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Is there a way to auto re-compute a Best Fit 3D (BF3D) Alignment?

CMM2020
Hi everyone,

I'm having a bit of an issue with regards to doing a Best Fit alignment to align a fixture, specifically the fixture datums (4-Way, 2-Way, A-Nets).

I like to start off my alignment with Iterative first off of the tooling bushings built into the fixture base,
Then I proceed to measure the datums in DCC, then I will proceed to create another Iterative alignment off of the datums if possible,
else I will do a Best Fit 3D using the datums and save my alignment to be recalled in another program.

I'm noticing that after I run the program the datums are sometimes out of spec. after BF3D,
taking a closer look I can see my Best Fit alignment needs to be re-computed! (it's even lost it's chosen directions).

Is there a way to set the Best Fit alignment to auto compute every time I run the program,
or is it rule of thumb that after performing an Iterative in DCC, that the best fit alignment always needs manual intervention?

My program is basically this:

1. Pick up 3 tooling bushings manually
2. Iterative pick up off of the same 3 tooling bushings using DCC
3. Measure datums in DCC (4-Way / 2-Way / A-Nets)
4. Create BF3D off of the datums
5. Re-measure datums to make sure they are in spec. within +/- 0.05 mm

Thanks for any help you guys can provide,
I don't want to have to re-compute manually every time I run my external alignment program.

- Pete

  • if you can do an iterative alignment of the nets and pins (locators) there is no need to do a best fit. The iterative IS a 'best fit' alignment that you can make re-measure to refine the alignment. No need for a best fit after. Simply report the datums AFTER the iterative alignment.
  • The BF3D should automatically compute each time the measuring routine is run. Something is wrong if you need to manually edit the alignment each time.

    I can't think of any reasons you should have problems other than to make sure you do NOT have the Iterate and repierce CAD option checked. I've been told that will cause issues like the ones you are describing.
  • Sounds like automotive work, if so never use Best Fit establishing datums on a fixture / gage. T-Balls / Bushings then A, B, C is the ONLY way to do it correctly!
  • in reply to Tater
    You are correct sir.

    How would you align a fixture, just looking for new ideas.

    I usually always pick up off of the tooling bushings using iterative alignment but then I find that when I measure the datums (4-Way / 2-Way / A-Nets / and sometimes D-Nets on a different plane) they are out more than 0.05 mm , some show .100+ deviation. When this happens I will do a Best Fit 3D off of the datums to get everything tight and in spec.

    Ideally I want to use Iterative exclusively but I'm having trouble aligning when I have to use nets and positional locating pins that are not exactly built square to grid.
    Even if they are exactly square to grid sometimes the Iterative does not want to work, especially if the datums are on different planes (example D-Nets),

    Thank you for any tips you can provide.
  • When establishing Datums on a "Fixture" or "Gage" that has Tooling Balls or Bushings, they are there for REFERENCE. Always remember the nets and pins simulate how the parts interface with the assembly
    of the vehicle. I always TRY to get the the net pads and the pins as close as possible to nominal . If the fixture is not good enough, get it fixed, with an agreed tolerance and proceed from there. Always show
    the datums in your report, that shows complete transparency to customer. Just make sure you have a good "foundation" for your measurements. Using a Best-Fit will shift datums and give false results!!!

    If someone wants you to Best-Fit, get the order in an e-mail or some paper trail to CYOA!!
  • in reply to Tater
    Tater,

    Say you've successfully established an Iterative alignment off of the 3 tooling bushings on a fixture,
    then when you go to measure the 4-Way / 2-Way / A-Nets you notice they are not within spec. but slightly out beyond +/- 0.05 mm (tolerance).

    How would you proceed to get the datums into spec.? Would you perform another iterative alignment but this time using the datum features? Or Would you update the nominals on the tooling bushings to "float/shift" the datums into spec?

    I'm trying to find the best iterative practice for solidifying a solid repeatable alignment.

    Thank you for any help in this matter, I greatly appreciate you sharing your wisdom with me.
  • Establish a new iterative alignment using the A-Nets (Level & Origin), B & C (align & Origin). The Tooling Balls / Bushings are REFERENCE ONLY!!
    To put things into perspective .100mm is approximately the diameter of a hair. These are CAR parts, if you are in sheet metal the surface profile default tolerance is 2.0mm and trim edges profile of 3.0mm along
    with holes having a True Position of 1.5mm. All this should be explained on the GD&T ( or Print as I call it ).
  • in reply to Tater
    what OEM are you working for? I'm automotive, and I see mostly profile 1.0 for surface, 1.5 for trim, and positions of 0.25 to 1.0 (currently I have several parts that surf & trim are prof 1.0 with postions of 0.1 (yes, 0.1mm positions)
  • We do welded assemblies for all. If you are in stamping, I agree it needs to be tighter tolerances for stack-up at assembly.
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