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Thread: stated accuracy of portable CMM arm and real world usage

  1. #1
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    stated accuracy of portable CMM arm and real world usage

    My company is considering the purchase of a Romer 7520 arm (2m range) for one of our plants to use for machine setup/alignment (non-critical) and some inspection. In their literature the repeatability is quoted to be +/-.016mm and accuracy +/-.023mm and certified per B89.4.22.

    Shouldn't the true reported repeatability be .032 and for accuracy .046, for example measuring the distance between two holes? Wouldn't the uncertainty be the sum of those? I find the +/- to be a bit misleading.

    Our other plant had a demo performed and the plant manager is sold on the machine. My plant currently has a smaller manual CMM. I obtained a quote for a larger DCC CMM as we do different types of work than our sister plant. Rumor has it however, that instead of upgrading it to a newer DCC CMM, some are suggesting the Romer could replace the CMM for inspection.

    I am having deep reservations about this because we routinely measure machined details with TP of .030, concentricity .013, perp .013, diameters <.025 tolerance and so on (metric).

    In a meeting I brought up the accuracy issue. The quoted DCC CMM has a MPEe= 3.2 + L/300 accuracy and MPEp=3.2. which would suit most of our needs and provide greater throughput. The probe system is the PH20 with TP20.

    The supporters of the Romer say the machine is more accurate than the spec when measured at closer ranges which is believable, however since Romer does not offer an accuracy statement for that, how can it possibly be known what the accuracy/repeatability is throughout the measurement range? Admittedly I've never used an articulating arm and do realize the advantages of them, however some tell me they are not well suited for close tolerance measurements, and since Romer only quotes the total volumetric error, how reliable can the measurement be? I don't think it should be any less than the stated accuracy in their documentation. If they are are much more accurate in the smaller envelope, than where is the cert to prove it? What do you do, measure the feature until it falls in tolerance and accept it as good sarc/off ?

    Using the 4:1 TUR (test uncertainty ratio), I can't imagine ever trusting an arm to measuring with any confidence closer than +/-.13mm tolerance.

    Anyone out there that can give me some insight into this would be greatly appreciated as I'm afraid by purchasing this arm my plant may be making a big mistake.
    Last edited by slimething; 03-05-2014 at 08:52 PM.

  2. #2
    Administrator Kelly Bramble's Avatar
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    According to ROMER website:

    +/- 0.016mm (+/- .0006”) = Point Repeatability Test
    +/- 0.023mm (+/- .0009”) = Volumetric Accuracy Test

    I find the +/- to be a bit misleading.
    I think it is different – not wrong.

    Point Repeatability test is the reference test to determine measurement arm repeatability with ball probe. The cone is in front of the machine. Points are measured from multiple approach directions. The average point and the deviation of each point to the average center are calculated. The result is the maximum range divided by two.

    Volumetric Accuracy Test most accurately represents the reasonable expectations for machine performance in practical measuring applications since it involves measuring a certified length standard many times in several locations and orientations and compares the resultant measurements to the actual length. The Volumetric Length Accuracy Test is the most appropriate test for determining machine accuracy and repeatability. The result is the maximum deviation of the measuring distance less the theoretical length.

    The calibration test report included with the standard ROMER Arm will give you the actual performance accuracy of the particular model you have. The accuracy is unlikely to be better than what the manufacturer states or the calibration certificate.

    I do think that most CMM’s become more accurate with shorter distances, however I do not see that data in ROMER specifications. If it is not stated then the accuracy you see is the best the CMM can do..

  3. #3
    Administrator Kelly Bramble's Avatar
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    Check out the following Document on CMM Arm Accuracy.

    (FARO)

    Accuracy_definition.pdf

  4. #4
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    Quote Originally Posted by Kelly Bramble View Post
    According to ROMER website:

    +/- 0.016mm (+/- .0006”) = Point Repeatability Test
    +/- 0.023mm (+/- .0009”) = Volumetric Accuracy Test



    I think it is different – not wrong.

    Point Repeatability test is the reference test to determine measurement arm repeatability with ball probe. The cone is in front of the machine. Points are measured from multiple approach directions. The average point and the deviation of each point to the average center are calculated. The result is the maximum range divided by two.

