Steel coffee table?

Hello,
I hope this is the right forum for this question, it is my first time posting here. A friend of mine would like me to make a coffee table
(45" long x 26" wide x 10" high) where the top is as thin as possible. I've only made furniture from wood, but I thought, why not make the table top from steel?
So the question is, how thick would a 45" x 26" piece of steel have to be to be rigid enough for use as a table top? I guess rigid enough so that a 200 lb person could walk around on top without much deflection. The plan is to use 4 legs with a 4" cantilever, so the central span will be 37" x 18". I calculated that if the top were 1/8" thick, then it would weight about 40 pounds, which is about the limit of feasibility.
Thanks, Wayne
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I'd use 1/8" plate but with a few pieces of 1/8x3/4" flat bar welded (on edge) below for stiffeners. Should make a remarkable difference in rigidity. Also, if the thing, completed, has some spots in it where if you put down something heavy it goes "ka-plonk" then the steel will need to be shrunk slightly in that area. This is done by heating a spot on the plate to cherry red (spot maybe the size of a 50 cent piece) and then quenching with excess cold water (a garden hose) and then repeating. In this fashion deck plates on ships are stretched like drum heads, and no more "ka-plonk" when officers (or Navy inspectors!) walk on them. - GWE
Wayne Whitney wrote:

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Wayne Whitney wrote:

OK, but the goal was to minimize the total thickness (profile) in the space between the legs, so the 3/4" stiffeners increases the total profile to 7/8". The one place that it would be OK to increase the profile would be the very center line of the tabletop lengthwise.
Would three stiffeners in an I pattern, one between each pair of legs, and one between those two along the center line of the tabletop, be effective? Also, it would be OK to use something thinner than 1/8" (125 mil) plate in the interest of reducing the total weight? Maybe 12 gauge (97 mil) or 14 gauge (68 mil)?
Thanks, Wayne
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Personally, 18 to 22 gauge should do it. Some 1/4" thick stiffeners (oh, 1/2" wide) around the edges and one or a few down the center will make it good.
Welding will cause some interesting distortions, personally I'd rivet it... but that's because I don't have a welder, my alternative being brazing with a propane torch....can you say massive warpage?
Tim
-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms
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Wayne Whitney wrote:

If you start with a 45"x36" sheet of 12 gauge steel (about .1" thick) and fold the sheet 9" in from each long edge 172, and weld at the bottom centerline, you would get a hollow triangular box beam that might be stiff enough, and being thin at the outer edge would have the appearance of being quite thin even though about 1.25" thick along the centerline.
For light weight and probably easier construction, a kevlar or carbon fiber construction probably would work better. You would use a sheet of perhaps .050" steel on top, a flat triangle of dense structural foam, then a bottom layer of kevlar or carbon fiber on the bottom.
I think a given weight of steel provides more deflection resistance than tungsten and about the same as aluminum and titanium. See eg http://www.mse.cornell.edu/courses/engri111/modulus.htm and http://www.mcelwee.net/html/densities_of_various_materials.html where you find steel density is ~ 7.8 and its Young's Modulus is about 200 GPa; tungsten, 18.82 and 406; titanium, 4.5-4.85 and 116; aluminum, ~ 2.7 and 69. The ratios are about: 25.6 steel, 21.6 W, 24-25.9 Ti, and 25.6 Al. Chromium (289/7.14@.5) stands out among modulus/density ratios, but perhaps is hard to find and expensive in unalloyed sheets. Note, at a given weight, less-dense materials like Al and W gain substantial deflection resistance by beam effects so the modulus/density ratio doesn't tell the whole story in general, but I think for your purpose, a thin table top, it does.

If you anchor the legs solidly in the floor, or have a frame that holds the legs vertical, you can make your table top out of foil, stretched like a drum head. I don't think it would take even one lakh pounds of tension to work pretty good. -jiw
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stands out among

In fact, it gets squared, if you increase volume in the direction of the bending. For a given volume, aluminum is 200/69 = 2.9 times more flexible than steel. But aluminum is 7.8/2.7 = 2.9 times less dense, meaning the beam can be 2.9 times wider, and hence 8.3 times stiffer than if it were in the same dimensions as the steel beam. This leaves the aluminum beam 2.9 times wider, but 2.9 times stiffer than the steel beam. A similar case can be made for the stronger aluminum alloys (tensile up to 80KSI). Of course, you would be using 4140 or so (250KSI isn't it?) in say, an aircraft, so steel [alloy] still wins in strength.

Now *that* would be an interesting table.
Tim
-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms
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Also see Blodgett, "Design of Weldments" for some good discussions on designing for stiffness.
Ted
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This in an interesting idea, a hollow triangular box beam. If I understand correctly, I'd need to start with a piece of sheet just over twice my desired table width, so that would be about 52.1" wide. How would I finish the ends? Would the thin edge end up about twice as thick as the sheet? Can my neighborhood metal shop bend a sheet 172 degrees?
Thanks, Wayne
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Wayne Whitney wrote:

Right - I said 36" since I thought 18" was table width - must have misread

The end cap could be a welded-in piece of steel plate, drilled and possibly tapped to bolt to end rails. This idea has some problems - 1/8" plate probably is thick enough structurally for the end cap, but too thin for adequate threads; 1/4" or more would allow enough threads, but might cause difficulty welding the thin skin to the thick end cap. So there probably are much-better approaches. Eg, run all-thread end-to-end inside the beam.

Right - if the bend were pressed down to close to a zero-radius

That probably is the hard part of this plan. I think most sheet metal brakes can bend to around 100 or maybe a bit more, so it would take either a special brake or a second bending step - eg, bend both edges to 90-100, then (somehow) use a press to close the bends. It might be more practical to use one flat sheet for the top, and a second sheet about the same size with 3 bends (say +8 a few inches from edge, -16 at middle, and +8 near other edge). Tig weld the two pieces together then grind the edge smooth. I've seen boxes made this way where the box looked like it was made of one piece of metal (ie, the weld wasn't obvious) and also some that looked awful.
It might be worthwhile and not difficult to make a full-size or half-size model from flashing, to see if the appearance would be ok. -jiw
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James Waldby wrote:

Welding and grinding would work fine and look good as long as the welds had good penetration.
I knew someone that had a steel coffee table the looked like a big I-beam (actually more like an H on it's side). It had two H-shape end plates and long plates welded on every edge of the end plates, connecting them together. I don't know how heavy it was. It took two to lift. I saw people dance on it with no problems. This was in a house full of party-loving college students. It was the ideal indestructible coffee table.
Greg
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James Waldby wrote:

Use riv-nuts in the 1/8" end pieces.
Ted
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On Mon, 20 Sep 2004 05:21:55 GMT, Wayne Whitney
I did a nice one a year or two back.
Three 12" square slate floor tiles. Welded 1" x 2" tube ("landscape" mode) to make a frame around them, with two 1/2" solid squares between them and a little angle beneath to support the tiles. Legs of 1" square tube. Flapwheel and hot oil blue to finish, wax on the slate tiles.
With just a bandsaw, a Mig, an angle grinder and a propane torch for the bluing, I did the whole lot in an evening.
BTW - the tiles are neither square nor identical. Be careful when laying out the frame - shim the tiles with a little file folder card and clamp up around them to make it self-jigging.
--
Smert' spamionam

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