Question: Twice turned bowls
When going from faceplate to a chuck, the bowls do not seem to run true afterwards and I have to return it. I often times have the same trouble when final turning rough turned bowls using a chuck. Anyone have tips to make sure they run true?
~ Darrell, AAW member
Answer: Shaping the Tenon
There are a couple of things that can easily cause this. One you may not be able to correct because it's the nature of wood but I'll give it a try. The others are the shape of the tenon, the size of the tenon and the length of the tenon.
I assume you are turning the outside with a faceplate and then turning a tenon, or rebate on the bottom. The first thing to do is to make the tenon the right size for the jaws you’re using. If you make a large tenon and the jaws have to open up, they are only being held by the eight corners of the jaws. These bite into the wood unevenly because of the difference in end grain vs side grain hardness. Wood compresses very little against the end grain and will compress more against the side grain. So the first thing to do is to choose jaws that will only have a small gap when fully closed over the tenon. This will tend to compress the wood more evenly and has more grip surface.
The next thing is to properly shape the tenon. If you have dovetail jaws, the shape of the dovetail can be a factor. Many turners custom grind a tool that will cut the tenon the exact shape of the dovetail jaws. If the jaws close on only the bottom or the top of the dovetail, they can crush the wood unevenly and make it shift off center.
The tenon should be shorter than the jaws are deep so that the wood rests on a flat area at the bottom of the tenon. This helps register the bowl square and also helps prevent the bowl from rocking and coming out of the chuck when you’re turning.
How you orient the wood in the chuck can cause problems. Let’s imagine the wood grain is running from 12 o’clock to 6 o’clock. If I mount the wood in the chuck with one jaw at 12, one at 3 one at 6 and one at 9, you will have end grain at 12 and 6 and side grain at 3 and 9. The side grain will compress differently than end grain and can easily throw it off. Rotate the grain about 45 degrees so it's at 1:30, 4:30, 7:30 and 10:30 or close to that. This will give each jaw a little end grain and a little side grain and can help with the compression problem. I'm assuming you have solid sound wood. If it's punky, it may compress unevenly no matter how you orient it but the next tip may help.
I start all of my bowls between centers so there is a center mark in the face of the bowl. When I remount it in the chuck, I always bring up the tailstock and push the bowl against the chuck while tightening. That can go a long way toward keeping the bowl aligning while tightening the chuck. If you don't have that centering hole, here's what you can do. I built a gadget on someone else's tip that goes into the threaded hole of my faceplate and fits snugly. It has a center hole. I put a long sharpened steel rod down that center hole and punch a mark in the wood exactly centered on the faceplate. I can use this to punch a mark that my live center will go into to align the bowl. If your faceplates are solid, simply drill a hole in the center while they are on the lathe using your drill chuck in the tailstock.
~ John Lucas, a retired photographer, has been working in wood for about 35 years and also dabbles in metalworking. He also enjoys modifying machines, making tools, and sharing his knowledge through written articles and videos. He has taught classes at John C. Campbell Folk School, Arrowmont, and The Appalachian Center for Crafts.