Getting the wall of a bowl the right thickness is very important. The wall doesn’t have to be the same thickness all the way down, but it must look right and feel right. An experienced turner would likely rely entirely on their sense of touch and not use calipers at all, at least for small bowls where their fingers can reach the bottom. But calipers can help, and simple, non-adjustable homemade calipers are useful. They are quick to make and use. Calipers are, of course, normally adjustable, and made of steel. But adjustability is only necessary if you need to get the wall to a specific thickness. I find that when using ordinary steel calipers to check the wall of a bowl, I usually set them to more than the thickness required anyway. By watching or feeling the gap between the point and the wall, I can tell how the thickness changes. I made a batch of these non-adjustable ones to help with the teaching I do at the Camden Town Shed, where money for equipment is limited. I often use them instead of normal calipers in my own workshop too.
Material for homemade calipers
I used scraps of 6 mm MDF. This is unlikely to scratch the finished surface of a bowl. But with care in use, other materials would be fine. Dimensions are not at all critical, as long as the central cut-out is big enough for the caliper arms to reach down to the bottom of the bowl. They could be made in any size.
Making the calipers
I cut the blank to a circle with a band saw and found the centre. I then pinned the disc to the jaws of a chuck with the tailstock. This is a very quick method of mounting thin discs in the lathe. Friction from the chuck jaws drives them. I trued up the edge of the disc. Turning MDF is dusty work, but very easy with a scraper. Then I marked an off-centre circle and cut the middle out with the band saw. The entry cut is at the narrowest point. I sanded the inner curve with a drum sander, but this, like the turning, is purely for appearance. Hand sanding, or no sanding, would work just as well.
Then I enlarged the entry cut and sanded the points. The gap should be a little more than the likely wall thickness, say 12 mm for walls of 10 mm. The size of the gap is not critical, but should not be too big. Although these calipers have no adjustment, it would be easy enough to cut them in half, glue a couple of scraps from the centre cut-out to sandwich the wide end of one of the halves, and drill for a small bolt, washers and wing nut.
When one arm slides down the bowl’s inside wall, the gap between the other arm and the outside of the bowl clearly reveals changes in thickness. I drilled a small hole for hanging the caliper on a nail – it slips off if the large hole is on the nail.