Which is best, twice the clamps with 1/2 the force, or 1/2 clamps with 2 times force?

[Jon Steffen]

i'm clamping down a large grid structure to a panel.

My budget is only so large.

Would it be better to have 64 clamps that are only rated to 150lbs, or have 32 clamps that are rated to 300 lbs?

I'm already anticipating the answer to be, that depends, how big is the thing you're clamping, does each board have 2-3 clamps on it in either scenario, use curved cauls instead, ect.


my thought was to use my parf mark 2 to drill a series of holes in the bottom skin of my torsion box, so that I can glue the bottom skin to the torsion grid structure, rather than using curved cauls to provide pressure to the middle part of the grid/skin.

[Steve Demuth]

As long as the clamps you're using develop the required force for the glue you're using, the more clamping points the better. The ideal is completely uniform pressure, using a vacuum or equivalent clamping mechanism. So, for clamping a grid to a panel, the ideal would be a platen atop the grid, compressed in a vacuum bag. But I can imagine if your panel and grid are very large, you wouldn't want to go that route. I would consider, before buying dozens of clamps, using a 3/4" sheet of plywood as a platen on the grid, and just sitting a bunch of flat sandbags on the plywood - unless of course, you have other reasons to want to buy a lot of clamps. If you're going with clamps, go with more.

[glenn bradley]

I use sand bags, gallons of finish/paint and jugs of water to clamp large panels to substrates like your gridpattern.

[Brian Tymchak]

I haven't built a torsion box but if were going to, I would probably use screws to attach the skins and avoid trying to clamp the skins to the grid. Instead of attching the grid to the skin it's lying on, lay the 2nd skin on the grid, glue an screw that down, let the glue dry, flip the assembly and attach the 2nd skin with glue and screws.

[Paul F Franklin]

Concrete blocks are an inexpensive way to get a lot of evenly distributed mass on top. When you're done, they can be repurposed as weights on top of a stickered lumber stack, or donated to habitat. But if you really want to go with clamps, then use more clamps to get even clamping pressure, as Steve said.

[Andrew Seemann]

For a torsion box, I would glue and screw the skins on rather than glue and clamp them, unless they were too thin to screw. On a torsion box, you usually have so much glue surface, that a super strong bond isn't necessary, kind off like a plywood back on a cabinet. The sand or cinder block method or your collection or books or FWW magazines should work adequate as well. Actually, and air nailer would work just fine, and is faster than screws or clamps.

With regards to clamps, I have found that more clamps are almost always better than better clamps.

[Jon Steffen]

you guys rock. This is exactly what I needed to hear. While I do need a lot of clamps, I may not need the specific clamps I would buy for this one project in the future. I'll save the money until another day.

I think i'm going to try making the saw horses with 2x4 grid (tracksaw cut on one side, then planed parallel on the other).
Set the top skin upside down on that.
Set Grid on that skin,
glue grid to 2nd skin that is layed on top (the bottom skin actually) with cinder blocks, I've got a bunch in the backyard for a few projects. I may then end up putting some screws through the top skin into grid if I feel the cinder blocks aren't doing the trick, but they should if the original saw horses/2x4 grid is in fact flat/plane/plumb/perfect. The whole sandwich with that much weight on it should also be flat/plane/plub/perfect.

then flip the whole mess over and glue the real top on.

Thanks again!
edit: i'm having 2nd thoughts on using the weight method. i'm too new to know if i need a ton of weight to squeeze the 2 pieces together, or if just enough weight to take out any bowing is fine as the glue dries.....

[Dave Sabo]

Your joinery should go together properly without clamps. If you need clamps to force things together, fix that.

This succinctly sums up all you really need to know.

With that in mind, more clamps are better. They will spread out the force applied more evenly.


and you can never have too many clamps.

[Andy D Jones]

The glue you use will dictate the pressure required to achieve the film thickness required for the rated bond strength.

If you want to use weights to provide the clamping pressure, you must provide better support for it while gluing than the torsion box would provide itself when completely assembled and the glue is dry. Otherwise, the structure will be perfectly glued together, with a permanent sag. The weight required to provide the adequate pressure will almost certainly be more than what even the finished structure would support with any usable flatness.

This is an advantage of using clamps. Or screws. Or both.

If you intend to employ parf clamps with the completed structure, you could use those holes (top and bottom) to insert pipe clamps for clamping the skins on the structure, with adequate cauls. You could also use threaded rod, washers and nuts.

-- Andy - Arlington TX

[Thomas McCurnin]

Besides traditional clamps, I have a supply of 10" concrete blocks and 4" red clay bricks which I use a lot for clamping. To keep the wood clean, I've wrapped the bricks in duct tape and use paper to protect the wood with the CMU blocks.

[Tom M King]

I parked a car tire on a good sized glueup once. It worked fine. It wasn't a big car.

[Mark Bolton]

If your talking PVA (white or yellow wood glue) more clamps closer together will ultimately give you better results given your not grossly over exceeding the clamps rating. Technically the most force you can apply, 6" spacing, will never over-clamp a PVA joint. Virtually any arrangement you choose can never over-clamp the joint. Your bigger concern with cheaper clamps would be trying to clamp more than you should and under-clamping.

There will be no competing on price with super cheap harbor freight style aluminum clamps, but you will have to keep your glue ups within reason. Pony/yost 3/4" pipe clamps are your best bang for the buck but still not super cheap.

In the end, pretty much anything including sitting on the boards will work. But tight strong glue joints,.. more is better.

[Richard Coers]

When I operated a custom woodworking business, I did a 4'x12' conference table that was built as a torsion box. I clamped the skins on by cutting some 1x4s a little long, then bent them and shoved them between the ceiling and torsion box. I was in an old concrete warehouse, so didn't have to be particular where I put them. But easy to mark where your ceiling joists are and do the same thing.

[Jon Steffen]

The glue you use will dictate the pressure required to achieve the film thickness required for the rated bond strength.

If you want to use weights to provide the clamping pressure, you must provide better support for it while gluing than the torsion box would provide itself when completely assembled and the glue is dry. Otherwise, the structure will be perfectly glued together, with a permanent sag. The weight required to provide the adequate pressure will almost certainly be more than what even the finished structure would support with any usable flatness.

This is an advantage of using clamps. Or screws. Or both.

If you intend to employ parf clamps with the completed structure, you could use those holes (top and bottom) to insert pipe clamps for clamping the skins on the structure, with adequate cauls. You could also use threaded rod, washers and nuts.

-- Andy - Arlington TX

am i thinking this correctly? if you have 10 clamps rated at 150lbs force, to have same force by weight only, you would need 1500 lbs to be equivalent? if that's the case, i can definately see how to achieve the required pressure, it would bend everything.

[Jon Steffen]

now i'm thinking the glue/screw route is maybe the right way for me. i'd have to spend a small fortune on clamps, even cheaper $32 4 pack type clamps, also I don't want to spend a ton on clamps and then find out they're not the size/ones I want for other jobs. Probably best to slowly aquire more and more. I already have like 12-16 irwin clamps, some medium, some light duty.

So what do i need to know about glue/screws? how do I make sure the screws I choose will pull the pieces together? i read it shouldn't be threaded the whole way, any specific brand you guys like for a project like this? i was thinking spax or grk like these with square or torx. how much "clamping pressure" does a screw provide?
T-StarYellowZincWoodScrew-MainImage.jpg