Torsion box bowing towards center, and creaking.

[Fierra]

Hi there,

I built out a CNCable 30" torsion box recently. I made a grid of 7 by 7 struts from 1/2" MDF, with cross-halving joints and I built the struts to total 2" in height. I then have a 1/2" face and base plate. The faceplate has holes to allow the struts to pass into the structure, locking the face in position. The baseplate does not have holes for every strut, but has some corner holes to locate the struts, so there's some inherent geometry difference there. Finally, a large grid of bolts and threaded inserts are used to 'sandwich' the plates together over the struts. No glue is used.

When attempting to mount routed panels to the side of the torsion box, there was an immediately obvious difference of about 1/8" between the corners of the torsion box, and the center, bowing towards the baseplate. I can rule out the CNC'd parts being bowed. So my assumption is that my design method introduces some level of stress into the structure.

The second issue I'm facing, is that when any adequate weight is placed on the structure, there is a large amount of creaking. It can either be the screws I use, or the MDF structure itself rubbing.

Does anyone have some tips for me to combat these issues?

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[ger21]

If the face panels are not glued to the ribs, then it's not really a torsion box. The only way to keep it flat is to glue it together.

[Fierra]

If the face panels are not glued to the ribs, then it's not really a torsion box. The only way to keep it flat is to glue it together.

Thanks for the response, ger. What about glueing as opposed to other fastening methods gives the box its rigidity and flatness? Could I continue using a grid of 9 bolts throughout the structure to adequately hold the piece together during gluing?

[ger21]

When either side bends, or bows, it's length changes slightly. The concave side gets smaller, and the outside gets larger. This changes their position relative to the ribs. Meaning the ends of the faces will move slightly relative to the ribs. Glue transmits the load from one face to the other face, and keeps the faces location to each other from moving. This is what makes the torsion box rigid and stable.

Screws or bolts will allow a small amount of movement in the faces, which is what allows the box to warp.