A sizing tool used with a beading and parting tool is a quick way to turn tenon after tenon on the lathe to within a couple thousandths of an inch in diameter. Precise sizing of the mortise and tenon produces very strong joints. Combine that with optimal grain orientation in the joint, assembly of the pieces with the tenon at a lower moisture content than the mortise, and a little hide glue; and you end up with chair joints that can withstand a couple of hundred years of abuse.
But getting the sizing tool set up to cut exactly the diameter that you want has been a frustrating process. Accurately sized set up blocks followed by taking measurements with a micrometer get you in the ball park, but getting the final adjustment from there has been a trial and error process: loosen the thumbscrews, attempt to move the sizing tool two thousandths of an inch closer to the cutting edge, and try again until you get the exact size that you want. Sometimes the set up goes quickly and sometimes you end up chasing your tail for a little while.
Last week while I was doing something else I had a head slap moment and realized how to make a simple adjuster that would introduce more control into the trial and error.
The adjuster has a square mortise to slide over the beading and parting tool, a set screw to secure it on the shaft, and a thumbscrew to press against the back of the sizing tool and aid in adjustment. In the photo above, note the counterbore that gives the thumbscrew more clearance. The adjuster has to be compact so that it fits in the space between the ferule and the back of the sizing tool.
I cut a notch out of the upper portion of the adjuster to give the thumbscrew on the sizing tool clearance to tighten. The thumbscrews of the sizing tool must be loosened for adjustment.
I tapped the holes for the screw threads.