joe greiner
13th June 2007, 10:53 PM
My Longworth Chuck variation, part 1
First, let's offer a toast to the late Mr. Leslie Longworth for developing the basic design. Scroll chucks have been around for ages, but Longworth was the first to incorporate two sets of arcs to establish the radial positions of the buttons.
On this one, the disks are 12 inches OD. My 12-inch lathe has the spindle 6 1/8 inches above the bed, so I opted for a minimum clearance of 1/8". It can hold rims of 96 to 248 mm (OD) in compression mode, and 144 to 296 mm (ID) in expansion mode. (I love to mix measuring systems.) I chose six buttons for a better grip than four. Eight buttons would have been too crowded at the minimum diameter. And it's easier to lay out a hexagon than a square or octagon - all done with straightedge and beam compass alone, without need for perpendiculars. The back disk is 23/32" plywood; the front is 7/32".
My mounting is via the intermediate jaws of my four-jaw chuck, in expansion mode. This was driven mostly because of my lathe's motor mounting - above the bed, and projecting beyond the headstock spindle. The stock faceplate has a long enough hub, but I would have had to cut down its diameter, and my after market smaller faceplates have shorter hubs. I eschewed a glued-on disk for compression mode, because I figured it would be another opportunity for misalignment. The back disk is from an old piece of plywood - almost no internal voids.
I made a plexiglass template to guide a router bushing with a dovetail bit; the dovetail angle isn't the same as the chuck jaws, but it's better than straight sides. I set the position of the jaws so that they would be in line with the body of the chuck. (See pic 4 in part 2.) The slots aren't concentric; they match the curvature of the jaws at their designated positions. I used the centre hole of the template only to establish the outer registration points; at 360-degree rotation, they missed closure by less than half the nail diameter - close enough. I drilled an access hole in each slot near the innermost excursion of the jaws; dovetail bits don't plunge very well. Most of these pics are after construction, not WIP's; in pic 5, pretend that the arc slots and the half holes on the rim aren't there yet. (I added more half holes later.) I dribbled some CA on the jaw slots for reinforcement. The jaw slots reach the inside of the far lamination of the plywood, and the bridges between slots further maintain the location of the centre anchorage for the front disk.
I mounted the disk sandwich on the lathe to sand the outer diameter and to establish the work point circles for cutting the arc slots. Runout was as close to zero as I could hope for - no need to face the disks.
More in part 2.
Joe
First, let's offer a toast to the late Mr. Leslie Longworth for developing the basic design. Scroll chucks have been around for ages, but Longworth was the first to incorporate two sets of arcs to establish the radial positions of the buttons.
On this one, the disks are 12 inches OD. My 12-inch lathe has the spindle 6 1/8 inches above the bed, so I opted for a minimum clearance of 1/8". It can hold rims of 96 to 248 mm (OD) in compression mode, and 144 to 296 mm (ID) in expansion mode. (I love to mix measuring systems.) I chose six buttons for a better grip than four. Eight buttons would have been too crowded at the minimum diameter. And it's easier to lay out a hexagon than a square or octagon - all done with straightedge and beam compass alone, without need for perpendiculars. The back disk is 23/32" plywood; the front is 7/32".
My mounting is via the intermediate jaws of my four-jaw chuck, in expansion mode. This was driven mostly because of my lathe's motor mounting - above the bed, and projecting beyond the headstock spindle. The stock faceplate has a long enough hub, but I would have had to cut down its diameter, and my after market smaller faceplates have shorter hubs. I eschewed a glued-on disk for compression mode, because I figured it would be another opportunity for misalignment. The back disk is from an old piece of plywood - almost no internal voids.
I made a plexiglass template to guide a router bushing with a dovetail bit; the dovetail angle isn't the same as the chuck jaws, but it's better than straight sides. I set the position of the jaws so that they would be in line with the body of the chuck. (See pic 4 in part 2.) The slots aren't concentric; they match the curvature of the jaws at their designated positions. I used the centre hole of the template only to establish the outer registration points; at 360-degree rotation, they missed closure by less than half the nail diameter - close enough. I drilled an access hole in each slot near the innermost excursion of the jaws; dovetail bits don't plunge very well. Most of these pics are after construction, not WIP's; in pic 5, pretend that the arc slots and the half holes on the rim aren't there yet. (I added more half holes later.) I dribbled some CA on the jaw slots for reinforcement. The jaw slots reach the inside of the far lamination of the plywood, and the bridges between slots further maintain the location of the centre anchorage for the front disk.
I mounted the disk sandwich on the lathe to sand the outer diameter and to establish the work point circles for cutting the arc slots. Runout was as close to zero as I could hope for - no need to face the disks.
More in part 2.
Joe