[delco]

I have a openbuilds workbee and am looking to upgrade the torsional rigidity using two 80/20 Beam extrusions rather than the standard 1 beam.

Question is should I put them back to back to give a 80/80 I beam or front to front to give a 80/80 tube picture of option attached.

[ger21]

I believe that the tube will be more rigid.

[Wrat]

I believe that the tube will be more rigid.

Rarely is the loading purely torsional, consisting of a moment around the beam.

However, if all one is seeking is increased torsional rigidity, then absolutely yes, the box beam will be best, exactly as ger21 says.

Whatever most-resembles a circle, basically. The square box-beam would be closest of the options presented.

[delco]

The x axis is 1m long and most of the deflection I can feel is a moment around the beam and am worried the cutting forces will make this noticeable in chatter.

the other option was to put a 80/20 on the back to make a smaller circle as per first pic ,as I am also worried about the width causing a issue with the longer bolts required across the gantry.

[Wrat]

Being a charter member of the "more is better" club, and a lifelong subscriber to "Overkill Monthly" I'm one to think that chatter is always present unless there's sufficient mass to dampen it.

In fact, a profilometer will bear this out. Even what a machinist calls a "perfectly smooth cut" is simply chatter amplitude below immediate detection. But it's always there.

I tend to believe there's not enough stiffness or strength that can be added to subdue chatter. Sooner or later the whole assembly will hit a harmonic. Mass dampens. Cast iron, granite, etc.

Of course, this isn't always practical. But i thought i'd bring that up before you launched into a endless pursuit. Loading is probably fine. If your gantry is a box beam, then your weak spot will probably be your X axis motive components. But for chatter, adding stiffness works only to a point. After that, you'd be ahead with a big lump of clay to take out the vibes :-)

[mactec54]

The x axis is 1m long and most of the deflection I can feel is a moment around the beam and am worried the cutting forces will make this noticeable in chatter.

the other option was to put a 80/20 on the back to make a smaller circle as per first pic ,as I am also worried about the width causing a issue with the longer bolts required across the gantry.

This is the problem with extrusion, if you look at the end view you will see the weakest points all the links between the holes going through the extrusion all have a wall thickness, this is as strong as it can be, your bottom snip is the strongest way to mount it for torsional rigidity, add an aluminum plate between the 2 extrusions like 10mm or 3/8" and this will help but won't solve the problem that extrusion has