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Thread: true position formula

  1. #1
    On the Hunt Hawk's Avatar
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    true position formula

    i do quality control at the factory i work at and my question is when reading a blue print, sometimes there's a callout for a true position for a hole or slot, coming from datum points A/B/C

    does anyone know the formula in finding the true position

    say you have a hole that is 3.0mm on your X axis
    and 5.0mm on your Y axis
    and you have a tolerance of say .3mm for your true postion

    what would be the formula to find this ?
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  2. #2
    SG Enthusiast IRS's Avatar
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    There is always a hard point to pull off of for any master set of prints, If your looking at a detailed view it more then likely wont have the measurements you want, but will have a line some where that says detail sheet M1(or whatever) If this is the case go to sheet M1 to find the measurements to the area you are working in then go back to the detail sheet for final verification of sizes, location and direction.
    Good luck... Brian
    "edit" I R not to brite, sorry you want the formula.... sorry me bad
    but will look.
    You make it, We take it !

  3. #3
    Quote Originally Posted by Hawk
    i do quality control at the factory i work at and my question is when reading a blue print, sometimes there's a callout for a true position for a hole or slot, coming from datum points A/B/C

    does anyone know the formula in finding the true position

    say you have a hole that is 3.0mm on your X axis
    and 5.0mm on your Y axis
    and you have a tolerance of say .3mm for your true postion

    what would be the formula to find this ?
    Assuming the X and Y are basic dimensions, using the amount X and Y are out the formula as is follows. 2 * SGRT( X to the 2 + Y to the 2)

  4. #4
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    The measured location must fall within a circle with a radius of .3 mm centered at x = 3 and y = 5.

    If Ym and Xm are the measured data, the formula is:

    error = Sqrt((Ym - 5)**2 + (Xm - 3)**2), must be < .3

  5. #5
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    I am not sure of Kip's formula (**2?) I do understand Kip is using actual vs nominal, below is finding vs finding.
    This is the actual formula:
    2*sqrt(x^2 + Y^2)
    "^2" is the term squared
    Example; 2*sqrt(.0084^2 + .0020^2) = .0173 (.01727)

    This can be confirmed via this online calculator:
    http://www.engineersedge.com/calcula...sition_pop.htm

    HTH
    Last edited by QC Rick; 07-16-07 at 03:00 PM. Reason: fixed error in example

  6. #6
    Best In Show Noevo's Avatar
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    whoah, old school geeks

  7. #7
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    I was just passing by and caught this old post and thought I could help.

  8. #8
    Best In Show Noevo's Avatar
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    lol I didn't even notice the date. just thought it was kinda cool

    And Welcome, pull up a stool and stick around

  9. #9

    Question True Position CMM

    well, this thread is old and probably retired, but maybe someone out here will catch it. I am having almost the same issue as the opening thread, but with a twist. I have a true position call out to a slot to A-B-C. Slot length is 7.6mm and width is 6.6mm. Now the twisty part, I actually have 2 TP call outs on the print, 1 to each of the dimensions to .5mm A-B-C. How do I TP a dimension on a CMM.
    thx much.
    Last edited by GeekyEasyRider; 10-24-08 at 09:39 AM. Reason: wrong dimension

  10. #10
    Certified SG Addict CableDude's Avatar
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    Math forum please

  11. #11

    Cool

    not looking for the formula to calculate True position, i got that. I just want to know if its possible to call out TP on a "dimension". How do i tell my cmm to go to a number?

  12. #12
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    Quote Originally Posted by GeekyEasyRider View Post
    not looking for the formula to calculate True position, i got that. I just want to know if its possible to call out TP on a "dimension". How do i tell my cmm to go to a number?
    Passing by again.

    Yes this is a legal callout, the TP is measured from the centroid of the width (a dimension) and again centroid of the length (a dimension).
    The centroid of each may or may not fall within the same exact number but by perfect nominals it does share the same centroid.

  13. #13
    Certified SG Addict CableDude's Avatar
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  14. #14
    Quote Originally Posted by QC Rick View Post
    I am not sure of Kip's formula (**2?) I do understand Kip is using actual vs nominal, below is finding vs finding.
    This is the actual formula:
    2*sqrt(x^2 + Y^2)
    "^2" is the term squared
    Example; 2*sqrt(.0084^2 + .0020^2) = .0173 (.01727)

    This can be confirmed via this online calculator:
    http://www.engineersedge.com/calcula...sition_pop.htm

    HTH
    Hello QC Rick, I understand how to calculate the true position as you explained above, but how does MMC factor in to that? Is there an additional calculation you have to make to get true position @ MMC? Thanks.

  15. #15
    Resident Rodent Randy's Avatar
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    I was going to post a link to that thread, but the SG search results for "bullsh|t" were too numerous

    sometimes you have to think outside the box to get inside the box .

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