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Machining convex profiles

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  • Machining convex profiles

    Spheres can be machined with a flycutter on a rotary table. Has anyone ever made a contraption on the lathe or the mill to machine convex profiles? It's for a guitar I'm building; the fretboard will be made from aluminium, and needs to have a specific radius.


  • #2
    Looks like You need to modify/ regrind a slab cutter with the proper radius profile, put it in a horizontal mill and let it rip.

    Steve

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    • #3
      You can do a convex surface with the top side of a flycutter for a short distance, maybe an inch or 2 with the head angled. This would not help on your project.
      On the mill you could use a ball end mill and do cuts lengthwise, stepping up to the centerline, then going down the other side and draw file off the cusps or high spots between milling cuts.
      On a lathe you could mount the fretboard on a cylinder with the top surface at the correct radius and simply turn a straight line or a straight angle. This would require a lathe with a fairly large swing and may not be practical in your case.
      Another way would be to have someone with a CNC mill do it for you.

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      • #4
        Plane it or sand it, like wood. Aluminum cuts and sands better than many hardwoods.


        Either make a blade for a plane (or planing jig) that has the curvature you want, or make a jig with a sander that will swing the fretboard past the belt in an arc, thereby cutting the curvature you want. If you go this way it's easy to make a compound curvature fretboard as well.


        A jig for the sander could be as simple as attaching the fretboard to a couple arms that rotate around the same axis, for example by being loosely bolted to a fixture. You'd then manually swing the fretboard past the sander.

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        • #5
          You'll also need to cut the fret slots to match the curvature- are you planning to do this by machine as well? I can see some ways to do it, but none of it is easy or simple. For my guitar project, I made up a right angle head for the mill so I could do the fret slots, but I left the fretboard flat because I couldn't come up with a good way to curve it nicely.
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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          • #6
            Paging Dr. Frank Ford, Paging Dr. Frank Ford to the guitar neck project.

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            • #7
              Mount it on a rotary table

              Or offset in a lathe. (might need to make a shaft with dogs on both ends to 'carry' the torque, and some counterweight on the other side of the shaft to keep it balanced)

              Other solutions include mounting it on the mill, doing multiple passes with setting it at a diffrent angle each time. Once its roughed out, some light sanding will take care of the facets without letting user error deform the curve much.
              Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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              • #8
                First I would make a piece like this on the 4-jaw chuck. You can do it as one or two pieces. Both the red and blue surfaces share a center point, with a radius less than that of your part. The yellow surface bolts to your guitar neck, and its height determines the radius of curvature.



                Next I would make this piece also on the 4 jaw. The red surface has the same radius as the other red surface.



                The base of this piece rests against the table. The other piece sits on top of this one, with the two red surfaces contacting. The usual t-slot clamp contraption rests on the blue surface.

                I would make three sets to support the guitar neck on the two ends and the center.

                Make a cut, unclamp, and rotate it over by a little bit at a time.
                You don't have to be super careful about the indexing or where exactly it rests on the table or the alignment. It is not possible to accidentally take off too much material.

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                • #9
                  That's easy (See Black_Moons post), just add a riser block to the headstock and tailstock to match the radius + a little more for clearance if your lathe isn't big enough.

                  Turning things like this in wood is fairly common. Do a search on multi-axis turning and also therming for some techniques.

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                  • #10
                    Aluminium...No.......................... Run....Run Quickly....

                    I used to have a Dan Armstrong Ally necked clear plastic guitar....and the neck was awful to play.(looked cool though).
                    Always going out ot tune with temperature and just not a nice material for a neck.


                    You would need quite a big lathe to turn a neck ..Older Fender ish necks were a 9-10 inch radius and more modern necks tend to be 12.

                    My favourite neck shape is the assymetrical C shape 10->16 inch compound radius on my Jackson Adrian Smith/San Dimas strat type....just fantastic...

                    Rob

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                    • #11
                      I am not a guitar maker so this may be totally off base, but does it really need to be an exact radius as in a segment of a cylinder? I have to believe that many guitars were made with hand tools and the curves were made by eye and sanding. So, could you not just start with two or more angled cuts and then plane and sand it to a smooth curve? I would have no hesitation in doing this in wood. I do not see how this would have any adverse effect on the performance of the instrument. The radius spec may just be a way of saying it should be this approximate shape.

                      As for the fret slots, could they not have straight bottoms and the inserts (frets) be shaped like skinny "D"s to fit?

                      As I said, I do not make musical instruments, but this seems obvious to me.
                      Paul A.
                      SE Texas

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

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                      • #12
                        May be off base here, but could you not use an indexing table (with a tail stock) on a mill and make a plate for the tail stock to hold the piece up so when the able is rotated you get the right curve ? Would probably require making a plate (like a face plate) to go in the tail stock to "lift it up" from the normal center of rotation.

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                        • #13
                          Seems pretty obvious that a jig of some kind would be required to control the neck, and multiple passes taken to rough out the shape and refine it as much as time and patience permits. If the neck blank was held firmly on the mill table, then the cutter would have to trace out the arc. The original question was basically how to achieve this.

                          It seems you can't have a cutter making complete rotations with the spindle and allowing this shape to emerge. But you could have the stationary spindle carrying a router of sorts, which would ride a curved track. It would make multiple passes along the track, and for each pass the table would be moved along some small amount.

                          This does give me an idea. What if you mounted a cutter on an arm which was free to pivot up and down to some degree. The arm is held in the spindle, and is spring loaded to remain 'up'. If you rotated the spindle, the cutter would sweep out a flat circle like any other trepanning cutter. But then above the arm, mounted stationary on the quill, you have a disc with a bump in one side. As the arm comes around, a bearing mounted on the arm rides the disc and follows the curve of the bump. You would create a path that pushes the cutter down, allows it to arc upwards then down again, then allows it to come fully up so it clears the workpiece on the second half of the rotation.

                          This is a rather crude description, but it gives the general idea. The shape you create in the disc determines the path of the cutter. I can envision a couple of ways of making this mechanism, but it would take a while to describe. I'll have to think on it more to see whether I can end up with a simplified version. Also, it would need to be very solidly constructed so as to resist the deflection effects of cutting the aluminum blank.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                          • #14
                            Use common wood tools and a basic jig: http://www.youtube.com/watch?v=T2NOFadrtKY

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                            • #15
                              Originally posted by dp View Post
                              Use common wood tools and a basic jig: http://www.youtube.com/watch?v=T2NOFadrtKY
                              Bravo!
                              Paul A.
                              SE Texas

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

                              Comment

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