Torsion box question...

[Jim O'Dell]

What would happen if I bored some holes in the bottom of a torsion box at the outside frame edge to be able to put nuts on some bolts going through the sides to attach it to my saw as an extension table? There would probably be 3 down the long sides, and 2 on the one short side. Should I plug the holes or just leave them open? I guess I could get some phillips head screws and sink them into the MDF sides, but I'm not confident that they would hold very well, or for very long. Jim.

[Scott Rollins]

I used some threaded inserts in the sides of a torsion box. So far no problems. I used some 5 min epoxy in the threads to help lock the inserts in place. The application was a torsion box platform 2" thick which was joined to a head board and foot board of a pirate ship theme bed. BTW the kid loves it. The platform torsion box holds the matress. It has taken alot of abuse from little pirates without any problems.

[Jim O'Dell]

Scott, that reminded me about using knife threads. Maybe that is the same thing you are calling threaded inserts. Thank you! I can get some that are the same threads as the bolts that I'm using now, and that should hold very well. Guess I'll need to go back to Lowes. ;) Jim.

[Tom Veatch]

What would happen if I bored some holes in the bottom of a torsion box at the outside frame edge...

It would slightly reduce the torsional stiffness in that bay of the overall structure. That's the same as cutting an opening for an access panel, landing gear door, etc. in the skin of a stressed skin aircraft structure. Normally, in aircraft, a doubler would be placed around the opening primarily to stabilize the edges and to possibly regain a small part of the stiffness.

Unless you really go ape and cut away "huge" amounts of the skin, I seriously doubt that you'll notice any difference between "before" and "after". These types of torsion boxes are built with much more margin (and weight) than are flight vehicle structures.

[Scott Vigder]

I'd be inclined to add additional internal panels surrounding the hole area.

[Bart Leetch]

I'd be inclined to add additional internal panels surrounding the hole area.


If you used a forstner bit to drill the holes you shouldn't have any problems as long as the hole is the minimum size needed to get you access. You won't need to re-enforce the area of the hole.

[Larry James]

It would slightly reduce the torsional stiffness in that bay of the overall structure. That's the same as cutting an opening for an access panel, landing gear door, etc. in the skin of a stressed skin aircraft structure. Normally, in aircraft, a doubler would be placed around the opening primarily to stabilize the edges and to possibly regain a small part of the stiffness.

Unless you really go ape and cut away "huge" amounts of the skin, I seriously doubt that you'll notice any difference between "before" and "after". These types of torsion boxes are built with much more margin (and weight) than are flight vehicle structures.

Gee, thanks Tom. You just shot my plans out of the air (pun intended). View this video: Bench Horse (http://www.youtube.com/watch?v=Lnul7hYbU5w)
I'm trying to design a torsion box assembly table and was going to include holes for pipe clamps, etc., but not as elaborate as the Bench Horse. How do you think his design works? What is different about this torsion assembly? The top is only 18" wide, is this a factor? How does the ratio of length to width influence strength? Is there an optimal ratio for strength?

Also, for example, if I used a 3/4" MDF / 1/4" hard board top and rout a slot in the top for T-track, or a track on the side like the Kreg Klamp Table, how would this effect the integrity and flatness of the torsion box? How does the ratio of length to width influence strength? Is there an optimal ratio for strength?

Thanks, Larry.

[Tom Veatch]

Gee, thanks Tom. You just shot my plans out of the air (pun intended). View this video: Bench Horse (http://www.youtube.com/watch?v=Lnul7hYbU5w)...

Hmmmm. Somehow, I think what I said in my post was misunderstood. Cutting a hole through the skin will slightly reduce the torsional stiffness. Emphasis on "slightly".

As I said in the prior post, unless you cut away a "huge" amount of the skin, I doubt that you'll notice much difference in the stiffness of the box with or without the holes. Any hole will result in some reduction, but for this type of construction, the reduction is likely to be negligible as long as the skin is continuous along and bonded to the stringers on all sides of each bay and the hole doesn't remove more than 60% to 70% of the skin in that bay. Openings in the skin that extend across stringers will have much more detrimental effect than openings entirely confined within one bay.

