UPDATE: I've since moved on to a design with tighter tolerances in the quest to get reliable operation with smaller extrusion diameters. This thing here worked well down to 0.35 mm for me though, and requires less print bed modification than the new one. However, if you want more precision (i.e. a stiffer adjustable heated bed), try this one: http://www.thingiverse.com/thing:43022
In the photos you'll see a couple of things that aren't strictly part of this mod, but are nice to have.
First, I have two 8" x 8" panes of glass from Lowe's secured to the heater PCB using 8 small binder slips with the handles removed.
Second, I have added some aluminum ribbing to the plywood bed to prevent it from bowing along the x-axis direction. The placement of the ribs caused me to have to carve the mounts to make room for the screws and nuts securing the aluminum ribbing, but you shouldn't have to worry about that.
The mounts replace the y-axis rod mounts that came with the Printrbot. First slide the y-axis rods through the mounts and secure them using a 6-32 1" screw. There is a hidden nut snugged up to the head of the screw to prevent the screw from turning. There is a washer and a nut securing the screw from the bottom. As designed, the y-axis rods were a little loose in the mounts. You should be able to fix that by filing the mounts in the gap that is closed by tightening the screw, but I also added a little bit of scrap plastic in the hole with the rod. I didn't adjust the depth of the rod in the back mounts, but you should be able to adjust the rod so it's at least flush with the front and back mounts now.
The front mounts are secured to the plywood bed using inverted 6-32 3/4" screws with the nuts on the top side of the bed.
The front of the heater PCB is secured using a 6-32 3" screw inserted through the mount through the inserted thumbscrew that encloses a hidden nut. The stack up on the front spring assembly is as follows: 1) screw head, 2) heater PCB, 3) washer, 4) compression spring*, 5) washer with shaved edge so it would lay flat nest to the nut holding the mount to the plywood bed.
- I actually used two nested compression springs here with two different diameters.
The back of the heater PCB is secured using a 6-32 1-1/2" screw. The stack up on the back spring assembly is as follows: 1) screw head, 2) nut to reduce the amount of screw protruding out the bottom, 3) heater PCB, 4) washer, 5) compression springs, 6) unmodified washer.
The back thumbscrew is different than the front. The part is designed to have the nut dropped into it as the part is being printed to effectively over mold the nut into the part. This makes it a little easier to use without dropping the nut out if you over-loosen the thumbscrew.
I leveled the carriage to the plywood bed using a business card. I then leveled the glass to the extruder nozzle using a piece of paper moving around the heated PCB repeatedly until the nozzle just drags on the paper in all four corners using the thumbscrews.