This is a very simple workshop aid. So simple that I hesitated to post about it. But it eliminates the risk and possible errors associated with using dividers. I made some of these chuck sizing templates to help beginners during my bowl turning classes, but they are so convenient that I now use them for my own work too.
A chuck sizing template helps you to make the chucking tenon or recess on a bowl the right diameter for the chuck. It should fit when the chuck jaws are almost closed, because that’s when they have the best, non-marking grip.
The usual method is to use dividers set to the correct diameter to scribe a circle on the work. If the dividers are set correctly, if both points fall on that circle and if the points are on the diameter (that is, not too high or too low on the work), the circle will fit the chuck. This method works well, but takes practice. It can be risky, because only the left point should touch and it’s possible for the points to catch in the spinning wood.
As an alternative, I made some templates from offcuts of 6 mm MDF. They are just small rectangles, with widths to suit the chuck jaws. You could add a millimetre or two to the ‘correct’ size to give some allowance for error when cutting the tenon or recess. They aren’t adjustable, so each set of jaws needs its own templates for marking a tenon and recess.
Using the chuck sizing template.
The end of the template has a mark in the middle. I hold that mark to the centre of the spinning wood, which can be highlighted with a pencil if necessary. Then I use a pencil to make a circle the same diameter as the template’s width.
It only takes a moment to mark the circle. And it’s the same each time with no need for measurement or setting dividers. You need separate chuck sizing templates for each set of chuck jaws. But a benefit of that is you can try the templates against the bowl blank to select the best size.