Laser Depth Gauge for a Hollow Form
Designed and Built by Earl Kennedy
When hollowing the interior of a hollow form, it is important to know exactly where the bottom of the hollowed portion is relative to the external profile. This fixture allows you to locate the bottom of the interior quickly and precisely.
Of course, if you are using a hollowing rig that has a built-in laser, this fixture is not necessary. But not all turners have, or even want, the fancy hollowing rigs.
A three-sided rectangular frame is glued together using whatever flat stock is available. A laser pointer mounted at the top is aimed at the end of a round-head screw on the end of the bottom leg. As an enhancement, a small bubble level is mounted on the top leg.
Building the Fixture
The first thing to do is buy the laser pointer that you will use. Pay attention to the location of the switch or button that turns the laser on; it must be accessible when the laser is installed in the frame. While you’re shopping, get the level if you want to include it.
Based on the hollow forms you plan to turn, decide upon the dimensions for the fixture you plan to build. You may be able to shorten the vertical member of the frame if you turn only smaller vessels, and the length of the legs can be adjusted to suit your needs.
There is one aspect of this design that is important: the laser beam should be aimed in a direction exactly perpendicular to the top surface of the top leg, where the level will be located. Any departure from this will introduce an error that could be significant on large-diameter pieces.
After cutting the pieces for the frame, prepare to drill the hole for the laser in the top leg. Measure the diameter of the laser and select a drill bit the same size.
Drill the hole. Locate the center of the hole 3/4” from the end of the leg. Use a drill press if one is available, or if you are using a hand drill, take whatever steps you can to be sure the hole is drilled at right angles to the top surface. Drill the hole almost all the way through, then finish the hole with a smaller bit.
Do a trial fit of the laser. If the fit is too tight for easy insertion and removal, use a bandsaw to cut a slot that will allow the hole to expand slightly. If the fit is too loose, wrap painters tape around the laser to compensate.
Install the screw at the end of the bottom leg. Drill a pilot hole to be sure the screw doesn’t cause a split in the wood. Try to get the screw in line with the axis of the leg, but a small error will not be important.
Clamp the frame together with wood clamps to check the length of the legs, and to be sure the laser beam hits the very tip of the screw. Make any necessary adjustments. When everything appears in order, glue the frame together.
Do the final installation of the laser pointer and mount the level on the top surface. The fixture should now be ready to use.
Earl Kennedy is a founding member of the Piedmont Triad Woodturners Association of Greensboro, North Carolina.