I wouldn't have the slightest hesitation about cutting holes for the purposes that Jim mentioned. And a "T-Slot" that doesn't penetrate the entire thickness of the skin isn't a matter for concern either. If I were you, I'd charge ahead with the plans and fully expect the result to be strong and stable.

Edit: If you have a square bay, I wouldn't start to become concerned until the diameter of a circular hole through the skin in that bay became greater than about 3/4 of the length of the side of the bay. On a thin skin construction, i.e. 1/32", I'd look for some kind of doubler or flange to reinforce and stabilize the edge of the hole. But that wouldn't be necessary for this kind of construction.

[Mike Henderson]

A torsion box can be viewed as a series of I beams. The strength of the I beam comes from the top and bottom being able to resist tension and compression. The center part just keeps the two pieces separated and keeps them from "sliding" relative to each other. As long as you don't compromise the strength of the individual I beams, the box should be okay.

Just a comment on torsion boxes, the center dividers can be made fairly small (and lighter) and the box will still have lots of strength. Just make sure to glue the dividers really well to the top and bottom.

Mike

[Larry James]

Hmmmm. Somehow, I think what I said in my post was misunderstood. Cutting a hole through the skin will slightly reduce the torsional stiffness. Emphasis on "slightly". ...

Thanks Tom, no panic, I was just trying to grasp the dynamics of the torsion box. I plan to build a 2'x4' "prototype" with material I already have. Working on the SketchUp drawing now.

Thanks for the input.

Larry

[Larry James]

...The strength of the I beam comes from the top and bottom being able to resist tension and compression. The center part just keeps the two pieces separated and keeps them from "sliding" relative to each other. ...
Mike

Mike,
In your I beam analogy, would the "top and bottom" be comparable to the sides of the torsion box, and the I beam "center part" be comparable the center dividers between the sides?

Could one look at it as an I beam on its side with the top work surface and bottom used to tie it all together? The center part transferring a downward force on the work surface to the sides, and the sides preventing the "sliding."

Answer may be obvious and irrelevant, but I'm retired and my mind has rid itself of worrying about work and interacting with too many people. I spend a lot of time thinking about how things work.

Thanks for sharing your expertise.

Larry

[Peter Quadarella]

By the way, the Bench Horse is awesome! I have two, one I leave folded and then attach it to the other for when I'm doing something large (for either a really long bench or a square one - these double as an assembly table for me).

The idea of using pipe clamps as vices is ingenious and works great. I can tell you that the holes in the Bench Horse have not compromised its flatness or rigidity. (Of course, I've never weight tested it.)

[Mike Henderson]

Mike,
In your I beam analogy, would the "top and bottom" be comparable to the sides of the torsion box, and the I beam "center part" be comparable the center dividers between the sides?

Could one look at it as an I beam on its side with the top work surface and bottom used to tie it all together? The center part transferring a downward force on the work surface to the sides, and the sides preventing the "sliding."

Answer may be obvious and irrelevant, but I'm retired and my mind has rid itself of worrying about work and interacting with too many people. I spend a lot of time thinking about how things work.

Thanks for sharing your expertise.

Larry
Larry, Normally when you make a torsion box, you put the dividers vertically which would make the top and bottom be the flanges and the dividers be the web. See Wikipedia (http://en.wikipedia.org/wiki/I_beam) for more on I-beams and their terminology.

The sides would contribute somewhat to the strength but to my thinking, the major part of the strength would come from the structure inside the box, and the top and bottom.

The concept of a torsion box (at least the way I think of it) is that it's a ridgid structure that won't deform easily (bend, twist, indent).

Wikipedia has an article on torsion boxes but it's only a couple of lines - so it's not much help.

Mike