Space bags?

[Michael Adelong]

I just got done laminating two 23"x12" scraps of 3/4" MDF together so that I can mount my new PC 4212 dovetail jig on in. (Mini gloat - I got 2 PC 690 routers and the DT jig for $240 after a $20 rebate on each of the routers). I remembered that I had one of those "space bags" in the basement, so I decided to give it a whirl at using it as a vacuum press. I rolled glue on both surfaces, put them together, and then put them in the Space Bag on my assembly table (I'll post pics on this project later - came out great). I then used my shop vac to suck the air out of it.

I can't see any reason why this won't work. I might not get the whole 1500 psi, but I should get most of it. What do you folks think?

Anyone else ever try this? I'll let ya'll know how it turns out tomorrow. I probably won't be able to get back on the Creek before then.

Michael

[Richard Wolf]

I'm not sure where you got the 1500psi, I think it is closer to 15 psi, which is the weight of the atmosphere on a sq inch.

Richard

[Vaughn McMillan]

Michael, I'd recommend checking the bag periodically to make sure it's holding the vaccuum. I've used them around the house (not in the shop yet) and have found they tend to leak...some slowly and others faster.

Nice stealth gloat, BTW. I really like my 690s. ;)

- Vaughn

[Lee DeRaud]

I'm not sure where you got the 1500psi, I think it is closer to 15 psi, which is the weight of the atmosphere on a sq inch.

RichardA decent vacuum bag will pull 1500 pounds per square foot (about 10-11psi). That's the number David Marks is always quoting whenever he's stuffing something into his bag...I think it sticks in everybody's head, but as (the more common) "PSI".

[Lee Schierer]

"Wood" magazine just did a test on this in their April/May edition. What they found is that the space bags did not hold the vacuum levels very long. My own experience here around the house has shown the same thing. By morning most of the air is back in. The one way valves they use are pretty cheap and don't seal all that well.

[Michael Adelong]

You guys were right. ALL of you.

Richard was correct, so was Lee. I misquoted David Marks getting the PSF / PSI thingy messed up. However, standard atmospheric pressure is 14.696 PSI, and there are 144 square inches in 1 square foot, so that would be 2116 PSF. Maybe we should let DM know that he needs to have someone at DIY edit out that mistake in every episode with a vacuum bag in it. In order to get to 15,000 PSF, you would need to be in a place with a standard pressue of 7 atmospheres (104.1667 PSI). I think oxygen liquifies at that pressure...

Lee and Vaughn (and "Wood" for that matter) were also correct. The bags leaked.

The 2 pieces of MDF are stuck together. The assembly is flat. There is a larger than usual glue line, so the pressure didn't get very high. There was no squeeze out even though there should have been given the amount of glue that I put on the two pieces.

I know that it held pressure for at least an hour, so I guess the MDF sandwich won't be falling apart any time soon. :rolleyes:

My "official" SMC recommendation is - Don't waste your time trying Space Bags. (unless you're killing 20 minutes in the shop on some unnecessary project waiting for your girl to arrive...)

Michael

[Lee DeRaud]

I misquoted David Marks getting the PSF / PSI thingy messed up. However, standard atmospheric pressure is 14.696 PSI, and there are 144 square inches in 1 square foot, so that would be 2116 PSF. Maybe we should let DM know that he needs to have someone at DIY edit out that mistake in every episode with a vacuum bag in it.DM has it right: he says "square foot", but I think everybody hears "square inch". Can the typical vacuum bag pump really pull 14+ PSI?

[Keith Barkhau]

Lee,

Most of the vacuum pumps measure their performance in inches of mercury (" Hg) and the conversion to psi is psi = (" Hg)/2.036. My pump routinely pulls 25"Hg (its rated for 28), which would be 12.3 psi or 1770#/sqft. If I ran it at 28"Hg it would be 13.8 psi. I think this is actually typical for most decent vacuum pumps/generators.

Michael,
I believe the average shopvac pulls less than 4"Hg, so you were probably actually getting on 2psi or so in your space bag.

[Michael Adelong]

Just chalk it up as a stupid noob trick. :rolleyes:

I always thought that the pressure came from the atmosphere pushing in, not the pump pulling out.

M-

[Charlie Plesums]

I always thought that the pressure came from the atmosphere pushing in, not the pump pulling out.

You are right. The atmosphere pushes in... on the bottom as well as the top, on the inside as well as the outside. So the vacuum pump removes the pressure on the inside, leaving the atmospheric pressure on the outside pushing in.

If it were a perfect vacuum, the pressure would be based on your altitude and the weather (as the weatherman said, "the barometric pressure, adjusted to sea level, is 29.97 inches and rising...").

In simple round numbers 15 pounds per square inch if you had a perfect vacuum inside. My bag and vacuum pump aren't perfect, so 15 or 17 hundred (1,500) pounds per square foot is a pretty good estimate.

[David Rose]

to let the trapped glue dry. I don't know what is reasonable, but I'll bet it will take a few days. If you hang the router too soon, you will likely put a permanent sag in it. I'm just guessing, but...

After I had an episode with shellaced MDF edges, the board seemed dry after three days. I laid it on a finished shellaced cherry panel for about an hour. When I picked up the piece of MDF, the panel was very 'effected'. :o Sooo glad shellac is easy to touch up! ;)

David