Vacuum chuck limitations?

[John Gornall]

I realize there are different circumstances based on the type of wood, the shape, and weight of the turning but what are the limitations of a vacuum chuck? Can you use a gouge or is it limited to a light scraper cut?

I've got all the parts and will give it a go next week.

[Bob Hamilton]

Hi, John:
The "grip" of a vacuum chuck is dependent on two things: the degree of effective vacuum developed and the area of the workpiece that the vacuum is actually working on. The effective vacuum takes into account the efficiency of the pump drawing the vacuum and the total amount of leakage in the system. The area of the workpiece that the vacuum is acting on is only that area that is inside the gasket of the vacuum chuck, so it really becomes a question of the diameter of the chuck. The area is the product of the square of the radius times pi, so increasing or decreasing the diameter of the chuck has a very large effect on the grip.

The forces exerted on the workpiece by the gouge are not trying to pull the piece straight off of the chuck but trying to make it stop turning, or slip around on the chuck, so the gasket material plays a part in things as well. A rubber type compound (closed cell foam weather stripping works well) is better than a soft plastic that might tend to be a bit slippery.

So, yes, you can make gouge cuts using a vacuum chuck. The larger the diameter of the chuck and the higher the developed vacuum, the heavier the cuts you can take. Keep in mind, though, that you have a huge amount of mechanical advantage on your end of the lever that is formed by the cutting edge, the tool rest, and the handle of the gouge. A heavy catch can still dislodge the workpiece.

Good Luck!
Bob

[Keith Burns]

All that Bob says is absolutely true. That being said, I always use the tailstock to help hold the piece in place and turn a "sub-tennon" from the base doen to about 3/8" or so. Then after I finish everything alls thats left to do is part the small tennon off and clean up whats left. Hope this makes sense. I've added a pic which I have started doing the bottom. I have not turned the sub-tennon yet.

[Bob Hamilton]

Yowza, Keith! I admit I hadn't even considered workpiece LENGTH when I was composing my reply. Distance from the chuck would greatly magnify the forces of the gouge. I am amazed you can do ANY turning at that distance from the chuck without tailstock support. I do use the tailstock for all but the last nub, but failed to mention that. Thanks for pointing it out.

Take care
Bob

[John Gornall]

Bob and Keith, thanks for the replies.

I got another idea from a turner. He had lost vacuum when the vac pump which he had sitting on the floor walked away by vibration from the electric outlet and pulled the plug. He lost a nice piece that he had a lot of hours in when it flew off the vacuum chuck and broke on the floor. He of course bolted the vac pump down but the other idea he had was a good one: he wraps the workpiece and vacuum chuck with plastic stretch wrap so the piece will stay on the chuck should he lose vacuum.