Calibration houses

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  • Calibration houses

    This is not strictly a pcdmis question but it is important if you are forced to use a third party calibration house.

    This little tip will work best if you have previous calibration history on what you are sending out.

    When you send out gage block sets, take a very good look at the blocks in the best shape IE the ones less used. Make a chart of those block deviations from nominal, better yet keep the old copies of the calibration report.

    If your set comes back with the note that many blocks are being replaced because they are out of tolerance, be VERY careful about the house you are using.

    Master sets of blocks that are used to transfer readings to working sets tend to be VERY stable. If the calibration reports for blocks seldom used is changing a lot ( 3-4 millionths) you most likely have a serious problem with your calibration house. This is especially true for new or near new blocks with minmal usage. If your calibration house is turning the blocks over, it is most likely THEY do not have a set of master blocks that THEY use to transfer measurements to YOUR blocks.

    If you have history on plain cylindrical rings. Hold on to that data.

    Find the largest ring you have and preferably one that is an uncommon size.

    This will require the calibration house to make a gage block set up to measure the ring. The more blocks they have to use the better for you and more of a problem for them.

    Most houses will normalize the ring but when they have to make a gage block stack up to set their internal comparator they will impart a lot of heat into the gage block set up.

    If they do not let that heat soak out, guess what?......your large ring comes back in SMALLER than it was when it went out!!!! Why? Well when a hot gage block setup is placed between the anvils and zeroed out, the block set is "long" to the true size. The ring that is at temperature will then check small.

    The more blocks you force them to use to make the setup to measure an uncommon size, the worse the error will be.

    I have seen rings on the order of 8 inches or so shrink by as much as .0014 in. simply because the calibration house was in a hurry.

    If you do not have history on your gages, you are pretty much at the mercy of a third party calibration house. You had best believe that they make their money on volume.

    If you need to survey them, for goodness sake send someone down there who has a whole lot of bench time in a gage lab. They will know what to look for.

    For the PCDMIS community, you need to know whether the standard you are using either to set a bore gage or calibrate your tools on the machine are really what they state on the certificate.

    I hope these two tips will help.
    Last edited by Hilton Roberts; 01-31-2007, 12:26 PM.
    Hilton Roberts

    "Carpe Cerveza"

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