I have an old Powermatic 90 lathe that’s as old as I am (pretty old) and I like to turn bowls. For those that don’t know, a common way to hold a bowl is with a 4 jaw chuck, and we use it to grab a tenon on the bottom of the bowl. After finish turning the outside and inside of the bowl, that tenon must be removed.
I’ve always struggled with how to hold the bowl with the open side towards the headstock so I can turn off the bottom. Enter the Longworth chuck. Not my design, it’s been around for a long time, and there are a lot of free videos and plans on how to make them. It has two round plates with arcs cut into them. The front and rear plates are opposites, so when you rotate one the intersection of the arcs increases or decreases in radius. A bolt through both plates and some rubber stoppers move in and out to allow you to hold the bowl. This one is 16" in diameter and can hold a bowl from about 14.5" to roughly 4.5".
I found free plans and redrew them in FreeCad then followed a pretty typical workflow that I use. Export as dxf, import into Inkscape for some touch up, then save that as svg for use in Estlcam. I’m going to try to link a video I took. In the video you can see a cut after I mount the bowl. During the pause I tightened the 8 wing nuts on the back of the chuck before spinning the lathe. Also, you can see my lowrider 2 table in the background of the video.
To be honest my lathe doesn’t have a tach so I don’t know how fast it turns, I’ve always just gone by what feels right. Well I didn’t know until tonight… you got my curiosity piqued so I skipped some other chores and went down and learned something.
I had a cheap mag pickup tach squirreled away so I stuck a rare earth magnet on the spindle and mapped it. The lathe has a reeves drive for variable speed. The speeds on it are for a factory motor pulley, but the guy I bought it from put a smaller pulley on it to slow it down for bowls.
I put a piece of blue tape around the rim and mapped the speeds with the tach. At the “STOP” mark, it spins at 275 rpm and that’s the speed I used in the video I uploaded before. I haven’t actually used this chuck yet but I don’t think I’d go much faster than that. Maybe 500 or so at the top end. The outer rim at 16" gets moving pretty fast. At 275 rpm the edge is moving at 13 mph.
The grippers are just #4 rubber stoppers in which I drilled 1/4" through holes for the 1/4" bolts. When you tighten the wing nuts on the back it sort of squishes them against the bowl edge to improve the grip.
I was really happy that the thing is very balanced. MDF seems to be of very consistent density and is very flat. With the precision cuts made on the cnc it seems perfectly balanced. I have a foot pedal that lifts the motor so the belt slips. I step on it, start the lathe, then ease it up so the belt engages like a soft start. I eased into it at first to make sure it was running true and am happy that it does run very true.
