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Need Advice: trouble milling arc on small mill

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  • Need Advice: trouble milling arc on small mill

    I need to mill multiple 1/8" wide arcs in 1/16" brass plate. The challenge using my rotary table is that the position of the arc requires a deeper throat than my Sherline mill provides even with an extension. While not sleeping last night I came up with this possible solution and would appreciate your input including alternatives.

    The idea is to use an auxiliary table mounted above the rotary table with a cam in between. The cam is essentially two disks fixed one to the other with their centers offset. The bottom disk would be centered on and attached to the rotary table. The top disk would sit in a cylinder of the same diameter attached to the bottom of the auxiliary table. Then turning the rotary table would cause the table and any point on the table to describe an arc whose radius is the offset distance.

    To keep the table from moving other directions than desired a second mounting point is required. This would consist of a pin attached to the underside of the table. This pin would ride in a cylindrical groove in a block set below one end of the auxiliary table. The radius of this cylindrical groove is the offset plus/minus the radius of the pin.

    Finally, to keep the auxiliary table from tipping and pinching the cam, the end of the table opposite to the pin would need to be supported on a screw jack with a ball on top. Hopefully the photo of my sketch clears up any misunderstandings created by my description.



    I appreciate any comments, suggestions, alternatives, etc.

    John

  • #2
    I think you're overthinking the process. Make a sub-plate out of 3/4" thick aluminum, bolt it rigidly to the top of the
    rotab, clamp your brass to the sub-plate and have at it...
    Keith
    __________________________
    Just one project too many--that's what finally got him...

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    • #3
      Get rid of the jack, cylinder and rotab.

      Space the subtable up with just enough space for the cutter. put part on at desired radius, and use a lever to move the subtable and part.

      That saves needing to find a way to crank the rotab. under the subtable. If that is not an issue just mount the subtable on the rotab as suggested.
      4357 2773 5647 3671 3645 0087 1276

      CNC machines only go through the motions

      "There's no pleasing these serpents"......Lewis Carroll

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      • #4
        How many of these do you have to make amd are there more than one arc on each piece? Meaning do they get progressively larger or is it just one cut? I agree with the last two guys. Unless you're making hundreds and hundreds of these you are over thinking it. 1/16 could easily be cut with a lever. Unless it's a repeat job or large quantity, keep it simple.

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        • #5
          Thanks for the input. I may be overthinking the problem and/or I may not have been sufficiently clear with the problem statement. Maybe this is clearer:

          The arcs I need to cut are not centered on the plate, so one part of the arc is significantly farther from the edge of the part than the other. This larger distance is more than the distance between the spindle center and the mill column. Consequently, I cannot rotate the part all the way through the cut without the part hitting the mill column. That is why I think I need to use the cam approach (or something similar). If cam approach works, it will always keep the narrowest side of the part toward the mill column as set, but still move the part in a circle under the spindle allowing me to cut the full arc.

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          • #6
            You haven't given us a sketch with the viewpoint from above. I think that would clarify your problem a lot. And frankly, I am not even sure where the two cams you are talking about are in the front or side view sketch you did provide.

            If the arcs are short, you can probably mount the RT on one end of the table and crank it out from the spindle far enough to get your radius sideways instead of front-to-back (X axis instead of Y). If the part still hits the column, perhaps you can offset the spindle toward the RT to allow it to clear. Of course, this would probably require a round column mill or one with a head that rotates in the horizontal plane.
            Paul A.
            SE Texas

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

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            • #7
              Not really grasping the cam , but a sliding pin in the subtable, fitting in the t slot of rotab, will usually do many of those arcs..

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              • #8
                Make a plywood template of the arc you need and clamp it to the brass sheet and use a wood router with an endmill in it.

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