Torsion box design

[Dan Chouinard]

I need to build a new torsion box to put on a scissor lift table that I just purchased. Idea is to have height adjustable table/bench for cabinet assembly. The top on lift measures 32" x 72". The new torsion box will be at least 42"x 8' but may be as big as 48" x 10'. Torsion box needs to be as thin as possible. Weight of it is a non issue, lift has 3 ton capacity.
Any thoughts on how to build this new torsion box would be greatly appreciated.
Thanks
Dan

[Kevin Jenness]

In have built a number of torsion boxes of that scale with various scantlings. Given sufficient glue area the thickness determines the stiffness. Almost all have been 4" thick 4' x 8' and are very rigid torsionally- if set on a non-flat surface they will touch at three points even under normal loads. The one outlier is a 2 1/4" x42" x 90" box and that one requires a flat support although it is reasonably stiff for its weight. You could compromise, depending on your tolerance for flatness and the rigidity of the scissor lift support.

I used one for many years that was built with a core of 5/16" bassswood strips about 4" o. c. and a rim of 3/4" thick basswood, skinned with 1/4" lauan and covered with p-lam. It probably weighed about 60 lb. The only weakness was the skin deflecting under clamp pressure between the core grids. You can use thicker skins and a core with wider spacing, especially if you want to screw into the surface.

I have found it is easiest to assemble the core with half-lap joints.

[Frank Pratt]

I built some torsion box shelves that had to be very light span 7'. They had 1/8" plywood skins top & bottom with a 1x1 solid wood perimeter frame and 1" blue extruded insulation filling the center cavity. These had no noticeable sag with 30 or 40 lbs on them. This is not to suggest you should build yours so light, but to give an example of how little material needs to go into a very strong torsion box. If you went 1-1/2" thick & 1/4" skins, I bet it would be rigid enough.

How big is the lift? With a 10' long table there might be issues with tipping if the table is leaned on or not loaded just right.

[lowell holmes]

See this site:
https://www.google.com/search?q=torsion+box&sxsrf=ALeKk02_4XTBaGPu2PKSOCmVIUGo-6Un_Q:1612877398495&tbm=isch&source=iu&ictx=1&fir=9PR9OdbsHY9DrM%252CPLef_S79ZY3pOM%252C_&vet=1&usg=AI4_-kRHKRYmtERljCrkSYaEv1b3xcyKxw&sa=X&ved=2ahUKEwio25zD9NzuAhVLOs0KHbn7CswQ9QF6BAgKEAE&biw=2133&bih=1076#imgrc=9PR9OdbsHY9DrM

[Harvey Miller]

Here's one describing a torsion box made from baltic birch 72"x24" and 2 1/2" thick :
https://www.canadianwoodworking.com/plans-projects/rolling-shop-storage

[scott lipscomb]

Here is mine, its about 78"x38"x5", with MFT style holes. Ribs are in between the the holes and totally 3/4" birch ply. Weighs probably 180lbs. Its absolutely as dead flat as I can tell, which is what I really like about it. Its my go to for building/checking/clamping all kinds of things that need to be flat. I used the the Parf guide for the wholes and used the "every other" holes to screw it together for glue up-then I took the screws out. I use the Festool ratcheting clamps on this table a lot.

BTW, I use that vise made from a pipe vise on the end all the time. It clamps things that my two more traditional vises won't.

If I build another one, I'd probably do it this way again.

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[Richard Coers]

I need to build a new torsion box to put on a scissor lift table that I just purchased. Idea is to have height adjustable table/bench for cabinet assembly. The top on lift measures 32" x 72". The new torsion box will be at least 42"x 8' but may be as big as 48" x 10'. Torsion box needs to be as thin as possible. Weight of it is a non issue, lift has 3 ton capacity.
Any thoughts on how to build this new torsion box would be greatly appreciated.
Thanks
Dan
It would be helpful if we knew how much weight you intend to put on the box and if you intend to bolt it to the lift. I did a 4'x12' torsion box conference table with a minimal base under it. To handle the loads I made an angle iron structure inside the base. To mount to that steel, I used 1/4" steel plate with bolts welded to in and installed inside the torsion box as I built it up. The biggest hurdle though with a huge torsion box, is have a flat surface to build it on. Very easy to build a curve into a torsion box. I knew the floor in the rented shop was not flat enough for a table. So to get a flat torsion box, I built a 16" tall grid from particle board to bridge gaps in the floor. It worked great. With the concrete ceiling in the shop, I used slightly long 1x4s to bow between the torsion box and the ceiling as clamps.

[Dan Chouinard]

Lift weighs 965 pounds.
Thanks guys for your guidance.

