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Good way of marking the center line of flat stock?

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  • Good way of marking the center line of flat stock?

    Hi Everyone,

    Does anyone know of a clever way of scribing a line down the exact middle
    of a piece of flat CRS stock?

    I've got a method that works in theory but it requires a lot of very fiddly alignment and then three hands to scribe it.

    Oh and by the way, I've started working on my milling attachment. However I may have to start over again because I've had a lot of trouble drilling the holes in the right place. I got my stock marked and punched well enough but on my first attempt the holes were all over the place.

    On my second I used a center drill to start and had the same problem.

    Then I discovered that my cheap Harbor Freight benchtop drill press has about 3/32" play in the spindle!

    So I moved over to the lathe to do the drilling, and that was very precise. But with having to jury rig faceplate clamps and align the punch marks it took about 2 hours to drill 3 holes. Faceplate clamp setup for your amusement:



    Here is a shot showing how far off the hole was when I used the drill press. The punch mark was centered on the line, and the center drill started off in the punchmark.

    Lee

  • #2
    Too many square brackets on your image links. Remove the ones before and after the http://... string. Never mind!

    I use a compass from both edges at two points along the edge to scribe intersecting arcs at two places then draw a line between them.

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    • #3
      I use a square with a ruler blade that's held in place with a thumb nut. After measuring and checking the CL mark from both sides of the stock, I slide the square down the side while holding a scribe against the end of the blade.
      Any products mentioned in my posts have been endorsed by their manufacturer.

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      • #4
        Generally I would use my height guage to do this. Set stock on side and measure full width then set and lock the guage at 1/2 that reading and slide the scribe on the guage down the length of the stock
        Ernie (VE7ERN)

        May the wind be always at your back

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        • #5
          Originally posted by dp
          Too many square brackets on your image links. Remove the ones before and after the http://... string. Never mind!

          I use a compass from both edges at two points along the edge to scribe intersecting arcs at two places then draw a line between them.
          You'd think a programmer could get it right the first time!

          I've used that method too but in this case the stock is only 3/4" wide so I didn't think it would work well. I think that it also requires precisely measuring in an equal amount from each side to place the compass point, which it what I was trying to avoid.

          Now that I think about it, I've got a tool for marking model airplane hinge slots that is exactly what I need -- if it wasn't made of out nylon and too imprecise.

          It is a short bar with 2 pins at each end. In the middle is a hole for a pencil. You just place it on the stock, twist it so the pins touch both sides of the stock, and the center hole lines up automatically.

          Maybe I could make something like that out of metal with a hole for a scriber... Sounds like making it would require more precision than I am currently capable of.
          Lee

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          • #6
            Originally posted by winchman
            I use a square with a ruler blade that's held in place with a thumb nut. After measuring and checking the CL mark from both sides of the stock, I slide the square down the side while holding a scribe against the end of the blade.
            What a coincidence, that is almost exactly what I did... I held the scribe against on of the tick mark indentations.
            Lee

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            • #7
              Centreing a line on bar stock

              Get an adjustable square from a combination set:


              Set the rule to project as near as you can to half the width you want to "centre". Ie for 4" bar, set the rule projection to 2"

              Put the square against one side of the 4" bar and place a scriber against the end of the rule. Drag the square along the side face of the 4" bar while holding the scriber onto the end of the rule.

              The scribed line will be close to centre.

              Now change he square to the opposite side of the 4" bar (do not alter the ruler setting in the square) and repeat as before and draw another line alongside the first line.

              The centre is dead between the lines.

              Adjust the ruler so that its end is exactly mid-way between the lines.

              Scribe two more lines as before.

              Those lines if right on each other are dead on centre. If not dead on centre, no matter as the centre is still dead between those lines which are now closer together.

              Re-try if necessary.

              It is a process of (re)iteration where the "error" gets successively less/smaller.

              I usually leave two lines just a bit less than a 1/64" apart as it makes it easier to measure to or centre-punch than a single line does.

              It is very easy to get the "hang" of, and once you have it, it is very easy, very quick and very accurate to use.

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              • #8
                Originally posted by oldtiffie
                It is a process of (re)iteration where the "error" gets successively less/smaller.

                I usually leave two lines just a bit less than a 1/64" apart as it makes it easier to measure to or centre-punch than a single line does.
                Thank you, that's exactly what I was looking for, a method that naturally finds the center and doesn't require precisely lining up the scribe with a tick mark on a ruler.
                Lee

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                • #9
                  Height gauge then center punch, do a ton of stuff this way. Old Rockwell drill press, spindle has play side to side, still does the job quite well.

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                  • #10
                    Originally posted by ldn
                    You'd think a programmer could get it right the first time!

                    I've used that method too but in this case the stock is only 3/4" wide so I didn't think it would work well. I think that it also requires precisely measuring in an equal amount from each side to place the compass point, which it what I was trying to avoid.
                    I push the work against a straight edge so the compass has something to work against, but you can scribe a witness line parallel to each edge and use that, too. It's one of the best uses I've found for my odd leg calipers.

                    Comment


                    • #11
                      Originally posted by dp
                      I use a compass from both edges at two points along the edge to scribe intersecting arcs at two places then draw a line between them.
                      Could you elaborate on this please.

                      At the first place you make your intersecting arcs, unless the line joining the two centrepoints of your arcs is perpendicular to the long axis of the bar, neither arc intersection point will lie on the centreline of the bar.

                      Similarly with the second place you try.

                      You will have four points defined by arc intersections, two of which lie one side of the centreline and two lie the other side. Pick any pair of these and join them up; none of the lines so produced will lie on the centreline of the bar.

                      One way to minimise the problem is to use a square to scribe a line across the bar and then make sure your compass point is on that line.

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                      • #12
                        I'll lay up an example tomorrow and take pictures. It's pretty easy to set up.

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                        • #13
                          Use an oddleg caliper, guess at the setting and mark from both sides. You'll see how much the error is, adjust the caliper to reduce it, try again. Continue until you have it as good as you need.
                          Same principle as Tiffie's method.

                          Tim

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                          • #14
                            Originally posted by Timleech
                            Use an oddleg caliper, guess at the setting and mark from both sides. You'll see how much the error is, adjust the caliper to reduce it, try again. Continue until you have it as good as you need.
                            Same principle as Tiffie's method.

                            Tim
                            Yes - that's a good approximation and can get you really close if not spot on. One more step with a compass and straight edge, though, and you are actually spot on.

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                            • #15
                              I use tiffie's method, works very well.
                              Peter - novice home machinist, modern motorcycle enthusiast.

                              Denford Viceroy 280 Synchro (11 x 24)
                              Herbert 0V adapted to R8 by 'Sir John'.
                              Monarch 10EE 1942

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