My 4' x 6' torsion box

[Paul Simmel]

This is going to be an assembly table/work surface. Currently it is supported on saw horses, but I will eventually build some sort of rolling under carriage.

It has just about a playing card's dip in the middle according to a Menard's 4' straight edge. I'm not going to sweat it.... should I?

[Eddie Darby]

When you build the under-carriage, just make sure that it's flat, and then the top will flatten out once attached........hopefully?:confused::eek:

[Paul Simmel]

Probably so. Length to length, BTW. Also, since it's a Menard's straight edge, I get slightly different readings as I flip it over... which begs two questions.

Split the difference?

Since so many people say aluminum machines "well" with WW'ing tools, and since my jointer's knives are already in need of sharpening...

Can I joint my straight edge?

[Eddie Darby]

Use a reference surface, such as a jointer table, and a feeler gauge to verify what you are seeing.

If you want more accuracy, then LV and others sell straightedges that are very tight in there tolerances.

http://busybeetools.ca/cgi-bin/product10?&NTDESC=straight%20edges

http://www.leevalley.com/wood/page.aspx?c=2&p=50074&cat=1,240,45313

http://www.leevalley.com/wood/page.aspx?c=2&p=56676&cat=1,240,45313

[Roger Warford]

I was under the impression that by design a torsion box would not flex at all if properly constructed. Is that not the case?

[Joe Chritz]

It won't flex under its own weight.

I expect that the "dip" is actually in the straight edge or at least mostly.

If you are concerned make a temporary straight edge from a piece of jointed wood and check it against a known surface (jointer fence or table) and then against the torsion box.

I just made a 3x5 box that is on sawhorses still too. With a 5' jointed piece I can't see light under the piece anywhere on the table. I have access to a 48" precision ground Starrett but haven't bothered to get it to check yet.

Don't sweat the small stuff.

Just wanted to add (as an edit) that the box was one of the nicer things I have built in a while. I don't know how I survived this long without one. If you are on the fence about doing one it is a good idea.

Joe

[Chris Padilla]

This is going to be an assembly table/work surface. Currently it is supported on saw horses, but I will eventually build some sort of rolling under carriage.

It has just about a playing card's dip in the middle according to a Menard's 4' straight edge. I'm not going to sweat it.... should I?

I would but I'm about as anal as they come! ;) You have some good suggestions already to verify how flat your box is so have at it! Pick up a set of feeler gauges...handy suckers to have around and the aluminum straight-edges from Lee Valley are great!

[Paul Simmel]

I was under the impression that by design a torsion box would not flex at all if properly constructed. Is that not the case?

I don't know about that. I'm sure there is some flex. In my case it is probably more the way it was constructed, than flex between the two horses. I could always shove them closer to see. The hardest part about building this was getting a flat reference surface... which I did as closely as humanly possible.

10-thousandths to me over a 6' length is really not that big of a deal. I was just wondering how many of you have a 6' TB within 10-thousandths... I guess at any point... and I was wondering if you feel this is a big deal or not. 5-thousandths is acceptable for a TS top.

I feel this can be corrected if need be. I have a hardboard top screwed down, which can be replaced. I could simply lay in a few shims in the low spots, and re-screw the hardboard down.

Edit: I said playing card, not playing card deck. (Smile)

My main question is, should I sweat it? Or fix it?

[Al Willits]

Wait for a rainny weekend and fix it, sounds like it'd be easy with your set up, if all ya have to do is unscrew the top.

Al

[Jim O'Dell]

Sounds like a good use for a skim coat of Bondo in the low spot and sand it out smooth. At least if you're going to put something on top of it, like laminate. Jim.

[Paul Simmel]

Thanks everyone. My Menard's straight edge is actually rocking on one side and crowned on the other. Very good news for me, actually. So I'm off to start a thread on jointing my Menard's straight edge.

[glenn bradley]

Funny. All this discussion and I still don't see any proof that the thing even exists let alone exists in a non-perfect state ;-)

[Joe Chritz]

I'm not sure that jointing the straight edge will work all that well. Tools designed to cut wood and to cut metals are made differently. A mill with a fly cutter would work if you can find one with enough travel, or a surface grinder or a CNC router.

