One of the defining characteristics of the 2013/early 2014 versions of the Printrbot Simple was the use of Kevlar fishing line for the motion transfer on the X/Y axis. A rubber hose gets superglued to the stepper shaft, a Dremel sanding wheel gets glued to that, and the fishing line gets several wraps around it. It kept cost down (the original was just shy of $300 in kit form) and it works surprisingly well. Mine has held up for quite a lot of printing over the 13 months I have had it running.
However, it did have a couple of disadvantages. It required tightening every now and then. It can result in a loss of precision, because the fishing line can walk back and forth on the drum. And frankly - it's just not very dignified looking. Here is a view of the X axis drive with the bed removed.
Thanks to the work of Thingiverse contributor iamjonlawrence there is a printable conversion to GT2 belts for both the X and Y axis. Newer Simple models come with belts, though they cost more than the original Simple kit did.
Jon is a mechanical engineer, and it shows in his hobby work. He has released a number of upgrades for various versions of the Printrbot Simple, and it is accompanied by professional drawings and detailed bills of material. The parts in these kits were very well thought out and fit perfectly, I highly recommend his work.
I printed two sets of the parts - I was concerned that I would have my printer torn apart, and if I messed up a part I would not be able to print replacements. This turned out to not be necessary, but I still think it is worth doing.
I made sure my printer was calibrated well before printing the parts. The tolerances are snug, but if your printer is printing accurately it will fit with only minor brushes with a file to remove burrs or other loose material from the print.
In addition to the McMaster Carr part numbers called out on Jon's BOM, I used the following components from Amazon:
Belts and pulleys (there is plenty of belt for this conversion - I had enough left over to replace one of the belts if I ever need to)
Note that FLDM printers tend to print holes and slots slightly small. I calibrated mine to accurately print outside dimensions, and just drill my holes to the right size. With the hardware specified on the BOM, a 7/64 bit will make the hole sized nicely for the screw to thread into. A 1/8" bit will allow the screw to pass through smoothly.
You'll need to recalibrate the X and Y axis since the pulleys are slightly larger than the original drums.
Also, have extra zip ties handy, you'll need them to put things back together.
Procedure - Y Axis
First, I clipped the zip ties holding the Y axis carriage to the motion rods. I then removed the stepper.
Next, I installed the new motor plate and bearings, and aligned the pulley.
I fed the belt in and checked motion, and secured one end of the belt to the stop.
The stop gets belted on.
The second stop gets attached.
Securing the belt to the tension block is a little tricky. I had to remove material from the slot that the belt passes through to let it pass through twice. The drawing shows clearly that the belt should just fit through the slot when folded back on itself. A short length of filament acts as a pin to hold the filament in place. There are detailed shots of this in the X axis section.
Tension is adjusted by turning the screws in the tension block. At this point, I connected to the printer and tested the motion. All looked good, so I moved on to the X axis.
Procedure - X Axis
The X axis is more involved because you have to install a replacement motor mount plate for the X axis stepper. This is not a trivial process, but it went pretty smoothly.
First, the bed is removed and the X carriage is removed by clipping the zip ties holding it in place.
The side opposite the control board is removed.
The bottom plate can now be pulled free and the X axis motor mount is removed.
The new bearing plate is assembled.
Carefully align the pulley with the bearings. They are a snug fit, but they do fit, and don't allow any slop when assembled.
Install the new motor plate and reassemble.
The new belt ends are held in place with the zip ties securing the carriage to the motion rods. As the drawings call out, the carriage is flipped over and a new hole drilled for the X axis end stop screw.
Here is a detail shot of how the belt tension blocks work. I had to open the slots a bit with an exacto knife, just enough to pass the belt when it is folded back on itself.
Test it! I had to remove a little material from the carriage to get it to run smoothly, just rounding over an edge.
I had to modify my back clips that hold my heated bed on. I just bent and cut office clips into a z-shape. Details on the heated bed installation is here.
Initial test prints look really good. I am in the processing of recalibrating the X and Y motion in the Printrboard, since the pulleys are slightly larger than the original sanding drums. I will post the final values once they are determined.
Update: X and Y values for M92 are both just a hair above 80. I have mine printing to within .001" on a 2.000 inch square test model.