How much weight for glue up top on torsion box

[Paulh Tremblay]

I am getting ready to glue up my top to the web of my torsion box. Since the frame sits on the floor, I cannot use clamps. I will put weight on top of the MDF skin instead.

Can anyone give me a tip on how much weight to use? Bricks (seem too light)? Cinder blocks (seem too heavy)?

[John Lemke]

I believe I've handled things like this with bags of water softener salt.

[Rick Christopherson]

A pickup truck is probably a good start. Add up the surface area of your web and multiply by the desired pressure for the type of adhesive. You'll quickly exceed 10,000 pounds.

[Jamie Buxton]

The key is not so much huge weight, but rather getting weight distributed all over the surfaces being laminated. In glue-ups, what clamps and such must do is squeeze out excess glue and bring the wood parts close together. That's all that's necessary.

For instance, say you're using 1/4" ply for the skin. You can put a zillion pounds in the middle of the area, and the edges of the skin are going to be flapping in the breeze. That's not good. But say you put your hand down on some part of the skin and push down firmly. Right under your hand, the plywood will be contacting the spacer grid. You might only be pushing down with a few pounds, but that would still make the plywood contact the spacer grid. Now if you had enough hands to cover the skin with that same fairly-small force, you'd be making the skin touch the spacer grid everywhere, and you'd get a good torsion box. So what do you do if you don't have a hundred hands? John Lemke's suggestion of bags of water softener salt is a good answer. He's distributing pressure all over the torsion box. A somewhat different approach is a caul. A caul is a piece of wood which distributes pressure. An example is a couple of sheets of MDF. Put pressure in a single point on the caul, and it will get spread out to some larger area below the caul. It still won't get to the edge of the MDF sheets, but it will get spread out some ways. So instead of John's bags of salt all over, you could use a caul stacked with other heavy things -- workboxes, engine blocks, cinder blocks, a couple boulders, what have you. The key is to try to put pressure everyplace.

[Keith Westfall]

Curious as to the "levelness" of your floor. Any error there will be built into the top. Appreciate it's easier to clamp with weights, but you do want to make sure it's glued up straight.

[Art Mann]

I have built a couple of torsion boxes for special applications; one of which I used a 16 gauge pin nailer to hold the top on. As someone already said, it isn't really possible to apply significant clamping force with weight because of the surface area involved. The other one was a router table top that was only 2 by 3 feet or so. I used a hydraulic jack and some oak 4X4 blocks to clamp the top on. I have a beam running down the center of my shop and I used the jack and a 4 X 4 to generate the force. I would estimate the force was around 2000 pounds. Having said all that, I don't think you need a lot of clamping force but you do need to have the sheet and the web to make good contact at all points of intersection.

[Paulh Tremblay]

I'm getting two different responses: can't be done (need too much weight); and make sure your weight is evenly distributed.

My only other options besides weight is buying a brad gun and compressor or using screws. I would have to pre-drill the holes before I applied the glue, and then use some referencing system to find the same holes. If I couldn't find the same holes I'd have a mess on my hands.

[Paulh Tremblay]

Curious as to the "levelness" of your floor. Any error there will be built into the top. Appreciate it's easier to clamp with weights, but you do want to make sure it's glued up straight.

Not having a planer, I could not plane 2 X 4's to even thickness. So I cut 8 pieces of MDF 100mm (4 inches) thick. I laid them on the floor (edge up) and then used 3 packages of shims, placed 6 inches apart, to make everything level. I have a piece of MDF on top of this frame, and on that MDF I will build my inner web.

[Jamie Buxton]

..I'm getting two different responses: can't be done (need too much weight)...

I'm trying to delete this reply, but can't seem to make it work.

[Art Mann]

I'm getting two different responses: can't be done (need too much weight); and make sure your weight is evenly distributed.

My only other options besides weight is buying a brad gun and compressor or using screws. I would have to pre-drill the holes before I applied the glue, and then use some referencing system to find the same holes. If I couldn't find the same holes I'd have a mess on my hands.

I'm sorry if i seemed to be saying contradictory things. I should have been more specific. At the time I built my torsion boxes, I was under the impression that good strong glue joints required really strong clamping force. Since that time, I have done some reading on the subject and done some destructive testing just to see for myself. I am now convinced that strong joints don't necessarily have to have extreme clamping pressure and in fact, too much force can have a weakening effect. As far as torsion boxes go, their stiffness and strength depend on keeping the skins in tension or compression with no slippage with respect to the interior structure. That isn't a very challenging thing to do because of all the glued contact surface area. You do need enough weight or clamping force to keep the web structure and skins in intimate contact until the glue cures.

