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Cutting a radius, not a sphere

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  • Cutting a radius, not a sphere

    Hi to all. This is my first post. I'm in the process of making an endoskeleton arm and I need to make a clean radius for the knuckles. I have tried the Sherline radius cutting tool, but the results are as you would imagine on the lathe: the radius has a beveled circular edge. Since I am a machining novice, I've been unable to figure out making the clean radius for the knuckle. I would appreciate any advice on the calculation theory for making the radius precise in using the milling machine and the rotary table, or any other method. This is escaping me. By the way, I am not set up for CNC.

    [IMG][/IMG]

  • #2
    Do you have a mill?

    I don't think you can make that on a lathe - unless you put an end mill in the chuck and a pivot pin for the part in the tool post.

    You'd normally do that on a mill with a rounding off jig or a rotary table.

    The jig is just a vertical pin that the part can pivot on and two stops.

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    • #3
      There's an easy way to do this.
      Drill the hole in your part first.
      Then place the part in a vise, on your mill, with a pin thru the hole.
      Now just endmill or facemill the part, rotating it after every pass to achive your desired radius.
      Raise or lower the table to get the desired radius.

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      • #4
        End rounding is usually done on a pin like KidZima showed. I like to do mine with my disc sander as opposed to the mill, but either will work.

        Be careful!

        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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        • #5
          I remember why I don't bother posting on this board - it's like I said nothing at all, next person says the same thing I said.

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          • #6
            There's nothing wrong with identical answers.

            Perhaps the next person typed out their post before you typed yours but took longer to submit it because they were making an accompanying picture?
            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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            • #7
              That image appears to be an artist's conception of a robotic arm, not an endo skeleton. Further, it wasn't designed but only drawn and with no concept or consideration of how such a thing would actually be assembled. If you are trying to duplicate the drawing in metal then you are in for some real challenges, especially if you want it to work to any degree.

              I would suggest you do some looking online for real robotic devices that have pick and place grippers as well as robotic prostheses for humans. The Japanese in particular are very good in this field.
              Free software for calculating bolt circles and similar: Click Here

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              • #8
                Originally posted by Evan
                . The Japanese in particular are very good in this field.
                The are definitely advancing the mecha and robo-maid technology.

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                • #9
                  Peter,the sketch made the process of using the pin much clearer to the novice.

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                  • #10
                    Originally posted by gwilson
                    Peter,the sketch made the process of using the pin much clearer to the novice.
                    I agree, sketches are usually a great help. I was responding to ScottM.
                    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

                    Comment


                    • #11
                      Originally posted by Evan
                      That image appears to be an artist's conception of a robotic arm, not an endo skeleton. Further, it wasn't designed but only drawn and with no concept or consideration of how such a thing would actually be assembled. If you are trying to duplicate the drawing in metal then you are in for some real challenges, especially if you want it to work to any degree.

                      I would suggest you do some looking online for real robotic devices that have pick and place grippers as well as robotic prostheses for humans. The Japanese in particular are very good in this field.
                      I'm attempting to fabricate the endoskeleton arm from the movie, Terminator 2. This is an image that someone on another forum put together based on the movie scene clips. So yes, it is an endoskeleton. I'm not as concerned with it being a functioning arm as I am with it being a display piece.

                      Thanks.

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                      • #12
                        Sorry, I often mix up exo and endo. I was thinking exo since I need to build one that actually works for myself.

                        It looked familiar but if the joints don't need to function you can get away with a lot. Makes some round pieces with holes and cut them in half, make some rectangular pieces with holes and cut them in half and soft solder them together with silver bearing solder.

                        edit: To make things easier I would build it up from brass and then electroless plate when finished. You can buy a nickel plate kit from Caswell.
                        Last edited by Evan; 08-22-2009, 08:16 PM.
                        Free software for calculating bolt circles and similar: Click Here

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                        • #13
                          Scott M, if you want to be the one and only to give someone the answer, you are going to have a hard time finding any forum that suits you. There are lots of reasons for answers to double up, one that has not been mentioned is sometimes someone has not refreshed the page and does not even know your have posted. I suggest getting a thicker skin.
                          James

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                          • #14
                            If you're comfortable using the lathe radius tool, you can first turn a hemisphere, and then flatten the sides using your mill. Of course, this will mean that the half-circle tab will be more or less centered in the part unless you start with a large diameter bar.

                            You can see how we did something similar in the following video. The nose of the part was turned to the desired profile and then milled flat.

                            Video Link

                            Alternative youtube link
                            Last edited by Glacern; 08-23-2009, 02:23 AM.

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                            • #15
                              You say the joints do not need to be functional. And, even if they do, the radius feature shown would not be a functioning element. It is only a way of providing clearance for the joint to pivot on the central pin which is the real hinge there without hitting the other side.

                              Soooo, the machining does not have to be exact. You could simply draw the pattern and use a file or a band saw or other kind of saw to cut to an approximate shape and then take it to a belt sander and make it "round". This would only take a few seconds, even with steel, unless you use some super hard alloy. In short, woodworking or modeling techniques should surfice here. I do this sort of thing all the time and with a little practice you could produce a finish that would be hard to tell from a precision ground part.
                              Paul A.

                              Make it fit.
                              You can't win and there is a penalty for trying!

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