Sam passes on an approach for determining the depth or thickness of a bowl or a hollow form. This simple shop-made device is easy to build and set up----and, it is very accurate. (This is a jig demonstrated at the AAW Symposium in Atlanta). Sam passes on an approach for determining the depth or thickness of a bowl or a hollow form. This simple shop-made device is easy to build and set up----and, it is very accurate. (This is a jig demonstrated at the AAW Symposium in Atlanta).