I use playing cards for shim stock sometimes and I can't think of anything that a few thousandths off on an assembly table would ever make a difference.

Joe

[Paul Simmel]

Lol.

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[Paul Simmel]

I'm not sure that jointing the straight edge will work all that well. Tools designed to cut wood and to cut metals are made differently. A mill with a fly cutter would work if you can find one with enough travel, or a surface grinder or a CNC router.

I use playing cards for shim stock sometimes and I can't think of anything that a few thousandths off on an assembly table would ever make a difference.

Joe

Joe I agree with you 100% re the wood/metal point, but hey, what the heck. Seriously I have read a lot of threads where aluminum has been machined by WW'ing tooling. I was very surprised, but people are doing it. I really did start a thread on this. I am VERY curious to see what comments come from this. I have a Makita knife sharpening system, so I am really not concerned about my knives being damaged beyond repair... one pass would be enough to straighten that thing.

And I agree with you on the assembly table as well. I'm pretty happy with such a large surface to work on (finally), and it is really very flat and it seems to be very strong as well. .005 or so here or there. Wood is going to move on its own more than that, even with the most careful care.

[Art Mann]

You are assuming that your jointer will machine any material, be it wood or aluminum, to an accuracy of better than 0.005". I do not think that is at all a good assumption. There is a reason why companies manufacture, and many people buy precision straight edges at a relatively high cost. Achieving that kind of accuracy is not that easy.

By the way, if I built a torsion box assembly table that was withing 0.005" of flat, I wouldn't change anything as it will make no practical difference in the projects you assemble on it.

[Joe Chritz]

I would be more concerned with a blade snagging the aluminum and not cutting it cleanly. Hence tossing it rapidly into some part of your anatomy you probably aren't willing to part with.

Aluminum won't damage HSS knives but they will dull them faster than wood by a long shot. Anyone who has used a mill has used HSS to machine all kinds of metal. Thats a lot different than try to machine it hand held however.

Joe

[Gene E Miller]

Greetings & Salutations,

Your TB looks great and I just finished on myself.

I made mine using David Marks Wood Works show and
information off the DIY website to pattern off of.

I made mine 4'x8' and 4" thick. I put 1" oak around the
outside edges and a 45 degree on the top edge.

When I put a straight edge on it in any direction and used
a feeler gage I could not see any light or get a .001 feeler
under the edge so I think it will be great for making square
and plumb assemblies and glue ups.

85188

[Tom Veatch]

I was under the impression that by design a torsion box would not flex at all if properly constructed. Is that not the case?

Any real world material or construct will deform under load, any load, no matter how small, even if the load is only its own weight. The deformation may be very small, but there will be deformation.

Nothing in the real world is infinitely stiff.

[Todd Bin]

If you want your torsion box to be stiffer then upgrade the skin from 1/2 inch to 3/4 inch.

[Will Blick]

Do you have an estimated weight of the top? I would assume even though its somewhat hollow, its still damn heavy... thanks to MDF :-)

[Joe Chritz]

Making the grids taller will do much more than thicker skins.

The facts are technically correct, everything will deflect some. However, if it takes major effort to measure it isn't worth messing with.

I have a 3x5 and a 4x 4 /12 foot box. Weight on the big one has to be around 150 lbs or so. I haven't weighed it but it is all 1/2 MDF (the big one has 3/4 sides) and has around 2 1/2 sheets since one side is double layer to make room for miter slots and a replaceable work top.

The little one is fully movable but only because it is on wheels.

Joe

[Tom Veatch]

Making the grids taller will do much more than thicker skins.
...
However, if it takes major effort to measure it isn't worth messing with....

Concur completely with both statements. And, considering that woodworking is not the same as high precision machining and that wood is nowhere near as dimensionally stable as metal, IMO, a playing card thickness over a 4' distance is in the "isn't worth messing with" category.