torsion box with basic Festools

[paulhtremblay]

I wanted to build a torsion assembly table with somewhat basic tools. Many of the plans on the internet require that you have a planer, a nail gun and a table saw. Could I construct the same with a Festool track saw, the parallel guides, an MFT3 table, and little else?

Many torsion box plans call for planing 2 x 4's to exact thickness and using these to create a level platform. Not having a planer, I ripped 8 pieces of MDF 100 mm thick. I would double up on each ripped piece to create the equivalent of a 2 x 4. However, when I straddled these pieces across 2 saw horses, I realized I had underestimated the weakness of MDF. Each piece sagged too much.

Instead, I placed the pieces on the floor and shimmed them to get a flat surface.

I stabilized the grid by screwing in two pieces of at the ends, seen in later picts.

(If I were to make another, I would use 4 pieces of MDF 200 mm wide; tall beams provide much more stiffness than wide ones.)

Next I cut the strips with the parallel guides. I measured the strips with a caliper and noted they differed by only about 2/10 of 1 millimeter.

Next I cut the blocks using the MFT3 table and the stop flag. Finally, I cut my two skins a bit oversize.

I used a glue gun to assemble most of the grid. I started using regular yellow glue, but feared some of it might lead on the other side of the web and compromise the glue up for the second skin. I'm glad I followed my reservations: in fact, when gluing the second skin, I saw some glue had gotten on the the other side of the web.

One problem I had was keeping the inner web square. Even though I used cross pieces, the flexibility of the MDF caused the end pieces to bow. If I were to build another, I would set blocks in the bottom skin to make a perfect square and build my web around that. Since I wanted the table perfectly flat, not not necessarily square, I decided to accept the bowing at both ends, though it did leave less overlap for the skins than I wanted and made the glue up a bit trickier.

Since I had built my frame on the floor, I could not use clamps to hold the skins in place. I did not have a nail gun, either. I could not have screwed in enough screws in time. I debated if I should buy bags of sand to place on top. On another forum, I was told I would need at least 5,000 pounds of weight to get enough pressure. Other posters questioned this. I did a small experiment by placing a flat piece of mdf on to the edges of two other pieces, in order to mimic my glue up job. I put only 3 books on top. The next day I could not break the glue bond.

Based on my experiment, I decided to use boxes of books. Quite a lot of glue dribbled on to the sides. Were this a finished project, the glue would have marred the perfect face of the MDF and made finishing very difficult.

The top came out perfectly flat.

 

[GarryMartin]

(If I were to make another, I would use 4 pieces of MDF 200 mm wide; wide beams provide much more stiffness than wide ones.)

That won't help if I've understood you correctly, although I admit I don't understand the "wide beams provide much more stiffness than wide ones" bit of that quote...  [wink]

Rather than doubling up the core pieces (2 pieces), or doubling again (4 pieces) increase the width (depth) to maximise strength and under load, flatness.

The best information I ever found on Torsion Boxes came from More Woodworkers' Essential Facts, Formulas & Short-Cuts which is a superb reference book.

Here are the important bits, copied from the book;

"To make a torsion box stiffer and thus capable of supporting a heavier load, increase the thickness of the skins, and/or the width (depth) of the core parts. These two factors are much more important than changing from one wood species to another, changing the thickness of the core parts, or changing their spacing."

"In a torsion box, space the long pieces about 3 inches apart for 1/8 inch skin, 4 inches for 1/4 inch skin, 5 inches for 3/8 inch skin, and 6 inches for 1/2 inch and thicker skins."

"The width (depth) of the core materials is important for stiffness. The thickness and grid spacing are less important. The glue bond between the core and skins is most important."

