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

[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?
Attachment 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/p...l-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!

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.

[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.