Hmmm... I'm not too convinced. I see your point with un-supported rails etc.

But (and this is where we're going to get a bit geeky with our engineering here lol) the rails are 350mm long and 16.0005mm diameter, we're guessing their toughened steel. Meaning the Transverse loading weight (which would actually have to be made by the machine it's self, unless you are putting something heavy on the machine) would have to be 96kilo's per 100mm to get a flex of <1mm. devided by the 350mm rail length would mean an actuall transverse loading of 27 kilo's from the part that moves back and forth (Y axis?) and even more on the side to side axis as it is only 300mm (32kg-<1mm)

We couldnt find any stats on the density of the HDPE frame it's self that we have.
But the way the machine is setup, from what we can see all the "shear" strength is taken by the rails.

Given that the distance between the rails on the weakest part of the machine, the side to side (it's the weakest as the most energy is being put there when in motion and cutting) is 80mm, and the risen height from the bed (the place where the force is generated) is 100mm, theres a good ratio for ultimate strength.

It hasn't been used yet, but a simple (by pushing it with an arm) the uniaxial compression from the top right of the machine on a 45 degree angle is pretty solid with no flex up until about 15 kilos.
If i could remember the formular to workout what kind of force in kg that would be at the cutting edge then i would let you know.

We only have to PCB engrave with it ayway, so by rights a machine made from cardboard would probably be strong enough lol.