Torsion Box - Alright you aviators . . .

[glenn bradley]

I'm after an assembly table. My rolling worktable is made to skew a bit as it rolls across the none-too-flat garage floor. Drawers and bins were made with a little more clearance than usual so when things shift, drawers still slide. That's fine but I want a stand alone, solid, flat surface.

How tall do the grid pieces need to be? What I'm asking is; can I use 1" grid pieces between 2 sheets of MDF and still get the benefit? Do the grid pieces need to be at least 2 inches or so high to create the stability effect?

I'm attempting to control the height of the table to match it to an existing leg frame without having to cut the frame down.

TIA!

[tod evans]

glenn, a torsion box gets its strength fron the webbing more than the skins. if i was trying to build a 2" thick torsion box i`d opt for 1 1/2" webs and 1/4" skins not 1/2" skins and 1" webs.......02 tod

[Ian Abraham]

The taller the webs, the stiffer the torsion box.

Imagine sitting the 1" beam between to end supposts and pressing down. You can probably bend it a bit, then try with a 2" beam, wont bend as much. A 4" or 6" beam will hardly move.

So to answer your question, yes you will get some benefit from a 1" torsion box, as opposed to just 2 sheets of mdf alone. It wont be as stiff as 2 or 4", but it will be a torsion box :)

Maybe you could add a deeper 'skirt' around the outside of the whole top? That would help stiffen the whole thing up.

Cheers

Ian

[glenn bradley]

Thanks Tod and Ian,
I figured as much just based on my non-existent knowledge of physics. Sounds like if I'm going to go through the effort I may as well use a minimum 2" webbing and adjust the leg frame as required.

Thanks again!

[Al Willits]

If ya check Dave Marks site on the DIY website, he makes a torsion table.

He uses 6 ? long lengths of 1/2" MDF and a bunch of short ons for the rest of the grid, I make it that way but would think about using nothing but long lengths both ways and inner-locking them

Al

[Gary Curtis]

An assembly table with plans and photos is shown in the ww book, Smart Shops. The top is a torsion box design. I'm away from home so I can get the author and publisher info. But I'm sure Highland Hardware still sells it.

As I recall, their dimensions were at least 8" tall.

Gary Curtis

[Roger Everett]

I have a portable assembly table of torsion box design, that I built 14 years ago. Stands against wall and I just get out and pop on a couple saw horses, when needed. It is 43 1/2" x 95", with 1/4" ply on one side and 1/2" on other. The frame and ribs are of 3/4 x 1 3/4" poplar. Soon after I built it, I needed to make a similar type bottom, for a buffet ( that was to be a wine cabinet ), I was making for a client. Figuring it would have a lot of weight in it, and to get an idea of how strong the assembly table configuration was, for comparision. I mounted it on 2 saw horses ( one at the very end of each end ), and got 2 friends to set in middle ( each about a 200 pounder ). It deflected about 1/16" in middle. It's the fact that it is totally glued and you're dealing with stretch and contraction, that gives torsion boxes there stiffness.
Roger

[glenn bradley]

Al, I agree on the interlocking thing. I can't see dealing with all those small pieces and giving up the built in surface tension of a continuos piece. Thanks.

[glenn bradley]

Roger, Thanks for the real world info. As the goal for me is a nice flat surface that won't deflect under "household goods" sized projects, it sounds like I can stay on the shorter side with the grid pieces.

[Al Willits]

Might add, even though I built it with the small pieces, this is one very strong table, I just think the interlocking strips would be easier to deal with.

I have mine sitting on a 3.5x3.5' by 3 foot high aluminum table with rollers and just roll it under one of the work benches I have along the wall, sticks about about 18" but its out of the way when I'm not using it.

The lower table height works well for me, as a foot injury makes sitting down when possible much easier for me.

Al

[Phil Maddox]

I made a 2 1/2" torsion box about 5x5 with 1/2" skins and 2" wide x 3/4 poplar grid. This baby was STRONG!!!!

1/4" skins seem a little too thin, although I've never tried it. I like something that will take a bit of pounding.

Good luck

Phil

[Alan Schaffter]

I made a torsion box assembly table along the lines of the David Marks DIY table. I used 1/2 MDF web and skins and 3/4" edges. I also covered the top with laminate. The height is easily adjustable using legs I designed. Here are the links to The Wood Works threads with pics- post is at bottom of each thread so you'll need to cursor down.

http://members.cox.net/aschaffter/table-15.jpg

The web grid (http://www.woodworkslive.com/index.php?topic=713.0)

almost complete table (http://www.woodworkslive.com/index.php?topic=738.0)

adjustable legs (http://www.woodworkslive.com/index.php?topic=682.0)

finished table (http://www.woodworkslive.com/index.php?topic=825.0)

[glenn bradley]

I've seen your adjustable leg design on another page while browsing. I am very impressed! Very clever and beautifully executed.

[John Stevens]

I figured as much just based on my non-existent knowledge of physics.

