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  • need machining help/ideas

    My current project is a 1/6 scale artillery piece and I am presently working on the barrel. On the top of the barrel are two guides which hold the barrel in alignment when the barrel, on a full size piece, recoils. The bottom of the guides are constantly changing curves and diameters which need to be rough machined to shape to match the top of the barrel so they can be soldered in place on the barrel and then finished machined into their final shape, Any small imperfections where the guides meet the barrel can be filled with solder and finished to look like they were a casting. I don't have any CNC or fancy copying equipment to work with, only a lathe and vertical mill. Any ideas on the best way to do this job?

    the first picture is a cad drawing of the finished barrel



    This picture shows the barrel in place in the gun cradle (barrel liner is not installed) , the slots that hold the guides can be seen on the cradle



    the next picture shows the barrel with a 1" mic for scale



    the last picture is a cad drawings of the bottom of the guides, the complex shape can be seen

    Last edited by loose nut; 01-10-2008, 01:16 PM.
    The shortest distance between two points is a circle of infinite diameter.

    Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

  • #2
    My thoughts are along the lines of making the guides the same size full length and then mill 2 slots in the barrel where the gets wider you would have to round the ends of the guides where they meet the widest part of the barrel as the milling will leave a round hole.
    Peter
    I have tools I don't know how to use!!

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    • #3
      L S nice workmanship well done we boys need our toys no doubting thatAlistair
      Please excuse my typing as I have a form of parkinsons disease

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      • #4
        Just an idea, and possibly not a good one, but here goes:
        Assumes you have a good sized lathe and mill.
        Start with a piece of flat stock thicker than the distance from the top of the barrel to the bottom of the slides. get a piece of round stock a reasonable amount bigger than the diameter of the barrel. mill a slot across it to accept the piece of flat, at the correct place so the flat ends up where it would be relative to the center of the barrel. Add the block to the end of the piece.
        now set the piece up in the 4jaw on the lathe. use the circular block in a steady at the tailstock end. Now 'just' bore the correct shape into the flat plate. The reason for using a thicker piece is to keep the 2 rails attached whilst you do this boring job. Interrupted cut, but not a blind bore, so hard but simple all at once
        You can test fit the rails to the barrel and once happy with the fit remove from lathe, remove end block. then bolt down to the mill, curved side down and face of the top to seperate the rails and thickness them to the correct size. You can add the slots / rails whilst they are still attached together.

        hth or at least provides some inspiration

        Dave
        Just south of Sudspumpwater UK

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        • #5
          Is that a 13-pounder? Very nice!

          Walt

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          • #6
            18 PDR actually but their basically the same, 13 PDR is a 3" caliber and a shorter barrel,to keep the weight down to allow fast movement, the 18 PDR is 3.3" cal., and was the standard field piece of the British and empire army's, also used in the US as the M 1917 A1 chambered to fire the French 75 round.
            The shortest distance between two points is a circle of infinite diameter.

            Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

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            • #7
              Originally posted by ptjw7uk
              My thoughts are along the lines of making the guides the same size full length and then mill 2 slots in the barrel where the gets wider you would have to round the ends of the guides where they meet the widest part of the barrel as the milling will leave a round hole.
              Peter
              I don't know why I didn't think of that. Do you hear that, it's the sound of my head hitting the desk over and over. I'm going to model it on the cad program to see if it will work out. Small Planes, I was thinking of something sort of like your idea, but I wasn't sure on how to machine the varying angles to get the right ID of the "bore" so I'm saving that idea as plan B. Thanks for the help.
              The shortest distance between two points is a circle of infinite diameter.

              Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

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              • #8
                It wasn't clear if this was to be a working model or not. If it were, I thought that milling grooves could weaken the barrel too much.

                Since it appears that grooves will work for you, when you get near the end, try switching to a smaller diameter cutter to finish up, minimizing the radius needed on the end of the rail.

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                • #9
                  It's a non-firing model, in Canada making your own firearms (it would be classified as a pistol/restricted weapon because of the barrel length) is a serious NO-NO. I don't know if the grove cutting method will work after all. I tried modeling it in cad and the breach end of the grove was very large. I'm going to try it with a different orientation and see if the grove becomes reasonable. I have also been working on the idea that small planes suggested to see if it will work out but some tricky machining there.
                  The shortest distance between two points is a circle of infinite diameter.

                  Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

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                  • #10
                    You only have to slot the tricky bits. The part near the breech is looks tubular, not conic and thus shouldn't be too hard to hand-fit.

                    That's nice work.

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                    • #11
                      If you've a suitable grinder, it could be hand ground to the profile scribed on the part. Knife makers do that sort of thing all the time. A belt grinder would be ideal. For something this size, one of the inexpensive combo belt disc grinders would do the trick. You could even hand file to the profile if you're patient.

                      You could even do a combination of the two. You needn't slot the barrel to full depth, but just enough to hide the profile work going on.

                      Best,

                      BW
                      ---------------------------------------------------

                      http://www.cnccookbook.com/index.htm
                      Try G-Wizard Machinist's Calculator for free:
                      http://www.cnccookbook.com/CCGWizard.html

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                      • #12
                        Originally posted by loose nut
                        I tried modeling it in cad and the breach end of the grove was very large. I'm going to try it with a different orientation and see if the grove becomes reasonable. I have also been working on the idea that small planes suggested to see if it will work out but some tricky machining there.
                        Could you use a combination? If you 'bore' the plate for the smaller diameter then you only need to slot the breach end by the difference between the diameters. that should be a smaller slot, and just runs out into the barrel dia. I can see it in my head, but have no drawing tools here to draw it.
                        I assumed it was a 'live' model in my suggestion, where a slot that long would probably weaken the barrel substantially, as a dead model such considerations dont apply.

                        Dave
                        Just south of Sudspumpwater UK

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                        • #13
                          After looking at every method mentioned and a few that were not, I think that the easiest (not necessarily the best) way to do it is to lay it out as close as possible and then cut ,chip saw, beat, grind and file until it's as close a fit as possible . At least it will help me work out some pent up aggression. Thanks for the help.
                          The shortest distance between two points is a circle of infinite diameter.

                          Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

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                          • #14
                            What kind of stress will the part be under? Is it literally holding up the barrel or is it decoration, more or less, since the cannon won't really fire?

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                            • #15
                              I see three possiblities-

                              1-make a dummy barrel out of something easy like wood,slot it and use it as a scribing tool to scribe top and bottom of the finished piece and then cut and file to connect the lines on both sides of the part.

                              2-set up on center and parallel the existing barrel to the slide stock and use a pair of fine dividers to scribe each side to the profile then file to shape.

                              3-make a plaster of paris mould of the barrel and use it to scribe the slide stock.

                              Either way I would use a Dremel and a burr to relieve the area between top and bottom of the slides so they fit up razor tight to the barrel.
                              I just need one more tool,just one!

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