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Cylindrical vs. spherical tip probes on sheetmetal thru holes?

Alex B-shift
I have a disagreement with a customer as to how I'm measuring their part on my CMM (this is the same part I was posting about back last October). I have about 3 dozen 10mm PUNCHED thru holes on a sheetmetal weldment (.050 sheet copper, tin-plated). The assembly is actually 2 layers of sheet copper, and the holes are one layer or the other, with embossments around the holes bringing them to roughly the same datum plane.

In my past, places that I've worked where sheetmetal was measured on the CMM, we always used to use stainless-steel cylindrical "barrel probes" with a radiused end, and left the spherical end probes for measuring machined parts. This was (as I was told) because of the nature of sheetmetal in general giving you a very small 'window of flat surface' to hit accurately with a ball probe, and also because of the unpredictability of 'blowout' from punched holes, or the raggedness of lasered holes. Even a machined hole on sheetmetal only gives you a target area the size of the metal width.

My customer, OTOH, has decided to measure the part with THEIR CMM, using a (I believe) 1.5mm spherical tip probe, and they are wondering why they are getting different results than us. Their CMM guy's take is that spherical probe tips are more accurate, and that somehow using 'pre-hit' before measuring each hole makes z-axis mishits non-existent.

Also, about a dozen holes are filled with M8 PEMS, which he is also measuring on the threads with the same tip (whereas I decided to measure the PEMS around the outer diameters). These PEMS are only being held to a tolerance of TP 040, so it's not like they are 'tightly toleranced'. The thru holes are toleranced to TP's of .010-.020.

So, oracles of the CMM world, who is right? Who is wrong? What would you do in a similar situation? We need to make a single program that we can both use, but we seem to be going at this from different styles and don't agree on much. FWIW, their programmer is a guy with 20 years experience on CMM's in machining, whereas I only have about 14 years (on many different softwares), but I'm also a licensed mechanical engineer and have been involved with sheetmetal for about half of my career.

There were also other disagreements in our programming style; he believes in using only his take on a 'progressive alignment', which in his world means: Measure plane, open alignment box and level plane, then set as Z origin. Close alignment box. Measure X-axis line, open alignment box, rotate line to X+, set as Y-origin, and close alignment box. Measure Y-axis line, etc.... He claims that my 'full alignment' which I accomplish in just one alignment box session (in the order of Level, rotate, origin z, origin x, origin y) the same thing AFTER measuring all the elements is invalid.
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  • They are 'right' in using the ball and the prehits. Using a cylinder is ONLY going to be correct IF the shank of the cylinder is perfectly perpendicular to the surface (hole axis) being measured. While you can 'calibrate' the shank of the probe, there is no place in Pcdmis where you can tell Pcdmis that you are using the shank and not the tip of the probe, which is where ALL data comes from.

    As for the alignment, it will 'depend' on what the alignment is done to. If you are measuring a holding fixture that sits flat on the table, then yes, you are correct, the full alignment (manual) followed by a DCC alignment (ALWAYS do a DCC alignment after a manual alignment) is perfectly fine. If, however, the plane you are measuring is NOT flat to a machine axis, then you will get probe comp error if you don't level first before measuring a hole or a line.

    For the 'stepped' alignment he does, it does sound INCORRECT to me. Yes, measure the plane, level & origin to it. Then measure a line, but then you SHOULD level & rotate in the next alignment. Then measure the last feature, then LEVEL, ROTATE, and set origins in the last alignment. It has been proven that Pcdmis CAN SOMETIMES muck up the THEO values in a program if you do NOT have 1 alignment that locks all 6 DOF at once.

    Remember, NEVER inspect a part with just a manual alignment, always follow a manual alignment with a DCC alignment. Always.
  • in reply to Matthew D. Hoedeman
    "It has been proven that Pcdmis CAN SOMETIMES muck up the THEO values in a program if you do NOT have 1 alignment that locks all 6 DOF at once.

    Remember, NEVER inspect a part with just a manual alignment, always follow a manual alignment with a DCC alignment. Always."

    Yes, that is what our differences boil down to; He thinks doing piecemeal alignments is somehow better than addressing all 6 degrees of freedom in one single alignment. And, no, I never run with only a manual alignment on any DCC machine.
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  • in reply to Matthew D. Hoedeman
    "It has been proven that Pcdmis CAN SOMETIMES muck up the THEO values in a program if you do NOT have 1 alignment that locks all 6 DOF at once.

    Remember, NEVER inspect a part with just a manual alignment, always follow a manual alignment with a DCC alignment. Always."

    Yes, that is what our differences boil down to; He thinks doing piecemeal alignments is somehow better than addressing all 6 degrees of freedom in one single alignment. And, no, I never run with only a manual alignment on any DCC machine.
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