How to Cut a Recess for Chuck Jaws:
Philip Greenwood looks at fitting chuck jaws
I am often asked how to cut the recess for chuck jaws. The first important thing to know is the correct recess size for your chuck. The jaws may be able to expand from a diameter of 50mm to that of 70mm, but
there will be only one diameter that will allow the jaws to form a perfect circle. A recess can be used to hold a bowl while turning the inside, but also for the initial mounting of the bowl blank. Here I will show you some of the ways to cut the recess.
Measure the jaw size: where do the jaws sit?
Close the chuck jaws until there is only a small gap between the jaws. You will see the jaws form a perfect circle. Now take a measurement to establish the recess size needed. This will ensure that you have maximum jaw contact with the recess. The photo shows how much loss of contact you have if you expand the jaws too far. You can expand into a larger recess, but do be aware that you will not achieve the maximum grip. The next question is how deep to cut the recess.
The general rule is that the larger or heavier the item is, the deeper the recess. I cut mine to around 4mm but up to 8mm for larger items. The maximum depth depends on your jaws. The top of the jaws need
to sit on the bottom of the recess. Make sure you have a clean corner for the jaws to sit into or you will experience vibration when turning.
Bowl gouge and skew
After marking the diameter of the recess, I use a bowl gouge to remove most of the waste first and then use the skew chisel to clean up the sides and bottom of the recess. To cut a dovetail, lay the skew chisel flat on the tool rest with the long point to the left side. Next, slide the skew down the side of the recess at an angle to match angle of the jaws. Take several cuts if needed. Be careful when sanding not to round over the
recess outer corner with the sandpaper. If you do, simply take a small cut to redefine the dovetail.
Parting tool, skew and bowl gouge
Use the parting tool to cut down to the right hand side of the pencil mark until you have reached the correct depth. Now take a second cut to the right of the first groove and keep repeating until you have removed all the waste. Alternatively, you could use a bowl gouge to remove the waste and then use the skew chisel as before to cut the dovetail.
You can buy a dovetailing tool that matches the angle of chuck jaws, or make your own tool. However, with
most designs you first need to remove the waste from the recess before you use the tool. This can be carried out by any of the methods above. Some tool designs are are hybid of an end scraper and dovetail cutting tool, so can remove waste as well as cut dovetails, some both internal and external.
Saw tooth bit, hold on chuck jaws
If you are turning a thin piece of timber, you can drill a recess by using a pillar drill. Use a saw tooth bit to drill a recess that is just larger than the outside of your chuck jaws, then you can expand the chuck jaws into the drilled recess. This enables you to turn a spigot on the bottom of your piece to hold in the chuck. The piece of timber must be clamped to the drill table. This method is not suitable with a hand held drill due to the torque encounter. You won’t have a dovetail on this but your jaws will still grip. Use the tailstock to give additional support if needed. If you have a set of small jaws, you may find you can use a smaller flat bit drill which could be used in a hand held drill.
There are many ways to cut a recess that will hold your work. I have shown a few in this article but there are other methods. Use a method that works well for yourself, providing that it is safe. We all have a preferred way of working and know our favourite tools to use.