Vacuum and Jam Chucking
Vacuum and Reverse Jam Chucking
In a vacuum chuck, air is pumped from a cavity behind the workpiece, and atmospheric pressure provides the holding force. The vacuum should produce a hold down pressure of 14.7 psi at sea level. Setting up a Vacuum Chuck
A vacuum chuck is a very simple way to reverse mount your work for cleaning off the bottoms. I use a normal faceplate with a wooden (3/4' MDF) disk attached. A one inch hole is drilled in the center of the disc to allow a clear passage through the faceplate and the headstock when the face plate is attached. The surface of the faceplate is covered in thin rubber about 1.5 mm thick. The film used to wrap computer equipment is ideal, or else something similar to a very thin wet suit material. A corresponding hole needs to be cut in the film as well. You do not want the covering to be too thick or else even though the work is attached to the vacuum chuck, it will not be as stable as possible.
The basic principle of vacuum chucking involves the science of the atmosphere around us. normal atmospheric pressure, at sea level, is just under 15 pounds per square inch. That means if we create a vacuum, such as a vacuum chuck, the barrier between the atmosphere and the vacuum, in this case our turning, has a pressure exerted upon it by the atmosphere. Vacuum is measured in inches of mercury or hg. A perfect vacuum is 30 inches of mercury (30 hg) and cannot be obtained on the surface of the earth. Most of the small pumps we use for our type of work are able to provide 22 to 28 hg, and this level of vacuum is more than sufficient for our purposes.
I made my own vacuum chuck set up for the most part. I ordered the kit for the gauge and valve from Veneer Supply. For the shaft to go through the headstock I made that myself with lamp rod. First try I used 1 bearing with a brass fitting JB Welded into place. That seemed to work but pulled 22 inches of vacuum. Then I made another one with 2 bearings and it pulled 26 inches of vacuum, so I am using the 2 bearing model. I just used Corian to make the bearing holder block from. Then taped the end so I could screw the lamp rod into place.
I've always wanted to adapt my vacuum press so I could use it for vacuum chucking small bowls on my lathe. This way, I can finish up the bottoms without showing any machine markings. One nice thing about this add-on is that it allows you to easily monitor the vacuum pressure on the work piece. When the needle on the gauge is above 15" of Hg, you know that the project is ready to be turned. And with the vacuum valve and bleeder, it also allows you to pre-fit the work piece with light vacuum pressure so you can center the project on the jig easily without having to have an extra set of hands to control the vacuum source.
I often turn small oil candle holders that hold glass confetti oil bottles. These usually require a 1 ½” diameter hole for the glass insert. After turning and parting off the holder, I like to reverse chuck the piece and finish turning the bottom. One method of doing this is to use a “jam chuck”. There are other methods that could be used to hold the work piece for this including using various jaws available for a scroll chuck. I don’t want to take a chance of marring the now almost completed piece. Vacuum chucking would be the ideal way to hold the piece.