Torsion box material for mobile base?

[Josiah Bartlett]

I'd like to build a mobile base for my Unisaw + router table that would consist of a torsion box for the bottom and one for the extension table with a storage cabinet. I have a fairly uneven slab floor in my shop so I don't want to worry about it flexing and throwing the table out of alignment, but I also don't want to have the thing sag. I'm using a shop fox mobile base right now and I'm not happy with the rigidity and mobility of it.

I'm thinking of using 3/4" skins for the base with 1/2" material for the grid, with a 2" spacing between, and for the top using 1/2" mdf skins on 1.5" spacing. I'm more-or-less following David Marks' torsion box design but the base has to support the unisaw without sagging. The fence rails (Shop fox Bies clone, 7') can support the sides of the top, so I'm thinking MDF would work well for that, but should I use plywood for the base? I plan to put heavy duty casters on the ends with screw jacks for leveling and locking.

Or would I be better served making the base out of box section steel a. la. a truck frame and bolting the saw to the base?

[John Lucas]

I am not going to try to answer your questions but I will suggest adding another area to your thinking: wheel size. The casters that are in the t ypical mobile bases are too small for bumpy floors. For all my shop buiilt bases I try to use 3 or 4" wheels. It makes a lot of difference when moving it around. Even power cords are not a problem.

[Jack Ellis]

Spacing between grid elements is less critical than the height of the torsion box. I think you could get away with 6" between grid elements but you should make the grid 3-4" tall. Also, make sure you glue the grid to the top and bottom skins.

Beam strength is proportional to the cube of the beam depth (akin to the height of the box) and proportional to a fraction of the beam width. That's why the height of an I beam is more critical than the thickness of the web. If not for the fact that 1/" material is probably easier to work with, you would not lose much strength by using 1/4" material for the grid.

[Josiah Bartlett]

Spacing between grid elements is less critical than the height of the torsion box. I think you could get away with 6" between grid elements but you should make the grid 3-4" tall. Also, make sure you glue the grid to the top and bottom skins.

Beam strength is proportional to the cube of the beam depth (akin to the height of the box) and proportional to a fraction of the beam width. That's why the height of an I beam is more critical than the thickness of the web. If not for the fact that 1/" material is probably easier to work with, you would not lose much strength by using 1/4" material for the grid.

What about resistance to creep when using MDF?

I already plan to use 4" casters. Fortunately I'm tall so 6" of added height is a good thing for me with an old uni.

[Jack Ellis]

Any material is susceptible to creep, even steel. The ribs of the torsion box will add a lot of stiffness. Gluing everything together helps distribute the load throughout the structure.

Remember that an airplane wing is essentially a torsion box. On large transport airplanes, the skin, the spar and the ribs are relatively thin but they support around the same load per square foot as the floors in your home.

You might want to think about attaching the casters to the side of the box rather than underneath so that the saw table isn't too high. If you make the ribs 3" tall and use 3/4" material, you could rip down a 2x6 to 4 1/2" for edging and attach the casters to that.

[Mikail Khan]

I would go with a steel base and the large diameter wheels as suggested above. I also had problems with store bought moblie bases because my garage floor is uneven.

A steel base will be:

1.Stronger.
2. Easier to make (for me anyway). A simple rectangle welded together from 1.5" to 2" box section steel on 4" casters will give you the extra height you need. Or you can mount the casters on outriggers to keep the table height to a minimum.

I used 1.5" square steel (1.5mm thick) to support my 800# jointer and the weak point is the casters.

MK

[Mark Grotenhuis]

If you do end up making your torsion box mobile base ... post some pics! I want to build one for my Powermatic 15 planer. When I run long pieces (8' or more) through my planer I run out of room and they hit my table saw extension table. I want to raise the whole thing up 4 or so inches so that the pieces will go over top my table saw extension instead of running into it. I've been looking to find a summary of how thick a torsion box can support different weight. My planer weighs a hefty 423lbs. Metal isn't an option due to the fact that I don't have a welder. I really like the woodcraft 4" double locking casters so I'm going to use those.

[Barry Vabeach]

Josiah, I didn't follow all your dimensions, but I have built a few torsion boxes handling loads up to 700 lbs. You may already know this, but the stiffness of the box is largely due to the height of the grid or spacers - If I use 1/2 skins on top and bottom, with 1/2 thick material for spacers that are 2 inches high on a 5 " by 5" grid, it will be much stronger than the same box with 1 1/2" high spacers and 3/4 skins, and not as strong as one with 2 1/2 inch spacers and 1/2 skins. I think the 3/4 bottom is overkill - unless you expect to have a concentrated load in a very small space that you can't predict. If you intend to mount casters in certain locations, you can glue additional support there when you are doing your glue up. I also think your spacing would work but is far too conservative - try a little experiment that I do when laying one out, take a sample of your bottom skin, and place it on 4 scraps set up with the proposed grid spacing - and then apply loads to the middle of the skin ( the furthest point from the grid ) to see if you get deflection - if no - space them 1/2 " further apart and try again - note that the actual assembly will be much more rigid since it is glued together. As to the upper skin, I don't know what the underside of a Unisaw looks like, but if it spreads the load along the entire base, I would feel sure you can get by with 1/4 ply ( though again I would test it based on your grid spacing). IIRC I have made ramps for walking on with 1/4 skins with no problems so as long as you don't have any point loads, 1/4 should work. Again, I am trying to minimize the height of the skins so that I can maximize the height of the grid.