I have updated my Chuck Sizing Guides. The “new” shape is simpler to use.
These are not marking gauges – that is, I don’t use them to mark the blank. Instead I use them to check my marks. So first, I guess at the right diameter and mark the blank. Then I check it with the gauge and re-mark. Usually that’s all it takes, but a third adjustment will get me there in any case.
I have never liked holding metal calipers against spinning wood. These seem safer, easy to use, and I don’t have to remember or look up numbers to set the calipers.
I cut guides for each of my chuck jaws from scrap 1/4″ plywood. There is a point on one end and a flat on the other. From the point to one corner of the flat is the right size to start a tenon, to the other corner is for a recess.
Getting the measurements right took a bit of trial-and-error. The measurement I wanted was where I start cutting the tenon or recess. The diameter where you start cutting ends up on the inside of the jaws when you’re done.
So to get the measurements, I removed two jaw segments from the chuck and placed them in their perfect-round diameter (where the gap between the segments is about 3/16″). That makes a half-circle and I can measure between the jaws, near the bottom.
For the recess, the starting point can not be any smaller than with the jaws fully closed, otherwise you won’t get the jaws into the recess. That turns out to be about the right diameter when you expand the jaws.
I made a guide for one of my jaws, and tested it out by making a tenon. Then I tweaked the guide to get it just right. I used that “tweak” to help me know how best to measure for the other jaws. I feel this is important, as the guides have to match my style and how I use them.
I marked the measurements I ended up with on the guide, so I can re-make it if it breaks. They are painted blue so I can see them amongst the chips when I drop them.
The guides also have a small slot cut on the side to indicate the maximum chuck jaw depth.