[Paulh Tremblay]

I'm sorry if i seemed to be saying contradictory things. I should have been more specific. At the time I built my torsion boxes, I was under the impression that good strong glue joints required really strong clamping force. Since that time, I have done some reading on the subject and done some destructive testing just to see for myself. I am now convinced that strong joints don't necessarily have to have extreme clamping pressure and in fact, too much force can have a weakening effect. As far as torsion boxes go, their stiffness and strength depend on keeping the skins in tension or compression with no slippage with respect to the interior structure. That isn't a very challenging thing to do because of all the glued contact surface area. You do need enough weight or clamping force to keep the web structure and skins in intimate contact until the glue cures.

Thanks. That makes sense. I had read some other info on the web that said the same. For example, a vacuum press is considered an effective way to glue pieces together, and it uses only 15 psi, not the 150 psi often cited. (Though, maybe a vacuum press is meant only for veneer?)

Time to find some cinder blocks...

[Rich Enders]

Mine was made in two steps.

1) Inner web glued and screwed down to one of the surface sheets which was placed on a flat surface.

2) Second surface sheet glued and screwed to the inner web.

Clearance holes predrilled through the web for step one. Clearance holes predrilled through the second surface sheet for step 2.

Enough screws and no clamping required.

[Rick Christopherson]

Thanks. That makes sense. I had read some other info on the web that said the same. For example, a vacuum press is considered an effective way to glue pieces together, and it uses only 15 psi, not the 150 psi often cited. (Though, maybe a vacuum press is meant only for veneer?)

Time to find some cinder blocks...

No where in my post did I mention 150 psi, or any other pressure for that matter. What I was pointing out is that with almost any pressure, even as low as 15 psi, you can quickly reach the GVW of a pickup truck due to the large surface area of your web.

Add up the surface area of your web. I don't know what this is because I don't know their spacing nor the total size of the box. Then multiply this by your target pressure. 15 psi should be an absolute minimum, but 50 psi would be better.

So let's say you had 300 square inches of web edge to glue and wanted to stick to the minimum of 15 psi. That still gives you a required weight of 4500 pounds. However, a 4x8 box could easily have 1800 square inches of web edges, so now the total weight balloons up to 27,000 pounds at only 15 psi.

Just for comparison, a 4x8 face-to-face glue up (i.e. no web) has 48 x 96 = 4608 square inches. So at 15 psi, this yields 69,120 pounds via a vacuum bag.

[glenn bradley]

The key is not so much huge weight, but rather getting weight distributed all over the surfaces being laminated.

My experience echos Jamie's thoughts. I have overloaded or over clamped such assemblies and forced them out of flat. I am curious where you found a floor flat enough to be a reference surface though. I assume you have it shimmed somehow(?). at any rate, with a known flat surface as a reference and a good grid and frame (consistent wide parts) I put only enough weight to hold the piece being glued in place. For smaller thick pieces of MDF, the weight of the material is fine. For a large panel of thinner stock I would set small weights distributed across the surface or better yet, small weights on long cauls to distribute the weight and avoid "wrinkles" dues to pin-point weight locations.

[ian maybury]

The real key is probably a grid of accurately levelled support points on the floor or whatever - placed to fall under the internal beams/webs rather than just at the corners. Wood blocks cut to thickness and stuck to a floor with double sided tape? It'd also need a good long (and accurate) level which can double as a straight edge Maybe a laser leveller would be good, but i've not used one.

Next up is the structure. If it's naturally inclined to lie flat - the beams/webs are straight/accurately cut and the skins flat (not to be assumed with ply or MDF these days - the sheets are likely to need hand picking/be of a very high grade) then it's probably not going to need (or be desirable to use - risk of distortion) an enormous amount of clamping force. I've had good luck using a couple of layers of our local peat/wood fibre briquettes from the stack of firing outside of my door - centred over the webs/support points. Too much or not over webs may cause the skins to sag under load and be glued in that position.

The glue up probably matters - ideally use a slow adhesive so that the webs can be tacked in place and the top skin added before the earlier joints start to cure and mean that the construction is starting to take a set before it's all in it's final position...