Something you definitely don’t want to do when hollowing a bowl is cut too deep. Less disastrous, but still a problem, is not cutting deep enough, because too much wood in the bottom leaves the item too heavy. Many turners assess the thickness of the bottom by feel. But on larger items, fingers don’t reach far enough. Even calipers, like these, don’t measure the thickness where the chuck prevents access. So a depth gauge can be useful when making bowls.
The simplest depth gauge is just a pencil. You sight across the rim of the bowl, put the pencil in contact with the deepest point, then grip it at a point that is in the same plane as the rim. Then you hold the pencil against the rim and sight across the bottom of the item. This works, but parallax error means it’s not very accurate. And if the chuck jaws are in a recess, you can’t see how deep that recess is.
A sliding dowel set in a cross bar will accurately measure the depth. Such a simple depth gauge is very easy to make, and lots of turners use one. But there is still the same difficulty when assessing the amount of wood remaining in the bottom.
Make a measuring stick to suit the chuck
Using the dowel gauge with a separate measuring stick can greatly reduce error. Cut a wooden rod to length so that when placed in contact with a suitable fixed point, it extends about one inch beyond the chuck jaws. That fixed point may be most convenient if it is on the headstock, but it need not be as long as the distance from it to the jaws is constant. The contact point must be far enough off the lathe axis for the rod and dowel to clear the rim of the bowl while they are parallel to the axis. For wider bowls, or if the front of the headstock does not have a vertical surface, something standing on the lathe bed could act as a reference point for the stick, as long as it’s always put in the same position.
Mark a scale with 1/8 inch intervals on the last inch of the rod, beginning exactly level with the outer face of the jaws. You will need a separate rod for each different chuck.
Using the improved depth gauge
Place the cross bar across the rim of the item and adjust the dowel to touch the deepest point. This tells you the internal depth. Move the gauge so the dowel is outside the bowl, with the cross bar still touching both sides of the rim. The cross bar must be long enough to enable this. Put the measuring stick next to the dowel. The scale will then show the thickness of wood remaining above the chuck jaws.
You can also use the rod to preset the dowel in the cross bar. When the bar touches the rim, the bowl depth is complete.
If the jaws are in a recess but don’t bottom out in it, or if the inner surface of the recess is not flat, an allowance has to be made.