Newbie question about bottom Z limit switch.

Building my first reprap, a mendel 90, and I'm having a little bit of confusion about the z limit switch. Based on my research of all different scattered info,it appears that the z limit used to be at the bottom, but nowadays there does not appear to be one at the bottom, only at the top.

Are there pros/cons to having the switch at the bottom vs the top? Is there a particular reason for the change?

As a newbie, it seems like you'd want one on the bottom, because what if there was some sort of glitch or error (or more likely user error), which tried to drive the extruder down through the bed?

Also, I'm wondering, how exactly does a properly calibrated printer determine where it is on the z axis, and locate itself to the proper distance of 1 paper width above the print bed each time its powered on? Does it need to travel all the way up and hit the limit switch, then travel a predetermined distance back down with perfect accuracy? Thats the only thing I can think of, but it seems like there would be room for error, when dealing with only a paper's thickness. Especially when temperature factors like the expanding and contracting of metal or other materials are at play.

See [hydraraptor.blogspot.co.uk] for the justification for having it at the top.

The extruder cannot be driven through the bed on a Mendel90 because the nuts in the X ends are not fully captive so they can only lift the axis. Gravity is the only downwards force. Once the axis has been homed the soft limits in the firmware prevent the nozzle going below 0.

The Z axis lives at the top between builds. When you start a build it homes and moves down to the bed. At the end of the build it returns to the top. There is no time wasted as the bed is warming up during the decent and cooling during the accent.

Edited 1 time(s). Last edit at 11/04/2014 05:47PM by nophead.

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[www.hydraraptor.blogspot.com]