17 Share Posted February 17 Since length of cylinder is 0.119" ( cca 3mm ) then i woul measure that as circle. I think you have problem with calculating vector ( cylinder axis ). Look at that. On mine version i have selection for cylinder in TP where you select (P) or feature with not straight lines - that would be like that - (P) will evaluate as nominal cylinder length, otherwise it will evaluate as measured length. That (P) is often used for evaluating threads - projected tolerance zone Link to comment Share on other sites More sharing options...
[Do...] Posted February 17 Author Share Posted February 17 Please sign in to view this username. if I measure the hole as a circle, then doesn't that defeat the purpose of checking the second tier of the FCF since it is really checking the orientation back to datum A only? Link to comment Share on other sites More sharing options...
[Ma...] Posted February 17 Share Posted February 17 Yeah - that would be useless. You can create new alignment with that datum A as primary - just this, nothing more is needed and recall that cylinder into this new alignment - it should tell you, that problem will be Y/Z X/Z angles. Having .0256 ( thats 0.65mm ) deviation should be that vector. Have you used symmetry plane or symmetry from planes? Using symmetry plane when you don't have same opposing points can be bad - using symmetry from planes will not affect that. Link to comment Share on other sites More sharing options...
[Do...] Posted February 17 Author Share Posted February 17 Please sign in to view this username. I have only really used symmetry plane, not symmetry from planes. Link to comment Share on other sites More sharing options...
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