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How to accurately measure a long rod?

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  • How to accurately measure a long rod?

    I was just given a job of facing-off 4 long 1-1/4" dia. Aluminum rods to an overall length of 33.500" +/- .005.
    My dilemma is, that I don't have any 36" verniers handy.
    How can I accurately measure the length of these rods?
    I was thinking of using my magbase/dial indicator attached to my mill head, and using the lowered table as a reference.
    But then I'd have to stack a bunch of 'something' to set zero.
    Any other ideas?

  • #2
    I don't have any suggestions on how to measure it, but be wary of the temperature. A 12 degree F temperature change will eat up all of your +/-.005 on an aluminum rod of that length.
    Mark




    Originally posted by KiddZimaHater View Post
    I was just given a job of facing-off 4 long 1-1/4" dia. Aluminum rods to an overall length of 33.500" +/- .005.
    My dilemma is, that I don't have any 36" verniers handy.
    How can I accurately measure the length of these rods?
    I was thinking of using my magbase/dial indicator attached to my mill head, and using the lowered table as a reference.
    But then I'd have to stack a bunch of 'something' to set zero.
    Any other ideas?

    Comment


    • #3
      Make two pieces as close to six inches long as you can. Within a few tenths at most and pay attention to the temperature. If you can find some, carbon fibre/epoxy rod is the best since the thermal expanision coefficient is nearly zero, about .03 that of steel. If not then use 410 SS or just make sure the temperature of the piece doesn't change from handling or machining before measuring.

      Just place a stick even to the faced surface at the start on the work. While keeping that stick in place butt the next to the end of stick A and do so until you have 30 inches. Measure the remaining distance with calipers. With care you should be able to hold tolerance easily.
      Free software for calculating bolt circles and similar: Click Here

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      • #4
        If you have a DRO on your mill and enough travel you could set a stop, zero out the DTI then put one end of the workpiece against the stop. Move 33.500" and check the other end. However unless you have something much larger than a BP you will not have enough travel.

        So, if you have a DRO on the lathe, a steady rest and enough travel I'd try the following.

        Face one end to clean up.

        Make a shallow (say 1 to 2 inches) deep fixture in the lathe with a slip fit and a couple of nylon tipped set screws to hold the work in place. Make sure you know exactly how deep you make the hole. Set up the steady so you can face off the other end. Move the steady out of the way. You could then use a DTI to set zero on the face of the fixture and on the DRO. Then put the work piece back in and check the length.

        I realize this would be a real PITA but once you knew the length of the first piece you should be able to just repeat the facing of the 2nd end on the other 3 pieces.

        You do not say how big of a lathe is available, but if the spindle bore is over 1 1/4" you could also make a stop for the inside of the spindle similar to the ones used in RC collets. You'd also want to make slip fit bushings so the part would be running fairly true inside the spindle.

        BTW, how will your customer check the length of the finished rods? Do they have a 36" vernier you could use?

        Comment


        • #5
          Originally posted by Dr Stan View Post
          .........BTW, how will your customer check the length of the finished rods? Do they have a 36" vernier you could use?
          Location: Saskatoon, Saskatchewan, Canada

          Comment


          • #6
            WHY, would anyone need that kind of tolerance on 33"? As was mentioned, the temperature factor is quite important in this instance. I, myself, would question the need. It could be just someone writing specs that is not familiar with the actual need or use. Bob.

            Comment


            • #7
              Before you even think of making the part make sure you have agreement on what temperature it is to be measured at. Maybe that will wake them up to what they are asking for.
              Free software for calculating bolt circles and similar: Click Here

              Comment


              • #8
                Standard temperature (ambient) is 20 Deg C or 68 Deg F.

                Make 5 rods 6" +/- 0.001" and 1 rod " +/- 0.001".

                The stack limits are +/- 0.006" but if all are not all at one limit the 33.5 +/- 0.005" stack will do the job as a reference as some "plusses" will be cancelled out by some "minusses".

                Comment


                • #9
                  Get three micrometer calibration rods that add up to the desired length for your new "standard" length.

