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Precision plate protocole for part measurement!

Discussion in 'Sampling, Standards and Inspection' started by Joel Lapointe, Jan 5, 2021.

  1. Joel Lapointe

    Joel Lapointe New Member

    Jan 5, 2021
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    I saw alot of videos over YouTube that explain how to clean, lap, calibrate, manufacture a precision surface plate. However, I have some difficulty to get a general manipulation protocol for part inspection/measurments on a precision plate. Metrology training is important.

    Is it only necessary to put the part over the precision plate and begin to measure readily?

    Is there any measurment training, advices, written protocol on which I can rely?

    I'm baffled by that lack of proper info on usage of a such fundamental instrument for metrology! (it provides a datum plane)

    I know that a precision plate has a very "passive" behaviour, it stays there, well put by its own weight. The only action is : receive and stabilize the inspected part so you can provide a good datum feature plane. There is not much to say. Most normal people might say :

    - BAM! Put the part on the surface plate and proceed with measurements... nothing less, nothing more!

    However, there might be some guidelines to follow which I don't know. Some of them might be :

    - Clean the measuring instrument's bottom to ensure good mating with the datum feature surface (e.g. height micrometer, any other instrument which touches the plate)

    - Clean the part so the dimension isn't affected by contamination.

    - Place carefully, with no percussion/bump/shock, the piece as wanted on the plate. Chose a different place for each part so the wear is well distributed on the plate over the years.

    - Once the part is placed on the precision plate, let it thermally stabilize X minutes to ensure dimension repeatability... (heat from the hand is heating the part and chilling by solvant evaporation is reducing the accuracy/repeatability in measurements)

    - any other thing...
  2. Miner

    Miner Moderator Staff Member

    Jul 30, 2015
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    Greater Milwaukee USA
    You raise many valid points, which seem to be assumed today. Plus, so many are using CMMs now that the old school methods appear to be dying out. I learned hands on over the years from more experienced people.

    I found one book that may help: Metrology and Measurement. While it doesn't go into the detail that you want for the how to, it does a good job of covering the precautions in use. It also has a good section on the use of sine bars.

    I assume from the areas that you brought up that you are primarily using instruments like height gages and transfer indicators. Cleanliness of the plate, bottom surfaces of height gage/indicator stands, surfaces to be measured, etc. are important. Temperature, soak time, etc. may or may not be important. This would depend on the tolerances that you are trying to hold as well as the material's coefficient of thermal expansion. The tighter the tolerance, the more important this becomes.

    If you are using transfer indicators, read up on the concept of cosine error. These indicators are designed to work at a specific angle to the surface to be indicated.

    It is difficult to anticipate all your potential questions, but if you ask specific questions, we can probably help you.
    Atul Khandekar likes this.
  3. Gejmet

    Gejmet Member

    Jul 23, 2019
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    Hi Joel,

    The aspects you are referring to are fundamental to good metrology practice.

    This link www.npl.co.uk/resources/gpgs will give you access to many free guides you can download about many aspects of metrology and if they don't answer your question specifically they will help you to healthily question what you do and figure out a way forward. I assume you live in the US, you may even find a company out there who do virtual training under license from NPL, the advantage being that these courses are properly accredited.

    Hope this helps and good luck!
    Atul Khandekar and Miner like this.