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Alignment/Measuring

Hello all,

I'm back with another rapid-fire set of questions:

I'm having a lot of troubles getting my alignments down. So, basically where i work, we make exhaust tubes. I am almost always supplied a cad, with a predetermined origin and rotation orientation for my coordinate system. Because of this, I was told in the PC-Dmis 101 class, I would have to almost exclusively use best-fit alignments. Now, to do my alignment, I typically take four separate hits (using my mouse on my CAD) on each end of the tube in a kind-of plus pattern. Then i start a new alignment, click best fit, select all 8 of my hits, click compute, then ok, ok. So that's my initial alignment. Then I select auto-trigger mode and place 4 hits on either end of the tube (The cut part of the tube, by the way; what will eventually be measured as a plane soon), and create a second best fit alignment using the 8 new auto trigger hits.

After this is done, i shift-left click all my features starting at one plane, then all the cylinders down the part, then the last plane. I construct a pierce point between the first plane and first cylinder, and then the second plane and the last cylinder.

Here's where i get into trouble:

By blueprints typically call out distances between these two points across the X, Y, and Z axis, all separately. X normally goes down the length of the part, Y for the width, and Z for height. My X measurement is 'almost' always in spec, and then my Y and Z measurement is be off, sometimes significantly. This leads me to believe that there is a rotational issue present, likely due to how i built my first alignment. On occasion, my x measurement will be near perfect, then Y and Z be off my 8-10mm. Yet when i check the part to the check fixture the issue isn't present, which only further confirms that i am doing something blatantly wrong. In the PC-Dmis class, we only used the training block, so it was incredibly easy (relatively) to visualize my alignments, and make sure i had all degrees of travel and rotation locked.

Please, someone help me understand what I'm doing wrong, and what i can do to fix this issue. I have been doing this for around 6 months, (did the PC-Dmis 101 course this last December) so I don't know all the lingo or shorthand names for some of these actions i see some people talk about. Please explain how i should be doing it, as step-by-step as you can as though you were instructing an infant, because I'm definitely missing something here.

With my last post after the guy helping me realized I'm an incompetent newb at this, and he broke down what i needed to do step-by-step, it almost completely solved my issue that i was having.

Thank you.
Parents
  • We measure a lot of tubes (probe) here and the way we do it is by measuring two 3D-circles on each straight section and then create a 3D-line between them. This method is less prone to throw your cylinder measurements off due to any (large) variation in form. The ends are measured as planes. All lines are then intersected with each other and with the planes of course. Nominals for the intersection points are checked and changed accordingly (if needed). Depending on the datum structure, either 3D or 2D bestfit is used with the intersection points.
  • Could you explain what you mean by measuring a 3D circle? I though circles were 2D features.
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