[OP]
Contributor
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
Contributor
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.
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.
Member
See this site:
https://www.google.com/search?q=tors...9PR9OdbsHY9DrM
Member
Here's one describing a torsion box made from baltic birch 72"x24" and 2 1/2" thick :
https://www.canadianwoodworking.com/...g-shop-storage
Just a Duffer
Member
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.
torsionmft.jpg
Contributor
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.Originally Posted by Dan Chouinard
[OP]
Contributor
Lift weighs 965 pounds.
Thanks guys for your guidance.
Contributor
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.
[OP]
Contributor
I have no interest in 1/4" skin. I need to be able to clamp work down to this thing.
Contributor
So why don't you go ahead & let us know all your criteria, rather than having everyone trying to guess how to help?
[OP]
Contributor
My apologies Frank for omitting the need for clamping capabilities. Have a good evening Sir.
Moderator
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.
--
The most expensive tool is the one you buy "cheaply" and often...
Contributor
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?
Contributor
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
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