One of the great features of the Printrbot Metal Simple is its automatic bed leveling sensor. When properly calibrated it greatly reduces bed leveling hassles. The sensor works by measuring the bed height at three points and then calculating z height offsets for every location on the bed. The sensor uses induction to sense the location of the bed without touching it. This works very well in the printer’s stock configuration, but if you want to print on a glass build plate it calls for some trickery.
Everyone has their own favorite build surface, but for my money you can’t beat glass. Glass is smoother and flatter than any other material you can easily find, and combined with a bonding agent such as Aquanet hairspray or PVA glue (either glue stick or white glue work) it provides an ideal combination of stick, while you are printing, and release once you are done. Aquanet has enough stick for most PLA and ABS prints, while PVA can give you a stronger stick useful for printing troublesome materials like nylon.
The problem is that the sensor used in the Metal Simple doesn’t have enough sensitivity to pick up the height of the bed when there is a sheet of glass on top of it. The inductive sensor can’t “see” the glass, but with the glass in the way it can’t get close enough to the bed to see it. You could replace the sensor with a longer range unit, but the range of error of the sensor is a percentage of its total range, so longer range means less accurate.
The simplest solution I came up with was to increase the inductance of the bed. The Printrbot Metal Simple is beautifully made of very precisely fabricated aluminum, so I didn’t want to permanently modify it. I tested a bunch of different materials and discovered the best one was 24ga. galvanized steel. I sheared a piece big enough to fit under the build plate. The steel has enough inductance that the sensor can see it from a distance greater than the thickness of the added glass build plate. You will need to adjust the z height offset in the firmware the same way you probably adjusted it when you first got your machine. Once you get it dialed in it is very reliable.
Andy came up with a great trick using crimp-on electrical terminals as hold downs for the build plate. Use slightly longer screws and a couple of washers to shim them up and you can hold your glass build plate in place without making any permanent modifications to your printer. Just cut to steel sheet and the glass build plate to fit between the bed mounting screws. With this solution you don’t lose any of your build area.