by dirktheeng » Sat Sep 11, 2010 4:56 am
I would be a bit concerned about the straightness tolerences that one can get with extruded parts. I looked into PCB Integral-V and talked with the engineers there about being able to hold tolerances to make a home made CNC router earlier this year. Basically, they make the extrusion and then mill the guide rail slot and fix a hardened steel V into the milled slot to give a high wear surface. That's why the stuff is so stinkin expensive. For short runs, extruded parts may be able to hold >.01" per foot without milling, which may be good enough for some small cnc projects, but not so good for something when you want to make a 4ftx8ft router.
The other thing to consider is deflection under load and how far you can span without support. I was doing the design calcs for a CNC router earlier this year and I wanted to be able to mainain a 0.001" deflection standard (really helps reduce chatter when milling hard aluminum). I was going to use 8020 extruded parts and soon found that I had to use the 2040 profile supported every 2 feet to take a load of 80lbs from the gantry, z axis, motor, etc (live and static loads). It could be done, but it gets expensive. It was cheaper to use an I-beam and mount round rails and pillow blocks.
Granted, for a laser these may work ok because the runs are shorter and the loads are very low. All that is to say, beware of the potential for low straitness tolerances and the higher complience of extruded aluminum parts.
What kind of tolerances are you expecting to be able to get out of this?