Self-Supporting Depth Gauge
(An original design)
The gauge described in this article may be used to measure the bottom thickness of small bowls, vases, or hollow forms. It rests on the ways of the lathe and is therefore self supporting. A laser is used but in a way somewhat different than the customary method. The achievable precision is on the order of 1/16”, perhaps better.
You might say that such precision is not required and the depth can be measured with a much simpler gauge. For the most part, I agree. However, if you have accidentally hollowed a vessel to a depth greater than what you intended, you need greater precision in order to find how much, if any, of the tenon can be removed before you cut through the bottom – and make a funnel. This gauge will do the job.
Making the Gauge
My prototype fits a Powermatic 3520B and will measure depths to 12”. The laser is positioned so that it will slide past a vessel 8” in diameter, but the gauge can be used for vessels somewhat larger, depending on the outside profile.
The base. Make the base from 3/4” plywood with a length equal to the maximum depth you intend to measure. Its width should be very nearly the same as the width of the ways of the lathe.
Attach the alignment strip to the bottom of the base. Position it so the base is centered on the ways when the strip is pulled against the near side of the gap between the ways. Make sure the strip is parallel to the long edges of the base.
Dowel holder. Measure the distance from the ways to the center line of the lathe, which will be nearly the same as one half the swing of the lathe. For the PM 3520B, this distance is 10.25”. Make the length of the dowel holder 3/4” more than what you measure.
Make the cut at the bottom of the dowel holder as shown in the drawing. Drill the hole for the dowel. (See below for details of the dowel.)
Install a point center in the tailstock. Place the base on the lathe. Pull it toward you so the alignment strip makes contact. Position the dowel holder on the base so the hole for the dowel is centered on the point center in the tailstock. Mark the position on the base for the dowel holder.
Attach the dowel holder to the base. (Drill a pilot hole for the screw.) For extra strength, epoxy a small brace between the holder and the base.
The dowel. I used a 1/4” dowel, which works fine. It should slide freely in the hole in the dowel holder. If it’s too tight, sand it down. Smoother operation will result if you coat the surface with a bit of paste wax. It doesn’t matter if the fit of the dowel in the hole is such that the dowel wiggles a small amount.
I turned a fancy knob for the end of the dowel. However, I now realize that the knob gets in the way of a machinist’s scale that might be used to measure the distance between the surface of the dowel holder and the indicator mark. You can omit the knob. It won’t look as pretty but will be more functional.
Support for the laser holder. The length should be such that the laser winds up almost level with the spindle. The angle cut on the bottom causes the support to lean out, which allows for a larger diameter workpiece. None of this is critical and can be changed to suit your needs.
It is important for the support to be square to the sides of the base, or parallel to the end, which amounts to the same thing. Two screws hold the support on the base.
Laser holder. The block that holds the laser is attached with a single wood screw so that it can be aimed directly at the tip of the dowel. Once aimed, the screw is tightened.
Be sure the hole is drilled parallel to the sides of the block. Then, if the support is square to the base, the laser beam will be aimed square to the base, which will be at right angles to the center line of the headstock spindle.
Indicator mark. Once everything is put together, position the dowel so that the laser beam hits the very end of it. Then take a fine pencil and make an indicator mark where the outer end of the dowel enters the dowel holder.
Using the gauge. Place the base on the ways of the lathe with the dowel rod projecting part way into the vessel. Pull the base toward you until the alignment strip makes contact.
Slide the base sideways until the laser dot falls on the reference point you desire, such as the shoulder of the tenon. Advance the dowel until it makes contact with the bottom of the hollowed interior. The distance between the side of the dowel support and the indicator mark will then be the distance between the reference point (laser dot) and the bottom of the hollow.