Hi, Curious Aardvark!

Thanks so much for the feedback!

I wanted a larger build table than a standard 220 mm table; so I chose to go with a table twice the depth, and twice the width. That took me to 440 mm, and 18" is 457 mm so I just rounded up.

I could have had a custom heater pad made, but for a 457 mm sq. build plate it would have cost me about $140. Instead, I chose to go with 4 standard 220 mm heater pads, at about $8 each. The pads want 200 watts each, so that means I need 800 watts for the bed, and the rest for running the board and extrusion heater cartridge. I could have gone with one 900 watt power supply, but the cost per watt rises rapidly; two 450 watt power supplies are actually cheaper than one 900 watt power supply. 450 watt psus are actually the cheapest psu on the market, when viewed on a cost per watt basis.

Thanks for the feedback on the rod diameter. I like bowden feed, but I don't want to be restricted in the future, so I think I'll opt for 12 mm rods and bearings.

If I mount the drive motor for the bowden extruder at the top of the printer, on the back and half way between the left and right vertical support columns, then the greatest bowden tube length would be 20 inches plus a generous amount to allow the tube to transition from vertical to horizontal to vertical again. Call it 28 inches. I think that's not an impossible tube length. I could be wrong, which means I'd have to go direct drive, which means 12 mm rods are a good idea.

Some ultimaker-style designs use a single drive at the back of the build platform, and support the platform from the rear only. There might be chrome guide rods at the front to keep the build plate from shifting left or right, but all the weight is being supported from the back. I think that with a machine of the size I'm building - an 18 inch square build plate, and a maximum load of 1 kilogram of plastic sitting on it - supporting it from the back only is unwise. I think there could be problems keeping the build plate perfectly flat, and long term wear issues at the back screw drive. So I see a minimum of 3 drive screws being required: one at the center back, and two at the front left and right corners. But this means that those rods must be driven; and that means a circular drive belt driving all three rods at the same time, or three stepper motors working in unison. But modern boards (melzi, ramps, rambo) can drive a maximum of two stepper motors on the Z axis. This means that the problem can be most easily solved with one largish stepper motor driving a circular belt (a piece of continuous drive belt in the shape of an elastic band or a doughnut, as opposed to a linear piece of drive belt like a shoelace).

The problem I'm having is that I haven't been able to find drive belts, with the exception of the really tiny ones that ultimaker itself uses for transitioning the drive motor to the Y axis on the print head. It's only 3 or 4 inches in size, and I need something way, way bigger.

If there's a way of supporting the build platform in three points without resorting to stepper motors or drive belts, please tell me. Maybe I'm missing the obvious.

Thanks again, CA - I appreciate your help!

>Charlie1