    Volumetric Accuracy Test most accurately represents the reasonable expectations for machine performance in practical measuring applications since it involves measuring a certified length standard many times in several locations and orientations and compares the resultant measurements to the actual length. The Volumetric Length Accuracy Test is the most appropriate test for determining machine accuracy and repeatability. The result is the maximum deviation of the measuring distance less the theoretical length.

    The calibration test report included with the standard ROMER Arm will give you the actual performance accuracy of the particular model you have. The accuracy is unlikely to be better than what the manufacturer states or the calibration certificate.

    I do think that most CMM’s become more accurate with shorter distances, however I do not see that data in ROMER specifications. If it is not stated then the accuracy you see is the best the CMM can do..
    Thanks for your response.

    I find it misleading because if measuring for example the distance between two holes (round within 1 micron for the sake of argument), Hole A true value is x-100mm and Hole B true value is x+200mm from zero. Going by the Romer spec, Hole A could be measured at x-100.023 and Hole B x+100.023 for a total error of .046. Then there is the issue of the form of the hole which also contributes to error, a whole other issue. Say then the part is measured again, according to the Romer spec, repeatability is +/- .016, further adding to the uncertainty.

    Or another example could be measuring the distance between two lines. A line requires a minimum of 2 points, so a total of 4 points is used. Again, for simplicity's sake the total error can be .046 because each independent point can have a +/- .023 error. If the drawing requires those lines to be parallel within .05, or even one line to be straight within .050", how can the result be trusted?

    Just based on the accuracy statement alone, I would find it difficult to accept these AACMM for use as an inspection tool for anything less than .010" (or +/-.005" if you prefer) tolerances.

    I've talked to people whose company bought these arms thinking it would replace their aging CMM or negate the need for one and save a lot of money. It now sits in the corner unused. To me they would be excellent tools for getting ballpark figures for verifying large details or aiding in machine assembly, but to actually use to accept/reject machined details?

    There aren't a lot of published papers on AACMM, however reading this one did not leave me with warm fuzzy feelings. http://www.mdpi.com/1424-8220/13/8/10430

    Someone is telling our management these arms are the greatest thing since sliced bread. I'm trying to convince them with Romer's own data they are severely limited in accuracy/repeatability for what we measure in the inspection department. There's a reason why a AACMM is not certified to ISO 10360.
    Last edited by slimething; 03-07-2014 at 08:21 AM.

  5. #5
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    Quote Originally Posted by Kelly Bramble View Post
    Check out the following Document on CMM Arm Accuracy.

    (FARO)

    Accuracy_definition.pdf
    Interesting. Thanks. What do you get out of their statement:
    As an example in the case of a silver 8’ FaroArm we report single point +/- 2 sigma repeatability of +/-0.003”.
    The expected +/-(max-min)/2 would be approximately +/- 3 Sigma or +/- 0.0045”. Another way to express this
    Issue #: 21 Revised: 8-28-02 Page 2 of 5 f:\control\referenc\06servic\english\06ref176-003.doc
    ©1999 FARO Technologies, Inc.
    is to state the total error band for a single point of 0.009”. We chose the +/- terminology since the true value is
    at the middle of the band and therefore the greatest deviation from the true value is 1/2 of the band.
    The accuracy of a linear displacement measurement is the sum of the deviations of the single point. The
    standard deviation is the square root of the variance. Therefore, if the variance is the same for each of any two
    points, the standard deviation of the linear displacement is the square root of 2 (or 1.414) times the single point
    standard deviation. Therefore the displacement +/- 2 sigma is +/- 0.0042 and the displacement +/- 3 sigma is
    .0063” , or total band of 6 sigma .0127”.
    In my view, if the true zero is probed and zero is established by the arm, that is the first error. It would of course be +/-.023 (in the case of the Romer). Then every point taken after that would fall into the +/-.023 error band as well, however because in the first measurement point there is error, the maximum potential error must be .046. No?
    Last edited by slimething; 03-07-2014 at 10:56 AM.

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