It's a common misconception peope seem to have that steel bar / RSJ etc, hardened or otherwise "doesn't bend". People see bridges and buildings made from the sort of size steel that we sometimes see on CNC router frames and assume that if it can hold up a bridge it will hold a cutter steady. The critical difference is the acceptable magnitude of deflection is markedly different in these circumstances. For any CNC router worth owning 0.05mm (perhaps less) of deflection is likely to be disastrous, but in a bridge a few millimeters of deflection won't be noticed. For CNC machining the permissible bending thresholds are very small, hence even though the forces are generally not that great you still need a very rigid structure, which includes the rails. You're not going to actually see the deflection in an unsupported rail until you put a dial indicator on it as the deflection will be small, but you only need a very small amount to make a difference.

Quote Originally Posted by Tenson View Post
A lot of the DIY machines on here are built with Chinese ballscews, ballnut and linear bearings. Do you have evidence the ones in the 6040 are inferior? The rest of the structure is metal and screws, probably machined on very good industrial kit.
I doubt there's anything wrong with the ballscrews and I'm sure that's not what Jazz is suggesting. The main problem with the 6040 is it uses unsupported rails on Y and Z. There's a huge overhang on the Z-axis, relying on the spindle for strength, very small spacing between the linear bearings on all the axis', combined with only one ballscrew on the X-axis, sub-optimal direct drive not timing belts, bed is proabably weak towards the center, tiny ER11 collet, etc etc ... all things that are easy to do better yourself. You only really need to read a couple of the build logs on this forum to learn most of the things required to make a good machine as Jazz and I mention the same weaknesses and how to solve them every single time.

I'm also happy to talk to anyone about how to design/build a machine. I regularly talk to several people on the forum via Skype/wlm/email ...

Jazz is right about people being to greedy when it comes to selling machines in the UK. The capitalise on the ignorance of the customer by selling machines which are barely adequate and as a result overpriced. I'd happily be making good CNC routers, but as I'm currently at University my time is somewhat limited. Having said that I will be making a CNC Router, primarily for cutting aluminium and steel, in partnership with a friend from school over the next couple of weeks. I will post a build log when its done to demonstrate what can be achieved if you're prepared to do some background reading and think for yourself.