Opinions/Advice on my 800 x600 design

[davegrennan]

Thanks for the feedback folks.
shoeswith: Sorry but its not clear from the pics. I already have a VERY heavy and sturdy bench. Its got 10mm plate steel sitting on top of 75mm of hardwood joist. You could run a railway over this thing. The plan is to drill and tap the rails from underneath so the rails and the bed are permanently fixed to the bench. I guess that I can use a 'suck it and see' approach to whether I need dual ballscrews. If it turns out I do It won't be a huge deal.
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Clive: One of my constraints is that I already have a 2000mm x 1000mm piece of 15mm alu plate. I could double it on the gantry sides, I certainly have enough of the plate to do that. I certainly take your point about the horizontal box sections.
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The supported rails in the design are going to be 20mm. Do you mean a different type of rail would be better? This machine will be for 3d profiling/ cutting wood, cutting aluminium and *possibly* drilling mild steel. Its all hobby stuff. I don't need micron precision, I just need it to be better than what I got. e.g I did a complex profiling op last weekend. The machine whirred off for about 30 minutes after I finished and sent the machine back to zero, the cutter was close to 3mm away from where it began. I'm putting this down to backlash and flexure etc. That's what I'm looking to avoid with this machine.
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Also my existing machine, being a fixed gantry only allows me access to around 400mm x 300 mm on the table. I found this to be too limiting. I'm delighted I went with the MDF first and would recommend any first time builder to do so as your mistakes wont be too costly. Imagine if I had cut up that 2m x 1m 15mm plate and then realised I'd made some serious mistakes!
Thanks again for the feedback. I'll certainly start a build log for this one.