Not sure, but perhaps this little idea will be able to help someone.
I
occasionally get a pot that goes every so slightly oblong after turning, slightly cups, or otherwise freaks out. Occasionally it is my fault for turning wood that I thought was dry that actually wasn't quite ready, more often it is dry wood that just relaxes after being turned. I fit my surfaces relatively tight to the edge of my pots, leaving a business card's thickness around the perimeter. So, if the wood moves after turning I'm in trouble because my slate won't fit in, or fits too tight. But it is a risk I take because I'm a freak about not seeing gaps filled with goop. Several pots have gone into the trash can in the last couple years after this happens, long before they ever got surfaces put in them.
Had it happen on a piece of very dry African mahogany last week. I decided to create a fixture to save it. Cole jaws just are not accurate enough and there is just about zero repeatability with them, so I quit messing with them a long time ago. I took a piece of 8/4 ash, screwed it to my lathe's faceplate (which I don't use for anything), turned it round, drilled a 1/2" hole in it on the lathe to perfectly center the hole on the faceplate and then made it slightly concave. I drove a 1/2" o.d., 3/8" i.d. nylon bushing into the hole, and epoxied a 3/8" hex bolt into the hole from the back side.
Now I can slip a screwy pot onto the fixture and use a parting tool to remove a tiny amount of wood from the circumference of the inside where the slate sits. I can also flatten out the top if there is minimal cupping, after chucking it in my 4-jaw and flattening the bottom of the pot. The nylon bushing perfectly fits the hole drilled in the middle of my calls so there is no play, which is critical. Here's a picture of a pot being resurrected. The flat washer puts pressure on the soundboard pedestal and everything is secured with a wing nut.
This pot warped over a year ago and it is such beautiful claro walnut crotch I just didn't throw it away. Glad I didn't because tonight I saved it by re-shaping it with a carbide tool, then sanding, then cutting a better fitting slate ledge. Wait until you see it with finish!
