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  • Home shop metrology..

    Following on from a discussion on the Shop made tools topic...

    Even a home shop requires the best metrology we can manage, calipers, micrometer, various gauge blocks and of course the 'engineers' square.

    I have always understood that a square can be proved to be square (i.e. 90 degrees) but testing against two other squares which in turn could be tested against each other and that only truly square squares could past this test. This is one of the most basic metrology tests we can do in a modest home shop but maybe I am wrong?

  • #2
    The same way you can test a flat surface against 2 others to verify it/them.

    Mike

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    • #3
      One of the articles in one of the volumes of The Machinist's Bedside Reader is about making squares from basic principles. It goes through a process of such comparisons to produce as exact a square (pair of actually, the second is a byproduct of the making of the first one) with no fancy tooling. I believe that all which is required is a good straight edge. I'd have to dig it out and read it again.
      Chilliwack BC, Canada

      Comment


      • #4
        I've made a set of squares. OK, ONE square, and two objects which were square when they were first scraped.... as part of making the actual one.

        Yes, it was a pain to do, and I won't do it again without a good reason. I did it as an "apprentice exercise", self-assigned. I did cheat a bit. The classic way gets them flat as well as aligned ONLY by the comparison and scraping. I used the flat I set them on as a flat reference so that I could shorten the process, as opposed to doing both at once. Every so often I would spot them on the flat itself to keep things from going off-kilter.

        The square and the "objects"



        They were all sawed out of one CI angle plate. And I did NOT square them up vs the sides. Just the main 90 deg angle was all I wanted and all I did.
        CNC machines only go through the motions

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        • #5
          Got a new CMM at work,
          and the setup tech had a
          white ceramic right angle plate
          for setting the axis' square.
          It was soooo sexy, and square
          within some crazy amount
          with lots of zeros before the number.
          It was like the rare albino angle plate
          that most of us never see.

          --Doozer
          DZER

          Comment


          • #6
            In my humble shop a granite surface plate and a set of good machinists squares together with a set of Jo blocks serves me well.
            I don't work to microns and I'll bet almost no one else here does either. (Except Evan)
            I do this as a hobby. I can understand people who, as a hobby try to have the best possible metrology standards but can they really employ them?
            I do have precise electronic standards such as a standard cell but that's how I made my living.
            Bill
            I cut it off twice and it's still too short!

            Comment


            • #7
              I don't even have the jo blocks in mine. I've got an import surface plate and I'd love to have a right angle for the plate which I could know is square. There's a thread running currently about a guy that made a cylinder square. I might just have a go at making one myself since as such things go it's relatively easy.

              I suspect that many of us work to "good 'nuff". But now and then we need to up the game for something specific. And when we do that it's nice to know we can trust our metrology gear even if it is limited.
              Chilliwack BC, Canada

              Comment


              • #8
                Originally posted by Doozer View Post
                Got a new CMM at work,
                and the setup tech had a
                white ceramic right angle plate
                for setting the axis' square.
                It was soooo sexy, and square
                within some crazy amount
                with lots of zeros before the number.
                It was like the rare albino angle plate
                that most of us never see.

                --Doozer
                A picture of this mysterious device (albino angle plate), or it doesn't exist!

                Comment


                • #9
                  Originally posted by Seastar View Post
                  In my humble shop a granite surface plate and a set of good machinists squares together with a set of Jo blocks serves me well.
                  I don't work to microns and I'll bet almost no one else here does either. (Except Evan)
                  I do this as a hobby. I can understand people who, as a hobby try to have the best possible metrology standards but can they really employ them?
                  I do have precise electronic standards such as a standard cell but that's how I made my living.
                  Bill
                  Last use of the square was to align a mill I am scraping-in. Relatively recently.

                  Microns? EVERYBODY WORKS TO MICRONS...... the variance is in how many microns......

                  Your standard cell is likely out of cal, and may have fallen in voltage enough to be worthless. If not yet, then soon. Mine is, I know that, I just have it still as a curiosity. They only last so long, and a Fluke calibrator is likely more accurate.
                  CNC machines only go through the motions

                  Comment


                  • #10
                    Like I keep a couple of old vacuum tubes around as a reminder of my early days?
                    Chilliwack BC, Canada

                    Comment


                    • #11
                      Originally posted by BCRider View Post
                      Like I keep a couple of old vacuum tubes around as a reminder of my early days?
                      I have boxes of them.

                      A former employer, in their new, post-takeover incarnation, still makes vacuum tube products. And they are in demand.
                      CNC machines only go through the motions

                      Comment


                      • #12
                        Originally posted by Seastar View Post
                        I can understand people who, as a hobby try to have the best possible metrology standards but can they really employ them?
                        Bill
                        For me it's been a mixture of wanting to improve my abilities, "apprentice exercises", and despite and because of old habits.

                        My background in machining is very mixed. My career is/was as a chemist, and so I did a lot of small shop projects to facilitate performing lab experiments. I made some pretty nifty stuff, but I also developed a lot of bad habits. For example - when I made flanges for pressure devices the bolt hole circles would only fit one way. As long as I was the guy working with the apparatus it didn't matter if things were a wee bit funky - it did a great job of holding pressure if you assembled it right. At home I did woodworking, where precision is measured in 16'ths...

