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Linear scales...

Discussion in 'Gage Calibration and Uncertainty' started by Chris Glover, Aug 24, 2015.

  1. Chris Glover

    Chris Glover Active Member

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    (in the other world known as rulers)...

    We have many instances where a "scale" is called out as the gage used to confirm product quality. How best to "calibrate" or "verify" such an instrument?
     
  2. BradM

    BradM Moderator Staff Member

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    Hello Chris!
    Not sure if you recall the discussions we had at Elsmar on this, but they were quite lengthy and varied in opinions. :)

    So, let's dispense with the argument of whether rulers should or should not be verified, and due to your question, state that they are to be verified.

    I would say it depends on how accurate you need the ruler to measure. If you have a ruler that only needs to be 1/4" inch accurate, then a comparison to gauge blocks, or another ruler (with better resolution, that has been verified against a standard) could be used for verification.

    If you are needing your ruler verified to 2 or more places, I would suggest sending it to a competent vendor. They can verify the ruler using a profile comparator. They can verify from zero on all readings. So... 0 to 1 inch; 0 to 2 inches; 0 to 4 inches; etc.

    i have yet to see a ruler vary much (expansion; contraction), so depending on your needs/ risk/ compliance requirement, I would set the recall to 3-5 years, or only perform an initial calibration of it.
     
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  3. Chris Glover

    Chris Glover Active Member

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    Yep..I agree..there is not really much a steel rule can vary..most of the problems arise from damage and wear.

    What I have done in the past is purchase steel rules with certs (these are available for a few dollars more), set calibration frequency for one year...then at the end of that year replace them with new ones. In the long run this proved to work..and be cheaper than having them "calibrated" by an outside vendor
     
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  4. BradM

    BradM Moderator Staff Member

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    Not sure how many of these rulers you have. But what you could do...
    Is instead of disposing of them, send them off for calibration. What this does is 1) saves the transaction costs of purchasing, disposing, etc.; and 2) now you have data. If you analyze after a year calbration and none of your rulers are changing (as if... they would... :D :) ), then you can justify extending the interval to 2-3 years or longer.
     
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  5. Eric Twiname

    Eric Twiname Well-Known Member

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    Howdy all,

    I had posted my approach on the "old" thread as well...we use hundreds of rulers, so periodic recheck or even buying a "calibrated" cert cost quite a lot.

    I contacted the ruler manufacturers and they were willing to send me their QC tolerances for manufacture and end of line test.
    The resolution they use for quality acceptance is so much better than we use the rulers for that it is laughable.
    Anything that you might use a ruler for...the mfg specs are way tighter than that (many measured in microns).

    Johnson level & tool
    Starrett
    GEI International
    all were willing to provide product acceptance specs if you ask nicely.

    We "re-verify" based on visual inspection for damage or wear prior to each use (takes ~ 1 second).

    The manufacturer's logo on the scale serves as the calibration mark...no other brands are allowed on site, and no calibration stickers are placed on the scales which might impede use.

    Full defense of approach is kept on file, together with mfr specs.
    Easy, defensible, and has been adopted by some of my customers as well (as opposed to them complaining, they adopted it and saved a lot of money)...

    Note that this approach would never work for end hooks or the like.

    HTH,
     
  6. BradM

    BradM Moderator Staff Member

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    I like that, Eric! The reasonable/ rationale side of me would say that should be good.
    The "I hate being beat to death in audits" side of me :D would prefer to have something a little more concrete on file; such as a calibration certificate or something from the mfg. for that particular ruler. :)
     
  7. PaulJSmith

    PaulJSmith Well-Known Member

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    Ah, the rebirth of the ruler/scale thread. It had to happen sooner or later.

    I, personally, love Ninja's idea. Think I'll propose that for our scales.
     
  8. Eric Twiname

    Eric Twiname Well-Known Member

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    I hate being beat to death in audits too...I think if this scale topic ever reached that stage, I would remind them that I had a 24" x 2" steel ruler that I would be glad to show them repeatedly and forcefully for durability testing...o_O
     
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  9. ncwalker

    ncwalker Well-Known Member

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    Another "Clouding the issue with logic" thread ... :)
     
  10. drgnrider

    drgnrider Member

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    We had an auditor ask us this, our response was basically: Tape measures typically don't last more than 9-12 months and we have at least three different tape measures check any given part/product, (not limited to: operator, supervisor, inspector), if these agree within 1/16", then all is good. If any is off, a NEW tape measure is obtained and then any that are in error are discarded.
     
  11. Eric Twiname

    Eric Twiname Well-Known Member

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    Does this not still require the "NEW" tape measure to be calibrated/validated? It must cost quite a bit...
     
  12. RoxaneB

    RoxaneB Moderator Staff Member

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    In a previous life, we also applied the 1/16" tolerance perspective. ... but ... it was discovered during our certification audit that the 1/16" increments on the tapes only went up to a certain length and then, for some reason, the increments changed to 1/8". Unfortunately, the dimensions that the tapes were being applied to were up where the tapes were in the 1/8" increments. :oops:
     

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