Granite as Simulated Datum Opinions

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  • Granite as Simulated Datum Opinions

    Hey all!

    I have an engineer that would like me program using the CMM granite as a simulated datum. The part will not be restrained. It's been requested to measure the granite with an unspecified probe. And use that measured granite feature(plane) to dimension other measured features(true position). The granite is not calibrated and has been severely abused. How do you guys feel about that?
    Last edited by A_Shomshor; 11-14-2017, 04:32 PM.

  • #2
    It really depends on how the part is made, what callouts are relative to that datum, and the allowable tolerances...
    -If you have a part that ends up looking like a potato chip on occasion, and has a tolerance of .003", you won't have any chance of passing a GageRR by simulating the datum.
    -If you have a part that is almost dead-nuts flat by design, and tolerances are .010 or higher, simulating the datum wouldn't induce nearly enough variation to fail a GageRR.

    Other factors are obvious: outliers like dings or chips in your granite, the ability to keep the part from moving while probing (don't know the size or weight of your products).
    Last edited by louisd; 11-14-2017, 03:35 PM. Reason: added otehr factors

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    • A_Shomshor
      A_Shomshor commented
      Editing a comment
      It will be multiple parts with multiple features and FCF's using the simulated datum. They are not extremely flat and wouldn't come close to passing RR.

  • #3
    Terrible idea. Is your quality system registered? Pull out the requirement that all gages be calibrated.

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    • A_Shomshor
      A_Shomshor commented
      Editing a comment
      They are aware. Sighting that the calibration is done when I measure it. No procedure/standard for a nonconformance either.
      Last edited by A_Shomshor; 11-14-2017, 03:51 PM. Reason: added explanation

    • RandomJerk
      RandomJerk commented
      Editing a comment
      I personally wouldn't do it. If you're forced to do it, make sure you report the flatness of the granite each time you run it, discard the report if it's above a certain threshold.

      Any chance the parts are small enough you can get a small, calibrated surface plate to put on the granite? How about a fixture? If they're gonna make you eat a turd sandwich they can at least offer some mustard.

    • A_Shomshor
      A_Shomshor commented
      Editing a comment
      I program with calibrated fixtures on a regular basis. I have always programmed with fixtures in these scenarios.

  • #4
    How do measure on a granite that's not calibrated? I agree with RandomJerk....TERRIBLE IDEA!
    Darroll

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    • A_Shomshor
      A_Shomshor commented
      Editing a comment
      Much appreciated.

  • #5
    Just a thought :
    Do you remember why the flat datums are planes, tangent on the free side of the material, which minimizes the max gap (or which are balanced depending on the curvature) ?
    It's just because, long time before the first CMM, the control was done on a granit... And there were no hesitation between algorithmes, least square or not

    Using the granit as a datum is not necesseraly a good idea, nor a bad, it just depends on the granit and on the part...
    I repeat, it's just a thought !

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    • A_Shomshor
      A_Shomshor commented
      Editing a comment
      Thank you.

  • #6
    If I need to use the granite as a datum plane (and I have on a few occasions) I put a pause in the program, instruct the operator to place a 1-2-3 block under where I have positioned the probe, continue the program, probe the block, another pause to instruct to remove the block, then rinse/repeat for however many points I need on the granite (I usually use 3 points). Then I just translate down 1".

    This way you avoid the divots that all CMM granite blocks end up with.

    It's kind of a pain, but it works.
    Lately, it occurs to me
    What a long, strange trip it's been.

    2017 R1 (Offline programming)

    Comment


    • A_Shomshor
      A_Shomshor commented
      Editing a comment
      Thanks. My blocks are "calibrate per use" with no procedure for a parallelism/flatness nonconformance.

    • John Riggins
      John Riggins commented
      Editing a comment
      Well, you could use a jo-block.

  • #7
    Place the friggin' part on the CMM, plane the granite (avoid craters) , output the flatness, print results, compare on a calibrated granite and inspect the old skool way. If it matches the CMM output...put your stamp on it. The CMM is designed to do what our fathers did on the stone (well most of it). A good competent Inspector could confidently call it good or bad CMM or not IMNSHO. CMM is fast and accurate (depending on the programmer) you can also be accurate (but not so fast). If my programs don't match the granite (within reason) I go back to the CMM to debug.

    Focus! You're in a money making business, if that engineer is harassing you get your higher up and leave it up to him to decide what should be expected of you AND email, email, email can't say that enough. Shady people can only get away with crap for so long. Keep the parts moving. 2 departments in companies Revenue and Cost centers. IF you're in revenue center, the goal is to shovel as much money into company. IF you're in a cost center SAVE as much money for the company, move the part to scrap or move it forward in production.

    my 2 pesos
    Last edited by Kp61dude!; 11-14-2017, 08:45 PM.
    PcDmis 2015.1 SP10 CAD++
    Global 7-10-7 DC800S

    Comment


    • A_Shomshor
      A_Shomshor commented
      Editing a comment
      Thank you.

    • newaircrafthomebuilder
      Editing a comment
      I will have to kindly disagree with this philosophy. I consider quality to be the check in the system such as the Supreme Court is supposed to be the check system in the US government. While every other department in the company is asking to get this out on time or get this done at a lower cost the quality department is there to ask is this right per whatever rules we are using? If you allow group think of getting the parts out as fast as possible or making it as cheap as possible, then you will get burned as a company. Quality is there as a check in the system not a rubber stamping theater operation to make it appear the company cares about themselves or their customer. That is why I'm a firm believer in not putting quality in the manufacturing department on the company tree as it does not give quality the anonymity it needs to function as necessary for the company.

