Why do you need fixtures. Just use iterative alignments!

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  • Why do you need fixtures. Just use iterative alignments!

    My group does not want to make fixtures for parts to be checked. THEY tell ME that using the iterative alignment eliminates the need for fixturing.

    Any help delivering a clever response?


    I haven't been on for awhile as I was told that this website was not approved by management.

  • #2
    You do not need fixtures to measure parts.

    However, fixtures will do the following for you (plus probably more)
    • Give a repeatable location for placing parts so many alignment steps only need to be done once rather than everytime you execute.
    • Can simulate a datum when it may not be acessable.
    • Save time on complex set ups
    • Keep the part stable
    • keep the part orientated in a manner that prevents shanking out


    I'm sure the list goes on and on but they are not mandatory, they will make life easier and CMM time more efficient.
    <internet bumper sticker goes here>

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    • #3
      Originally posted by craiger_ny View Post
      You do not need fixtures to measure parts.

      However, fixtures will do the following for you (plus probably more)
      • Give a repeatable location for placing parts so many alignment steps only need to be done once rather than everytime you execute.
      • Can simulate a datum when it may not be acessable.
      • Save time on complex set ups
      • Keep the part stable
      • keep the part orientated in a manner that prevents shanking out

      I'm sure the list goes on and on but they are not mandatory, they will make life easier and CMM time more efficient.
      Excellent summation Craig.
      sigpic

      James Mannes

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      • #4
        Having fixturing allows a quicker set up time.

        If you have more than one part, ummm say ten, the that would be ten set ups, because you have a very small chance of setting the parts in the same exact position.

        With fixturing it allows you to position each part in the same position.

        Less set up time = higher productivity.

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        • #5
          Fixtuing is most often used to present the part to the CMM in a manner that theCMM can obtain the most and the most repeatable measurments. AND/OR to hold the part in a restrained state.
          If you can glue the part to the same spot on the table in the same orentation and reach every thing you need to your fine without a fixture.

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          • #6
            Originally posted by Fraen1 View Post
            Having fixturing allows a quicker set up time.

            If you have more than one part, ummm say ten, the that would be ten set ups, because you have a very small chance of setting the parts in the same exact position.

            With fixturing it allows you to position each part in the same position.

            Less set up time = higher productivity.
            And higher productivity means more money in the mans pocket and thats what's important.
            sigpic.....Its called golf because all the other 4 letter words were taken

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            • #7
              Cmms are not the only place fixtures are used. Yes we use them too, but most shop floor checks are done on fixtures. If you have a very thin part that even gravity deforms you have to have a fixture, nothing else will do.

              Iterative alignments are great for us, personally I love them, but what do the guys on the floor use for checking parts?

              Bill

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              • #8
                An iterative alignment is very useful in determining the output of a process.

                A composite part layed up on a bond jig is a good example. The iterative alignment will align the part as best it can. Some thin wall composite and sheet metal parts can achieve a solution but will fail contour measurements many times because the thin walled parts are not pinned to datum points from which the contour tolerance is derived. One thing you can do is actually look at the variation of selected common points from part to part and use the ranges and standard deviations of those points to look at the capability of the process. If that number is more than one sixth of your part tolerance, you have a problem if you consider the process of manufacturing that part to be normaly distributed.

                I use iterative alignments a lot on tools but I also measure hard tooling points to get my final alignment because without doing that, I cannot provide actual tooling ball measurements to the shop so the tools can be reworked later should an engineering change come up.

                Most of the time when I measure contour, the shop will ask me two questions:

                1) Is the tool within tolerance for contour?
                2) Is the contour correct in aircraft coordinates?

                They are two different things and I use more than one approach to get what the shop wants.

                HTH
                Hilton Roberts

                "Carpe Cerveza"

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                • #9
                  Fixtures are good for some parts not good for others. It all depends on the part.

                  IF you are going to use fixtures, make sure they are designed and constructed well. When you get a new fixture, check it out. Make sure the pins are where they should be. If the fixture simulates datums (not just hold the part) make sure it is repeatable. Measure a part on the fixture. Remove the part, put it back on and measure it again. Do that quite a few times and see if the part changes with each time.

                  They built me some nice looking fixtures. They hold the parts well. They simulate the datums. Problem is that the same feature on the same part will measure as much as .015-inch different (.005 tol) from one run to the next. Not repeatable at all. The fixtures are junk and all they do is take up space and collect dust.

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                  • #10
                    I use bench centers bolted to the table and a suburban tool as my 2 fixtures. Operators can walk in and check a part without ever having to touch the wrist control. As long as the centers and sub do not get moved no one ever has to do a manual alignment. When time is money, manual alignments are a waste.
                    I talk dirty to my cmm. Justn

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                    • #11
                      I'd love to have more fixtures for parts, but since 95% of my work is first article and small sample sizes....that won't happen.

                      Repeatability is key for fixtures. Prints (since CAD models are taboo) which are intelligently drawn by someone other than a college co-op with data that can be used is even more important. Usually we have neither....

                      Tom H>

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                      • #12
                        Originally posted by craiger_ny View Post
                        You do not need fixtures to measure parts.

                        However, fixtures will do the following for you (plus probably more)
                        • Give a repeatable location for placing parts so many alignment steps only need to be done once rather than everytime you execute.
                        • Can simulate a datum when it may not be acessable.
                        • Save time on complex set ups
                        • Keep the part stable
                        • keep the part orientated in a manner that prevents shanking out


                        I'm sure the list goes on and on but they are not mandatory, they will make life easier and CMM time more efficient.
                        Great list.
                        Add this, which is sort of like #2
                        Fixtures can simluate datums that are impossible to reproduce with CMM touches such as flat or radiused locater. In this case it is almost mandatory.
                        Links to my utilities for PCDMIS

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                        • #13
                          Yeah, what they said. I would definitely recommend fixtures for production inspection. We couldn't live without them. The operators never have to do a manual alignment. Our operators load their parts, select their program, run it and walk away to load / unload their CNCs. They come back in 10-15 minutes and they have a printout waiting.

                          We do gauge R&R studies on our fixtures before we release them to the production floor. This has saved our butts a few times.

                          This may go into a slight hijack...

                          I've measured one-offs or product engineering parts that I thought that I would never see again. Inevitably, months or years later someone will come up to me and ask me to measure another one. I save all my programs, so that's never an issue, but usually I have forgotten how I had it set up and find myself wishing that I had used a fixture. Being that having a $5-10K fixture made would be impractical, I've found a solution that works for me. I take digital photos of the set up and create an externalcommand/display to pull those pictures up if I need them. Plus, if something happens and I can't get to the parts, another one of the programmers can and this helps them out.

                          Yeah, it's pretty much common sense, but thought I would throw that in in case you don't go the fixture route.
                          PCDMIS 3.7 MR3, 2010

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