Parallel Straightedge Build
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  1. #1
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    Default Parallel Straightedge Build

    I have a 18" cast iron level body I am going to use as a parallel straightedge in a measurement tool. It will be suspended vertically so sag will not be a problem. Measurements will be with either a test indicator or dial so no large lateral forces. Hopefully that covers most of the naysayers inputs. I have surface ground the two faces to within about 3 tenths and was going to scrape it to as close as I can get then lap for final accuracy. I am now tempted to use adhesive to attach shim stock (waterjet cut .03" spring steel?) to the parallel faces then lap them to final accuracy instead.
    Easy to regularly check it for parallel on surface plate with my Mahr gage head..... Easier to re-lap the bare cast iron than the shim stock if it needs rework.....
    I would appreciate everyone's input/pro's/cons. Suggested adhesive? Differential expansion issues? Etc. Etc.
    Thanks.

  2. #2
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    1st questions I would ask are how stable & rigid is the adhesive layer, and what is the differential thermal expansion of the shim/adhesive/iron sandwich going to do?

  3. #3
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    Gernoff, What is the purpose of the spring steel? Wear resistance?

    Charles

  4. #4
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    No actual need for the strips. Just looking for a consensus. Surely it will be far easier to lap in the bare cast iron to final accuracy. Then if it creeps etc. for any reason it will be easiest to bring back into accuracy. I am shooting for a sub tenth over the 18" length. I know, I know, virtually impossible w/o lab conditions, etc. But I intend to map it on a 1.5" step distance with a 1/10 second accuracy autocollimator. Honestly ought to probably use a solid piece of durabar well annealed etc. But I scraped one of these levels in to within about a tenth (DTI on grade B surface plate) 6 months ago. Still measures the same and prints just as it did at the start. Considering how thin the webs are etc. I have definitely given up on the idea of the steel strips.

  5. #5
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    I have found a great super glue a gun-smith (PM Member ) told me about. He uses it to glue sights on rifles. Loc-Tite 380 Black Max. Be sure to wear rubber gloves when using it.

    I guess I am getting old as I can't visualize what you're making. A picture or sketch would be handy. Rich


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