Squircular circles and hot X stepper?

The belt tensioning was just trial and error.  Just when I thought it was as good as it can get, I would change it again and get different results.  I got the best results by putting pretty tight tension on the long belts, and not too much tension on the short belt (from the motor to the rear shaft).  Also, I noticed that the pulleys in the front of the machine are mounted on screws instead of a shaft, and the tension would bend the screw mounts to the point that the screws were no longer parallel to the rear shaft.  This would cause the pulleys to bump up against one side of the screw mount.  I was able to jam some washers between the case and the screw mount to force it parallel again and I think this helped a lot.

For the hot x motor, I printed this out, http://www.thingiverse.com/thing:32994, which worked great.  You need to buy a 40mm fan to use with it though (and possibly a 12v power supply if you can't hook it into the motherboard supply).  My x motor temp went from 120C down to 60C.

sounds like maybe the belt is too tight and the clunking sound is cogs of belt riding up the side of the toothed pulley... if this is the problem the belt will eventually shred itself.

Many people overtighten their belts thinking it will cure problems not related to the belt tension, if the belts are bending plastic or metal parts then they are way too tight... the belts should only be tight enough that there is no droop in the free span length for the y axis belts, another thing to remember is the tighter you make the belt the more it will be stretched and this will cause backlash because you are stretching the spaces between the toothed parts of the belt and it will not match the spacing on the toothed pulleys.

Right, I loosened up the belts significantly, and applied some lithium grease at the insistence of a youtube comment.
Seems to have gotten rid of the lovely grinding sounds.

Seems to be printing rather well at 1mm, even though my calibration isn't that great.

Thanks for the help guys

This is a crowd funding thing that I'm running: http://www.gofundme.com/bvi140 It's for pretty selfish reasons