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Vacuum clamp question

Robert Feingold

Robert Feingold

Joined
Jun 14, 2011
Messages
119
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64
Location
Brooklyn, NY
I have a Jet 1642 evs lathe and a compressor driven vacuum pump that i used with a vacuum bag for veneer and curved clamping. Will this be usable for a vacuum clamp system on my lathe. I suppose you need more specs but I don't see specs on the pump. What else will i need to set it up. I imagine that i just need to have good connections to the head stock but is there a kit I can buy?
If this pump doesn't work what will a new system cost. please excuse the broad questions.
Thanks,
Rob
 
You can probably Google the pump model and find out the maximum in/HG and CFM.
CFM isn't as important as you are only evacuating the line and chuck bell, but more is better.
For in/Hg (vacuum is measured in inches of mercury not PSI) it should be at least capable of 15" but 25-27" is better.
Easy way is to put a vacuum gauge on it and see where it goes.
 
Rob,

Your compressor driven vacuum system is actually a venturi. Its advantage is that it does not cost much. The disadvantages include the following:

  1. Your compressor will be running a lot so if it is noisy, you might consider relocating it away from your lathe.
  2. The venturi is also noisy.
  3. The achievable vacuum level is somewhat limited compared to a real vacuum pump. The vacuum level that it could achieve with a vacuum veneering system is much greater than what it can reach for vacuum chucking because a vacuum veneering system has essentially no leakage while, by comparison, there is always considerable leakage with vacuum chucking.
  4. A rotary vane type vacuum pump has the capacity to handle a considerable air flow while maintaining a respectable holding vacuum. A venturi has only a very small flow rate capacity while maintaining a decent vacuum level.
In addition to the other advantages of a real vacuum pump, they are generally quiet.

There are several types of vacuum pumps. The most desirable for woodturning use is a rotary vane pump. Diaphragm vacuum pumps are the lowest cost, but are not much better than a venturi because of their relatively low vacuum level and limited flow capacity. They are also a bit noisier. The other type sometimes used is a piston pump. They achieve the highest vacuum level, but generally do not have a large flow rate capacity. Their vacuum level also is not smooth, but has a large ripple in the vacuum level. An accumulator can help a lot to smooth out the vacuum pulses.

You can give your venturi system a shot -- a number of turners use them. Just recognize its limitations and be very gentle when turning the bottom of a vessel.

If you buy a vacuum pump, I would recommend the rotary vane type -- specifically a 1/4 or 1/3 horsepower Gast pump. And make sure that it is the dry vane type and not the lubricated vane type. You can buy one from Craft Supplies for about $400, but they are about the highest priced place that you can find. A lot of turners, including me, have bought used Gast rotary vane vacuum pumps on eBay for much less than the cost of a new one. I was very fortunate and got a barely used one for about $25 from a company that makes orthotics. The typical eBay price is $75 - $100. Make sure that you thoroughly check out a seller before buying.

Of course, the pump is just one part of the system. You will need a vacuum chuck, rotary coupler, hose, manifold, valves, filter, and vacuum gauge. I made my own vacuum chuck and rotary adapter and saved a lot of money.
 
Steve Worcester said:
You can probably Google the pump model and find out the maximum in/HG and CFM.
CFM isn't as important as you are only evacuating the line and chuck bell, but more is better.
For in/Hg (vacuum is measured in inches of mercury not PSI) it should be at least capable of 15" but 25-27" is better.
Easy way is to put a vacuum gauge on it and see where it goes.
Click to expand...

That is not quite true, Steve. There is normally significant flow at the bleed valve unless you run full vacuum on your turning. Additionally, the rotary coupler has leakage, most vacuum chucks have at least some leakage at the interface of chuck to spindle, and there is always significant leakage around the seal between the chuck and vessel as well as air flow through the vessel walls, especially if the turning is thin. I use quick disconnects on my hoses and although they work quite well, they can have a slight amount of leakage (usually of no consequence). Although it is an apples to oranges comparison, I consider flow rate to be just as significant as vacuum. Gast specifies their pump performance giving open port flow rate in CFM as well as blocked port vacuum in in-Hg. They also provide performances curves for vacuum vs. flow rate.
 
My vacuum system has almost no leakage. I worked very hard to get this. However wood itself often leaks. So I'm with Bill on this one. I think the flow rate is also important. You often have to turn the vacuum down on larger thin vessels to keep from collapsing them. These thin vessels tend to leak the vacuum more than thicker ones. I usually get around this by putting tape or cling wrap around the bowl but of course you can't do that where your going to turn it.
 
