**UPDATE** Printrbot has created a kit to add a heat bed to your Simple Metal Here: http://printrbot.com/shop/simple-metal-heated-bed/  you can also buy an aftermarked adhesive one at http://3dpaftermarket.bigcartel.com/product/printrbot-metal-bundleThe stock Printrbot Metal Simple is a fantastic little printer with an exceptional degree of accuracy for its price, I really wouldn't change a thing about it. Recently I had to print a few large flat designs and wanted to try attaching a heat bed to get them perfectly flat. The ideology of mounting the heat bed on a raised platform design posted here is that you are able to insulate the moving parts from heat, use less power and keep a more constant temp - I have no idea how this mod stacks up against other designs. Your Printrbot Simple Metal can print PLA super flat, and experiment other filaments like ABS and Nylon. If you plan to print with high temperature filaments, I recommend printing these parts in ABS for long term durability. Instructions:Print four of these brackets and attach the 6X6 aluminum print bed heatbed and thermistor from http://printrbot.com/ See BOM for details and instructions.Assemble as shown in photo, mounting holes in STL file are tight but allow for better self tapping into the plastic, you can also insert a nut into the captured space. Using the aluminum heat bed from printrbot you you won't need to make any changes to your z sensor (I would use caution the first time to make sure you didn't move anything while installing this mod - go through the M212 calibration process).  Once installed you can easily add or remove the bed without making calibration adjustments. Remember to do all of your z height calibration with the heat bed at temperature.I have included a PDF file that shows you where to plug in the heat bed, thermistor cable, and power supply.Finally, you will want to send the following code to your machine M211 X150 Y150 Z140 to compensate for the slight decrease in Z travel because of the heat bed thickness then M500 to save and M501 to check your work. - WARNING - You must replace the laptop style power source and add your own 12 volt power source with enough amps to run the heated print surface. An ATX power supply that you would find in an old PC tower is a really good option. BOM: -2 each of the provided stl files -4 small binder clips 20mm wide -4 3mm bolts ~10mm long and nuts -6X6 printrbot heat bed PCB from http://printrbot.com/shop/6-6-heated-bed/ or http://3dpaftermarket.bigcartel.com/product/printrbot-metal-bundle (this one includes thermistor) -printrbot aluminum print bed 6x6 from http://printrbot.com/shop/printrbot-aluminum-6x6-build-platform/ -thermistor with cable http://printrbot.com/shop/thermistor-with-cable/ -ATX power supply or other 12 volt source capable of running the higher draw of a heat bed. -"Y" adapter for the ATX power supply from printrbot http://printrbot.com/shop/y-power-adapter/ If you are handy you can make the Y adapter part with the existing connectors that comes with the ATX power supply and jump the green wire so the box will power up http://help.printrbot.com/Guide/Power+Supply+Cabling/16 This video shows you pretty clearly what the connection looks like and how to connect adapter https://www.youtube.com/watch?v=bqF0m7xMMms -Optional: You may want a box for your ATX power supply, there are lots around. This one is a spool holder too http://printrbot.com/shop/power-tower-with-spool-rack/ -Some cable wrap to keep the heat bed and thermistor wires neat and tidy.-Finally you may want to insulate the bottom of the heat bed.  I used some metalized bubble wrap designed for insulating duct work, cork could be a good option too, it makes the heat bed much more efficient.Instructions: Assemble as shown in photo, mounting holes in STL file are tight but allow for better self tapping into the plastic, you can also put a nut in the captured nut space. Using the aluminum heat bed from printrbot you you won't need to make any changes to your z sensor (I would use caution the first time to make sure you didn't move anything while installing this mod - go through the M212 calibration process). Once installed you can easily add or remove the bed without making calibration adjustments. Remember to do all of your z height calibration with the heat bed at temperature. I have included a PDF file that shows you where to plug in the heat bed, thermistor cable, and power supply. I used a pencil to lightly mark all four corners of the mounting brackets so I can easily install and center my heat bed so the Z sensor reads the bed perfectly.  The last spot where it reads the bed is close to the edge and you don't want the sensor to read off the bed.Finally, you will want to send the following code to your machine M211 X150 Y150 Z140 to compensate for the slight decrease in Z travel because of the heat bed thickness then M500 to save and M501 to check your work.