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  • #16
    Any angle at the upper end will reflect down to the drawing rulers. So if you make it so the anchor clamp can be adjustable to a small degree then you can adjust the rulers to be parallel to the sides even if there is a slight discrepancy. That may be a feature worth building in?

    You'll have 8 pivot points which will all have a touch of play. And in this sort of design all 8 points will stack up like a line of rail cars changes overall length as the play is taken up in all the car couplers. And on top of that your ruler will be longer than the spacing baseline of the joints which is variable. The amount of play will be minimized when the support rods are all spaced apart as wide as possible. But when the arms are angled so the rods are as close as possible this becomes the new leverage ratio between bushing play stacking and movement at the tip of the ruler.

    All of which is to say that you want those pivots to be as precise as you can make them. I would even go as far as to suggest splitting the brass bushings and have an easy way to easily adjust how much compression is on the bushing to "just" close to where there's no sticktion in the joints but play can be reduced to the barest possible minimum. Perhaps the brass bushings are threaded into holes that are made using only the starting nose of a taper tap? And slotted so you can adjust the play?

    After all if the base ruler(s) wobble around by any amount at all you may as well just use a T square and triangle.
    Chilliwack BC, Canada

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    • #17
      Thanks BC- in typical home shop machinist fashion, I'm making these brass pivots to .00003 tolerances I did space the arms apart as far as practical just for that reason. I spent a while figuring out the length for the arms so that in the center of my drawing area the arms will all be pretty much at right angles. I'm currently looking at how the screws that hold the pivots in place will distort the brass as they become tight- and take up the play in the pivot by expanding the brass slightly. The pivots are mostly made now, and I'm chamfering the beginning of the threads a tad at a time to get a compromise between easy action and zero play. This takes time- I was in the shop until about 1 am getting one of the pivots prepared- five more to go.

      I screwed up one of them, I think. Didn't leave enough length so the arms will clear the screws when the angles get small- I'll be figuring that out in the next few minutes.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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      • #18
        Sounds like you already had a good handle on the important details of the design.

        It was interesting to note that on some of the more serious versions that they used taut bands of steel with an adjustable strut in the middle to preload the bands and remove ALL play. The trick thing with that is that then the leverage ratio didn't change through the whole range of motion.

        Hmmmm…. I wonder if a low profile smaller version of that idea could not be done using small steel cable? Don't see why not..... Although cable stretches more easily than a flat band due to the strand winding. Still for a smaller flat unit like you're looking for..... Hold that thought for a Mark II/Plan B if required.... 😁
        Chilliwack BC, Canada

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        • #19
          Plan B held in mind- I did look at that, and while I could have done that it would have been more trouble- and it probably would have been better.

          I re-made the pivot I screwed up, and now another problem showed up- two of the pivots are underneath the center plate, and now they are hard to get at to epoxy. Not a big deal- I'll run a drop of CA into the junction and let that set. Then I'll turn it upside down (carefully) and run the epoxy in.

          The fixing screws run in easily up to the point where they begin to get tight- then the have to go in another turn and a half to set them where they need to go. That does expand the brass a bit, and I can feel the interference when I turn the pivots. That's probably a good thing as there is now no play, and it will wear in. But I can't pull the pivots out now without force- again not a big deal because the center plate is finished and nothing more needs to be done to it.

          The highest point on the back pivots just barely rubs under the desktop when the shelf is pushed all the way in, and the highest point on the center plate does the same. So far this couldn't have worked out any better. On to the next part- cutting the short arms to length.
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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          • #20
            Geez.... sounds like you'll need to worry about the thickness of the paper! Any chance of moving the slides a bit to make at least a touch more room?
            Chilliwack BC, Canada

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            • #21
              Ok, short of fastening the square to its plate, it's done. All the pivots worked out nicely, aside from having to remake one of them. It is not zero friction, but it is zero play. There is no stiction effect at all, and it glides beautifully. I have to attribute much of this to the TriFlow- the rest to the ridiculously high tolerances of machining.

              I haven't attached the square because I want to drill some hole layouts into it first. Pin spacings for ICs, resistors, capacitors, semiconductors, potentiometers, etc. I'll have to try and think of other layouts that would be useful. Looks like I have 12 sq inches of clear, unmarked plastic to work with.

              I've taken to building my electronic circuits in surface mount style- but using regular parts with pin spacing that I can still see. The rows of pads for an IC becomes a little wider to accommodate the folded-out pins that allow a regular part to surface mount. I'll be able to use this machine to help me lay out resist patterns on pc boards.

              I'm also in the middle of rigging up a couple of lights at this desk so these tired eyes can see what I'm doing. I'll get a picture in once that's done.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #22
                Originally posted by BCRider View Post
                Years ago just before I self taught myself TurboCAD for 2D drawing...
                Yay, a fellow TurboCAD driver! What version do you use Bruce? I'm still using my $10 NIB ebay version 9 (on CD).

                Milton

                "Accuracy is the sum total of your compensating mistakes."

                "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

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                • #23
                  I see I'm a little late to the party, and you've progressed significantly from your first post on the subject. As an FYI the machine you originally described looks a lot like either a K&E (Keuffel & Esser) or a Vemco. There are currently over 100 listings on eBay for drafting machines and parts. Many are going for $50.00 or less.

                  There was a time when both drafting machines and boards were so cheap companies couldn't give them away. When the company I worked for started introducing a CAD system in the late 1980's all the boards, attached desks, drafting machines, and lights went into storage. They stayed there nearly a dozen years with a few being donated to the local technical college. When the powers on high were finally satisfied CAD was here to stay they started trying to sell the old machinery.

                  The first 50 or so went like candy. After that nothing for nearly a year. They finally decided to scrap the last 30 or so unless some employees were interested. I enquired as to the asking price and the response was "We'll make you a deal you can't refuse". In the end I purchased a Mayline Hamilton 36" x 72" bboard with a return desk, a Mutoh drafting machine, Vemco light, about 30 different scales, and all the remaining drafting supplies in the department for $50.00.

                  It truly was a deal I couldn't refuse. I still have everything and once again it gets used on an almost daily basis. I did have Auto Cad on a computer for several years. Unfortunately the computer died, and the software wasn't compatible with the new computer's operating system. A new version of the software was outrageously expensive so I literally went back to the drawing board.

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                  • #24
                    Any brand of 3D cad system is totally fabulous compared to the pencil.
                    I can't believe anyone wants to play with a pencil any more.
                    And I am old school as they come. But 3D cad is such a time saver.

                    -Doozer
                    DZER

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                    • #25
                      Originally posted by DICKEYBIRD View Post
                      Yay, a fellow TurboCAD driver! What version do you use Bruce? I'm still using my $10 NIB ebay version 9 (on CD).
                      I've upgraded a few times over the years mostly to get the additional file format compatibility and now running TC2015. It does all I could ever wish for so as long as they keep giving me new start up codes as my computers fail and get replaced I think I'll stick with it.

                      And with the new Samsung 26 inch monitor I also now have way more drawing area. It's not just bigger than the old one. It has more resolution too so when I fired it up the old toolbars that tool up the whole length of the edges are now only just over half the width and height of the screen.
                      Chilliwack BC, Canada

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