Thickness Gauges for Small Hollow Forms
(An original design)
Being able to turn a hollow form to a wall thickness of 1/8” or less depends on being able to measure the thickness of the wall. Otherwise, you will be working blind and very well may cut through.
A thin wall is desirable for globe-type tree ornaments, for example, to reduce the weight of the ornament. The gauges described in this article allow you to turn a 2” globe to a wall thickness of about 1/16” with confidence if the wood is reasonably sound.
More than one gauge is needed in order to measure the wall thickness at different points, like near the opening, middle, and bottom of the piece. The nail must contact the surface at very nearly a right angle to give an accurate indication. I made three gauges, each having the nail set at a different angle.
Make a Gauge.
Cut out the body. I cut mine from thin, scrap paneling. Plywood 1/8” thick can be purchased from a hobby shop and will do nicely. The diagrams below show each gauge laid out on a grid of 1/4” squares that should serve as a guide.
The nails. Locate the nails to be used. Mine are about 0.120” in diameter. Clean up the surface, if necessary, with a file or sandpaper. You can cut them to a suitable length a bit later.
The part that holds the nail. The easiest way to make the nail holders is to turn them on your lathe. This amounts to turning a cylinder 3/8” long with an outside diameter of about 5/16”. And, of course, a hole for the nail is drilled through the center.
To turn a nail holder, install a short section of 1/2” hardwood dowel in a scroll chuck. Drill a hole into the dowel about 1/2”. Make the diameter of the hole just a few thousandths of an inch larger than the nail you plan to use. Here is an instance where having a set of number drills is a time-saving convenience.
If you don’t have a micrometer or caliper to measure the diameters of the nail and drill bit, do it by trial and error. Select a bit you think will work and then drill a hole in a piece of scrap and see if the nail will fit. If it does, you’re in business. The nail should slide easily in the holder but not tend to drop out.
Once the hole is drilled, turn a 1/2” length of the dowel down to about 5/16” diameter. Make a mark 3/8” from the end. This is where the cut will be made to separate the hollow cylinder from the rest of the dowel.
Before you make the cut, take a drill bit slightly smaller than what you used to drill the hole and mount it backwards in the Jacobs chuck. Insert the smooth end of the bit into the hole in the cylinder so that when you cut it away from the dowel, it will remain on the drill bit. Otherwise, it will go flying into the shavings, forever lost.
Use the sharp point of a skew, a parting tool, or a fine-toothed saw to cut off the cylinder.
Attach the nail holder to the body. The tricky part of this operation is holding the small cylinder in place while you apply the glue. Here’s a method that works for me.
Put a spacer about 1/8” thick under the body and clamp it near the edge of a worktable. Place the cylinder against the body where it is to be attached. The body should contact the cylinder at near its centerline. Adjust the thickness of the spacer as necessary. A stack of old business cards makes a good spacer, easily adjusted by adding or removing cards.
Insert the nail into the cylinder and again place it against the body. Check the alignment of the nail. Does it make contact with the lobe on the other arm where it should? Trim the body as required to get the proper alignment. The fit does not have to be perfect.
After you are satisfied with the fit, tack the cylinder to the body with a drop of thin CA to hold it in place. Then, without moving anything, fill the cove between the cylinder and the body with epoxy. After giving the epoxy time to set up, remove the clamp, turn the assembly over, and apply epoxy to the cove on the other side.
Indicator. Now is the time to grind the point off the nail and adjust the length if it is longer than it needs to be.
I used black electrical tape wrapped around the nail as an indicator. Once the tape was in place, I ran a narrow bead of medium CA around the tape and then hit it with accelerator to make sure the tape stays put.
Once you have the indicator in place, the gauge is complete and ready for testing.
Test the gauge. Go turn a globe for a tree ornament and see how well your gauge works.