At present I'm going for E3D chimera/cyclops, and a mad-cap cross between ultimaker mechanism and core-xy fishing line arrangement on printer 2. But I'm keeping it small (fast and accurate), so different design goal I suppose!

I actually would find it hard to justify a dual-extrusion system. It is nice to have when you need it, but I rarely need it enough. If building from scratch, sure put one in. Trying to print with 4 different heads? I can only imagine doing that with something I would need 4 colors for. In that case, trying to get the slicing right would make my head spin.

I am regretting not getting a dual head.  There is a lot you can do with one, but find I do wish for more fairly commonly.  Even changing colors between layers is a hassle and I am always afraid I will bump the head - slipping the stepper.  I wanted three because I want at least one to be a volcano or equivalent with a larger nozzle for speed.  Number four was specifically for a Wades extruder which I am convinced is required for flexible filament.  Maybe it's an interchangeable head.. a multi-bowden *or* a single Wades.  Regardless, I want the board to support 4 extruders so I have the flexibility without buying a new complete machine.  At the same time, boards are cheap so I can start small for now.

I had been assuming (perhaps incorrectly) that all I had to do on my models is identify different volumes as different materials in the drawing and the slicer will automatically assign a different nozzle/color.  Is this not right?  If I have to manually edit the gcode, I agree, that would be limiting.

I looked at Smoothie, but right now I can only find ones with up to 5 stepper controllers.  I need 6 (7 if I am really going to have 4 extruders).

There are a slew of extra general purpose I/O pins available on the smoothie, and you can hook them up to extra external stepper controllers and tell the smoothie config which pins you used. (So the web pages claim - I haven't done it yet, but may need to if I want to go to a dual extruder because I'll need to slave two stepper controllers together to run my dual Z motors, which will leave me only able to run a single extruder).

• controller board:  I looked at Smoothie, but right now I can only find ones with up to 5 stepper controllers.  I need 6 (7 if I am really going to have 4 extruders).  Duet+Duetx4 expansion or AZteeg X3 PRO seems to be the only way without designing my own expansion.

• coreXY:  I was thinking along this lines, too (I see coreXY as a subcategory of Cartesian) but also looking at ballscrew

• heater bed: use AC+SSR.  Good idea. 

• hot end: use Kraken (lots of votes for this).  I hear there are challenges getting all four heads perfectly aligned.  Or is this just the folks trying to sell the Diamond head?  :-)

You might want to look at a smoothieboard. If you have a heavy duty frame which can run fast without flexing, the smoothieboard will be able to deliver gcode fast enough to release the full potential.

Personally, I wouldn't mind having a dual, but cant really ever see myself needing more than that. I rarely have the need for more than one color at a time, so more than a dual would be wasted on me.

Larger build platform/volume is a big one on my wishlist. 8x8x8 just isn't big enough. =P And it MUST be stable!

Second, I like the Rambo boards. Just IMHO, I know you cannot replace the stepper drivers on them, but not having trimpots to deal with is really nice.

Thirdly, I actually would find it hard to justify a dual-extrusion system. It is nice to have when you need it, but I rarely need it enough. If building from scratch, sure put one in. Trying to print with 4 different heads? I can only imagine doing that with something I would need 4 colors for. In that case, trying to get the slicing right would make my head spin. If you need 4 colors, you can print separate pieces and glue them together. For simplicity's sake, check what you really need. Also, if going with 4 heads, I would prefer an integrated 4 head system like the Kracken instead of trying to piece together a 3-head system with an extra head. It will make your life much easier.

Finally, on the issue of simplicity, I am big fan of having a machine that does one thing and does it well. When you start getting machines that do more than one thing you have more problems (think TV/VCR combos), and may not be able to any of them well due to the compromises needed to add the versatility. Why create a removable head for CNC, laser, etc? When a fixed head for 3d print will provide more stability and accuracy. I imagine there will be at least some slop with anything that fits in and is removable later. Not just for 3d printing, but the laser and CNC as well. With all of that being said, having a laser, CNC, and plasma cutter is cool.

[*]quick heating bed (yes, I will need a huge PSU).  Thinking I could experiment with fluids for quick heat up and cool down.[/*]
[*]high-end, expandable controller board (suggestions, anyone?)[/*]
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I have a suggestion for your bed heater (check my signature) I have a lot experience with this.
Controllers seems to be a matter of how much you want to spend.
Rumba is my go to controller
Ramps1.4 is well documented
Azteeg is well regarded
Smoothie is considered the gold standard (to some)