The stiffness increases proportionally to the cube of the thickness of the ribs plus skins. So if you double that thickness, it'll be eight times as stiff, assuming that the materials and length between supports is the same. The stiffness decreases proportionally to the cube of the distance between supports, so if you cut the span between supports in half, it'll be eight times as stiff, assuming the thickness and materials remain the same. The type of material also affects the stiffness, but to a smaller degree. Very roughly speaking, if you make the table out of MDF, it will flex about twice as much as if you'd made it from softwood, and about three times as much as if you'd made it from hardwood. And the number of internal ribs affects the thickness in a 1:1 proportion...double the number of ribs, and you'll cut the flex in half.

You can use these concepts to guesstimate how thick you want the skins, how thick you want the internal ribs, how far apart you want the supports, how many ribs you want, and what material you want to make the table from. And if you really wanted to, you could get the formula itself, plug in some numbers, and compare different designs.

The formula is probably in some message here in SMC. Values for the stiffness of various types of wood can be found in this message:

http://www.sawmillcreek.org/showpost.php?p=123801&postcount=9

Value for MDF is here:

http://www.sawmillcreek.org/showpost.php?p=123624&postcount=7

[Rick Lizek]

Ken Horner's book "More Woodworkers Essential Facts, Formulas &Short Cuts" has the definitive chapter on torsion boxes with calculations on strength and best ways to build them. A must have book in my opinion and truly the best info on torsion boxes since Ian Kirby's article over 20 years ago in FWW. Even better info than David Marks DIY on torsion boxes although his method of leveling is worth noting.

[Jeff Cord]

I'm sure there is a simple answer to this question.
A friend at work loaned me his HF HVLP sprayer to try out. He told me it did a good (enough) job and, after trying it out this weekend, I tend to agree.
(I was expecting it to lay down the paint a bit quicker but after passing over the same spot a number of times I got good coverage.)
I also know that you get what you pay for in quality.
However, after looking at the HF sprayer and the PC sprayer they look identical to the untrained eye (mine).
What is the true difference between these?
Are the parts really that much different from a quality standpoint?
Does the PC do a better job? faster? smooter? ???
I'm curious to know what the real difference is.
thanks,
Jeff

[Jeff Cord]

It's completely in the wrong place.
Sorry.

[Rick Lizek]

Al, I agree on the interlocking thing. I can't see dealing with all those small pieces and giving up the built in surface tension of a continuos piece. Thanks.

It may appear easier to use half laps to make a torsion box but in reality its far easier to use long lengths and short butted cross members. The half laps only add more time and do nothing to contributr to the strength of a torsion box. The key is to use a staple to bridge the pieces before gluing the skins on. A torsion box is quickly made this way. It becomes even more apparent that this is a better way to make them when you start making curved and shaped torsion boxes. Ian Kirby paved the way with his original article over twenty years ago in FWW. It's interesting how the process has been made more complicated by folks over the years. Ken Horner's book "More Woodworkers Essential Facts, Formulas & Short Cuts" is a must for torsion box building. I've been building torsion boxes for years and you would be amazed how 1/4" skins on a 3" tall frame with 4" grids seems like solid wood. It's really a combination of the thickness of the frame and the glued on skin that gives it the strength. The technology is based on the construction of aircraft wings.

[Al Willits]

Rick, find it funny you think adding all them short pieces would be easier, personally I found it a pain and whatever time it took to make a simple cutting jig would still be less than trying to get all tham pieces glued and nailed it.
Different strokes I guess... :)

I think you can probably get as carried away with what kind of box your gonna build, but the Marks one will work for most anything that will fit on it, and it is still portable enough to deal with.

I suppose you could take it one step futher and make it out of aluminum griders, machince the top perfetly flat, then put a wood laminate over the top, be light, stong, flat and probably not much over a grand invested when ya got done...I think I'll stick with the Marks version fwiw...:D

Al

[tod evans]

one thing that rick forgot to mention, when building torsion boxes thought should be given to how the box will be loaded and supported. if a fellow does halflaps he effectively weakens the boards he cuts by the depth of the cut because in order to assemble them there must be several thousandths clearence whereas if a fellow does butt joints it`s pretty easy to jamb the glue joint tight. run full length boards between the supports and the cut pieces parallel to the supports. glue every joint in the core then glue the core to the skins.....02 tod

[Rick Lizek]

"Rick, find it funny you think adding all them short pieces would be easier, personally I found it a pain and whatever time it took to make a simple cutting jig would still be less than trying to get all tham pieces glued and nailed it.
Different strokes I guess..."

You are missing my point on how to assemble the grid. You simply lay out your long pieces and set your short pieces in between. There is no need to glue the grid pieces together. A simple hand stapler is all you need to connect your grid pieces together. Then flip it over and staple the grid pieces on the other side then you glue your skins on. It is very fast and simple. David Marks makes it more complicated with his nailing process. Ian Kirby and Ken Horner are the best sources for torsion box construction.

I don't think you lose any strength with half laps. It's the combined thickness of the grid and the glued on skins that gives the torsion box it strength. Gluing the half laps doesn't add strength, just time.

