January 2011

It is a new year and I have been crazy busy in the Workshop, but I wanted to take the time to answer some of the questions I have received regarding the vacuum chuck I used in the Square Bowl Video.

You have several options on how to mount your bowls to the lathe when turning. Many of those options have been around for as long as people have made bowls. The current standard is the scroll chuck. But what do you do after the bowl is finished except for the tenon on the bottom. Depending on the bowl type, you can reverse this with a jam chuck, cole jaws, a longsworth chuck, a donut chuck, or a vacuum chuck.

Vacuum chucking:

Vacuum chucking is done by drawing a vacuum, usually through the head stock spindle to hold the bowl in place while finish turning the bottom. It can look a little complicated at first, but once you understand the concept, it is actually a simple thing to do.
You can purchase a complete vacuum chucking system so all you need to do is hook it up and go, but if you are willing to do a little work, you can save a lot of money by making your own.

Let’s start by identifying the components used in a vacuum chuck.

Important Safety Tip:
The total pressure is not just the diameter of the Drum, but is the total area of the bowl exposed to vacuum. If you have a hollow vessel with a small opening and cover that opening with the 3 inch drum, the total pressure is the total number of square inches exposed to vacuum multiplied by the PSI created by the amount of vacuum. This can quickly become several hundred of pounds. And that is a lot of Golden Retrievers!

Some bowls leak!
Vacuum chucking a bowl with the finish applied and cured on the interior is a piece of cake. If the bowl does not have a finish you will probably be loosing some vacuum through the bowl wall. As you start to remove the tenon from the bottom of the bowl, you may see the vacuum start to decrease as the bowl wall gets thinner. And REMEMBER! Unlike a mechanical mount like a donut chuck, if you go all the way through the bottom, there will be NOTHING holding on to your turning.

Scotty! We need more volume...
This is why you need to consider the volume of air the vacuum pump can draw. It is commonly agreed that 4.5 CFM is the ideal rating for vacuum chucking. This is not absolute, the pump I currently use is rated at 2 CFM. My shop is 350 feet above sea level and the majority of my vacuum chucking is to bowls with a finished interior.
With larger bowls, there is more surface area to create force, but with an unfinished bowl there is also more area capable of leaking. This is where more air volume is important. 

There is a lot more that can be covered. I am going to hit the high points: 

Vacuum pump:
A vacuum pump is basically an air compressor where the connection is made to the intake side. The pressure side of the pump is vented to the atmosphere. Vacuum pump designs vary, and may use a piston, diaphragm, rotary vane, or turbine to create a vacuum. Some designs as with some rotary vane pumps may use an oil bath, or multiple stages to create a higher vacuum. A venturi system can also create vacuum using high pressure air. The thing to consider is the operating voltage, vacuum capabilities and the volume of air the pump can move.
Vacuum gauge:
This is needed to measure the amount of vacuum being applied to the turning. I will explain why this is important later in this article.
Bleeder valve:
Used to control the amount of vacuum being applied to the turning by creating a controlled leak of ambient air into the area under vacuum.
Vacuum adapter:
This is used to connect the vacuum hose to the lathe's rotating headstock spindle. This is usually one or two sealed bearings mounted in a fixture that attaches to the spindle. A hose connector is set in the center of the bearing. Some lathes do not have a hollow spindle. In this case an adapter can mount between the Vacuum Drum and the headstock.
The manifold contains the Vacuum Gauge, Bleeder Valve, and connections to the Vacuum Adapter and Vacuum Pump.
This is mounted to the spindle, and is the surface that holds the bowl using vacuum. This can be made from a variety of materials. The chuck will have a soft gasket material to seal out air leaks to make a secure mount to the surface of the bowl.
What does Mercury have to do with this?
Vacuum is measured several ways. For our purpose, I will use the standard of Inches of mercury. Why mercury? In 1644 an Italian physicist named Evangelista Torricelli concluded that if you could measure a one inch diameter column of air that ran vertically from sea level to the edge of the Earth’s atmosphere, it would have the same weight as a one inch column of mercury that is 29.92 inches tall. Mercury is listed on the periodic table of the elements as Hg. This is why your vacuum gauge will likely read from 0 to 30 Hg.
So when your system is pulling 15 inches of mercury, you know that the vacuum is 50% of a perfect vacuum.
How does this apply to vacuum chucking?
It is the difference between the ambient pressure and the vacuum you create that determines the holding pressure against the turning. The total area exposed to vacuum, is also exposed to pressure. More specifically, a one inch area exposed to 29 Hg of vacuum has 14.24 Pounds per square inch pushing from the other side. These numbers multiply quickly as you increase the diameter of the vacuum drum used to hold your bowl.
A 3 inch diameter drum with a 27 Hg vacuum will produce about 94 Pounds of total force. The same vacuum using a 5 inch drum results in 260 Pounds of force. That’s the same pressure as me standing on my bowl while carrying my Golden Retriever. And let’s face it. My bowl, my Back, and my Golden Retriever don’t really need to do this. You can see how a thin walled vessel can be collapsed under this kind of pressure.
Control your Vacuum:
This is why you need a bleeder valve. You hold your bowl on the drum with the bleeder valve wide open. (no vacuum) Then slowly close the valve to start increasing the vacuum. Once you have only enough vacuum to hold the piece in place, you can reposition it to be centered and then increase vacuum until it is secure. In the case of a 3 inch drum, you may go to 20 Hg. When using a 5 inch Drum, 15 Hg may be enough.
There are a lot of variables. If you are not at sea level, the pressure is lower, so more vacuum is required.

The Drum:
These can be purchased ready made. Oneway ® offers beautiful aluminum drums.
You can also make your own with a face plate and sections of drain pipe. I made mine by gluing up scraps of wood, then drilled and tapped with a Beal ® Spindle Tap. Screwed it onto the spindle and turned it to shape. You want the outer rim to be smooth and have a soft radiused surface. I like to use Hobby Foam to make the gasket. I picked up mine at a craft store.

The Pump:
Look at surplus centers for this. A decent Gast® rotary vane pump can be found for $100 USD. Less than half of the original price. Look for 3.5 CFM or better.
The Rotary Adapter:
Many people make there own by turning them out of Corian® or out of wood. Some are sized to fit the size of the opening on the outboard side of the spindle, and others use a threaded tube that runs the length of the spindle with a treaded nut/seal on the face plate side. This is good universal solution that will also work if your spindle can not be used to hold a vacuum. Most retailers selling woodturning supplies will offer spindle vacuum adapters.
The Vacuum gauge:
Got mine at a surplus store for $5.

The Manifold:
All the parts for this can be found at your local home center in the pluming department.

I hope you find this helpful. You can search the Web for "lathe vacuum chuck" and find all kinds of information.  You can see more of my vacuum chuck in the "Turning Cabinet" and "Square Bowl" videos.

Keep an eye on my News and Projects page. I will introduce my first set of plans in the near future.
Have fun, learn new things, be safe,
Larry Marley