[Richard Coers]

Lift weighs 965 pounds.
Thanks guys for your guidance.
The weight of the lift makes no difference. I'm talking about the weight you will be putting on a corner of the torsion box, as well as the overall weight being placed on top of the torsion box. No one wants to recommend a 1/4" skin if you put a 400 pound item on it with a high point loading through 4 legs.

[Dan Chouinard]

The weight of the lift makes no difference. I'm talking about the weight you will be putting on a corner of the torsion box, as well as the overall weight being placed on top of the torsion box. No one wants to recommend a 1/4" skin if you put a 400 pound item on it with a high point loading through 4 legs.
I have no interest in 1/4" skin. I need to be able to clamp work down to this thing.

[Frank Pratt]

I have no interest in 1/4" skin. I need to be able to clamp work down to this thing.

So why don't you go ahead & let us know all your criteria, rather than having everyone trying to guess how to help?

[Dan Chouinard]

My apologies Frank for omitting the need for clamping capabilities. Have a good evening Sir.

[Jim Becker]

The currently popular "paulk" type top with dogs and clamping slots may be a good way to go here, Dan. You do have to give in to a little bit of thickness to accommodate your requirement, but the utility will be there that you need. One thing...try to find the flattest surface you can to build the torsion box on. It can be as simple as sawhorses as long as you can carefully level and get them all in the same plane. Borrow or rent a laser system if you don't have one to get that perfect and your resultant torsion box will be da bomb.

[Frank Pratt]

My apologies Frank for omitting the need for clamping capabilities. Have a good evening Sir.

Sorry Dan for the snarky post. I honestly am interested in your project. When you say you want the top to be thin. What thickness are you thinking?

My outfeed table is a torsion box that is 2-1/2" thick, using 1/2" Baltic birch for top & bottom skins & the same for the grid. It is very flat & robust. I use it to clamp to & it stands up very well. Is that the sort of thing you're wanting?

[Dennis Yamamoto]

I have found it is easiest to assemble the core with half-lap joints.

I got this suggestion from Jamie in the Bay Area Woodworking Assoc: you can gang cut all of the half-laps by stretch wrapping all of the grid boards together that are the same length. In addition, once you cut one half-lap, you can put that half-lap over a fixed pin for even spacing. Something like making a box joint.

Dennis

[Dan Chouinard]

I do have the luxury of making the new box on my current 32" x 8' torsion box. :)

[Dan Chouinard]

Sorry Dan for the snarky post. I honestly am interested in your project. When you say you want the top to be thin. What thickness are you thinking?

My outfeed table is a torsion box that is 2-1/2" thick, using 1/2" Baltic birch for top & bottom skins & the same for the grid. It is very flat & robust. I use it to clamp to & it stands up very well. Is that the sort of thing you're wanting?
No worries Frank. Its all good. I could have been more clear. Its the overall thickness I was hoping to minimize. I will be clamping to it frequently. The weight of it is less important and I should not have mentioned it at all.

[Jim Matthews]

If you're building this with an air nailer, leave enough space between dividers for the "gun" to fit.

[Jim Becker]

No worries Frank. Its all good. I could have been more clear. Its the overall thickness I was hoping to minimize. I will be clamping to it frequently. The weight of it is less important and I should not have mentioned it at all.
From a practical standpoint for both clamping and use of dogs for alignment purposes, I think you have to expect that probably about 3" is about the minimum you'll get away with. The shorter internal structures will also have less weight deflection capacity where the assembly isn't supported by the underlying lift-table...the overhang, in other words. The more-bigger you make the torsion box for your application, the thicker it necessarily has to be to insure it stays flat around the unsupported edges.

[Robert Engel]

Check Mike Farrington. He has a video on building exactly what you're talking about.

[Dan Chouinard]

Sorry Dan for the snarky post. I honestly am interested in your project. When you say you want the top to be thin. What thickness are you thinking?

My outfeed table is a torsion box that is 2-1/2" thick, using 1/2" Baltic birch for top & bottom skins & the same for the grid. It is very flat & robust. I use it to clamp to & it stands up very well. Is that the sort of thing you're wanting?
Yes Frank that is the sort of thing I have in mind. If I can get away with 2 1/2" thickness that would be great. In all likelihood the torsion box will be 4' x 8'. This will produce an overhang no greater than 12" beyond lift top. I have had very limited exposure to baltic birch ply. I will price it. My previous torsion box was made with 1/2" MDF with a sacrificial 1/8" hardboard on top. What would you recommend for a internal grid layout if I use plywood? Should that layout be any different if MDF is used?

