Cutting Rings and Templates
on the Lathe
It’s a fairly simple matter to cut an accurate circle in a thin, soft, or flexible material on the lathe. Some preparation is required but the results are excellent. Of course a pair of scissors will often do the job, but a lot of us lack the skill to do it with any sort of precision.
The idea is to sandwich the material to be cut between a mandrel and a disk, one (or both) of which is the diameter of the cut to be made. This fixture not only holds the material but also provides a guide for the cut.
This technique is applicable to thin plywood, plastic, Plexiglas, and so forth, as well as to softer materials like thin foam, rubber, or leather. The mandrel is held by a scroll chuck; the disk is jammed against the mandrel by the tailstock.
You can find many uses for the circles and rings. One that is obvious is a circular
template. Another is to make a foam seal for the rotary adaptor for a vacuum chuck.
This technique lends itself well to making the semi-
Making the Mandrel and Disk
To make the mandrel, just form an appropriate tenon on a circular blank and then true it up. Making the face ever so slightly concave will ensure that the clamping pressure is applied near the cut.
To make the disk, jam the blank against the mandrel and true it up. Then turn both the mandrel and the disk to the diameter you desire for the cut in the thin material.
Making the Cut
Make the cut in rigid materials using a small, fingernail-
The photo at right shows the position of a spindle gouge ready to make the cut in a thin plastic lid.
After the cut, the ring will slip to the side a small amount.
Making a Circular Template
For an inside template, this amounts to no more than making a disk as described above. However, for an outside template such as might be used to check the surface of a sphere, the focus shifts to the part that gets cut away from the material held between the mandrel and the disk.
A plastic lid from a food container makes a good blank for a template. For an outside template, it is desirable to center the lid between the disks. A small pin at the center of the disk held in the chuck is helpful in this regard. Just use an awl to punch a small hole at the center of the lid and then position the lid over the pin.
A Padded Fixture
One design for a kaleidoscope uses an object box made of a PVC coupling and two “lenses” of 1/8” Plexiglas about 2” in diameter. To avoid scratching the Plexiglas, I made a fixture with thin leather padding to hold the blanks for the lenses.
The leather is attached to the mandrel and disk with contact cement. The fixture itself was used to turn the leather pads to the proper diameter after the contact cement had set up.
Cutting a Ring from a Flexible Material
Forming a ring simply amounts to making two cuts of different diameters on the same blank. In this case it may be convenient to press the material against a large disk instead of a mandrel. Doing so will eliminate having to prepare and use mandrels of different diameters.
A separate disk for the tailstock side is required for each diameter you wish to cut. And, as will be obvious, you make the cut for the largest diameter first.
Attach the blank to the large disk using double-
Next, switch to the smaller disk and make another cut. This will cause the ring to pop away from the center of the blank.
I think that’s about it. Even though the examples shown here are just a few inches in diameter, the technique can be scaled up to cut disks and rings that are much larger, such as for sealing rings for vacuum chucks. The largest disk I’ve cut from a thin foam is about 10” in diameter.