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Precision layout

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  • Precision layout

    I am trying to do a layout where the dimensions are 45.788" for example and other dimensions larger than the largest measuring instruments I have. The largest steel rule is only 18" long; the largest vernier caliper is 12". Is it possible to use the steel rule and mark off the dimension in 18" increments, using a magnifing lens and scriber to get close to the 3 decimal number? If it is off a couple thous, I can live with that. The layout is for a CNC router I'm building. The material is plywood, actually MDO, a resin impregnated material, very stable dimensionally and strong. The dimension in question can vary by a 1/16", but I have to be able to read the dimensions, as other parts must mate up, such as bolt holes.

  • #2
    Would be hard but not impossible to do with a 18" ruler and come out to within three decimal points.

    If I understand correctly you said they have to be within a 1/16" if that is true just get a good steel tape measure. The printed lines on these can be rather wide so a trick to keep thing as accurate as possible is to also mark on the same edge of a line - example always mark on the edge of the line closes to the end of the tape.


    • #3
      The best solution would be to buy a 48" steel rule, but at 125.00 for a cheap one, I can understand why you wouldn't.
      Shop around for a tape measure blade with 1/32" graduations on the first foot of length (probably a 1/2" wide x 12' blade)
      Take the steel tape measure blade and secure it (contact cement) to a piece of steel (prefered) or aluminum angle stock that's the length you need. (48" ?)
      Don't trust the hook on the end of the tape, remove it and position the end of the tape with the 1" mark precisely aligned with a scribe line 1" from the end of your stock.
      That should let you layout within 1/64" with a little care.
      I'm sure the more experenced around here can offer you with a more precise solution. So I'll be curious to read their suggestions.
      I cut it twice, and it's still too short!


      • #4
        How about facing three rods slightly less than 12" in length. Measure them with your vernier and calculate the length needed to make your 45.788" measurement. Face a fourth rod to that length. Lay all four rods in a 45" length of angle iron with the rod ends projecting from both ends of the angle iron.
        Regards, Marv

        Home Shop Freeware - Tools for People Who Build Things

        Location: LA, CA, USA


        • #5
          Burn an inch

          A quality tape measure should do the job. Start at the 1 inch mark (burn an inch) and add an inch to the desired length. Also stay on the same side of the marks.

          When I started at my last teaching job I waas sure that it was the dumbest bunch of students I had ever met. Not one of them could measure within a consistant 1/16 of an inch. Then I found out I couldn't measure either. I got out all the combination squares and rules in the shop and set my vernier height gauge at 2 inches. Lo and behold not one ruler/scale measured 2 inches. I found out later that a angry student the year before had carefully cut off somewhere between 1/32 and 3/32 inch from every ruler in the shop.
          By starting at the 1 inch mark they could all measure.

          The end of tape measures are where the most wear is going to be. Rulers from combination squares are going to be damaged from dropping on concrete .

          Use Trig and the tape measure to obtain any angles needed.


          • #6
            +/- 1/16"? I would definitely use a tape measure. Actually, I would probably use three or four of them and go for the average of the lot.

            Do use the one inch mark as the start and add one inch to the distance.

            Do use a magnifying glass to make reading it easier.

            0.788" is an odd decimal equal to 50.432 1/64s or 12.608 1/16s. That's about half a 1/16 past 3/4" and should be easy to eyeball with a magnifier.

            But do use more than one tape.
            Paul A.
            SE Texas

            And if you look REAL close at an analog signal,
            You will find that it has discrete steps.


            • #7
              Length of all thread rod,something like 3/8 or 1/2 and some nuts.Jam two nuts against each other,run two more down and jam them together 12" from the last set and so on.Use your Vernier to accurately set the distance between each set.The closer you fit that distance the less cumulative error will be present in your final result.

              Long lengths of all thread rod can be had at the local big box store in the electrical hardware section usually near the Uni-strut rack.

              If you wanted to refine it you could pick up a cheap pair of dial calipers and screw them to the end of your new gauge so long as you accurately set the zero point.
              I just need one more tool,just one!


