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?
438957

[Andrew Seemann]

Any flat head screw that doesn't cause bridging is fine. You can also clearance drill the top/attaching piece so that it clears the threads, that is what usually do in when I use 2" and under screws that are threaded all the way.

I normally use those gold construction screws from Menards in the shop. They are relatively cheap and work for most everything. The phillips head versions tend to stay on the bit better, even though torx have a little better contact with the driver bit.

Not sure how big your glue up is, but if I were screwing it, I would predrill all the holes ahead of time, including the countersink. On a really big glue up, sometimes you can be racing the open time of the glue. When you predrill, temporarily put the screws in the corners right away, that will keep everything lined up while you are drilling the rest. If you drill it flat and level, it then should say level when you glue and screw it together.

[Jon Steffen]

Any flat head screw that doesn't cause bridging is fine. You can also clearance drill the top/attaching piece so that it clears the threads, that is what usually do in when I use 2" and under screws that are threaded all the way.

I normally use those gold construction screws from Menards in the shop. They are relatively cheap and work for most everything. The phillips head versions tend to stay on the bit better, even though torx have a little better contact with the driver bit.

Not sure how big your glue up is, but if I were screwing it, I would predrill all the holes ahead of time, including the countersink. On a really big glue up, sometimes you can be racing the open time of the glue. When you predrill, temporarily put the screws in the corners right away, that will keep everything lined up while you are drilling the rest. If you drill it flat and level, it then should say level when you glue and screw it together.
i'm planning to dry fit the whole thing, pre-drill and countersink all the holes on the bottom skin of the torsion box. not sure what i'm doing yet on the top skin.
i was also thinking of using titebond 3 to lengthen my working time, as this may take some time to glue it all up and screw it all down.

[Andrew Seemann]

You'll want the top figured out before you do the bottom. You don't want a lot of time to elapse between gluing on the bottom and the top, otherwise it can warp before you get the top on. Ideally you would glue the bottom and then immediately turn it over and glue the top.

[Jim Matthews]

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?
438957

You're over thinking this.

I would recommend buying a "Pin Nailer" and quiet compressor for this, and future cabinetry builds.

Pin nails leave an undisturbed surface and hold pieces together while the glue dries. Using screws for the task *would* work, but screws require predrilling clearance holes AND a countersink to get the head flush.

https://www.californiaairtools.com/

(The compressor package I chose came out of their bundled offerings, UPS.)

https://www.popularwoodworking.com/projects/q-a-why-predrill-screw-holes/

[Bruce Lowekamp]

A pin nail won’t be strong enough to fix any sort of bowing you might get with e.g. plywood, so your top & bottom would need to be perfectly flat.

For screws I normally use McFeely’s or the coated exterior screws HD sells. Both are only threaded 2/3 of the way up so would be fine in this application.

Another way to apply weight would be to get a kiddie pool and fill it with water.

Bruce

[Prashun Patel]

I would use screws. Form follows function here.

[Kevin Jenness]

I've done a number of torsion boxes like yours with cauls and clamps. I sandwich the work between 3/4" mdf cauls and 5 or 6 pairs of 2" x 4" hardwood cauls, so only 10 or 12 clamps are needed. The cauls are slightly curved or flat with sandpaper pads at the center and quarter points for the same effect. You don't need bone crushing force, just enough to get the parts in contact. Epoxy or ppr glue will give plenty of working time, though I have used yellow glue when working with a partner. You can also use clamp with vacuum by sealing a sheet over the torsion box and down to a benchtop underneath.

[Jon Steffen]

I've done a number of torsion boxes like yours with cauls and clamps. I sandwich the work between 3/4" mdf cauls and 5 or 6 pairs of 2" x 4" hardwood cauls, so only 10 or 12 clamps are needed. The cauls are slightly curved or flat with sandpaper pads at the center and quarter points for the same effect. You don't need bone crushing force, just enough to get the parts in contact. Epoxy or ppr glue will give plenty of working time, though I have used yellow glue when working with a partner. You can also use clamp with vacuum by sealing a sheet over the torsion box and down to a benchtop underneath.
Do you put the cauls accross the long way or the short way? If this was a 4x8 table, would you have 4.5' cauls, or 8.5' cauls?

[Jon Steffen]

Thanks everyone for your suggestions! This surely shows there is more than 4-5 ways to skin a cat! :p

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 grid and skin together, or if just enough weight to take out any bowing is fine as the glue dries.....

this is why i'm leaning back towards the cauls, screws or clamps. HA HA. I promise when its done it'll be better than average. :eek:

[ChrisA Edwards]

Cauls would go across the 4' width. I would put a pair of cauls over each width ways rib, so the number of cauls and clamps needed with vary with the number of ribs. Also a dual pair on each end.

Cauls can be made out of 2"x4"s of 2" x 6"s. I would buy 10ft lengths and cut them in half, so you can get 2 pairs out of a 2"x4" x 10', 5' long and 3 pairs out of a 2"x6"x10'.

If you are not using construction grade plywood, the sheeting will be very close to flat without much tendency to warp of twist. Getting a support for the base, that's flat, both X and Y, will be the most difficult job.

