The mechanical endstop is a simple solution to a simple problem. We want to be able to detect when an X/Y/Z stage has reached its minimum or maximum. Instead of messing with flags or complicated light beam interruptors, we use a mechanical switch. If we place the switch in the path of the stage, then the stage itself will simply close the switch when it moves against it. Other than properly positioning the switch, we do not need to modify the stage at all. If you're worried about reliability, you can sleep well at night. The switches we use are rated for 1 million operations before failure. Since we only use the switches once per print, that means you'll be able to do one million prints before having to replace the switch.