[Dan Chouinard]

I may actually have time to build my new torsion box this weekend. It will be 48 x 96, bolted to scissor lift measuring 32 x 72. I cant imagine ever having more than 350 pounds on it. Its only purpose will be to serve as a flat assembly table for cabinet assembly. I would like to keep it 3" thick, using 3/8 mdf skins. Therefor internal grid will be 2 1/4 rips of 3/4 birch ply or 3/4 mdf.
If this is a viable plan, what would the recommended spacing be for the grid?
Thank you

[Kevin Jenness]

4-5" o.c should work for overall stiffness ,maybe more . Do a small sample to see if the skins deflect significantly under clamp pressure between the grid elements. You could do variable spacing, closer at the perimeter where it overhangs your lift.

[Alan Schaffter]

I wrote a two-part illustrated article about my adjustable height assembly table for American Woodworker many years ago. In the article I discuss a little bit of the theory and construction considerations.

A torsion box has three inter-related properties/goals. They are a lightweight, strong, and have a flat surface. How you build it depends on your needs- do you need a hard tough surface that must withstand hammering and other punishment, will the surface have dog holes, how big must the surface be, how will it be supported, is weight a consideration? If all you need is a flat surface then a torsion box with thin 3/6" or 1/4" skins and internal (shear) webs will suffice. Resistance to bending depends on the lateral strength of the skins so you don't want skins that will stretch in the surface plane. The web only serves to keep the skins apart (it also helps the skins to resist stretching and compression) so it mainly needs to resist edge to edge compression.

There is no need to go to all the trouble making the web like David Marks or Mark Spagnuolo. Instead just make a simple indexing jig to cut half laps in web strips using a dado blade on a tablesaw. No need for precise spacing- the indexing jig will do that for all but the last cell, and it doesn't matter if the last one is a bit smaller or larger than the other cells. Using notched strips ensures the lengths of the web pieces will be the same after assembly (unlike the Marks' crazy piecing method!!!). There is no need to staple the web together, glue will be fine. No need to dado the skins for the web- attach the web to the skins with glue, a few brads, and weights until the glue dries. For heavier duty (and heavier!!!) table, consider 1/2" MDF skins and web. I'm sure my AWW article is hiding out there on the web somewhere. I think I found it, but couldn't view the whole thing.

https://s3files.core77.com/blog/images/2014/01/TorsionBox3.jpg

[Dan Chouinard]

Thanks for the input guys. I had stated earlier in this thread that I had no interest in 1/4" skins. I am now open to the idea of 1/4" mdf skins. This thing does not need to withstand a beating. It just needs to stay flat and tolerate clamping at the edges. Like my current torsion box, it will have a 1/8" sacrificial sheet of hardboard on top. Four to five inch grid spacing has been suggested. Any other feedback in regards to this would be welcome and appreciated.

[Kevin Jenness]

My old 4"x48"x96" bench surface had 5mm luan skins with a p-lam overlay and 5/16" thick core strips on approx. 4" centers with a 3/4" thick core frame. The box was very stiff torsionally but could have used closer spacing for clamping around the edges. There were a couple of cracks in the laminate from errant mallet blows over 30 years of use. I liked the laminate for glue and wear resistance but if you plan to screw into the surface a waxed hardboard or melamine coated particleboard sacrificial sheet makes more sense.

[Ole Anderson]

My first intro to torsion boxes was when David Marks had a show on TV. Hoping to pull up a YouTube video I only found this one on his website, $4.99. https://www.djmarks.com/store/woodworks-digital-downloads-seasons-1-thru-7-individual-episodes-digital-downloads-4-99-each/season-4-episodes/season-4-episode-9/

[Alan Schaffter]

Thanks for the input guys. I had stated earlier in this thread that I had no interest in 1/4" skins. I am now open to the idea of 1/4" mdf skins. This thing does not need to withstand a beating. It just needs to stay flat and tolerate clamping at the edges. Like my current torsion box, it will have a 1/8" sacrificial sheet of hardboard on top. Four to five inch grid spacing has been suggested. Any other feedback in regards to this would be welcome and appreciated.

There is no reason you can't make a hybrid- use more closely spaced, and possibly thicker, webs (1/2" or thicker MDF* or wood) around the periphery where you might be clamping, then 5" to 6" spacing of thinner web pieces in the field. I also used extra pieces to reinforce leg sockets so I could use bolt on legs and removed them if needed. FYI, my article contained a photo of a 30" X 8' experimental torsion box I made using 3/16" hardboard and leftover 3/16" to 1/4" peg board. I used it for a scaffold when cleaning my porch ceiling. For the article photo, I supported the ends on saw horses and loaded it with over 350 lbs of bricks stacked in the center of the span. While it looks greater, deflection in the center was only about a 1/2" (I measured it before and after loading)!! It was strong, rigid, and incredibly light- I could pick it up with one hand.

