Getting a flat surface when vacuum bagging?

[ian maybury]

John's thread on his veneer press screws reminded me of an issue I ran into recently when laminating a DIY rip table (top and bottom skins in 18mm birch ply, with a honeycomb like core from 24mm ply strips) for my Hammer K3 saw. I've extension tables to do next.

I ended up using weights (the compressed peat bales we burn locally) to clamp the glue up down flat on a machine table, but a vacuum bag would have been a lot easier than hulking the weights about. I don't really have the space or the budget to build a veneer press - although I could bite the bullet if it's the way to go.

Is there a technique for getting a dead flat glue up or lamination using a vacuum press? (the vacuum could press the skins down in between the ribs/honeycomb)

One option would be to use the vacuum, plus weights to hold the assembly down flat on a flat table, but that's adding to the complexity.

ian

[HANK METZ]

Here's an alternate construction method for creating a machine tool table with minimal tooling. It is inspired by the Mr. Sawdust design for radial arm saws, which depend on dead- flat surfaces for accuracy:
Torsion Box (http://www.youtube.com/watch?v=9euQNBStAG8)

I also own a vacuum veneer press but found this method needs no improvement with such a device.

- Beachside Hank
Do not use remaining fingers as push sticks.

[Jamie Buxton]

There's two ways. One is to do as you suggest: use weights to press the vacuum bag and its contents down to a flat surface. The other is to put a flat platen inside the bag, so the vacuum presses the workpiece against the platen.

[Chris Padilla]

www.joewoodworker.com will provide you hours of reading on vacuum pressing and veneering.

[ian maybury]

Thanks Jamie - there's no magic I guess, just the two basic options. The issue being that vacuum doesn't inherently pull anything flat. There's a risk too that it could pull down the skin between the ribs on a honeycomb.

That site is a great resource Chris, thank you.

I went down a very similar route to yours Hank, except that my top and bottom skins were 18mm phenolic birch ply, and the core strips in 24mm birch ply. The whole lot was glued up with liquid polyurethane. I wanted a ding proof table surface, hence the thick skins. I chickened out and put black laminate over the phenolic as it seemed a little soft.

My core wasn't a rigid framework as such like yours (just short pieces), so the thinking was instead to use the flatness of the shaper table underneath as a reference surface.

ian

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[Jeff Duncan]

Vacuum presses are amazingly handy tools. Yes if you must have something perfectly flat you'll want to use a nice flat platen inside of the bag. I usually use one anyway as it makes it much easier getting things in and out of the bag, as well as making it easier for the press to get all the air out, (because you cut a very shallow grid pattern in the surface of the platen).

Also the vacuum on a good press is adjustable....meaning you don't have to crank it so that it sucks things too tightly on a honeycomb. You drop the pressure so there's just enough to get a good even pressure. Then on your next project when you need to face glue some solid stock for a thick glue-up, you crank the pressure up and watch it work!

good luck,
JeffD

[Richard Dragin]

I use a platen inside the bag. Melamine with the edges eased and wax the surface. As long as it's on a flat table everything stays flat. Breather mesh on top (From Joe WW) makes it easy.

[ian maybury]

Thanks guys, that's good to know stuff. Next up is to figure out a vacuum system from over here in ireland...

ian

[Chris Padilla]

Ian,

That is QUITE a bit of glue squeeze-out you have there!!! LOL

[Alan Lightstone]

I use a platen inside the bag. Melamine with the edges eased and wax the surface. As long as it's on a flat table everything stays flat. Breather mesh on top (From Joe WW) makes it easy.
+1. It's always worked well for me.

[ian maybury]

:) I hadn't used a liquid polyurethane over a biggish area before Chris, and wasn't taking any chances on either time or coverage - but even so didn't expect the stuff to foam quite so enthusiastically. I used a brush to spread it too when in retrospect a toothed scraper would have been better - it would have given a more consistent coating.

Luckily enough it seems like it settled down all right - the excess squeezed out. The squeeze out saved my ass - ease of spreading it turned out is one positive with a liquid PU, but it was helped too by the fact that the honeycomb ribs were as luck would have it not too wide. Plus it was still fully wet and hadn't started foaming when it was weighted down.

On platens. The question quickly follows on how best to make a flat and rigid platen that can easily be handled. I'm guessing that two skins of birch ply over a fairly deep honeycomb/vertical webs of the same might not be a bad shot. It sounds like the vacuum bag plus weights and a good surface approach has to take over once the platen gets too big to handle and/or stay flat.

It looks like the Hammer F3 (shaper) is going to get use again as a surface table. It checks out as very flat, and the cutter hood lifts out of the way very easily to leave a clear surface..

Thanks again for the input, it really helps to wake up the grey cells and the insight. An out feed table is next, although another double skinned job would probably be overkill for that. Then the DIY extension tables for the planer, shaper and band saw which definitely will be double skinned.

ian