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Home Articles Page David@DavidReedSmith.com This article was published in the Spring 2007 edition of Woodturning Design.
Backup
Plate IntroductionThere are three traditional ways I knew of turning rectangular objects. One is to turn it round and cut it to a rectangular shape, which wastes a lot of prime wood. Another is to simply turn the project as a rectangle, which makes tool control difficult with the constant transition between turning wood and air, hard to sand without dubbing over the lead edges, and dangerous to your fingers. A third method is to glue waste wood to the edges of the project, which requires precise joinery and cleaning up the edges afterwards. The project I had in mind, a notebook cover, would have been nearly impossible using any of these methods. Not only did I want a large rectangular shape, but I wanted it centered eccentrically and it had to be very thin. The first two requirements made it difficult using the above methods, but the third was a deal breaker—can you imagine turning a flat disk nearly 20” in diameter only 1/8” thick and have it stay together much less turn chatter free? The solution I found was to use a Backup Plate. I cut a disk out of sheet goods and screwed it to a Mounting Disk which could be mounted on the lathe. I mounted the rectangle I wanted to turn on the disk with hot-melt glue and then filled in the rest of the circle by gluing waste wood to the disk with hot-melt glue. This turned an almost impossible task into an easy one. The only remaining problem was the shear size of the disk, which only required moderating the speed to solve. The waste wood fillers eliminated the air/wood transition, the rigid Backup Plate suppressed chatter, and the gap-tolerant hot-melt glue made precise joinery unnecessary and clean up easy. The only drawback is that the technique is limited to objects that are turned as more or less two dimensional. MakingThe first step in making the Backup Plate is picking out suitable material. What you want is some cheap, because it should be regarded as a semi-consumable, and something flat and stable. Sheet goods are the obvious choice, but which ones? Stable leaves out cheap Luan plywood in my experience. The first thing I tried was some ¾” MDF and that worked fine, although removing the hot-melt glue did tend to take chunks off with it. Certainly if you’ve got MDF or decent plywood on hand you should use it. But if you have to go out and buy something Melamine would make a good first choice. It’s stable (except when overloaded as shelving) and the coating mostly releases the hot-melt glue without harm. Besides, I still have a goodly bit of melamine left over from the Shelving Incident (a particularly plural violation of the “measure twice, cut once” rule). The second step is figuring out how big to make it. This will depend on the turning capacity of your lathe, the size of your project, and how off center you want the project to be turned. Sketching it out full size on the material you select is a reasonable way to figure this out. I picked 20” for the one in the pictures. I’ve made several Backup Plates, as smaller ones are better for smaller projects. You don’t want to deal with 20” of swing if you’re only using 4”. Layout where your center will be on the material you’ve selected. As you’re unlikely to have a compass big enough if making a big Backup Plate, find a strip of scrap and drill two small holes, as far apart as the radius you want. Loosely peg the strip through one hole to the sheet goods with a nail at the center. Put a pencil in the other hole and trace the circumference of the disk. Then cut out the disk on your band saw.
Next you must figure out how you’re going to mount the Backup Plate to your lathe. You could use a faceplate, a wooden faceplate, or turn a disk to mount in your chuck. I don’t recommend screwing the Backup Plate directly to your faceplate, nor turning a recess in it for chuck jaws as sheet goods aren’t particularly strong perpendicular to the face. It’s better to make a smaller disk and screw the Backup Plate to that, with the screws running through the Backup Plate into the smaller disk. Use a hardwood such as Maple. Temporarily mount a Mounting Disk, about 3” in diameter to your lathe. I’m using a faceplate in the pictures. I’ve used wooden faceplates more often. Turn the disk round and the face flat. Drill a pilot hole for the size screws you’ll use in the center of the disk. Remove the Mounting Disk from the lathe. Drill a clearance hole for the size screws you’ll be using through the center of the Backup Plate. Stick a nail or screw through the center hole in the Backup Plate and use it to center the Mounting Disk. Trace the outline of the Mounting Disk on the Backup Plate, and then remove the disk. Alternatively, you could use a compass to draw a circle the size of your Mounting Disk. Mark locations for three evenly spaced holes about ½” inside the Mounting Disk diameter, and then drill clearance holes for your mounting screws. Countersink all the clearance holes so that the screw heads will be below the surface. Then mount the Backup Plate to the Mounting Disk using screws. Optionally you can reinforce the joint with glue. Polyurethane sticks to Melamine okay. Start by driving in a screw through the center hole into the pilot hole drilled in the Mounting Disk. Then drill pilot holes in the Mounting Disk for the remaining three screws through the Clearance holes in the Backup Plate. Drive screws in the remaining three holes.