              • #8
                You need to determine what your tolerance is. You say you can be off "a couple " of thou. Is it .002? .005? .010? There might be someone who can layout to .010 but I've never seen it done. Maybe one of these wizards posting previous to me will come do it for you. Basically there is no such thing as layout to those tolerances. If really needs to be to .002 then you need a machine that can measure it.
                Last edited by tdmidget; 12-23-2011, 02:05 PM.


                • #9
                  Surely if you have a positional requirement as loose as 1/16" but need corresponding parts to within a couple of thou then you should make an appropriate jig, or just machine them as a pair.



                  • #10
                    How about two known blocks, say 18" long (since he can get that measurement) and "hopscotching" end for end until the rule would be used for the final dimension?

                    I too would use a tape measure. I can cut hardwood lumber quite accurately with a good tape (not the dollar store brand). It is fairly easy to get within a 64th, and a 32nd would (should) be child's play.



                    • #11
                      Very interesting post. I assume that there are other parts that you plan to make in this size range (greater than 36").

                      If so, it maybe expedient to have comparable measuring devices beginning with a precision 48" steel rule as well as trammel points and spring-style calipers. BTW you could make your own trammel points and calipers. The trammel and calipers would be used to transfer measurements to and from the rule.

                      The problem I see laying out 45" with a small 12 inch rule by stacking or chain measuring is the errors introduced with each step. Add to this, the confusion in avoiding the first inch or half inch of the shorter rule. It is just too easy to make a mistake.

                      Another one of your considerations should be thermal expansion. Forty Five inches of steel will change in length 1/16 with 20 F°

                      Thank you for the post. It is these kinds of problems that challenge our minds. I am looking forward seeing other solutions.


                      • #12
                        Just a thought here, .788 in = 20 mm. Perhaps you should be measuring in metric instead of imperial. Are the rest of your diminsions converted from metric as well? It might be easier and more accurite than trying to "guesstimate" it's fractional equivelent. It shouldn't be too difficult to find a tape measure that has both metric and imperial scales.
                        (I always seem to find them when I don't want them)
                        I cut it twice, and it's still too short!


                        • #13
                          I once needed to machine a couple dozen 60" + shafts to an exact--and consistent--dimension. Having no means of accurately measuring that length I took a piece of cold finish flat, set it up in the mill and machined one end square. From there I machined a precise 1" slot with it inner edge exactly 18" (I have an 18" vernier) from the end. I stepped over another 18" and milled a second slot. Doing this a third time gave me a slot with its outer edge exactly 57" from the end. From this edge I machined the end of the bar to the exact length I needed.

                          To use the gauge I faced both ends of each bar--leaving them about 1/16" long--and then, using a stop and a depth mic, I determined how much to shorten each in the final operation.

                          Using this method does raise the possibility of having incremental errors creep in but I'm sure--given the care I took when machining the bar--that I was much closer than I'd ever get with a tape measure. And of course this method ensures much greater consistency, something which was--in my case at least--as important as the actual final dimension...
                          Just one project too many--that's what finally got him...


                          • #14
                            Pick a division of that final number that you can accuratly 15.262...use your mill to drill 2 holes in a piece of stock, exactly 15.262" apart. Size the holes to accept a couple of transfer punches with a light press fit...stick a transfer punch in each hole..this is your tool.

                            We'll call one end of the tool "A" and the other end "B".

                            lightly center punch the location of the first hole in your workpiece, set the transfer punch tip in end "A" in your dent. Apply a bit of pressure to punch "B" to make a little dent, then pivot your tool around "B" to put "A" now 30.524" away from your first hole. Repeat the procedure and "B" should be exactly 45.788" away from your first hole.

                            Use a long straight edge as a guide or put marks at your intevals in order to keep all 4 dots in a straight line, and your distance should be fairly accurate.


                            • #15
                              Originally posted by tdmidget
                              There might be someone who can layout to .010 but I've never seen it done. Basically there is no such thing as layout to those tolerances. If really needs to be to .002 then you need a machine that can measure it.
                              What? Have you been watching fabricators layout with soap stone and tape measures? What is your definition of "layout"?