  • #8
    As others have said it depends. It depends on the tolerances you're working to and the quality of the granite.

    I've heard people say many times 'Oh you can't use the granite as it;s not calibrated'. However using a CMM to measure a plane as a datum plane isn't perfect either. You won't end up with the same exact plane as you would placing the part on a perfectly flat surface. (i.e. you cannot guarantee hitting the three high-points no matter how many hits you take). But we'll happily accept the measured plane and use it as a datum.

    You do your best to measure the part with the tools you have - if you're aware it's not perfect cover your **** (in writing).
    Automettech - Automated Metrology Technology

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    • A_Shomshor
      A_Shomshor commented
      Editing a comment
      Thank you.

  • #9
    In the real world: Depending on your tolerances, you can get repeatably good data. I have done this in the past & have been just fine.

    In the world of quality management systems such as ISO 9001, AS9100, ISO 13485 and etc: No. The granite of a CMM IS NOT a calibrated instrument, it is a table. Using un-calibrated instruments to accept product is a no-go. If you get permission from your customer IN WRITING...this is a different story. Everything is all about covering your behind.

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    • A_Shomshor
      A_Shomshor commented
      Editing a comment
      Thank you.

    • DAN_M
      DAN_M commented
      Editing a comment
      No problem. Goodluck!

  • #10
    When I am in need of a datum simulator I use matched V-Blocks or parallel bars to support the parts.
    Some of my parts have features underneath therefore the workpiece has to be elevated.
    I never hit the granite.
    sigpicIt's corona time!
    737 Xcel Cad++ v2009MR1....SE HABLA ESPAÑOL

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    • A_Shomshor
      A_Shomshor commented
      Editing a comment
      Thank you.

  • #11
    Originally posted by NEWAIRCRAFTHOMEBUILDER
    I will have to kindly disagree with this philosophy. I consider quality to be the check in the system such as the Supreme Court is supposed to be the check system in the US government. While every other department in the company is asking to get this out on time or get this done at a lower cost the quality department is there to ask is this right per whatever rules we are using? If you allow group think of getting the parts out as fast as possible or making it as cheap as possible, then you will get burned as a company. Quality is there as a check in the system not a rubber stamping theater operation to make it appear the company cares about themselves or their customer. That is why I'm a firm believer in not putting quality in the manufacturing department on the company tree as it does not give quality the anonymity it needs to function as necessary for the company.
    No worries, no tears over here. You might of taken it over way too far to the other side of the spectrum but I didn't do a good job of explaining myself either. DO NOT RUBBER STAMP ANYTHING WITHOUT FIRST FEELING CONFIDENT ABOUT THE PART! (by bumping it up against every call out/ spec on the print/contract/etc.)

    I've spent a lot of time chasing the money around SoCal and as a result have been to over 10 different companies in just a few years. I've seen a bunch. I've seen very dishonest inspectors that if and when they have personal problems with others who are presenting a part will REJECT GOOD PARTS just to make the person feel bad. My point is simple, don't make it an act of congress trying to pass a bill, they sit on their behind and waste MANY taxpayers money. What I'm saying is don't spend too much time politrickin' with the engineer get your supervisor to do that for you while you figure out what IS best inspection method and if the part really is GOOD or BAD. Don't get punked around! nor punk others because you feel like the judge.

    Don't do anything dishonest or something that makes you feel uncomfortable. Do the RIGHT thing but do it as if it's your company and hustle to better your department/company and to rake in more cash, the hard working way, so there can be more toys (for some, to make payroll and keep the lights on maybe). Some inspectors like to reinvent the wheel every single time they get the same simple part and loose focus, if it takes you 3hrs to do a 30min inspection uh you're probably farting around instead of working. Some are very hard workers and not lazy but loose focus others are plain ol' LAZY! I've supervised a crew of 40+ inspectors in a very shady environment where QC dept was hated by everyone from the Owner to the Janitor. All it took was that one lazy/careless/or what ever inspector that did X wrong and bam just like that our department credit down the drain. The hard workers didn't get raises bonuses etc... but wouldn't complain and kept working hard. Most wouldn't stick around...they would move on to better bigger things.


    I'm not slapping a label on the OP I don't know him/her. No pun intended. I don't know the company environment nor the situation. All I'm saying is that, if it applies to you, stay focused as an inspector do the right thing and learn as much to do more quickly without cutting corners. If it's good PASS it and if it's not FAIL it, don't sit on it to collect dust.

    Keep it pushing,
    KP
    Last edited by Kp61dude!; 11-15-2017, 01:00 PM.
    PcDmis 2015.1 SP10 CAD++
    Global 7-10-7 DC800S

    Comment


    • A_Shomshor
      A_Shomshor commented
      Editing a comment
      Thanks again.

  • #12
    About granite quality :
    When I measure a plane on the granite (not "calibrated" one, just the cmm table !), I never get more than 2 or 3 µm of flatness, whatever the size or the number of points, ttp or scan...( I clean it before measurement !)
    And I never know if those µm come from the real flatness or the probe !

    Another time, just a thought !

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    • A_Shomshor
      A_Shomshor commented
      Editing a comment
      Ok. Thanks again.
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