* I use hardboard or MDF on my torsion boxes because it is dimensionally stable and doesn't warp like wood and ply. I never liked the idea of using hardboard, etc, as a protective overlay, so I applied high pressure laminate. I use thinner, acetone, etc. to remove paint and a cabinet scraper, paint scraper, or chisel :D to remove hardened glue. It doesn't look pristine any more, but has held up extremely well over the years.

I found an MSWord editor's cut of the article which I can send if you want. I'm not sure my current level of SMC membership will allow me to post photos but I'll try (most are in my media albums on NC Woodworker).

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[Michael Drew]

I built an outfeed table with a torsion box top. I got the idea from a YouTube video.....(two parts). https://www.youtube.com/watch?v=H6TctYtT6WY&ab_channel=HighlandWoodworking

You may find his methodology helpful.

[Alan Schaffter]

Boy that is a lot of talk and drawings for a non-problem!! He could have built any thickness torsion box and attached it to the outfeed of his Sawstop using a spacer made from wood to rest on the angle!

Since my extension, outfeed and drop-down outfeed tables are not huge, don't ever support heavy (point) loads (unlike in some shops where they are a catch-all :rolleyes: ), and are fully supported at their perimeters, when I designed and built a new mobile base for my tablesaw, I just made the tables from half torsion boxes (no bottom skin). I made the webs from 1/2" oak and the skins from 1/2" MDF. The lower edges of the web pieces provide enough resistance to tension (stretching). I also covered the top with high pressure laminate. All are still good almost 15 years later.

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[Bert McMahan]

A point to consider for anyone designing a torsion box. For a given thickness of skin, its effective strength goes up by the cube of the total table height.

In other words, doubling your thickness means an 8x increase in stiffness. This of course ignores local effects like point loads between spans; it's just a general mathematical term.

Doubling your skin thickness will double your stiffness. Doubling your height will give you an 8x increase.

If you have a reference design- say, a 2" table with 1/4" skins that you find "nearly adequate" you can estimate how much more overbuilding you need to do with those scaling factors.

Another way to think about it: a 4" thick table with 1/4" skins has the same overall rigidity as a 2" thick table with 1" thick skins.

(it's Friday, forgive me if I goof a number here or there :))

[Dan Chouinard]

I have searched the archives and found mention of gluing both skins at the same time. Anyone here done it this way?

[Kevin Jenness]

That's what I have done.

[Alan Schaffter]

I have searched the archives and found mention of gluing both skins at the same time. Anyone here done it this way?

I did not, but suppose it could be done. I would recommend using glue with an extended set time.

However you approach the glue-up, you will need a flat construction platform. You won't get a flat torsion box unless you start with an absolutely flat construction platform.

I built my platform starting with saw horses and attached the legs to the floor with dabs of bondo so they wouldn't move. Then I made two long stringers from ripped and jointed 2 X 4's and set them between the saw horses. I leveled and aligned the the stringers with winding sticks before attaching them to the saw horses with dabs of bondo. I placed shorter, ripped and jointed 2 X 4s across the stringers. I checked height, level and alignment of the cross pieces before attaching them with bondo. I placed the second skin on the frame and covered it with plastic wrap to prevent glue from sticking then proceeded to build my shear web frame. I added leg pockets and reinforcing for other hardpoints. When the web was done, I carefully marked the location of the web pieces and reinforcing on the skin so I would be sure of hitting them when brad nailing the skin to the web. I loaded the top edges of the web with glue and placed the skin- good alignment is important for gluing and nailing but the skin can be left bigger and trimmed flush with a router afterwards. When the glue had set I flipped it over and attached the second skin. For durability, I wrapped the edges of the table with oak srtips before applying the laminate to the top.

(I used bondo to hold the construction platform stationary, because it can be apply without moving anything and could be easily removed when I was done by knocking it off with a hammer.)

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[Dan Chouinard]

Only one local supplier had any ultralight mdf in stock. 3/4" or 1/2' was the choice. I decided to use 1/2" for the skins and 1/2" x 2 1/2" ribs spaced 5 1/2" apart.

[Alan Schaffter]

That should work just fine.

[Mark Hill]

This is pretty late to the party, but ...

https://www.woodworkingarchive.biz/saw-guards/info-tfz.html

[Dan Chouinard]

You are rather late to the party Mark. In fact the link you gave is to an article written by the gentleman who posted prior to your arrival here.

[Kevin Jenness]

In case you have not yet assembled your torsion box I will describe how I have gone about it.