The last step is to mount the Backup Plate on your lathe and true up the rim. Turn the lathe on, starting at a low speed, and turn the rim true with a bowl gouge. You’ll need to sharpen your gouge when you’re done, as the high glue content in most sheet goods is awfully hard on steel edges.
UsingThe first step in using the Backup Plate is to layout your project. You could draw it on paper on your computer using a CAD program, but one of the nice things about Melamine is you can easily draw and erase on it. Start by cutting your prime wood to size. You may wish to allow a small amount extra for cleaning up edges. You can plane the wood to near the thickness you want or plan on doing that on the lathe, depending on which is less work. Plop the prime wood down on the Backup Plate and move it around until it’s centered where you want it. Then trace around the prime wood so you can quickly relocate it. I’ve found hot-melt glue to work well for mounting wood to the Backup Disk. It’s cheap, gap tolerant, sets quickly, doesn’t require clamping or lots of pressure to activate, holds well enough to turn, but isn’t too hard to release or clean up. You could consider using double stick tape, but the tape requires initial pressure to bond, which will be hard to get if your project is large. A paper joint would be doable for small project you can clamp adequately, but a good bit more trouble mount and a lot more trouble to clean up. So heat up your hot-melt glue gun. Have an extra glue stick on hand. Run a quick squiggle of hot-melt glue on the back of your prime wood and without delay place it down on the Backup Plate where you traced its location. Press down on the prime wood so that it bonds evenly. Now find some wood to use as filler. I generally use pine because it’s cheap. Again, you can plane it to the thickness, or plan on turning it. It is easier to turn if the thickness is reasonably close to the thickness of your prime wood. Make sure the filler stock has a reasonably straight edge and a 90° corner. Put the Backup Plate on your workbench so that it overhangs the edge. Place filler stock onto the Backup Plate so that the straight edge is along one edge of your prime wood and the 90° corner is at a corner of the prime wood. The filler stock should continue until it runs off the Backup Plate. From underneath the Backup Plate, use a pencil to trace the outline of the Backup Plate onto the bottom of the filler stock. Cut just outside the traced line on your band saw. Quickly apply some hot-melt glue to the bottom of the filler stock (you can tell because the traced line should still be visible) and put it in place on the Backup Plate. Don’t bother to try to apply glue in the joint between the prime wood and filler. Avoid the temptation to cover as much Backup Plate as possible with the first piece instead of starting at a corner, as that will make the last piece much fussier to cut. Cut a new 90° corner on the filler stock, and place it on the Backup Plate at the corner formed between the prime wood and the edge of the filler wood that runs off the Backup Plate. Trace, cut and glue on as before. Continue to attach two more pieces in a similar manner and the entire Backup Plate will be covered.
Mount the Backup Plate on your lathe and turn your project. Start with a slow speed and take gentle cuts with just the tip of your bowl gouge at least until the surface is trued up. Keep the tool rest between you and any potential flying pieces. I’ve never had a segment come off of a Backup Plate, but there is an unavoidable possibility of this happening. If you glued the pieces on securely enough to survive a high energy catch then you would never be able to get them off when finished turning. Despite the mandatory safety paragraph above, I’ve felt quite comfortable using the Backup Plate. Turning a 20” disk on a Backup Plate feels easier and safer than turning a 3” bare rectangle. The Backup Plate keeps even extremely thin work from vibrating and sanding is much easier on your fingers. Really, give it a try. It will open up whole new avenues of design opportunity for you with a “how did he do that?” flavor. MaterialsSheet goods for Plate, size depending on your project size and desired eccentricity Scrap Hardwood for Mounting Disk, about 3” in diameter. AuthorDavid Reed Smith is a Basement Woodturner living in
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