How to ensure Torsion Box is built flat to begin with?

[Andy Sowers]

I'm considering making a torsion box for a assembly table. I've already searched through SMC and seen the TWW/FWW videos.

My question relates specifically to how ensure that its built FLAT when you are assembling the torsion box... Most of the info I've seen kinda skimps on this part. The TWW/FWW project shows how to do this with two sawhorses and a level... is this really a sufficient technique? I worry that the temporary wood supporting the table could sag during the glue up thereby creating a sag in the table itself... also worry about just how accurate/consistent I can be with a bubble level (but maybe that's just by ignorance)...

Can someone shed some light on this for me?

Thanks

Andy

[Dave Cav]

I built a 4 x 8 foot torsion box to form the top surface of a plywood/sheet stock storage cabinet. It's on casters and also serves as an assembly table. I assembled the torsion box on the concrete floor of my shop as it is pretty flat (but not necessarily level). I used 1 x 4 lumber for the structure and 1/4" plywood for the skins. It turned out pretty well.

You could probably also use sawhorses with a couple of 2x4 or 2x6 boards on edge, level them and use winding sticks to make sure they were in the same plane.

Dave C

[Brian Tax]

I agree, it seem so many plans, demos, etc. start by saying "On a flat surface do the following...". The problem is getting that initial flat surface, I try to use my table saw, but some things don't fit on there. Not really much help, but just saying.

[Alan Schwabacher]

This may be where you saw the sawhorses with cross supports carefully leveled:

http://www.diynetwork.com/diy/ww_materials_products/article/0,,diy_14442_2278181,00.html

Marks does use winding sticks and straightedge to check and fine tune flatness before building the torsion box.

[Myk Rian]

Figure I is a key to it. Your eyesight can detect problems.

[Tom Veatch]

Andy, I just completed a torsion box top for an assembly table appx 4" x 4' x 6' using a 4' level and winding sticks. Instead of saw horses, I built the base of the table first, leveled and shimmed it, and used it for support during assembly. I felt the cabinet structure was much stiffer than a few 2x4's. The top turned out quite flat and twist free.

Keep in mind, that it's just an assembly table, not a granite surface plate. It doesn't need to be flat within half a thousandth. Mine would be worthless for final inspection of high precision machined parts since I can lay a 6' Starrett "yardstick" (not a precision ground straight edge) across it and a little light peeks through in places. I'm not quite anal enough to break out the feeler gauges, but it's plenty good enough for assembly of wood structures.

[Red Dwyer]

Andy

Try this procedure from David Marks. I used it when I built mine a few years back. http://www.diynetwork.com/diy/ww_materials_products/article/0,2049,DIY_14442_2278181,00.html

Good luck
Red

[Myk Rian]

Andy

Try this procedure from David Marks. I used it when I built mine a few years back. http://www.diynetwork.com/diy/ww_materials_products/article/0,2049,DIY_14442_2278181,00.html

Good luck
Red
That's the same link Alan posted.

[Chris Tsutsui]

I have a wacky idea...

What if you poured self leveling floor resurfacer on a concrete area of your house like garage or covered patio. Let it cure to a dead flat surface.

Built your table on this and then keep or discard the thin level surface. (If you put it over a plastic barrier for temporary use or something)

It's either that or the DJM method that I can think of... Unless you have an ice skating rink to do the assembly on or a good quality metal construction ping pong table. heh

You can check the level of the surface by viewing reflections of straight line patterns. A new thick glass window is pretty flat. So is a large granite island.

[Matthew Hills]

The Wood Whisperer also has a video in which he builds a torsion box for an assembly table. probably a similar technique of shimming supports on sawhorses to get those level, then building from there.

http://thewoodwhisperer.com/episode-18-assembly-table-torsion-box/

[Jonathan Spool]

I built mine on my tablesaw. It's pretty big with the sliding table and router extension, and dead flat. I laid a slightly bigger piece of good MDF on it and that was dead flat. Volah! Perfect flat surface on which to build a flat surface!

[Steve Jenkins]

level really doesn't matter. It's just another thing to mess with. Make sure that your supports are in the same plane by using winding sticks and a straight edge.

[Jamie Buxton]

Crossed strings. I use them to ensure anything is untwisted -- a torsion box, a big plank, etc. In the following post I use them to ensure a plank is flat, but the exact same technique works for a torsion box.

http://www.sawmillcreek.org/showpost.php?p=971766&postcount=2

The last time I built a torsion box (about 4'x6') using this technique, I could clearly observe the effect of putting one business card underneath one corner of the box.

[Tony Bilello]

I used saw horses with 2 hollow core doors on top. That was flat enough for me.

[Brian Effinger]

I've been thinking of making a torsion box for workbench/assembly table top, but I'm not sure I have the patience for the winding stick thing (I'd set it up and it'd be off a little, then after fixing it it'd be off in another direction). If I do it I plan on using my table saw top as an assembly surface. It should be flat enough - just got to make sure the extension wings are in the same plane as the main surface.

Brian

[keith ouellette]

I'm trying to get enough time to build an extension/router table for my table saw. Because I want it to be a router table I want it to be within 3/1000ths of dead flat. I keep thinking about it and have been given all these same ideas

I doubt I will get it the way I want. The slightest thing can go wrong. I have a 5' straight edge and i want to be able to lay that across the finished table and and not see any light under it.

I just don't think I have the skill to build a 'temporary' dead flat surface to put my permanent flat surface together on.

If only i had a bunch of money. I could stack it all real flat and build my router table on it.

[John Schreiber]

Crossed strings. I use them to ensure anything is untwisted -- a torsion box, a big plank, etc. In the following post I use them to ensure a plank is flat, but the exact same technique works for a torsion box.

http://www.sawmillcreek.org/showpost.php?p=971766&postcount=2
+ 1

I've used the idea above and variations on it. Without a surface plate, I couldn't tell you if it would get to .003, but I cannot detect any error with my smallest (.0015) feeler gauge over eight feet.

I use monofilament and hang about 10lbs off each end. I did the math and the sag was insignificant over eight feet. I don't know the test of the line I use.

The variation which I add is to use a wedge shaped feeler gauge to measure the distance between the surface and the string. I use three 1" long finish nails. Two are at each end holding up the line. The other is tested at each end to make sure it contacts the line at the same point along its length. Then it is tested at various points along the line to see where the surface is high or low.

It's a little tedious. I'd rather have a giant granite surface plate, but the price is right.

[Will Blick]

This is a case of..... the largest reference surface wins...can't build anything bigger :-)

I would consider milling some poplar boards, as wide as your jointer will allow, plane to 3/8" thick.... this assumes your jointer and planer are FLAT (both the reference surfaces and the blades) ... thickness accuracy does not matter much, just thickness consistency between the boards. Now, glue up ASAP before they warp.... clamp the boards flat, like this, as well as the normal clamping arrangement.

http://www.woodpeck.com/media/main_flat-bars.jpg


This might be the flatest surface you can make, as you are using your highest and longest tolerance tools in your shop. Now have your MDF cross memebers ready to glue / nail ASAP... you can always add hardboard over the top for a more durable surface. This is sort of a reverse method of making a torsion box. I always found the winding sticks and levels much less accurate than using two solid tools. Of course, EXTREME care must still be taken to try to hold .003"...