Table saw garage in the garage - torsion box question

[Dale Murray]

I intend to build a wall mounted work surface that doubles as a shelter for the table saw. I have a Sawstop 52" ICS on mobile base - dimensions are 34.25" tall, 36" deep, 86" wide. When elevated to move the height of the extension table is 36" high.

My garage has foundation walls of poured concrete that extend 41.5" above the floor.

My intent is to build a torsion box 94.5" long and 36" deep with the bottom being 36.5" above the garage floor. That should give enough clearance to roll the saw in and out without much issue.

The torsion box I have drawn up would be made of 1/2" MDF.
The box would consist of 60 chambers each measuring 6-5/8" x 6-7/8".
I intend to add a replaceable hardboard top and solid wood front to the assembly.

The box would be fastened to the wall at one end and along its entire length, the open end would have a support of some sort and obviously have an open front.

What I have not determined is the required rib height to create reasonably stiff box.

I am 6'1" tall and have an actual woodworking bench inside the house. This surface would be for general garage type crap - sharpening lawn mower, fixing stuff, mounting a bench grinder, etc.

If I use 2" ribs the resulting bench height would be 39-1/2".

so the question is:
Are 2" ribs sufficient?
Should I add more ribs resulting in more chambers?

Any advice would be greatly appreciated.

[Todd Burch]

1/2" MDF top and bottom will make that one heavy sucker. 1/4" MDF is probably enough.

Your torsion box size isn't that much larger (if not smaller) that a good sized 8' door, 36" wide interior door. That interior door would be 1 3/8" thick, have 1/8" skins on it, and use cardboard for the internal ribs. That same door with an added piece of 1/4" ply or MDF as the work surface would be suitable for a work surface.

Todd

[Chris Padilla]

That sounds similar to the torsion box roll around bench I built many many years ago from plans in American Woodworker. Generally speaking, the taller the ribs, the stronger the box to a certain extent. I think I built mine all out of 3/4" MDF and let me tell you, that sucker is incredibly heavy so 1/2" would be a step in the right direction. As to the rib height, I think 2" is probably sufficient but consider maybe 2.5" or 3". I think that is closer to what I used in my MDF torsion box and what the plans called for. I never bothered to calculate anything...I just followed the plans. The torsion box is stiff and flat and remains that way some 10-12 years later. One thing I recommend with your MDF is to seal it with several coats of cheap polyurethane. MDF will absorb moisture over time and the poly will keep it sealed and make clean up easier.

[Dale Murray]

Using doors is an interesting idea very much worth considering.
Last night I came across a few people using foam as the core - the 2" pink board insulation - claiming excellent results and rigidity. The 2" board has 31psi compressive strength.

My reasoning for 1/2" MDF is durability - something to spread the load of a hard hit to a larger area. Certainly the bottom skin does not need to be 1/2". Also, I am not trying to build a reference surface so much as a durable rigid structure.

Currently I feel using doors or foam would be the more expedient solution so long as they are strong.

I have some 2.5" foam at home right now, I may make a lamination roughly 12"x36" tonight if I have the proper glue on hand.

I also have two hollow core sliding doors we are no longer using. They are roughly 36" x 72", I could use them end to end with top and bottom skins tying them together. I dont mind a longer surface.

[Dale Murray]

Well that did not work so well. I bought contact cement that, according to the label, is safe on foam - not so much. as soon as I put it on the foam it started melting.

I have some other foam on hand and made a test piece using construction adhesive. Not ideal but should give me some sense of strength.

after this setback I think I just may build a normal torsion box.

[Dale Murray]

The test piece I made used 24"x16"x2" foam sandwiched between 1/8" lauan plywood held with construction adhesive.

To test I placed the sandwich on two 2x4's on edge leaving an open span of roughly 21".

I weigh 234lbs as of this morning.

I stood on the lamination, nothing happened. It held and I had no sense of give or weakness. I give a bit of a push down while standing on it, still solid. A little hop, still held. A bit bigger hop and it failed.

I believe the construction adhesive failed. One sheet of lauan came off intact, the foam did not have a visible break. I did not have time to inspect more before work this morning.

[Dave Zellers]

Looks like you're on to something. Maybe try an adhesive specifically made for foam.

http://www.amazon.com/27425-Paneling...dp/B000W8PC26/

And give it a few days to fully cure.

[Charlie Plesums]

I made a test torsion box years ago to prove the concept to a customer. All 1/4 inch ply (which is really 1/5 inch or less). The top and bottom were 4 x 33 inches, same 1/4 inch ply all around the edges, plus 4 internal cross-members of the same stuff, all 1 3/8 inches high (so the outside dimension of the torsion box is 1 3/4 inches thick. When balanced on supports on each end, it holds my 200+ pounds in the middle.

That test allowed me to build bunk beds... queen size bunk beds ... where I did not want to give up thickness. They were queen size since the teenagers were very tall, and a spec was that they had to be able to sit comfortably and read on the lower bunk, so I made a 2 inch thick overall torsion box to support the mattress. The 1/4 inch plywood face would not support somebody walking on it without a mattress to distribute the load, but it had no trouble supporting multiple roughousing kids or adults on top of the mattress. The internal web was 1 1/2 inch plywood grid with large spaces between the grid. The upper headboard is 8 feet above the floor.



http://plesums.com/wood/bedroom/pic/BunkBeds0756.JPG

I also make Murphy desks like the one being opened below. I learned (the hard way) that the working surface had to be 1/2 inch plywood to resist the impact of a book or something falling off a shelf and landing between the web, but the bottom is only 1/4 inch. The overall thickness is 2 inches, so that suggests a 1 1/4 inch web.



http://plesums.com/wood/bedroom/pic/...losing4033.jpg

You should not have to glue the faces to the foam. The strength of the torsion box is that the bottom stretches and the top compresses across the face; the internal structure is only to keep the top and bottom a fixed distance apart. The greater the distance apart the stronger the torsion box, so if you will be putting heavy loads on it, you can get a lot of extra strength by increasing the distance between upper and lower skin. If you will be having high point loads, the upper skin needs to support any impact between the web.

There is no rule that the web has to be pretty (some people spend too much time on that). This is the torsion box for a different bed. The small compartments in the lower part of the picture were where I had to have a plywood joint, so I glue the plywood firmly to the web at that point. The cross pieces of the rest of the grid do not line up so I could tack them in place with a nail gun.



http://plesums.com/wood/bedroom/pic/PSch0112.jpg

[Dale Murray]

Today I did a bit of searching and found many hobbyist use pink foam insulation for their projects and one was nice enough to do a test of several different adhesives. The short version is Gorilla Glue and Glidden Gripper (primer paint) were in the top three is Glidden taking top honors. In both cases the foam failed before the glue joint.

https://www.youtube.com/watch?v=rnOegaOKu38

These were foam to foam tests, not foam to wood.

Tonight I intend to pick up Glidden Gripper and see how well that works.