When using a four-jaw scroll chuck for wood turning the best hold is achieved when the tenon or recess is matched to the “perfect circle” for that particular chuck. One way to achive this is to make a gage for each of your scroll chucks. I made it at The Tech Shop San Jose,, http:http://www.techshop.ws/,using the Epilog Laser Engraver to cut my gauges out of ¼” melamine faced MDF.1/4" plywood also works. Do not be tempted to use Plexiglas. If you offer Plexiglas to spinning wood, to check your progress, the Plexiglas will melt. Trust me.
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The diameter of this circle can be found by measuring between opposite jaws, both at the middle of their arcs and at the end points of the jaws. When these measurements are the same, the jaws are forming a “perfect circle”
This will be true for both the inside measurement, compression mode, and the outside measurement, expansion mode. Record these two numbers, the inside dimension (ID) and the outside dimension (OD).
Use these measurements to size the cutouts and compass point holes, at ½ the ID or OD, as shown here: