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What causes this? I haven't gotten a single good print tonight.

The infill and the surfaces on this model are not printing to the edges. I can watch it printing and from the very first layer, it simply stops the printing of the surface before it gets to the edge. look at the arm on the left, and you can see the huge gaps in it.

[img]http://i.imgur.com/nwYPjvz.jpg[/img]

I've been troubleshooting a number of issues all night; the closest I've come to a good print is printing a couple of calibration cubes. Those printed ok, but the top surface was really rough, and there was about a 16th of an inch gap on the bottom between the surface and one wall.

Try an extruder calibration.
https://www.matterhackers.com/articles/ ... r-extruder

You do look to be under extruding, but usually gaps like that in none direction are a function of backlash somewhere in the system.
Could be a loose belt, a loose pulley, a damaged belt, or a bad ujoint somewhere.
Can I assume you've never had a good print out of the printer, or did this just start happening?

That was it. Loose belts. I had changed a bunch of stuff on the printer recently (new bed, trick trucks) and apparently I didn't get the belts tensioned properly.

Here's the same part after tightening the idlers:
[img]http://i.imgur.com/SFW5Mlf.jpg[/img]

Good job in getting it diagnosed and working!

That reminds me. I still need to install the new cable tensioners...

Thanks for the reminder on calibrating the extruder as well. I was having some blobbing isssues on small parts. I calibrated the extruder last night and the problem went away completely.