Torsion Box Questions

[Jamie Smith]

Hi All-

I intend to build a torsion box to replace the sagging top of my SCMS stand/table saw garage, and I have some questions.

(I tried to find some sort of a calculator online, but couldn't)

1. The span needs to be greater than 8' (106x42). When building a torsion box, can I piece together the skins?

2. What material is best for the webbing? Plywood or MDF?

3. for something this size, what kind of material for the skins, and what thickness?

4. What thickness (height) for the filler web?

5. I plan to skirt it with ash or something- is there any problem with doing that?

Many thanks!

[Cliff Rohrabacher]

1. The span needs to be greater than 8' (106x42). When building a torsion box, can I piece together the skins?Yes but you'll likely have some design considerations to make. Possibly one of the first ones will be the question of whether you care about weight. Lots of guys build torsion boxes trying to maximize strength with a light a box as possible. I built mine to be a heavy box cause I just don't care about weight and wanted brute strength.

A torsion box can rely for the most part on the web members or the skins, or both. I made one here that relies on both the skins and the web members to result in a BEAST of a box - it is a very heavy torsion box.
http://www.sawmillcreek.org/showthread.php?t=64965


I am thinking that if you have to use two pieces of sheet stock for the skins that you may have to make a double layer of sheet stock to get the seams staggered. That way you won't have an opening in the skins.


2. What material is best for the webbing? Plywood or MDF? I don't think it matters so long as you use the same material on both sides for skins. For Web material: I made my own engineered trusses from pine and OSB and interlocked 'em by cutting notches. The result was that on assembly the skins and webs pulled themselves into a perfectly flat surface.


3. for something this size, what kind of material for the skins, and what thickness?Ya might want 1/2" birch doubled to eliminate the seam. Some guys 'll say I am talking over-kill. Maybe I am but, I don't like things that turn out to be too weak later on.


4. What thickness (height) for the filler web? The wider the web the sturdier the result. I'd make it as wide as will fit in your application.
Your app' does not sound like weight is going to be an issue so heavy may work out to be good thin.


5. I plan to skirt it with ash or something- is there any problem with doing that?A real man's torsion box should not be wearing women's cloths.

[Jamie Buxton]

Cliff's got it right, but I wouldn't double the skins everyplace. I'd piece together a single-layer skin, and arrange the seam to fall on one of the spacer-grid members.

[Greg Funk]

1. The span needs to be greater than 8' (106x42). When building a torsion box, can I piece together the skins?

It shouldn't be too hard to find 10' lengths of plywood or hardboard of some type. If you can't find anything long enough then I would use two layers of 1/4" with offset joints.



2. What material is best for the webbing? Plywood or MDF?

It doesn't really matter as the skins contribute most of the strength to the torsion box. MDF makes too much dust so I use plywood.



3. for something this size, what kind of material for the skins, and what thickness?

It depends what you plan to do with torsion box. Do you just need to support wood while cutting with the SCMS or do you plan on clamping to it. If you are planning to clamp to it then I would probably stick with 3/4" skins to prevent the clamps from breaking through the skins. If you are not clamping then you can use thinner skins.
As an example, for a 6' long x 10" wide by 2" thick torsion box with a 500lb load in the middle the deflection will be :
.46" (1/8" skin)
.35 (1/4)
.30 (3/8)
.28 (1/2)
.25 (solid plank)
So even with thin skins you will not see much deflection under normal loads.



4. What thickness (height) for the filler web?

It depends on how much weight you need to support and the distance between your supports. In general the stiffness of the box will vary with the square of the height so if you make it twice as thick it will be 4 times as stiff.



5. I plan to skirt it with ash or something- is there any problem with doing that?
No problem at all. I usually edge mine with solid wood and round over the edge.

If you want lots of formulas Ken Horner's Woodworkers Essential Facts & Formulas has plenty of detailed info on torsion boxes (http://www.amazon.com/Woodworkers-Essential-Facts-Formulas-Short-Cuts/dp/1892836211/ref=sr_1_1/105-1453943-4716469?ie=UTF8&s=books&qid=1191265009&sr=8-1)

Greg

[Jim O'Dell]

I made two 10' units using 4X8 MDF. (the boxes were 10' X 14") 1/2" skins and webbing. 5/8" skirts. I used plastic laminate on the top and 3 visable sides. I also used 5/8" MDF on the inside to span the joints in the skins, and staggered the joints so they were on opposite ends of the torsion box. They are about 2 months old so can't say for sure that they will stand the test of time, but so far so good. Jim.

[Bill Wyko]

I have purchased MDF in lengths longer than 8 feet. Check your local lumber co. Good luck.