Sphere-Turning Lathe Attachment
In 1946, a friend described a ball-turning device his machinist had made to fit an Atlas lathe. Unfortunately, I never got a chance to look at it. I’ve tried to design and make an attachment ever since, and recently I succeeded. Although the machinist had made the fixture entirely of metal, I used wood for the fixture’s core and reinforced it on the sides with sheet brass.
The fixture bolts to the lathe’s bed and holds a gouge at a height that’s on line with the lathe’s centers. Then, with the lathe running, the fixture is pivoted to make the final cleanup passes on a previously roughed-out ball. The final diameter of the sphere is determined by the projection of the gouge’s tip from the block.
To make the fixture, I bandsawed the wooden core from an old bridge beam and machined the top and bottom of the base flat with a rotary planer in a drill press. Then, I drilled a 5/8-in.-dia. hole through the base to take a bronze bushing. The bushing’s length is critical; it must extend from the tool-rest attachment bracket below the ways that I use for bolting the fixture to the lathe bed, up through the base of the fixture and 1/64 in. above the upper steel plate. If the bushing is shorter, the fixture will be clamped tightly to the lathe and won’t rotate; any longer and there will be too much slop as the fixture pivots. The steel plates are actually sawblade stabilizers from Sears (part no. 94952). They have a 5/8-in. hole, just right for the bushing, and act as large washers that the fixture’s arm pivots between. Following a cardboard pattern, I cut out the 3/32-in.-thick hard sheet brass that surrounds the wood core, and drilled and countersunk it for screws. I used 1/8-in.-thick brass for the cap, which I dovetailed into the brass sides and tapped for the setscrews that hold the gouge.
The gouge sits in a groove cut into the top of the core under the brass cap. It’s critical that the gouge’s cutting edge be the same height as the lathe’s center; so adjust the depth of the groove for the gouge you’re using.
Ralph M. Luman, Virginia Beach, Va.
Fine Woodworking Magazine, August 1990 No. 83
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