1.Prepare 10 or 12 clamping cauls about 2"x4"x48". Set half of them out on a flat surface and top them with a sheet of 3/4" mdf.

2. Assemble a perimeter frame and fill it with half-lapped core grid elements- no need for glue at this point. Pin through the outer frame into the core ends so that you can flip the whole thing over once the top sheet is glued on.

3. Run a fat bead of glue along the top edge of all the core elements. I would use Titebond Extend here for a reasonable working time. Set the top sheet on and pin it to the core in a few spots.

4. Flip over this sub-assembly, glue the core edges, pin on the top sheet, add another sheet of 3/4" mdf and the rest of the 2x4 cauls and apply clamps.

This scenario goes best with a helper, but it could be done alone using epoxy for a longer working time. Otherwise, you could do one face at a time.

You may want to insert some blocking into your core for secure fastening to your lift table below, ultralight being what it is.

[Jon Endres]

Am also late to the party but this thread has given me some good ideas - I want to toss another one out there. For years I've had a very lightweight, stiff and flat torsion box bench top in use, as a "temporary" top to throw on a pair of sawhorses. Kind of fell off my radar because I've been using it for so long. It's most recent use was supporting a stack of 3/4" plywood in my shop - 22 full sheets of 3/4" plywood. It's just a 36" x 80" lauan-skin cheap hollow core door. I've been overthinking what to do for my own assembly table/benchtop, and I am simply going to laminate a sheet of plywood to each face of this door and trim out the edges in some hardwood. Stiff enough, flat enough and strong enough to do what I need. Total thickness = 1-3/8" + 1-1/2" = 2-7/8", or I could use 1/2" ply on each side and save myself an inch of thickness.

Only downside that I see is that I can't really do dog holes because the hollow core of the door is inaccessible, but I was planning on using T-track all over it anyway.

[Alan Schaffter]

Don't give up so easily, if you want dog holes!

Once you have added heavier skins to the door, drill oversize holes 1-1/4" or 1 1/2" in diameter all the way through the door and both skins wherever you want dog holes. Then glue 2-7/8" long liners in the hole. For the liners you could use large pieces of dowel, closet pole, round stair handrail, etc. Though it would require more work making the openings, you could even use rectangular pieces of hardwood. You could drill out the center of the liners to make 3/4" or whatever size dog holes you want, before or after you glue in the liners. You might even consider lining the liner because the top opening may wear easily. To use a metal or plastic pipe inner liner, the initial hole would need to be a tad larger. Most of the WW benches I've seen have more dog holes than they will ever need. You won't be compromising the strength of the torsion box (door) you might even be improving it (cheap hollow core doors have a cardboard web.) You could add additional dog holes at any time using this technique.

For a neater appearance, you could install the liners first, glue on one solid skin, drill the dog holes through the skin afterwards from the other side. Or, since you will need a long drill or one with a long shank, you could drill a smaller hole with a spade bit and use a flush trim router to clean up the holes, then install the second skin and drill through it from the other side.

Also, though 36" is plenty wide, if you want a wider bench made from doors, but can't find one that is wide enough, just join two narrower ones together before you add the additional skins. Or . . . . make a hinged table- fold or remove the legs and let one side drop down when not needed. The possibilities are endless! :)

Whatever floats your boat. Think outside the (torsion) box :D

[Dan Chouinard]

Just a thank you to all that gave advice on this project, with special shout out for reminder to put blocking inside where it will get fastened to lift table. Although not perfect I am thrilled with the result. It has a perhaps 1/32" crown in the center but I will not loose any sleep over that. I will loose some sleep thinking of the cabinets being built on it. Its overall size of 97 3/4" x 49 3/4 will handle 95% of the cabinets I build. Edged it with poplar and has a sheet of sacrificial 1/4" masonite on it. Very excited to use it.

[Alan Schaffter]

No photos, didn't happen! :D

[Dan Chouinard]

Someone shows me how to post photos and I wills how them how to make a torsion box. Fair?

[Alan Schaffter]

Start your post. Put your cursor where you want the photo, then click on the "Insert image" icon in the tool bar (3rd from the right). That will open a dialogue box. Select "From computer" then select "browse". Find the photo you want then click on it, then click "Upload file". It may take a second or two. If the image is on the internet (any image), copy the image URL, click "From URL" and paste the URL in the dialogue box. I can get a web image to appear, like my third image below, but I have forgotten how to make the image instead of just the link appear when I upload one from my computer. It could have something to do with my low level membership. :cool:

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https://www.woodworkingarchive.biz/saw-guards/images/1351_65_106-torsion-box.jpg