Am I heading in the right direction?....

[EddyCurrent]

Lloyd,

I see you have been doing some reading up already and you have a good idea about what you need.
Starting from the top of your post;

Length of rails will be best determined once you have a drawing together but they look to be in the right area.

80x80x3 box section is fine but if you are bolting your rails to this then the wall thickness is insufficient. I used 50x50x3 for my 900x600 machine but used 6mm flat bar inside the box section where my rails bolted to so as to increase the thickness to 9mm, much more meat to tap holes into. Some people make a bed that can be moved up and down so you might want to consider that, it might mean you could reduce the stroke of your Z axis.

The epoxy method involves creating a moat around the top surfaces where the rails bolt to, the moat is then filled with low viscosity epoxy resin and like water will find it's own level thanks to gravity. Because the two X rails are mounted on two separate pieces of box section, an additional 'bridge' has to be created to join them so that the epoxy can flow between them like an aqueduct thus ensuring all top surfaces of the epoxy are at the same level.

It's a pity you mention SRB20 rails for the Z axis, you would be far better using profiled rails all round such as the Hiwin type.

Go with 2 ball screws on X right from the start, there are reported issues when using a central one. When cutting at each extreme of the Y axis there is a tendency for the gantry to twist or rack, pivoting on the central ball screw.

From what I've read I believe you would be better with 16mm ball screws, 10mm pitch for X and Y and 5mm pitch for Z

This is a good diagram I think to explain positioning of parts Do it yourself CNC router: Design Considerations, the Gantry
plus there is this calculator thanks to 'routercnc' http://www.mycncuk.com/forums/linear...html#post15791

Make a drawing of your gantry and post it for comment before starting, 20mm plate is the norm. Most people use the free Sketchup software.

3Nm sounds fine, I used these Nema23 Stepper Motor 4Nm

Digital drivers as supposed to be the best, it used to be Leadshine AM882's but I think the EM806 is taking over Stepper Motors & Drivers - Zapp Automation Ltd
Of course you could use servo's instead or a closed loop system.

The best power source is one you build yourself, it's very easy and all the info will be available here when you need it. The design will consist of, toroidal transformer, bridge rectifier, smoothing capacitor(s), fuses.

You also need to think about which spindle you are going for, most common is the 2.2kW water cooled ones from China that come with a variable frequency drive (vfd) e.g. WATER-COOLE MOTOR SPINDLE 2.2KW AND INVERTER VFD good | eBay

what other rules of thumb are there that you guys adopt when building a machine for the purpose of cutting those materials?

If you keep posting and refining your design according to forum input I'm sure you will end up with a suitable machine, I did for one.

[PAULRO]

hi EddyCurrent,
can you give me details of the epoxy mentioned above,( brand name . quantity required , suppliers etc.0 my m/c wil have a cutting area of 1200 mm x 1200 mm approx. cheers , Paulro