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new version of Caliper Accuracy article available

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  • speedsport
    OK tdmidget, don't hold back, tell us what you really think.

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  • ckelloug

    In my read of Rick's article, I see that he did some experiments trying to determine the ultimate accuracy and repeatability of the harbor freight calipers. I'm by no means one of the board's old timers but I know it is of great interest here to know whether inexpensive imported instruments are good enough for home shop use.

    I do not know Rick personally but I know he said somewhere that he is a retired electrical engineer. While few here are metrologists, many are quite interested in the ultimate accuracy of instruments. I don't think anybody here would try to use calipers for a critical measurement.

    I don't think Rick's point in this was either to advocate the use of calipers beyond reasonable accuracy, or to advocate using instruments not at thermal equilibrium with their surroundings. What he did show is that the accuracy specification for HF calipers is simplified somewhat from the actual performance and that they are not temperature compensated. The points are of academic interest mainly and demonstrate that when used for caliper grade measuring applications that the HF calipers do OK. This mini-paper increases the community's understanding of calipers and provided that silly inferences surely not intended by the author aren't drawn from it, it looks useful.


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  • tdmidget
    So, Rick I'd be interested to know what qualifies you to pen this article? For the most part it shows that you don't know much about measuring or the instruments to do so.
    1. You go on at lengths about possible errors of .001 or less in an instrument that no shop working to a serious level of precision would allow for tolerances of .002 or less, and therefore inconsequential. No slide caliper should be used for measurements closer than this as that is pretty much the limit of precision for the instrument.
    2. You cool the instrument, measure with it, then cool the part and measure it. One of the most basic principles of metrology is to have the instrument and the workpiece at the same temperature. All this shows is that metal contracts when cold, no error is shown. This demontrated by the fact that you claim the same "error" each time. And again beyond the capabilities of the instrument. It goes without saying that a steel caliper measuring a steel part at the same temperature has no error since the coefficient of expansion is the same for both.
    3. You used as a standard 1-2-3 blocks which are normally .0002 oversize unless you are using Starrett or Herman Schmidt blocks. If you could afford 1-2-3 blocks approaching $300.00 each you probably would not be testing a Horror Freight caliper. In any case 1-2-3 blocks are NOT gage blocks and should never be used as such.
    I wonder if you were a manufacturer of anything, calipers, wdgets, or what have you, would want someone "testing" your product with your level of knowledge? Should anyone make a buying decision based on this?
    Last edited by tdmidget; 06-01-2007, 08:18 PM.

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  • new version of Caliper Accuracy article available

    I have learned a lot since version 1 came out. For example, the name of this article is now Accuracy and Repeatability, not Accuracy and Precision.

    There is now a section that discusses the effects of temperature on the Harbor Freight caliper.

    If you are interested, see

    As always, comments are welcome and do have an effect on future versions.

    Rick Sparber
    [email protected]
    web site: