Bowl Depth Gauge

Ohhh, you want to turn a bowl toni-ight!

Ohhh, going through the bottom is a fri-ight!

Ohhh, you’re not sure it will come out,

Fat bottomed bowls, you make this turning world go round!

Turning bowls is both rewarding and tense. Often one is tempted to stop earlier than needed, or is fooled into thinking the bowl is thinner on the bottom than it really is. A bowl with a heavy bottom has its place, but more often than not it leads to a clunky feeling. Determining bottom thickness isn’t always easy, but here is a way to determine the bottom very accurately. I would like to point out that I did not invent this gauge, but as it has been passed about freely in our area, I have no qualms about publishing it.

This gauge is easy to make, and with a little instruction, easy to use. All you need is a piece of scrap and a couple of dowels. I suppose you could turn the round parts, but that’s more trouble than it’s worth, unless you live so far from a supply house that turning them becomes practical.

My young model insisted that I not pur her face in this, for all you who would write about my photographic skill, or lack thereof.

Here we have my lovely daughter Sarah (or part of her, anyway) displaying the gauge. There are very few critical measurements to this tool. The stick should be wide enough that the holes for the dowels will not weaken it, and the dowels should be thick enough that they don’t break every time you knock the thing over, drop it, or whatever, and long enough that they won’t fall out as you check the bottom of the deepest bowl you plan on turning. I think I used a ¾" x 1" piece of oak and two ¼" dowels for this one. The stick itself should be a little longer than twice the diameter of the biggest bowls you envision turning.

Mark the center point of the stick. Both dowels will go on one half, and should be spaced so that the stick can span the largest bowl you want to turn, with either dowel inside the bowl and no interference from the other dowel. For example, if you plan on turning 12" bowls, put the dowel closest to the end a little more than 6" in from the end, and the second dowel a little more than 12" from the same end. This allows for checking roughed out bowls, as well as the occasional fat bowl, which are fun to turn and have their place.

After drilling the holes, bandsaw a thin kerf from one edge of the stick to each hole on a gradual angle, starting an inch or so away from each hole. If you look closely in the picture below, you should be able to see the kerfs. This enables the stick to pinch the dowels, holding them in place while still allowing freedom to move them up and down. Insert the dowels and you are done. Now we move to the operations manual.

To check the thickness of the bottom of the bowl, place the gauge across the bowl with one dowel in the center. Push the dowel down until it makes contact with the bottom. Repeat the procedure with the second dowel, being careful not to bump the first. Bumping either dowel can change its position and make a very bad mess of things by giving a false reading.

Place the long flat part of the stick across the top of the bowl and sight down along the ends of the two dowels. Three point establish a plane, the third point being the actual bottom of the bowl. Its location is accurately transferred to the outside surface of the bowl, allowing the turner to determine how thick to make it with a high degree of confidence. This is as important in roughing as it is in finishing. A roughed bowl that has a bottom significantly thicker than the side will often crack from stress. For some reason, if the bottom is slightly thinner than the sides, there is less chance of cracking.

Used in conjunction with double ended calipers, bowls can be turned to whatever thickness the turner wishes with no fear of going through or leaving the bottom too thick.