Space Saver for vaccuum press?

[boblon]

David,
This seems to have almost grown into a discussion over theory and reality. Keith raises a lot of good points and I would take what he says to heart.

Since we really don't know what your intended use is, or whether it's a single project or something you plan on doing on an ongoing basis let me just throw a few things out there.

If what your doing is a small flat project I would say just use clamps and cauls and be done with it.

If your talking a rather large project then it seems to me that your investment of time in the design, implementation and materials could be costly. Especially if your using some nice matched veneer that you've either purchased or cut yourself. In this case I would not recommend trusting a space saver bag to it even though in theory it appears it should work.

If your trying to do a curved piece over a form things are a little different. Now you need enough vacuum not only to ensure a good even bond between the veneers and the substrate (if your using one), but you need enough pressure to ensure the glue up is held firmly and evenly to your form.

If your gluing up narrow pieces with curves in them you can glue some MDF together and cut the curve from that to create two mating pieces that can be used with clamps to shape the project between them. I think David Marks did this on one of his projects.

I believe in the 'old' days they created special clamping stations using veneer press screws similar to this:

http://www.woodcraft.com/family.aspx?FamilyID=4362

I suppose that 'could' be another option.

When I built my vacuum press I wanted to do curved work over forms (besides flat panels) so I just went ahead and built a vacuum system.
There are several sites out there on building your own vacuum systems, the one I used was this one:

http://www.joewoodworker.com

I got my pump off of EBay and most of the other parts from the link above. Here's the setup I made:



And here's the first test press I did. It's 4 pieces of maple veneer I cut, using a space saver bag by the way:



I used that very form to make this box:

http://www.bt3central.com/showthread...ght=walnut+box

The bottom line is I thing we've all been bitten by the 'Dang, I thought for sure that would work' syndrome (there must be a Murphy's law for this).
With anything other than a small flat panel glue up that can be easily clamped it would be very disappointing to put all the effort into trying to create a piece only to be dissatisfied with the results, or have a project delaminate later.

Anyhow, good luck with whatever your doing.

BobL.

[crokett]

Keith/Bob,

It's not a matter of not believing you. I've been told what I want to do won't work very well so I will find another way to do it - probably cauls since the first couple veneers I do will be small flat projects. What I want to understand is why my thinking was incorrect in the first place.

The evidence I have (visual) says my shop vac will remove all the air from a plastic bag and squash it flat. Therefore, it should do the same for a space saver. I am being told this is not so and just want to know where the air is that is creating that 5 atmospheres inside the bag.

[boblon]

David,
I'm still a newbie when it comes to this and I don't claim to have all the answers, but looking at it from your point of view I guess I can see what your saying. That is if this sums it up.

You lay a Space saver bag (empty) on the bench and hook up your shop vac. You turn on the shop vac and ALL the air is sucked out of the bag and both sides of the bag are sucked flat against each other. Since there appears to be no air left in the bag then how could hooking up a vacuum pump remove any more air that's not in there anyhow?

But, when you do a veneering project you typically use platens top and bottom with the veneer stack between them. These platens are flat and smooth on the side towards the veneer and have a crosshatch pattern of grooves on the outside to facilitate the evacuation of the air. That or a mesh material is used for the same purpose.

Now, with this stack of material in there, along with any slight abberations in the flatness of the veneer or curling from the glue application you no longer have both bag sides against each other with 'no air left'.

So now you begin to remove the air from the bag. I don't have any idea what level of vacuum a shop vac will create, but the vacuum created will need to compress that stack enough to flatten out any curl/abberations and ensure the glue gets into the pores of the wood to provide strong permanent adheshion. Due to the seperation of the bag sides by the stack inside just 'removing all the air now' becomes different as there will be spaces depending on what's in there.

Now all this need to sit untill the glue has formed a strong bond and this could be hours and hours depending on the glue you use. Ideally, if the bag you use has no leaks then everything is stable after the initial application of the vacuum you can just leave it and walk away for a while.

The problem is that small leaks seem to be a VERY common problem (not just with space bags either, although people here and those I've talked to have had problems with those bags even for thier intended use), one that can be exacerbated as the level of vacuum increases.

Now, like Keith said, you may try it on a small project and find that it works ok.

Is it the recommended way to do this?
No.

Can you get a way with it?
Maybe. Your results will probably vary depending on the circumstances and the dynamics of the particular project your trying.

If your curious and your material investment is small go ahead and give it a try. For large or serious veneer work I would recommend a different approach.

BobL.

[crokett]

Thanks Bob. Forgot about all the little spaces that exist. I think for now I will go with cauls on some small flat stuff more or less for practice.

[drumpriest]

For small flat stuff you can use cauls. One hint is ot make yourself some cauls that are curved on one side so that as you clamp down, they straighten and spread the load.