Using Dividers to Transfer Dimensions
A pair of dividers can be used to transfer a dimension from one part to another, such as is required when matching the lid of a box to the base. In this article, I describe an alternative approach to a technique that many turners use routinely and with confidence but which I believe to be dangerous.
The situation. The assumption is that we have a workpiece
several inches in diameter
and we want to transfer the dimension set on the dividers to the face of the piece.
For example, in the photo, the dividers have been set to the diameter of the opening
in the vessel, and we wish to transfer that dimension to the piece mounted in the
chuck.
The conventional method. With the desired dimension set, and with the body of the
dividers level and resting on the tool rest, the point of the left leg is touched
lightly to the spinning wood to scratch a very fine line on the piece. The objective
is to scratch the line at a point where the resulting circle will have a diameter
equal to the dimension set on the dividers. When this is the
case, the circle will
appear directly adjacent to the right point of the divider.
If the circle appears to the right of the right point, move the left point to the
right and try it again. This should bring the right point closer to the circle.
If the circle now appears to the left of the right point, move the left point to
the left. Repeat this trial and error method until the circle appears adjacent to
the right point, then apply more pressure to the left point to make a deeper scratch
that you will actually use. 
In this process, the right point must not be allowed to contact the wood. If it does, it may dig in and be jerked from your hand, with an uncertain outcome. Some turners dull the right point to make a dig in less likely.
An alternative method. The technique I use does not require touching either point of the divider to the spinning wood, and for me, it’s almost as fast and just as accurate. Here’s how to do it.
With the dividers set as desired and with the lathe Off, position the dividers by eyeball so they are centered on the piece. Note the position of the left point relative to the tool rest, then remove the dividers and start the lathe. Place the fine point of a pencil on the tool rest where the left point was, and scribe a line on the piece. Stop the lathe.
Now use the dividers to check the diameter of the circular pencil mark. If it’s too small, make another circle just outside the first, after removing the dividers and starting the lathe.
Again, stop the lathe and check the diameter of the second ci
rcle. If it matches
the setting of the divider, you’ve found your dimension. If it doesn’t, draw another.
With practice, you can hit it almost perfectly in two tries, three at the most. In fact, a circle of the exact diameter is not required. When you get a circle that’s close, you can make your cut a tad bigger than the circle, or just a skosh smaller if need be.
Using calipers instead. Some turners don’t bother with the dividers at all. Instead, they use a dial (or digital) caliper to measure the desired diameter, take half of it, and then mark this distance from the center of the piece, which can be located easily. They may then start the lathe and pencil a circle on the piece. There are many variations and preferences with this technique.
In the final analysis, the final fit is often achieved by a process of trial and error. That is, you cut close to “the line” and then test the fit, and repeat as necessary.