rick's Prusa Mendel Y Carriage

Update:
I made a better one. Please see Rick's [Prusa Mendel Y Carriage 2017](http://www.thingiverse.com/thing:2186790).

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Old:
Stable Y-carriage that can print on a 185mm x 185mm square without using much material.

I used to have 4 LM8UU bearings for my y carriage and 4 springs holding the heat bed but some clever forum posts explained to me that 3 points constra