                  Comment


                  • #10
                    Since you were "given the job", *someone* must be under the belief that you can do it and check it.....

                    Look around the shop and see if there is a tubular inside mic anywhere. The kind which has fixed length extensions which screw together Many of those will go up to 30". The remainder can be measured with whatever device is suitable, a depth mic, or a caliper with depth rod, whatever.

                    For any decent make, the fixed length portions will be very good in length, far better than 0.005". The adjustable head can be calibrated at a specific length with a shorter device, a large mic, or a good 12" caliper.

                    Just lay out the part and inside mic against a straight surface, and butt up to a machinist's square. measure from the end of the part to the end of the inside mic, and add the length set up on the inside mic. The errors should be virtually all in the measurement, if the inside mic is any good. For even better confidence, you can maybe only use the extension pieces (if they are long enough), which should be accurate to tenths or better.

                    Yes, temperature will be important.
                    CNC machines only go through the motions.

                    Ideas expressed may be mine, or from anyone else in the universe.
                    Not responsible for clerical errors. Or those made by lay people either.
                    Number formats and units may be chosen at random depending on what day it is.
                    I reserve the right to use a number system with any integer base without prior notice.
                    Generalizations are understood to be "often" true, but not true in every case.

                    Comment


                    • #11
                      I am wondering what machine will be used to face off the 33.500 in. rods. If a lathe will be used, I wonder if the lead screw could be used. The problem is still " how to check them" if no direct measuring instrument is available.

                      Jim
                      Jim

                      Comment


                      • #12
                        I'll present an idea as sort of a thought experiment. Not sure it is really practical, and I'm glad I don't have to do it.

                        Cut a shallow fine-pitch groove on the surface, say 28 threads per inch. This will effectively etch a scale on the length, with divisions of 0.036" between one groove and the next.

                        If the faces are true, you should be able to measure the angular difference between the point at one end where the groove starts, and the point at the other end where it runs off the piece. If you can measure this angular difference to an accuracy of, say, 20؛, then you will have 360؛/20؛ = 18 increments dividing the 0.036" pitch.

                        So with this method and these assumptions, you should be able to measure 0.002" increments on the length. Change the parameters to suit your fancy.
                        Last edited by aostling; 09-16-2012, 02:54 AM.
                        Allan Ostling

                        Phoenix, Arizona

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                        • #13
                          You could make your own measuring 'stick' using a section of lead screw. If the accuracy over length spec is suitable, then a pair of 'jaw' sections locked into the threads would give you a way to have variable but accurate spacings to compare with. One jaw would be fixed in place, and the other set onto the threads at some certain distance. An ordinary tape measure would be good enough to find the correct thread to drop the jaw into, since it's the thread itself that determines exactly what the spacing is. For instance, to check a rod 33.4 inches long, you would set the movable jaw at the 34 inch spacing, then measure the difference between that and the workpiece with a caliper.

                          The jaws would have to be in the same plane to give accurate distances, but this could be done on a surface plate or other non-warped surfaces.

                          Carrying the idea further, you could make up a long, narrow 'bed' for the lead screw to nest onto and for the jaws to come flush to when placed over the threads. You could also glue a section of measuring tape to this bed to make it quick and easy to position the movable jaw.

                          If the bed was say a steel tube, you could use magnets to hold the jaws to it. This would allow for the jaws to be held very consistently, for the best accuracy of measurements. Of course, as many have mentioned, you will have to account for temperature effects.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                          Comment


                          • #14
                            Are you aware of how some lead-screws are made? Amongst other methods they are "rolled" (like a bolt!! and straightened like an aluminium extrusion). They may not be too "precise" even when new.

                            And if the lathe bed is worn how confident of the lead-screw and the half-nut/s are you?

                            Otherwise think of something else to measure the rods with.

                            Comment


                            • #15
                              Could it be that they want 4 rods of 33 1/2", just all of the same length (ie. would they be just as happy with, say, 33.4" +/- .005? If so, could you clamp all 4 together and then take a skim across both ends?

                              Ian
                              All of the gear, no idea...

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