Vacuum question

[Dave Bagwill]

In a vacuum bag or press, does the bag material have to physically lay on the material being glued/clamped/veneered? Suppose for instance that I had only the rim of a drum, and I wanted to glue a thin piece of wood onto that rim, where the drum 'skin' would normally go, and I wanted to do it with a vacuum bag. Suppose that wood is fragile, and would fail under the pressure of the vacuum.

But as long as we are supposin' - what if I put a caul on the wood, following the rim, so that when vacuum was applied, the bag material would contact the caul only and not touch the rest of the wood?

Would the central part of the wood, inside of the rim, feel any pressure at all?

[Tim Benware]

From the way you're describing it, IMO, I would think so but it may disperse enough of the pressure to let it work depending on how fragile the piece is.

[John Parchem]

Be carful the vacuum will put pressure all around the drum. Using you drum example, it would be able to take a lot of pressure straight down as a ring makes a fairly stable structure. If air can escape the drum, the sides of that drum may not be able to take the pressure and the whole thing may collapse inwards. If the drum is sealed you may not need the caul as the pressure inside and outside the drum is the same and the rim will take the pressure of bag as it stretches over the drum.

[Dave Bagwill]

Time I fired up the cray supercomputer in my basement, and the NASA planetary mechanics algorithms again - see pic attached.

If the bag is kept away from the sides of the braces, is there pressure in the voids? In other words, is there any force against the sides of the brace as drawn in this example?

[John Parchem]

Time I fired up the cray supercomputer in my basement, and the NASA planetary mechanics algorithms again - see pic attached.

If the bag is kept away from the sides of the braces, is there pressure in the voids? In other words, is there any force against the sides of the brace as drawn in this example?

No pressure in the void, Just a lot of pressure against the bag. It is holding the whole atmosphere above it. If the brace is hollow and there is air on the inside there would be pressure from inside the brace as the air wants to expand into the void.

[Dave Bagwill]

Could, then, a top or back be glued on in a vacuum bag, if a caul was used around the rim to keep the bag off the middle portion, and away from the sides?
Would it not be less pressure across the back/top than using rubber bands, which is the method I currently use and which is perfectly safe?

[John Parchem]

Could, then, a top or back be glued on in a vacuum bag, if a caul was used around the rim to keep the bag off the middle portion, and away from the sides?
Would it not be less pressure across the back/top than using rubber bands, which is the method I currently use and which is perfectly safe?

I will let you try it first!