[tod evans]

[quote=Rick Lizek

I don't think you lose any strength with half laps. It's the combined thickness of the grid and the glued on skins that gives the torsion box it strength. Gluing the half laps doesn't add strength, just time.[/quote]


rick, although far from any kind of expert, it seems to me that by cutting halfway through the core boards even though they`re glued top-n-bottom that their structural integerity is compromised? i`d be interested in seeing some load data using all three methods, halflap, dry butt joints and glued butt joints....my gut says gluing the butt joints would yeild the strongest structure.....02 tod

[Stu Ablett in Tokyo Japan]

rick, although far from any kind of expert, it seems to me that by cutting halfway through the core boards even though they`re glued top-n-bottom that their structural integerity is compromised? i`d be interested in seeing some load data using all three methods, halflap, dry butt joints and glued butt joints....my gut says gluing the butt joints would yeild the strongest structure.....02 tod

If the half lap joint were tight and glued.....? :D

I made one and I just used butt joints, works real well, and it real strong.

Cheers!

[Rick Lizek]

I've given the definitive source for torsion box info for woodworkers...Ken Horner's "More Woodworkers Essential Facts, Formulas and Short cuts. I've been following torsion box info and building them for over 20 years. I have Kriby's original article and have seen the David Marks info. Considering how versatile the torsion box is I don't think most woodworkers really understand how a torsion box works or the best ways to construct one. You won't find load data comparing butt joints versus gluing butt joints or half lap comparisons but Ken's book has enough simplified engineering formulas for woodworkers to calculate the strength of a torsion box and answers all the questions on grid sizes, skin thickness, etc. The grid thickness and the glued on skins is what makes it work. Half laps or gluing the butt joint crossmembers shouldn't make any difference in strength based on the principles of torsion boxes.

[Al Willits]

Rick, I may be missing the point, but all my comments on torison boxes have been about the Marks version, and he nails the boards in place, good or bad, that's what I'm referencing to.

I haven't a clue what anybody else is doing, this way worked for me and in the event I make another (smaller) table, it will be with the innerlocking method, either way is going to be more than strong/flat enough for what I do.

Like I said, different strokes for different folks. :)

The end result and what your happy with is what counts.

Al

[Michael Adelong]

I made the Marks TB using half laps and it worked great. I used the thickest shims my dado set (Freud SD-508) came with. Felt too loose without glue, but the PVA expanded MDF and the gaps closed just fine.

As to torsion boxes and airplane wings. The inner structure is only responsible for a portion or so the total strength of the structure. The upper and lower skins provide the rest. Once assembled, if you want to flex a TB (or wing) by pressing down on an edge you must overcome the resistance of the top skin to stretch, the resistance of the bottom skin to compress, and the resistance of the grid structure to bending, compressing, and sliding on the inside surfaces of the skins in order to flex the whole assembly.

I made a Sketchup drawing of it so that I could save the measurements for future use. I'm not sure how long a TB lasts, but I didn't want to have to calculate the cut locations and such again if I ever have to build a replacement. I was also practicing creating a tour guide tutorial in SU, so I did that as well. Forgive the first cut at the commentary. I meant to go back and clean it up a bit, but I haven't had the time. I hope it speeds up your construction progress. (If you aren't going to do half laps, I also put the dimensions of the short pieces in the drawing. Hmmm... I guess I just came out of the closet. I wonder how my Dad will take it having an openly bidimensional son in the family? :cool: ). Comment welcome.

Michael

Edit: I should have called it "Modified DIY Torsion Box"... my apologies, DM.

[Chris Padilla]

Regardless of how you build the inner rib structure, all the methods outlined thus far will work and yield a good strong table top that will serve you well for many years.

I guess the application of the torsion box is where one might care about how strong it is relative to a different style of construction or different construction materials. I bet for most of us on here, MDF will work very well, is very stable, is dimensionally accurate, and best of all, it is very, very cheap! :D

I made a torsion box all out of 3/4" MDF and half-lapped the grid 'cause that is what my American Woodworker mag did. The half-laps were so tight when dry fitted, I didn't worry about trying to glue them because I couldn't even get my test pieces apart!!

Perhaps one negative thing about MDF: weight! Yeah, my torsion box weighs a frickin' ton. My next version will use thinner out skins.

I think the most important thing about building a torsion box was well-outlined by David Marks: it will only be as flat as the surface it is built upon. So get that surface as flat as you want/can!

Happy torsion box building, Folks! :)

[glenn bradley]

Thanks Chris! My TB will mount to a permanent set of steel legs so the weight will come into play mounting it and if I ever move it. Without trying to gloat, I have a 20 x 30 area all to myself (ah, the single life) so I can afford the room for a semi-permanent install if I plan carefully. My thanks to all who have participated here. I'll of course, appease the pic-gods upon completion.

[Chris Padilla]

Glenn, just get a few buddies over to help right it for you as I assume you'll want to attach the legs with the top down. If you are lag-bolting the legs in, be sure to fill up some of the grid areas with material so the lags have something to bite into. :)

[Bart Leetch]

The half lap method worked great for me when finished we put over 400 lbs on it & if it could have chuckled it would have maybe even laughed out loud.:D :D