Milling an S Shaped Crank

My first design for the reel crank (here in brass) was too “heavy” in the middle.
first design for s-crank

So back to the drawing board. Here I have reduced the diameter of the center circle, and moved one small inside radius to make the arm thinner. All curves are arcs of a circle.
new crank

Fabrication starts with drilling location holes for the handle, counterweight, and shaft.
drilling handle hole

Then I plunge mill four small holes which become inside radii.
plunge milling small radius

All the larger arcs are milled on the rotary table. The tool plate was carefully planned to hold the material in the correct positions.
cutting an arc on rotary table

This photo illustrates the emergence of the crank from bar stock.
s_crank emerging from bar stock

This entry was posted in Crank, Milling, Work Holding and tagged , . Bookmark the permalink.

2 Responses to Milling an S Shaped Crank

  1. i would sure like to know how you put S Shaped Crank blank on the The tool plate its real simple i bet thats why i can not figure it out what size is the tool plate?
    thank you
    richard westerfield

    • dave49 says:

      The initial step was to drill location holes for handle, shaft and counterweight. These are 0.116 inch diameter, a close fit for a 4-40 UNC screw. The drilled blank is then fastened into tapped holes in the tool plate with 4-40 socket head cap screws. I used brass washers as spacers to raise the blank off the face of the tool plate, but this is not really necessary. The tool plate is 3 inch diameter and 3/4 thick.
      The geometry problem here is to locate the tapped holes in the tool plate so the various finished arcs meet tangentially, so the arc transitions are smooth.
      After the crank is completely profiled, I make the center hole larger and square, to fit a square on the end of the crank.
      Hope this answers all,

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