If you make your ribs out of a good quality plywood or MDF, the sheeting will conform to these as you glue, screw and clamp, giving you a near perfect flat base.

[Jon Steffen]

Cauls would go across the 4' width. I would put a pair of cauls over each width ways rib, so the number of cauls and clamps needed with vary with the number of ribs. Also a dual pair on each end.

Cauls can be made out of 2"x4"s of 2" x 6"s. I would buy 10ft lengths and cut them in half, so you can get 2 pairs out of a 2"x4" x 10', 5' long and 3 pairs out of a 2"x6"x10'.

If you are not using construction grade plywood, the sheeting will be very close to flat without much tendency to warp of twist. Getting a support for the base, that's flat, both X and Y, will be the most difficult job.

If you make your ribs out of a good quality plywood or MDF, the sheeting will conform to these as you glue, screw and clamp, giving you a near perfect flat base.
Thanks Chris. I'll be using mdf for the skins and baltic birch plywood for the internal skeleton. i'm almost done making my 49x97 Parf top so I can get repeatable cuts on the skeleton grid, then I need to build the box.

This is a weird situation, i've said it before. I feel like i'm living the chicken and the egg, scenario. I need a big parf torsion table to build a big parf torsion table ...., lmao. This would be different if I had a nice table saw, but I don't. We'll see if it works out and WHEN IT DOES, i'll document it "With photos" and show ya'll what I did.

Can you make cauls out of plywood?.....i would assume so.

[ChrisA Edwards]

Can you make cauls out of plywood?.....i would assume so.

Yes, that first Torsion Box build is a bit Catch-22 or Chicken and Egg..

For Cauls, I think you could make them out of 2 layers of 3/4" ply in 2.5" to 3" strips. You want the very slight banana curve, with the convex side down on the work piece.

The clamps take out the gaps at each end, i.e. straightening the convex curve, by bending the caul, and this applies a fairly consistent pressure across the full length of the caul, especially in the middle.

[Jon Steffen]

Yes, that first Torsion Box build is a bit Catch-22 or Chicken and Egg..

For Cauls, I think you could make them out of 2 layers of 3/4" ply in 2.5" to 3" strips. You want the very slight banana curve, with the convex side down on the work piece.

The clamps take out the gaps at each end, i.e. straightening the convex curve, by bending the caul, and this applies a fairly consistent pressure across the full length of the caul, especially in the middle.
oh wait, i can't make them out of plywood. if I cut 8' in half, it won't overhang the width of the box, 2x?x10' it is. =)

[bill epstein]

I've done a number of torsion boxes like yours with cauls and clamps. I sandwich the work between 3/4" mdf cauls and 5 or 6 pairs of 2" x 4" hardwood cauls, so only 10 or 12 clamps are needed. The cauls are slightly curved or flat with sandpaper pads at the center and quarter points for the same effect. You don't need bone crushing force, just enough to get the parts in contact. Epoxy or ppr glue will give plenty of working time, though I have used yellow glue when working with a partner. You can also use clamp with vacuum by sealing a sheet over the torsion box and down to a benchtop underneath.

Second that. No different than veneering large-ish panels with PPR as I have done many times. 2 X 3/4" MDF as Platen, clamps every 6" on hardwood cauls.

[Kevin Jenness]

"oh wait, i can't make them out of plywood. if I cut 8' in half, it won't overhang the width of the box"

They need only be as long as the box is wide.

[Don Stephan]

When building my 4' x 8' torsion box assembly table I think I followed an article by Ian Kirby. I think my top, bottom are 1/2" MDF, and grid 3/4" MDF. If memory serves, first I very carefully cut the grid pieces. I think then I jointed and edge planed 4 or 5 2/x4's so they all were straight with parallel edges all the same width. A pair of sawhorses perhaps 6' apart were shimmed so the top top edges were coplanar, then spaced out the 2x4's across them. One sheet of 1/2" MDF was laid on the 2x4's, the grid was assembled with staples bridging the butt joints, a bead of glue on the top edges of the grid, the other sheet of 1/2" MDF laid on the grid. Several 4x8 sheets of plywood and melamine board were quickly laid on top to "clamp" while the glue dried. Several hours or the next day clamps held the sandwich together while it was turned over, then the gluing process repeated. A 2nd pair of hands was needed for the process.

I would not have the confidence that a sufficient number of screws could be driven, even in pre-drilled holes, before the glue started to set.

[Curt Harms]

You're over thinking this.

I would recommend buying a "Pin Nailer" and quiet compressor for this, and future cabinetry builds.

Pin nails leave an undisturbed surface and hold pieces together while the glue dries. Using screws for the task *would* work, but screws require predrilling clearance holes AND a countersink to get the head flush.

https://www.californiaairtools.com/

(The compressor package I chose came out of their bundled offerings, UPS.)

https://www.popularwoodworking.com/projects/q-a-why-predrill-screw-holes/

i've built a couple movable torsion box style work benches. I did 1/4" ply bottoms and 3/8" ply tops. Fastened with glue and narrow crown staples driven flush. I did check with string and winding sticks to make sure there was no significant bow or twist before stapling the top.