                        So as I started to do more in my home shop I realized that my sloppy habits were hurting my ability to machine things well. I decided to challenge myself to be able to achieve standard machinist standards. I'm not going for microns - but I do want to be able to do thousandths.

                        This immediately led to a different old habit. As a professional chemist I knew the importance of having standards in the lab (shop). I realized that my $15 digital calipers and Stanley combination square might be part of the reason my stuff tended to have a lot of slop in it. So, I decided that since my goal was to be able to work +/- 0.001", I needed to have standards +/- 0.0001".

                        That led me to buying a set of Jo blocks for length standards. I probably won't use them that often, but now I can use them as my primary standards for checking cheap calipers and micrometers, dial indicators, etc. I also bought a certified surface plate to be my main reference for planerity. Since then I got a couple of bigger but out of certification date surface plates from Craigs list... So the main reference is squirreled away protected except for occasional standard checking. Then I made the cylinder square to have my square standard along with the surface plates. Once I get that thing "calibrated" so I know where it is square, then it will go into the little box I made for it and only be used to check my cheap machinist squares every now and then.

                        Already this has all made a significant improvement in my ability to machine things. I'm to a point where I'm no longer astounded when I make something and it's correct to within +/- 0.003" I made a bolt circle flange/cover pair that fits nicely in any of the 12 possible rotations. I'm starting to make bolt clearance holes in terms of + 0.025" instead of + 1/8" inch.

                        So, it's been fun. Yeah, I might be going a little geekish when I use laser single-slit diffraction to check my cylinder square, but that's fun too. And that's the point now-adays... I'm retired, I'm having FUN.

                        Dan

                        Comment


                        • #13
                          I have a 9" x 9" x 9" granite angle plate I got off Ebay still in the felt lined box for $60 a few years ago. I use to align the axis on my homebuilt CNC. The table is an old hand scraped cast iron surface plate. Before I drilled and tapped 144 holes in the surface for hold downs the angle plate would float across the table when first set down. Then it was really hard to pull it up again! I looked for a similar one recently and was shocked at the prices!

                          Comment


                          • #14
                            The small discussion on the Shop Made Tools thread about making and verifying a cylindrical square suggested a need to talk about the requirements for verifying a tool or workpiece.

                            It's said that squares are self-checking, that is, if you have a flat reference (surface plate) and some adjuncts, you can prove that a square reference is in fact sqaure. The reference could be a shop or toolmaker's square, an angle plate, or a cylindrical square. And if you understand how to verify the geometry you can also then verify a workpiece, with or without an external reference.

                            There are some requisites that need to be in place, just like high school geometry where you could demonstrate that if A and B are true, then C is true. As in, "If angle A is equal to angle C, and if angle B is equal to angle C, then Angle A and B are equal to each other.

                            So, looking at the case of simple shop squares. If you have two of them you can compare them to each other by reversing positions.



                            If there are no gaps between blade and beam in both positions, you can be certain that blade and beam are both parallel, but it does not yet prove that they are square, only that whatever the angle it is identical.



                            If you see a condition like this where a gap appears, it proves they are not identical. Either could be out of parallel or out of square. It would require more careful checking to determine the fault.

                            However, if you have two squares which you can show are identical, and can set them on a surface plate, it will prove that they are also square, or will prove in what way the angle is incorrect.



                            Here you can see that if there are no gaps in comparing the two, and no gaps facing each other on a surface plate you will know you have two good squares. If you can see a gap at top or bottom as in the picture below, you can deduce that the angle is greater or less than 90 degrees.

                            .
                            "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

                            Comment


                            • #15
                              In another case, you may have angle plates you want to verify. You cannot check parallelism as in the case of the squares, but you can put them face to face on a surface plate and again check for any gaps. To check more closely than that there is SOME gap, you would normally use a spotting blue compound to see where the two surfaces touch. If you have two angle plates with good contact, you can be assured that they have complementary angles. For instance having one with 89 degrees and one with 91 might be good contact but not good squareness. To test that, you need one more square. If angle plate B and C are both a good fit to plate A, then checking plates B and C against each other will show a difference (or not). The only case in which all three will match each other is when all three are exactly 90 degrees. This is the three part method of creating a square reference without resort to any outside master other than the flat surface of the surface plate.



                              If you have a known master square and a surface gauge and indicator setup, you can test a single angle plate for squareness by comparing the two. Alternatively, if you have no master and only two angle plates, which match each other, you can use the indictor test instead of a third plate. If the angle plates match each other (complementary angles) then testing them each with the indicator will show double the deviation from square.



                              A further variation would let you check a single angle plate with a surface gauge and indicator if you can contrive parallels or a small flat plate surface.



                              Comparing the indicator reading on the face of the parallels to the back of the parallels, a proxy for the actual angle plate surface, you can again read any deviation from square. A zero change in the indicator reading "proves" the angle plate is square. And even if you can't contrive accurate parallels, you may be able to check the angle plate face against the opposite surface of a flat plate to get the same reading.
                              .
                              "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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