Popular Woodworking 2007-04 № 161, страница 40
The chuck is just a pair of counter-rotating disks. Their maximum radius is a smidgen less than the distance between the lathe's headstock and bed. Outboard turners could size chucks to accommodate still larger work, but they should consider increasing the thickness of the materials. One further outboard consideration is that the routed arcs should be cut in the opposite direction from those described below if the headstock is not reversed such that the workpiece is rotating counterclockwise. Some preliminary comments before starting to build: 1. Longworth's design shows only four jaws that are not well suited to handle side pressure. I discovered that it's just as easy to make a six-jaw chuck. 2. It's crucial to do the following steps in order. Trust me. 3. The spinning jaws and wing nuts can hurt. A lot. Start by cutting matching disks of 3/4" MDF for the back and V4" Baltic birch for the front. Other materials might work, but I know these stay flat. The next step is choosing how to mount the chuck on the lathe. You could turn a hardwood flange to fit your scroll chuck's jaws, and glue and screw it to the MDF disk. However, a self-mounting chuck is a convenience, so adding a small faceplate works well. It also ties up an expensive component, because removing and replacing the faceplate would soon wear the screw holes in the MDF. An alternative is a nut with the same thread as the lathe's headstock. (See ""Make the Headstock" on page 55.) Whatever you decide, tack the front disk to the back disk with four small finishing nails set about V2" in from the rim. Mount the assembly on the lathe, true up the rim, and round the edges. Now for the most crucial step: Use a chuck in the tailstock to drill a 1//16" hole through the exact center of both disks. This hole will ensure the chuck runs true after final assembly. Remove the chuck from the lathe and clamp it in a vise. Draw three circles. The smallest is the diameter of your faceplate or nut, plus about an inch; the largest is the diameter of the disk less about an inch; the middle circle is centered between the other two. At this point, if you're making a chuck larger than about 14", you have to decide how many j aws you want. I've made both, and while four are adequate, I recommend six. The photo at the bottom of the page shows the layouts for a four-jaw and a six-jaw chuck. For four jaws, draw two perpendicular lines through the center of the V16" hole you drilled earlier. For six jaws, draw three lines at 60° angles. I used drafting triangles. Dimple the intersections of the diameter lines with the middle ring, and then use the dimples to draw arcs from the outer ring to the tangent of the inner ring. (Drawing these arcs helps avoid mistakes while routing.) Keep the compas s set for the router j ig. I made a circle-cutting base (photo next page) for my router out of V4" Baltic birch. The hole is 3" in diameter for good vision, because all start and stop points are by eye. With a V4" bit in a router, set the compass pin against the edge of the bit closest to the wing on the router base, and draw a short arc on the wing. Drill a hole through that arc and insert a nail as a pivot pin. Push it through so that about V2" protrudes and set it in a dimple on the disk. Push the j ig flat and hammer the nail in at least V8" to ensure the j ig can't slip out of position. Routing the arcs is so easy that one can become a tad complacent and make a ruinous mistake. Take your time. Begin a cut by plunging the bit partially into the work about 1" away from an outside start point, back it up to that point -cautiously; this is a climb cut - and then pull forward to the end point. This 15" version of the Longworth chuck has six long jaws. Use your band saw to cut matching disks of 3/4" MDF for Here, it's being used to hold a small spalted maple bowl. the back and V4" Baltic birch for the front. The layouts for both a four-jaw and a six-jaw chuck. (You could use a trammel for both layouts.) 54 Popular Woodworking April 2007 |
