Crown Up or Dip Up?

[Paul Fitzgerald]

I'm mostly through with my 4'x4' torsion box assembly/outfeed table. I just need to attach some hardwood trim and put a protective finish on it.

I thought I was very careful to ensure flatness throughout the entire process, yet I somehow ended up with a very slight crown on one side and a very slight dip on the other. I haven't measured it, but I'd say it's probably somewhere around 1/64", maybe a tiny bit more.

I'm inclined to put the dip side up. Would you do the same? Why?

Thanks!

[David DeCristoforo]

Crown up. Material will "ride" over the crown. It will be hard to press pieces down onto the table if it is hollow. 1/64th of an inch over 4' is no biggie. You could probably "fix" it in a few minutes with a cabinet scraper.

[Lee Schierer]

I agree with David and would add that any weight on the surface will likely take out the crown effect making the surface flat.

[Tom Veatch]

Paul,

First I'll say that flat within an average of .0007 inches per inch is nothing to be ashamed of. Next I'll second (or third) the recommendation to mount it with the crown up.

A lot has been said about torsion boxes not flexing under load. Well, to be blunt about it, that's hogwash. Every real structural member flexes under load since no structural material has an infinite modulus of elasticity. Next time you're in an airplane, watch the wing. That's a torsion box.

Torsion boxes are good structural members because typically they are stronger and flex less than other structures with the same weight, made from the same material. They are noted for having a better strength/stiffness to weight ratio which is why that aircraft wing is constructed as a torsion box. But, if you load them, they deform. So, mount the box such that the load deforms it closer to flat rather than away from flat.

Same principle as installing joists and rafters with the crown up.

[Peter Quinn]

Crown up always. Over time gravity may just fix that one, where gravity will not likely fill a hollow over time. Or, put the crown down and BURY it in west systems to level it. Probably the stupidest waste of resources I can think of but it just might work.

[Paul Fitzgerald]

Cool, crown up it is! BTW, I measured it at less than 1/64".

I also just got done jointing a planing a bunch of stock that's 32" and less in length and having the assembly table is going to be GREAT!

[Peter Luch]

It's a total waste and you should just make another!

By the way I'll be passing by with my truck tomorrow and can haul it to the dump for you...for free!

:D:D:D:D:D:D:D:D

If I ever get to make one I hope I have your problem of ONLY a 1/64" over 4 ft.

Nice work!

Aloha, Pete

[Paul Fitzgerald]

It's a total waste and you should just make another!

By the way I'll be passing by with my truck tomorrow and can haul it to the dump for you...for free!

:D:D:D:D:D:D:D:D

If I ever get to make one I hope I have your problem of ONLY a 1/64" over 4 ft.

Nice work!

Aloha, Pete

Haha! That's very generous of you Peter! But I think I'm going to keep it. :D I put a lot of effort into getting it as flat as I could. :p

Paul

[Bill Houghton]

Every real structural member flexes under load since no structural material has an infinite modulus of elasticity. Next time you're in an airplane, watch the wing.

Useful advice for anyone not nervous about flying in the first place. I've watched those wings waggle, and have thought about a friend of mine who had to take calming drugs to fly; I don't think I'd ever have pointed the movement out to her.

[Tom Veatch]

...I've watched those wings waggle....

You ought to see one wave around during a static test to failure. It's amazing how far the tips move before you hear something break.

We'd generally run a static test article up to limit load, relax the load to make sure there was no permanent deformation, then run it to ultimate load (50% above limit load) and hold it there for a short time to make sure nothing failed, then load it to failure to verify positive margins. They move a whole lot farther than (hopefully) you'd ever see sitting in the cabin during flight.