Update Rationale.md

docs/machines/desktop_cnc/Rationale.md

@@ -95,3 +95,11 @@ Surface the following:
 - where the carriages will sit (every spots)
 - where the Z axis linear guides will sit.
 
+The aluminum stock for the bed should be a little bigger than the cutting area. For time constraints this first version will have the bed slighly smaller.
+
+Use ring-shaped tools to push the bearings in.
+
+Use 600mm rails for X.
+
+Use 400mm rails for Z.
+