Rule of Thumb - The Torsion Box

1. A strong bond between the skins and the core members is most important.
2. If possible, in addition to glue, also staple or nail the skins to the core members.
3. The thickness of the core members (3/8 inch, 1/2 inch or 3/4 inch) and spacing (5 inches, 6 inches, or 8 inches apart) of the grid makes little difference in the strength of the box.
4. The skin contributes most of the strength to a torsion box - the thicker the skin, the stronger the box.
5. The width of the core members (2 inches, 3 inches, or 4 inches) is very important in the strength of the box. The larger the cross section, the stiffer the box. All other things being the same, if the box depth is doubled, the stiffness will increase by eight.

There is a *lot* of torsion box math in the book too if you are interested, as well as a wealth of other information. The book is the companion book to the original Woodworkers' Essential Facts, Formulas & Short-Cuts.

 

[gippy]

Nice post. I will be doing something similar myself in a few months. I will be slowly moving into my new workshop in the country but will be doing it one pickup truck load at a time. I intend to take just enough tools to build an assembly table first so I can use that to build the rest of the shop furniture. I can't build the table here because it's going to be bigger than my current shop

 

[paulhtremblay]

[ That won't help if I've understood you correctly, although I admit I don't understand the "wide beams provide much more stiffness than wide ones" bit of that quote... 

Woops! I meant to say "tall beams provide more stiffness than wide ones." I know this both from reading about structures and first hand, working with a carpenter who kept doubling beams on a renovation in hopes of taking out the bounce on the floor, when he should simply have been making the beams deeper.

Incidentally, I was actually referring to the the grid *underneath* the torsion box, not the web itself. (See the pict with the MDF on the floor. Sorry if I was not clear in my original post.) 100 mm deep "beams" were simply not stiff enough to provide a reference face, not without many shims. 200 mm would have provided more stiffness and would have made the job easier.

"The width (depth) of the core materials is important for stiffness. The thickness and grid spacing are less important. The glue bond between the core and skins is most important."

2. If possible, in addition to glue, also staple or nail the skins to the core members.

These two points seem to contradict themselves. I really can't see how nails or staples or anything else would contribute to the bond between skin and web. That glue bond (especially on a material that soaks up glue like MDF) will be incredibly strong. Nails and staples are incredibly weak

5. The width of the core members (2 inches, 3 inches, or 4 inches) is very important in the strength of the box. The larger the cross section, the stiffer the box. All other things being the same, if the box depth is doubled, the stiffness will increase by eight.

Wow! I thought it was more like doubling would triple the stiffness. Wish I had added an extra 100 mm or so, since that would not have added that much weight, but would have meant much more strength. As it is, the top is very, very heavy.

Thanks for your feedback. Torsion boxes are kind of fascinating. I wonder, too, if I could have used pine for the web next time. I have read that pine won't move inside a sealed torsion box, though I seem to doubt this. It would be lighter and more stiff than using MDF for the web. Though, now that I consider, it would be too hard to get perfectly flat pine without a planer.

 

[roblg3]

I'm not versed in torsion boxes, but could you "I" beams from the mdf?  that would be strong.

 

[paulhtremblay]

Nice post. I will be doing something similar myself in a few months. I will be slowly moving into my new workshop in the country but will be doing it one pickup truck load at a time. I intend to take just enough tools to build an assembly table first so I can use that to build the rest of the shop furniture. I can't build the table here because it's going to be bigger than my current shop

Good luck. Don't worry too much about how you connect the inner web pieces or how to clamp the skin to the web. I worried too much about these matters. Any way you connect the inner web to itself will work. The strength comes from the glue bond of the skin to web, and just using ample glue with a bit of weight will work.

The most important part is getting a perfectly flat surface on which to build your web. Taking time to do this will pay off.

Oh, and make sure you put plastic on top of your bottom piece of MDF if you are going to use that on the top--glue will leak down on it.

 

[roblg3]

i see now that a torsion box IS a set of connected I beams when completed

 

[tiralie]

The best information I ever found on Torsion Boxes came from More Woodworkers' Essential Facts, Formulas & Short-Cuts which is a superb reference book.

Agreed, very good book.
Gary, thanks for the reminder.
Tim