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A Couple Newbie CMM Questions

zoebear
Good evening everyone!

I’m extremely new to CMMs, GD&T, and pretty much anything to do with this whole world. (>4 months) My employer found themselves in a tight spot when our old CMM Operator/Programmer left for another company. The previous employee was the only one who /really/ knew how to run a CMM and program it - of course there was one “back up” who got very small amounts of training.

So I’m interested in learning and I’ve gotten the job title and was trained on what could be - due to the current situation in the real world - no one is having in class training so I’m stuck with online videos and tutorials and this lovely forum.

I have a couple of random questions I’ve compiled since getting my job - they will be newbie questions and they will probably make people mad but I checked search and couldn’t find what I’m looking for.

1. I’ve noticed that an alignment shouldn’t have a circle as a rotation. Why is this? I have noticed nearly every single program made at my company has a circle - or even circle(s) as the rotational alignment.

2. What is the benefits of having a datum simulator? I understand a simulator is the perfect datum and that it reduces chances of positional tolerances to be out - but why exactly would I use them instead of taking hits from the actual physical part? Would it help if the part were (hypothetically) .005 in out?

3. What is the best method to learning programming for PC-DMIS without any type of real training or mentor? I have watched a lot of videos and I check the forum every day just for random knowledge. Are there any book, video, website suggestions anyone may have?

Thanks for any help and I’m sorry this post is super lengthy!!!
Parents
  • 1. A circle can’t be used for rotation because there’s nothing to grab hold of. Use a plane, a line or a cylinder.

    2. Explain Datum Simulator? Do you mean using a CAD model to program with?

    3. Please get the training. It’s not that expensive and it’s much better than watching a video or asking one of us. We will help you all we can but only when we have time.
Reply
  • 1. A circle can’t be used for rotation because there’s nothing to grab hold of. Use a plane, a line or a cylinder.

    2. Explain Datum Simulator? Do you mean using a CAD model to program with?

    3. Please get the training. It’s not that expensive and it’s much better than watching a video or asking one of us. We will help you all we can but only when we have time.
Children
  • in reply to Darrollh
    Thanks for the feedback! Would that mean any program made with a circle in the alignment is kinda garbage??

    I do everything with a CAD model but most of our programs just take datum hits on the actual part. I have an engineer telling me it would be more accurate if I were to use, for example, surface plates - as datum simulators and bank the part into those and take hits from that instead of the actual part. Which I feel like doesn’t make sense because wouldn’t that just cause extra error? I know datum simulators are “perfect” because our parts have a low and high point and who’s to say where those are - but I guess I just don’t understand how that would really make a significant difference.
  • in reply to Darrollh
    If the alignment is only using one circle then yes that is wrong, if it is asking to rotate using two circles then that is perfectly OK as it is the same as constructing a line between the circles and rotating to that.
  • in reply to Darrollh
    Taking hits on the granite would be a DATUM Simulator. Datums are supposed to be " perfect ". Just how you would place your part on the granite and call the granite " 0.000" when checking a part with a height stand ?

    Rotating to a circle only works with 2 circle as you can make a line from the 2 centroids.
  • in reply to Darrollh
    @Schlag

    In 99% of cases, using the CMM's granite table as a simulator is ok.

    In aerospace, you can't do it. The table is not a calibrated surface plate nor is the table calibrated along with the CMM. From a metrology standpoint, it is NOT a measurement instrument. I was caught by an aerospace auditor early on in my career with this. Have had to write up a process, as an auditor, over this as well. (supplier was probing their table to be a simulator for the bottom of their part, table had a flat spot that threw the flatness of the plane off, CMM accepted linear dimensions back to that face that were actually bad!!)
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