[TommyGunn]

I'm starting a new build an an integral part of that build is a 3" torsion box. I have never built one before and have been viewing different techniques, mostly on wood working sites. However, they all seem to emphasis the need to construct the box on a level surface and they go to great lengths to do so.

I feel like I am missing something. The way I see it is if all the ribs and sides are cut to exactly the same height and glue is applied to all the mating surfaces, when you apply the skin and nail it together with a pneumatic nailer, all the surfaces will pull tight and it will be flat and level to the skin. I think you should be able to assemble it on a gravel driveway if you wanted.

Does anyone have experience building one and did you go to great lengths to ensure a flat building surface?

[Trotline]

The problem you run into is that the ideal parts you refer to will not, in fact, be what you use to build the torsion box. There will be variations in both dimension and angle. Nails and glue allow for quite a bit of variety in joints, as well.

All the tiny variations from perfection add up. I have built torsion boxes, taking great care to start from a surface as flat and level as I could achieve, and the end result was slightly warped. You couldn't see it, by eye, but it was measurable, if you went looking for it. Once built, a torsion box will be extremely strong and rigid, but whether it's flat is up to you. I definitely recommend that you do everything you can to stack the deck in your favor.

Luke

[CaptainVee]

The surface you work on needs to be flat but it does not need to be level.

But for most it is easiest to set it up flat if you use a level

If it is not flat, the torsion box will not be flat either.

[ger21]

I feel like I am missing something. The way I see it is if all the ribs and sides are cut to exactly the same height and glue is applied to all the mating surfaces, when you apply the skin and nail it together with a pneumatic nailer, all the surfaces will pull tight and it will be flat and level to the skin.

You're assuming that every edge of your ribs is absolutely square and dead straight, and that the ribs are strong enough to resist any forces applied to them, including gravity, and the weight of the skins. Even the force of pressing the nailer against the skins can distort the box.

I built a 40"x90" torsion box. I built it on my table saw with extension side table, which is about 27"x 84", and appears to be perfectly flat when checked with a 4ft level. When the box was finished, one corner was 1/4" higher than the others.

A flat base is absolutely critical to the flatness of the final product. I'd recommend using string pulled to tight to check for flatness. One down each of the 4 edges, and two diagonal.

[Devastator]

I built a torsion box bed for my build. I used the flattest surface I could find, but it is still not perfectly flat. It came out with a low spot on one side and a hump on the other. Not drastic but not perfect. I just placed it hump side up and surfaced it flat after my machine was together.

[zool]

It is best to conceptualize your CNC as a dynamic object and not as a pile of parts.

The purpose of a torsion box is to absorb and dissipate force applied to it; in a sense, transfer the applied forces around the entire structure so the structure does not deform, while it, and all the other parts, are controlling the forces of the X- Y- and Z-axes that are in motion.

While building perfectly flat is a nice concept, in reality, don't hold your breath.

Here is reality: After your mechanical components are completed and then assembled into a whole, before the machine can begin production, the mechanicals, steppers, and software all must be calibrated; calibration being the most time-consuming; tedious; nerve racking; gut wrenching, etc etc, part of that calibration process, and probably the last step in this calibration process will be decking [milling flat] the table top so it is perpendicular with the chuck of your spindle [the X-Y-plane is perpendicular to the Z-plane].

Hence, and to conclude, building the torsion box absolutely flat is not necessary, though building it so it will stay stable and flat after being decked is necessary.

[PaulRowntree]

While it is true that machining it flat will remove many sins, this presumes that the torsion box does not control the rail alignment e.g., where the sides of the box have fixed pipe attachment points.

[CaptainVee]

A torsion box can be made to be warped. It would be a big pain to get the router set up correctly starting like that, the x guides would be misaligned.

You can not use the mill to align the misaligned sides. In fact, skimming the bed when the sides are warped gives a smooth but warped bed.

Do your very best to get it flat.

[radcliffe]

I feel like I am missing something. The way I see it is if all the ribs and sides are cut to exactly the same height and glue is applied to all the mating surfaces, when you apply the skin and nail it together with a pneumatic nailer, all the surfaces will pull tight and it will be flat and level to the skin.

I agree with Ger21, where he built his box on the table saw with extension and still had 1/4" twist. The box that you build will take the shape of the table or what ever you are building on, so take the time to make sure that it is dead flat.

Here is a link of how David Marks made his box.


Here is a video by the woodwhisper in using a string to check his workbench for being flat. Then he use a router to flatten it.

How to Flatten a Workbench and Wide Boards with a Router | The Wood Whisperer

Harold