Minimum strength vacuum for form?

[gannon]

Just wondering "how low can you go"
Mainly because I noticed 1HP shopvacs are fairly small and cheap (I have a 2.25HP one, but was looking at the smaller ones for the future)

[Life of Brian]

I've heard recommendations of 3hp or more. I personally use one in the 5 - 6 hp range.

[Triton]

a 1.5 or 2.5 hp shopvac should be adequate

[drcrash]

Horsepower isn't the right measure for strength of pull, and shop vacs aren't especially appropriate for vacuum forming.

DON'T buy a shop vac for vacuum forming. They don't suck any harder, on average, than similarly powerful canister vacuums you can get at a thrift store for next to nothing.

All other things being equal, you're at least as well off with a high-amp (10 watts or more) or high-watt (1200 watts or more) canister vacuum.

My 1000-watt Shark hand vacuum pulls a shade harder than my 5.5 HP shop vacuum, and it cost me a couple of bucks at the Goodwill Outlet Store.

You can test vacuum cleaners at a thrift shop by putting a piece of craft foam over the hose and seeing how deep a dimple they suck into the foam. Buy the one that sucks the foam furthest into the end of the hose.

The best vacuum cleaners have two-stage pumps, and can pull almost twice as hard as a regular vacuum (or even a high-watt high-end shop vacuum). Neither can pull anywhere near as hard as a good vacuum pump---even a converted bike pump that you can make into a vacuum pump for $13.

Good one-stage vacuums pull 4-6 inches of mercury, or 2 or 3 PSI. Philips makes a two-stage 1800 watt vacuum that pulls over 8 inches, but costs over $100.) A bike pump can pull around 22-24 inches of mercury, or about 11 or 12 PSI.

If you're out to spend money on a good vacuum, check out the surplus vacuum pumps Doug Walsh is selling at www.build-stuff.com They're under $100, shipped, and they really suck. (4.5 CFM free air flow, 28 inches of mercury max.) SurplusCenter sometimes has good deals on surplus vacuum pumps too.

[gannon]

Thanks for the input
I have an old vacuum strength meter, so I might try testing out a few of the vacuums I have around the house.

[drcrash]

I found out a bit more about this subject.

The horsepower ratings on shop vacs are totally bogus, and don't mean much of anything. Go by the amps or watts if that's the best kind of statistic you've got.

There is no way any of these machines actually produces anywhere near 6.5 horsepower. That's just the momentary inrush current from when you flip the thing on and before the motor spins up, converted from amps @ 120V to watts and then to horsepower. (746 watts per HP.)

6.5 HP, if it was real, would require 40 amps, and you'd flip a circuit breaker really quick. You can't get more than 2 or 3 horsepower on a 15- or 20-amp household circuit, so the "peak HP" ratings are a complete joke and a marketing scam.

The useful horsepower doesn't peak when you need it to---when the hose is essentially plugged and you hear what sounds like lots of raw power. The max watts is actually when pumping free air, not against resistance. Sucking hard is just not what vacuum cleaners are designed for, and shop vacuums are no better than canister vacuums. They just suck the air through a bucket to catch liquids. (Useful if you need it, but not for vacuum forming.)

I saw a pamphlet for 8 varied Shop*Vac brand shop vacuums that they sell at Lowes, which actually listed some useful numbers: sealed vacuum level in inches of water, and cubic feet per minute in free air.

All of the Shop*Vacs pulled between 56 and 60 inches of water. That is, the most powerful "6.5 HP" unit only pulled 11 percent harder than the weakest "2.5 HP" unit. There's a better correlation between amps and vacuum level, but even that's weak. The 12-amp Shop*Vac only sucks 11 percent harder than the 7- or 8-amp Shop*Vac.

The higher-powered shop vacuums are mostly optimized to pull more air than the lower-powered ones, without pulling it much harder. (Something like 50 percent higher CFM for the most powerful vs. the least powerful.)

That doesn't help at all with vacuum forming. (Unless you have a huge platen, say 2 x 4 feet; then it might.)

60 inches of water is only 4.4 inches of mercury, or 2.1 PSI. So the best of those machines pulls a few percent over 2 PSI and the worst a few percent less than 2. Not enough difference to notice in vacuum forming, and not worth paying good money for. A good canister vacuum, used, for $5 from the Goodwill Outlet Store is likely to do as well, and maybe significantly better.

My little 1000-watt $2-at-Goodwill Shark hand vacuum actually pulls just slightly harder than ANY of those Shop*Vacs , even the big expensive 6.5 HP unit.

Any good vacuum cleaner that draws 8 amps or more is likely to pull about as hard.

The hardest-pulling vacuum cleaners are certain canister vacuums: 18-amp monsters with 3-stage centrifugal blowers like Phillips makes. Those pull about twice as hard, but they're expensive; for the same money, you can buy a real vacuum pump, and get another factor of 2 or 3 in forming power beyond what even the 3-stage Phillips can do.

(Thanks to Doug Walsh for some insights about this stuff, in a recent thread over on tk560.com.)

[Sharp Sapphire]

daisy chaining like 4 of those 1 HP shop vacs might do the trick