I've turned several bowls and realized the need for a way to touch up the foot of the bowl. So I decided to build my own Longworth chuck. How hard could it be? Starting with some 3/4" Baltic birch plywood I cut two 17" squares and secured the corners and center together with brads. I was careful to keep the brads away from any cutting areas. I cut to within 1/16" of a 16" circle and mounted the blanks on the router mill (shown) and trimmed them round. I realized a dedicated face plate would take up too much of my face plate resources so using a billet of aluminum I turned a plate that would be permanently recessed into the wood 1/8" for strength and concentricity. I used an indexing head to locate the 8 holes for hex socket flat head machine bolts that locked the plate to 1/4" T-nuts countersunk between the wooden plates. Laid out the arcs as shown and using a double pivot arrangement carefully routed the curved slots. The round sub-table pivot established the curves and the other pivot centered the indexing for the 8 slots. (you can see the indexing tick lower left). Some TLC with sandpaper and two coats of lacquer made very smooth surfaces for a silky gliding action. Some #1 rubber stoppers on 1/4" bolts and washers completed the setup. The plate nests securely in the dovetail of the chuck jaws. Now to test it.
Dan is an experienced woodworker who is anxious to make an heirloom of the future for you.