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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!!!
  • Start by doing the courses offered through this site, start at the beginning Try to get into a class (Hexagon) or better yet see if the company will pony up to bring in a consultant / contractor for 1 on 1, if you do this put a big "DO Not Disturb" sign up and honor it. A circle can be used in an alignment, but a circle is a 2D feature, is work plane dependent and for alignment purposes you are using the center point (location) of that circle as the element used in the alignment. there are times when a datum simulator makes sense and when it does not, that will come with experience and practicality of what your trying to measure, correlation of results is huge to having bye in on the CMM accuracy and results. If you have some experience here great, if you do not find the crotchety old grey haired dude that is the toolmaker or reasonable facsimile and find out what kind of coffee he likes and plan on buying him a few dozen and hope he eventually becomes willing to share some "good fundamentals" and then some of his wisdom. Most people on this forum started where you are today, enjoy a chuckle, try to find what works for you and gives you results that can be repeated
  • My take on this is
    1. In a normal alignment the minimum requirements for your level, rotate, and origin are
    Level-3, Rotate-2, and Origin-1.
    You can use circles to establish your rotate, but you will need two circles. Imagine a flat part with two holes. If you pin one of those holes you can still rotate the part freely on a single pivot but if you pin both holes, the part will be secured and unable to rotate.

    2. I think it matters what you mean by Datum Simulator. Simulating the datum does “reduce error” within the datum feature but it should not help the location of a feature unless you have allowance for datum shift (bonus tolerance on a datum). The only times I have ever simulated a datum is when the datum is too small to take hits on or when my available fixturing does not allow me to take hits on that feature (Flat part with datum -A- facing down). If the feature is available to take hits on then I do, 100% of the time. If the person training you is simulating more datums than he is actually checking either you are in very special work that probably meets one of the conditions that I listed or you have a programmer that might be trying to cheat parts in.
    P.S. as I stated before I have simulated a datum before so I will not cast any stones, If your programmer is doing something unethical that is for you to decide because I don’t work close enough to have an opinion.

    3. If you have the means to go to the hexagon school it is a good option but there is only so much you can learn in a few weeks of classroom training. My best advice is to stay on the user forum and when you have questions feel free to post about it and ask the other programmers questions as well.

    Hope this helps! Good luck!
  • in reply to kingsld1
    This is very good advice and it's free! You can search for your interest or question and view as many times as needed to "get it". Good luck!
  • in reply to Darius11
    "at some point you will come across a time when a datum simulator is the way to measure a feature"
    I am using "datum simulator" once in a year, when I can't access datum feature and it working without high expectations.
    "and get good results"
    I like word "good" it is like to say "good guy". Could we say "right guy" ? I would invite good guy to pub, and right guy hire to do job. Speaking about parts I would buy right part and if I can't get right part I would buy good part. I have years of experience in quality and I can tell you, machinists are very keen to see good parts, Inspectors are keen to see wrong/right part. Good part not necessary meeting requirements (fit for purpose, good because could be fitted, good because only little bit out of requirements etc.) right part must meet requirements, otherwise it is wrong part.


  • Good evening everyone!
    Hi!!

    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. Welcome! I started my CMM programming career in a similar fashion 10 years ago. It get easier with time. You're in the right place to get help.

    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. Hexagon is holding courses. Suggest you look into it. Going to Hexagon has been better then the vast majority of my college courses. .

    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.
    When you're writing a CMM program, you're using rules of geometry to physically define where a part is for a machine. The machine has no brain. It only understands the mathematic implications of the commands that you tell it. PC DMIS will understand that a circle is an ORIGIN (imagine yourself standing at the center of a circle.. you can because that has definite location!) A circle CAN'T control rotation (imagine yourself standing at the center of that circle again..you know where the center is but it can still spin!) Telling PCD a circle is an origin makes sense but telling the software that it controls rotation is mathematically illogical. One circle rotated TO another will force the software to view that as working with a LINE..but thats a lesson for another day. It is time to start thinking in the TWO AND THREE DIMENSIONAL worlds. You're a programmer now! We REALLY need you to understand the SIX DEGREES OF FREEDOM and how making an alignment in PC DMIS works with those rules.

    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? I work in aircraft. Everything is regulated. For me, Datum Simulators are an acceptable method of measuring a part as long as is it is called out by the design authority of the part itself. I have some parts where -A- is a casting and that face bolts to an inspection fixture. Prior to measuring the part itself, the operator puts the fixture on the table and allows the CMM to measure it and create a plane that I use as my Datum -A- simulator. As ling as the APPROPRIATE people are telling you to use a Datum Simulator, it is a widely used industry norm and in a lot of cases WILL improve your results.

    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? Legit training at Hex is the best. Followed by working directly along side a senior programmer that wants to help you. Third best option..you're already here. Hope I helped, feel free to reach out if you ever need anything.

    Thanks for any help and I’m sorry this post is super lengthy!!!
  • Ok, I will put my two cents into the fray. First, there are lots of opinions on how to do something, as long as the results are correct at the end, that is what matters. Now, do you use prints? Do you know how to read a print? I am not trying to be insulting, but there are many cmm people that do not know how to read prints. Note your starting points, datums A, B, C, this most likely will be your starting points as many have mentioned above. Alignments are the start of any program, get it wrong, the rest follows. Program to the print and cad if available, if someone tells you different, get it in writing, you will be thrown under the bus a few times until you learn. I try to measure parts on the part fixture as a standard, if you have part fixtures, use them. As mentioned, parts are not perfect as cad, when you measure a part in freestate, that error can be introduced into the measured features. If you question a result, check it old school, height gage and or other measuring tools, all cmm operators should know how to use the tools. (showing my age) You will always learn a tip or tidbit if you pay attention, take notes, keep a folder with info, web links, what you might hear today may not be needed, but there's always tomorrow it will be needed. Good luck and always enjoy life as a programmer.
  • 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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