Refactored corners to position motor based on motor_shaft_xoffset; changed rail holder to use a rectangular block, added slots for belt tension adjustment

assembly.scad

@@ -57,7 +57,7 @@ module moving_parts(xpos = cur_xpos, ypos = cur_ypos) {
   translate([ yrail_offset, cur_ypos, 0 ]) ysled_slider(spread = sled_ysize);
 
   translate([ xsize - yrail_offset, cur_ypos, 0 ]) rotate([ 180, 180, 0 ])
-      ysled_slider(spread = sled_ysize);
+      ysled_slider(spread = sled_ysize, is_left = false);
 
   translate([ yrail_offset, cur_ypos, 0 ]) if ($preview) ysled_rails();
 
@@ -75,3 +75,9 @@ union() {
   corners();
   moving_parts();
 }
+
+*intersection() {
+  ysled_slider(spread=sled_ysize);
+  translate([0, -1000, -1000])
+  cube([2000, 2000, 2000]);
+}