mobile base question...

[Monte Milanuk]

Okay, newbie question here...

I've been looking at making various portions of my shop mobile, for the usual reasons. One of the common features I've seen is where some sort of torsion box bottom to support the main carcass, in the plan I have at hand here it's basically a frame made from 1x4 boards on edge fastened to the bottom of a piece of sheet stock, either 3/4 ply or MDF.

I get how the frame stiffens the sheet as a whole, and keeps it from buckling under the weight. What looks wrong to me is that you still have all the weight of the carcass (could be a lot, especially if built from 3/4" MDF) bearing down on the corners... which are unsupported out there at the ends, and never any sort of wheels or support in the middle.

What am I missing?

Thanks,

Monte

[Norman Hitt]

Okay, newbie question here...

I've been looking at making various portions of my shop mobile, for the usual reasons. One of the common features I've seen is where some sort of torsion box bottom to support the main carcass, in the plan I have at hand here it's basically a frame made from 1x4 boards on edge fastened to the bottom of a piece of sheet stock, either 3/4 ply or MDF.

I get how the frame stiffens the sheet as a whole, and keeps it from buckling under the weight. What looks wrong to me is that you still have all the weight of the carcass (could be a lot, especially if built from 3/4" MDF) bearing down on the corners... which are unsupported out there at the ends, and never any sort of wheels or support in the middle.

What am I missing?

Thanks,

Monte

Monte, you must be looking at a plan that was in Wood Magazine last year. I did not like that plan due to the corner problem you mentioned, and mentioned it on their forum but was kinda put down for questioning it. If I remember correctly, I saw a post from someone that had built that unit, and ........surprise.......surprise, he was asking questions about how to repair and beef it up. IMHO, there are a lot better designs than that floating around, and I personally would not build one using that plan, Especially with MDF, as ANY moisture that gets to it, will render it useless. I also don't want any kind of torsion box on the bottom either.

[Monte Milanuk]

Norman,

The one I'm looking at currently is Wood Magazine's 'Best Ever Workshops' issue that should be on the news stands now. Basically a compilation of various shop stuff, like storage, benches, mobile this-that and whatever. I think that particular article is probably from their Idea Shop series that the magazine ran over the last year or so, so it's probably the same one.

The torsion box bottom doesn't bother me so much as figuring out where they get a 600# rating! Granted, that is inclusive of the weight of the base, which given a healthy slab of sheet goods and the hardware, probably has to be over 50# in and of itself. I'm fairly new at this and all, but MDF isn't one of my favorite materials... routed some one time and the dust from it about kicked my tail... couldn't hardly breath for a couple hours. Definitely a lesson in dust collection and breathing protection!

I don't necessarily begrudge the use of the space way down there for a torsion box; heck at 6' 5" I'm not real keen on bending down there or getting on my hands and knees to get something out from the bottom area of any bench/cart.

I've seen other similar designs where they extended the center portion of the torsion box out to the ends, kind of leaving the wheels each in their own little pockets, which seems like it'd be somewhat stronger/stiffer... though again I worry a bit about the lack of any support in the middle, and also I hate making things (too much) more complicated than I really have to :D

Anybody out there have a thumbrule, or formula, or some sort of guideline as to what size/proportion things need to be to achieve certain levels of stiffness/ load bearing capability? How much would a person lose going from 1x4 frame members on the torsion box to 1x2 boards? What about going the other way to 1x6, or to 2x4 stock? Is there someway to roughly figure this out, w/o having to get too twiddly on the scientific calculator or spreadsheet, or is it more of a 'build it and try it and try again if it don't work the first time' sort of thing?

TIA,

Monte

[Vaughn McMillan]

Monte, you might get a rough idea of load bearing capacities using the Sagulator on this site: http://www.woodbin.com/calcs/sagulator.htm. It's really for figuring out shelf sag with different materials of different dimensions, but you can enter (for example) a 6" long red oak "shelf" 1" deep and 2" thick, and find that it will deflect 0.39" when center-loaded with 50 pounds.

You'd still need to guestimate quite a bit to extrapolate things into a torsion box, but you did say a "rough" thumbrule. (Like that term, BTW.)

HTH -

- Vaughn