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Kyle R
If I built a complete Ultimaker 2 (XY gantry, Z stage) is it possible for me to control it with a RAMPS 1.4 board instead of the Mainboard 2.1.1 from Ultimaker? Is it possible to control with Ultimaker 2 with the Ultimaker Original Arduino shield? I'm assuming the answer to both of these questions is yes, so how would the performance be affected?
Your assumption is right, stepper motor is stepper motor and as such you can use any controller which is capable of driving stepper motors. Same for extruder, heaters, thermistors. Performance is mostly affected by the type of CPU on that controller (ATmega vs. ARM, clock frequency) and the firmware in use. Typically, you can run all firmwares written for ATmegas on all ATmega based boards, it's just a matter of adjusting the pin layout.