Bill,
You stated that you used quick connect couplers in your vacuum lines. When I tried this I had two problems. 1. they leaked under vacuum as you stated. 2. when I tested my system vacuum levels with air flow (leaky bowls) I found a significant vacuum loss across the quick disconnects. I got rid of them.

John,
You indicated that you use cling wrap on your bowls to counteract leakage but were careful to not put it where you wanted to do your cutting/shaping. I found that is not necessary. The cling wrap is no barrier to my gouge so I wrap the bowl as needed. Any thing in the way just gets cut off during the normal turning process.
 
John Giem said:
Bill,
You stated that you used quick connect couplers in your vacuum lines. When I tried this I had two problems. 1. they leaked under vacuum as you stated. 2. when I tested my system vacuum levels with air flow (leaky bowls) I found a significant vacuum loss across the quick disconnects. I got rid of them.
Click to expand...

John, the quality of quick disconnect couplers vary all over the place. I used Speedaire connectors and with with all ports blocked after pulling a vacuum, it held for several minutes -- I don't remember exactly how long because I left the shop and came back a while later and the vacuum had only bled down about half way. This is significant considering that the volume is so small -- basically just the lines and filter internal volume. On the other hand, I have bought quick disconnects from Harbor Freight and some of them hissed like a snake while the best of them were just barely acceptable for use with my air compressor.
 
Bill Boehme said:
John, the quality of quick disconnect couplers vary all over the place. I used Speedaire connectors and with with all ports blocked after pulling a vacuum, it held for several minutes -- I don't remember exactly how long because I left the shop and came back a while later and the vacuum had only bled down about half way. This is significant considering that the volume is so small -- basically just the lines and filter internal volume. On the other hand, I have bought quick disconnects from Harbor Freight and some of them hissed like a snake while the best of them were just barely acceptable for use with my air compressor.
Click to expand...

Bill, it is good to know that there are some quick disconnects that can hold a vacuum.
As stated above the flow rate from the vacuum chuck to the pump is important. Do you have any 'feel' about how much flow restriction your quick disconnects have? A simple way to look for flow restrictions is to run the pump without anything on the vacuum chuck. The measured vacuum levels at different points in the system will give you a 'figure of merit' for your flow restrictions. When I did this test the first time, I had 6 in Hg at my manifold with nothing mounted on my vacuum chuck. After eliminating a quick disconnect and increasing the hose from 1/4 in to 3/8" the vacuum level dropped to 2 in Hg.
 
John Giem said:
... Do you have any 'feel' about how much flow restriction your quick disconnects have?...
Click to expand...
The quick disconnects have some pressure drop which is a function of flow rate. While they do not pose a problem when used with my air compressor, it might be much more important for a vacuum system.

I did a quick and dirty borderline scientific test roughly comparable to a open-circuit wind tunnel test this evening using the following equipment:

  1. stopwatch
  2. pair of lungs
  3. A mating pair of ⅜" Speedaire quick disconnect air line couplers
  4. A mating pair of ¼" Speedaire quick disconnect air line couplers
  5. A mating pair of ¼" Harbor Freight quick disconnect air line couplers
The lungs served the purpose as the accumulator of a fixed volume of air and the quick disconnects were the units under test. The objective was to empty the accumulator as quickly as possible and record the time. Measurements for each coupler pair were recorded first with the open coupler only and then with the two couplers plugged together to determine the effect of the primary flow restrictor -- the shutoff valve.

The ⅜" Speedaire quick disconnect air line couplers had very little flow resistance. The time for the open connector only was approximately 1 second and just slightly longer at about 1.5 seconds with both connectors mated together.

Next the mating pair of ¼" Speedaire quick disconnect air line couplers was tested. The time for the open connector only was approximately 2 seconds and just under 3 seconds with both connectors mated together.

Finally, with the mating pair of ¼" Harbor Freight quick disconnect air line couplers, the time for the open connector only was slightly longer than 2 seconds, but when the two HF connectors were mated together the time to completely exhale increased almost 6 seconds.

The conclusion is that the HF connectors have considerably more flow restriction that the US made Speedaire QD connectors. I have also noticed that many of the HF connectors leak air both when plugged together and also when disconnected, the shutoff valve does not completely seal. In contrast, I have always found the Speedaire connectors to provide an airtight seal.
 
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