There's some great design overall - I was really impressed how easy it was to get the tolerances tight on everything - but here is the list of design improvements I hope you guys consider:
- include two M5 washers for the Y axis mount screws. You said recently on the Frame construction page that Y-axis M5 8mm bolts had been delivered too long in some cases but I think you'll find that the bolts you did your original design with were delivered too short (unless my Misumi pieces are out of spec). The Mitsumi channel is 6mm deep and the lip of the Mitsumi channel on the other side is less than 2mm (about 1.9mm on mine) - so a standard M5 8mm screw is going to bottom out (the main problem however is if the bolt isn't held in the center of the channel and so the screw is at an angle - the nut slips down into the channel by a small amount). Cheap and simple solution: include two M5 washers, change build instructions to say to use them.
- even with the two washers, the Y axis joint just has no strength in a rotational direction. This would be easily solved with two small rectangular printed blocks which screw into the Y axis channels to keep the Y-axis perpendicular against the X-axis (one on each side). These can be quite narrow and so should be quite fast to print.
- increase the thickness of the bearing holder outer feet or redesign the location of the holes in the X & Y carriage pieces so that one foot doesn't hang off the edge of them.
- supply the Duo with two wider diameter split tubes rather than three thinner ones. The cables just don't fit well in the ones provided- most frustrating; other people have reported this as well. I found that tubing with 11-12mm ID worked well.
- longer extruder thermistor cables for the Duo. The currently included ones have trouble reaching.
- be a little more generous on the heated printer bed power cable (same length as thermistor cable) so that you can lay the cable better. If you cable tie the cable to the Y-motor mount piece (but on the other side of the Y-axis endstop), then this the cable feed very nicely backwards and forwards without catching on anything. From there it is nice to have the length to run the cable under Y-axis, and a long the bottom of the frame so that it is out of sight. I had to use the longer 16AWG 12VDC power supply cable piece to do this. Including two M2.5 x 16mm screws + two cable ties for this purpose would also help (I'll upload the pics soon - its very neat).
- make sure the 30mm screw supplied for the Z axis stop is threaded the entire length otherwise you can't fit the anti-rotation nut.
- include a minimal power-supply terminal cover. I mean come on guys - take responsibility with electrical safety. You should be able to print a very simple one based on http://www.thingiverse.com/thing:11821
except add two holes in the cover such that when you remove two of the low voltage screw terminals you can then firmly hold the cover in place by screwing two M4 screws into those terminal holes. The thing should should be very easy to print. It's irrelevant that some people might print a better cover themselves - its just not a proper product without providing a proper out-of-the-box solution here.
- the mains cable should be pre-crimped for people as well (as most people won't have a proper crimping tool and you don't want to have the case where it comes loose because of bodged up crimping). I think you can buy them like this.
- include a Bukobot sticker which people can attach to their frame or Y axis.
- include filament feed tube as standard or as an optional upgrade when you buy the printer (e.g., a "plus" kit which could include things like a spool holder, painters tape, tweezers, palette knife, small tube of bearing lubricant - basically everything you need to get started)