Backup Plate
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This article was published in the Spring 2007 edition of Woodturning Design. 

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Backup Plate as 3 page pdf

 

Backup Plate

 

Introduction

There 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 breakerócan you imagine turning a flat disk nearly 20Ē in diameter only 1/8Ē 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.

 

Making

The 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 ĺĒ MDF and that worked fine, although removing the hot-melt glue did tend to take chunks off with it.  Certainly if youíve 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.  Itís 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 ďmeasure twice, cut onceĒ 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 20Ē for the one in the pictures.  Iíve made several Backup Plates, as smaller ones are better for smaller projects.  You donít want to deal with 20Ē of swing if youíre only using 4Ē.

 

Layout where your center will be on the material youíve selected.  As youíre 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.

 

Fig01:  Using scrap wood, a nail and a pencil to draw a 20Ē circle on Melamine.  I actually do have a beam compass I inherited from my Dad.  I just donít know where it is.

Next you must figure out how youíre 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 donít recommend screwing the Backup Plate directly to your faceplate, nor turning a recess in it for chuck jaws as sheet goods arenít particularly strong perpendicular to the face.  Itís 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 3Ē in diameter to your lathe.  Iím using a faceplate in the pictures.  Iíve used wooden faceplates more often.  Turn the disk round and the face flat.  Drill a pilot hole for the size screws youíll use in the center of the disk.

 

Remove the Mounting Disk from the lathe.  Drill a clearance hole for the size screws youíll 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 ĹĒ 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.

 

Fig02:  The Mounting Disk next to the countersunk mounting holes on the Backup Plate.  The center hole in the plate and the matching hole in the Mounting Disk helps you mount it right in the center.

 

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.  Youíll need to sharpen your gouge when youíre done, as the high glue content in most sheet goods is awfully hard on steel edges.

 

Fig03:  The finished Backup Plate mounted on the lathe.

Fig04:  The finished Backup Plate mounted on the lathe viewed from the rear quarter.  The Mounting Disk is thicker than it ordinarily needs to be, but I have something in mind to try where Iíll need it.

 

Using

The 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 itís centered where you want it.  Then trace around the prime wood so you can quickly relocate it.

 

Iíve found hot-melt glue to work well for mounting wood to the Backup Disk.  Itís cheap,  gap tolerant, sets quickly, doesnít require clamping or lots of pressure to activate, holds well enough to turn, but isnít 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 itís 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 90į 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 90į 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.  Donít 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 90į 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.

 

Drawing01:  Doís and Doníts of fill patterns.  The pattern on the left is much easier.  The pattern on the right requires exact cutting and placement of the last piece.

 

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.  Iíve 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, Iíve felt quite comfortable using the Backup Plate.  Turning a 20Ē disk on a Backup Plate feels easier and safer than turning a 3Ē 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 ďhow did he do that?Ē flavor.

 

 

 

Materials

Sheet goods for Plate, size depending on your project size and desired eccentricity

Scrap Hardwood for Mounting Disk, about 3Ē in diameter.

Author

David Reed Smith is a Basement Woodturner living in Hampstead , Maryland .  He welcomes comments, questions, and suggestions via email at David@DavidReedSmith.com.  This article will be available on his web site at www.DavidReedSmith.com