Using a high cfm shop dust collector for vacuum table

[jsearles]

I see that many thermoformers use a shop vac for creating their vacuum tables. Is it advisable to use a high vacuum dust collector setup commonly used in a woodworking shop as the source of vacuum for a thermoforming table. They can be very high cfm blowers and thus produce a lot of vacuum. I already have this and was wondering whether a very high cfm blower would cause any unintended problems for a vacuum table.

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[txcncman]

When dealing with vacuum, you have 2 main considerations: Actual vacuum (inHg) and flow (cfm). For work holding, flow only comes into play when you have openings in the vacuum circuit. Actual vacuum is your holding power. Dust collectors are designed to have more flow and less actual vacuum. Vacuum pumps are designed to have more actual vacuum and less flow. If you can find a dust collector that gives you the holding power you need, by all means use it.

[jsearles]

Thanks for your reply. Should I throw a vacuum gauge on it to see what holding power it has? I have read that 29hg is optimal. Not sure. How much does a regular shop vac offer in hg and can I parallel 2 of them together since I have several that I am not using?

[ger21]

I'd be surprised if it pulled 5hg of Mercury. A shop vac might be close to 10 if your lucky.
Paralleling will increase the flow, but not increase vacuum.

[jsearles]

Hi Ger. Ok, I have a better understanding of the vacuum vs flow question, but please help me understand the best setup(device config) for maximizing vacuum. Is it a pump and tank or some other device?

[txcncman]

With a vacuum pump, you almost always want to use a accumulator tank. The accumulator tank provides the flow needed during the first "clamping" with the vacuum. 29 inHg is about the maximum you can expect from a well maintained vacuum pump and system. Anything over 26 would be considered good.

Dust collectors work from an impeller, when flow is blocked, they actually become less efficient.

For your work holding, you might want to look at: Flip-Pod Vacuum Clamp System by Carter Products

and

http://www.youtube.com/watch?v=ozkHmEN-TQg&feature=related]FLIP-POD.mpg - YouTube

This system, while much more expensive, would require much less actual vacuum than using a sheet of MDF and might actually work ok with a dust collector.

[jsearles]

Thanks a lot txcncman. Not sure how that Flip-Pod system would work with a vacuum thermo-forming table, but then, I did not specify. Thanks for your input.

[txcncman]

Well, you hinted at it, but I thought you were going a different direction.

Using a dust collector as the vacuum for your thermo-form molds might work as long as you are not trying to pull tight geometry or heavy gage materials.

[jsearles]

I have a 10 gallon pressure pot used for spray painting. Will that work as a vacuum tank?

All the vacuum pumps that I see on eBay relate to cfm and not Hg. Is there any way to know what the Hg would be?

[txcncman]

10 gallon might be on the small side. All of the vacuum accumulator tanks I have seen run 30 gallons and up. Again, CFM is flow. InHg is vacuum. There is no conversion between the two. They are independent measurements.

From a web search:

http://vacuumpumpsupply.com/

Vacuum Pumps | Vacuum Pump Repairs | Rebuilt Pump | Vacuum Tank System

[svenakela]

With a vacuum pump, you almost always want to use a accumulator tank. The accumulator tank provides the flow needed during the first "clamping" with the vacuum. 29 inHg is about the maximum you can expect from a well maintained vacuum pump and system. Anything over 26 would be considered good.
...
This system, while much more expensive, would require much less actual vacuum than using a sheet of MDF and might actually work ok with a dust collector.

I'm not sure I'm following you here, 26 inHg is not much. Did you mean absolute or relative pressure?
I have several vacuum pumps in my workshop, they are rotor disc-oil catchers and certified to 1.3x10^-20 mBar absolute pressure which is less than in outer space. They reach less than 100 mBar absolute pressure within a blink of an eye. Two blinks and a cheap manometer is at zero.

[ger21]

I'm not sure I'm following you here, 26 inHg is not much

I'm not sure I'm following you, as 29.92inHg is considered a perfect vacuum. I guess that's relative?

What's the formula to convert relative vacuum to absolute vacuum? Do you add or subtract 29.92?

I think your 100mBar (3inHg) absolute is equal to 27inHg relative? If I cover the inlet port on one of my pumps, it too can pull that high very quickly. But trying to evacuate a large canister, or bag, is another story.
I don't think anyone doing vacuum forming or vacuum laminating is using much more than 27-28inHg, as it's not practical.

[svenakela]

I'm not sure I'm following you, as 29.92inHg is considered a perfect vacuum. I guess that's relative?

What's the formula to convert relative vacuum to absolute vacuum? Do you add or subtract 29.92?

I think your 100mBar (3inHg) absolute is equal to 27inHg relative? If I cover the inlet port on one of my pumps, it too can pull that high very quickly. But trying to evacuate a large canister, or bag, is another story.
I don't think anyone doing vacuum forming or vacuum laminating is using much more than 27-28inHg, as it's not practical.

That explains it, thanks Ger. I never use relative pressure.
That's the beauty of a rotor pump, even though my pumps are small they work without a buffer can and evacuates fast by themselves. But I do use buffer cans as a trap for particles, dust etc to avoid pump damage.

BTW, there is no such thing as perfect vacuum.