Question for all,
Where do people seem to have the most problem with there systems flexing and racking please state where your problems are what your system is built out of and post pics of the weak spots if possible.
I am trying to design my cnc to over come many of the problems people have and I figure the best place to learn is from others ( as appose to making all of the problems myself) thank you. I hope this thread will grow and be a good reference for newbies as well as those who are upgrading there machines. :)

I decided to go mild steel and then got flex through using angle iron rather than square / rectangular section.

I tried bracing to strengthen but ultimately retrofitted square section. Also, flex arose from bolting together but have cured that through welding. I have used 3mm steel throughout. Any thinner could cause flexing within rectangular sections.

Now I am happy that there is no flexing and that the entire frame is rigid.

My next consideration is vibration which I believe I can manage through a variety of strategies when it comes up (I am expecting it to be a problem to a greater but hopefully lesser extent). I believe that mild steel has good qualities compared with aluminium with respect to vibration. I am happy to be corrected on that.

Racking should not be a problem for me as I have designed x axis with two timing belts (one each side) driven by a common shaft which runs through the frame of the gantry.

Andy

Assuming your building with wood, check out the machines of Joe2000 and Lionclaw:

http://www.cnczone.com/forums/showthread.php?t=15139
http://www.cnczone.com/forums/showthread.php?t=14331

Both do a good job of addressing a lot of the issues you mention.

The main issues encountered:

Rail flex, which can only be eliminated by using fully supported rails. The above machines use torsion box frames to support the rails.

Gantry racking, or skewing, when driven by a single screw. The easy way around this is to use two leadscrews, one on each side of the gantry, either driven independantly, or by a single motor through a belt and pulley arrangement.

Depending on construction methods and materials, the gantry uprights can also be a source of flexing, allowing the gantry to move side to side. Some have resorted to torsion box side condtruction, others have used thick aluminum for the uprights.

80/20 is nice and can be tailored to the size you need (you can use their software to size it - free download). It assembles like a big erector set and offers some design flexibility. I feel I've minimized the possibility of the Y axis "racking" by using a dual Y axis setup with the Router between the two.
Good luck - heaps of approaches and choices - Jim