Most of the frame up to the Y sleds is done

assembly.scad

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+use_colors = true;
+
+xsize = 500;
+ysize = 400;
+sled_size = 6 * 25.4;
+
+cur_xpos = 100;
+cur_ypos = 200;
+
+include <parts.scad>;
+
+
+csize = 15;
+
+module frame() {
+  translate([corner_offset, 0, 0])
+    rotate([0, 90, 0])
+    tslot(xsize - 2 * corner_offset);
+
+  translate([0, corner_offset, 0])
+    rotate([-90, 0, 0])
+    tslot(ysize - 2*corner_offset);
+
+  translate([corner_offset, ysize, 0])
+    rotate([0, 90, 0])
+    tslot(xsize - 2*corner_offset);
+
+  translate([xsize, corner_offset, 0])
+    rotate([-90, 0, 0])
+    tslot(ysize - 2*corner_offset);
+}
+module corners() {
+  render() {
+    translate([0,0]) flcorner();
+    translate([xsize, 0, 0]) frcorner();
+    translate([xsize, ysize, 0]) brcorner();
+    translate([0, ysize, 0]) blcorner();
+  }
+}
+
+module rails() {
+  frame_offset = frame_size / 2 + corner_size / 2;
+  colorize(stock_color)
+  {
+    for (xpos = [frame_offset, xsize - frame_offset]) {
+      translate([xpos,
+		 frame_size / 2 + wall_thickness])
+	rotate([-90, 0, 0])
+	cylinder(d = rail_diam, h = ysize - corner_offset * 2);
+    }
+  }
+}
+
+module ysled_rails() {
+  frame_offset = frame_size + corner_size / 2;
+  yoff = (sled_size + ysled_extralen * 2 - frame_size) / 2;
+  colorize(stock_color)
+  for (ypos = [-yoff, yoff])
+    translate([frame_size / 2, ypos, 0])
+      rotate([0, 90, 0])
+      cylinder(d = rail_diam, h = xsize - frame_size * 2 - corner_size);
+}
+
+
+
+union() {
+  frame();
+  rails();
+  corners();
+}
+
+translate([frame_size / 2 + corner_size / 2, cur_ypos, 0]) {
+  ysled(associated_parts = true);
+  translate([xsize - frame_size - corner_size, 0, 0])
+    rotate([0, 180, 0]) 
+    ysled(associated_parts = true);
+  ysled_rails();
+}
+
+  //translate([0, 0, -1000]) cube(2000, center=true);
+
+

parts.scad

@@ -0,0 +1,139 @@
+include <units.inc>;
+include <stock_parts.scad>;
+
+frame_size = 20;
+wall_thickness = 2;
+corner_size=30;
+// The length along each edge that the corner bracket takes.  I.e., if
+// you construct a frame of four corner brackets and 4 tubes of length
+// $L-$corner_size, the distance between opposite tubes' centerlines
+// will be $L.
+corner_offset = frame_size / 2 + wall_thickness;
+rail_diam = 8;
+
+ysled_extralen = frame_size; // on each end
+
+
+
+// Generates bar of length $l, centered on origin extending in +Z
+module tslot(l) {
+  colorize(stock_color) {
+    translate([-frame_size/2, -frame_size/2, 0])
+      cube([frame_size, frame_size, l]);
+  }
+}
+
+// A corner, with tubes extending in +x and +y.
+module corner_base() {
+  odim = frame_size/2 + wall_thickness;
+  ldim = frame_size/2 + corner_size;
+  cap_points = [[-odim, -odim],
+		[-odim,  ldim],
+		[ odim,  ldim],
+		[ ldim,  odim],
+		[ ldim, -odim]];
+  difference() {
+    translate([0, 0, -odim])
+      linear_extrude(odim * 2)
+      polygon(points = cap_points);
+    
+    // two bars...
+    translate([-frame_size/2, corner_offset, -frame_size/2])
+      cube([frame_size, ldim, frame_size]);
+    translate([corner_offset, -frame_size/2, -frame_size/2])
+      cube([ldim, frame_size, frame_size]);
+    translate([corner_offset, corner_offset, -frame_size/2])
+      cube([ldim, ldim, frame_size]);
+    //translate([-odim - 1, -odim - 1, 0])
+    //cube(ldim*2);
+  }
+}
+
+module rail_holder(in_x) {
+  odim = frame_size/2 + wall_thickness;
+  ldim = frame_size/2 + corner_size;
+  tube_offset = (corner_size) / 2 + wall_thickness;
+  tube_rotation = in_x ? 90 : 0;
+  translate([frame_size/2, frame_size/2, -frame_size/2]) {
+    difference() {
+      linear_extrude(frame_size)
+	polygon(points = [[wall_thickness, wall_thickness],
+			  [wall_thickness, corner_size],
+			  [corner_size, wall_thickness]]);
+      translate([corner_size / 2, corner_size/2, frame_size/2])
+	rotate([tube_rotation -90, tube_rotation, 0])
+	translate([0, 0, -corner_size])
+	cylinder(d = rail_diam, corner_size * 2);
+		
+    }
+  }
+}
+
+// centered on 
+module ysled(spread = 6*inch, associated_parts = false) {
+  sled_len = spread + ysled_extralen * 2;
+  sled_height = frame_size;
+  
+  colorize("green")
+  render()
+  translate([0, -sled_len/2, 0])
+  rotate([-90, 0, 0]) {
+    difference() {
+      union() {
+	translate([-frame_size/2, -frame_size/2, 0])
+	  cube([frame_size + 5, frame_size, sled_len]);
+      }
+      
+      cylinder(d = bushing_id * 1.1, h = sled_len);
+      
+      translate([0,0,-1])
+	cylinder(d = bushing_od, h = bushing_len+1);
+      translate([0,0,sled_len - bushing_len])
+	cylinder(d = bushing_od, h = bushing_len+1);
+
+      for (zpos = [frame_size / 2, sled_len - frame_size / 2]) {
+	translate([sled_height/2, 0, zpos]) 
+	rotate([0,90,0]) {
+	  cylinder(h = sled_height + 1, d = rail_diam);
+	}
+      }
+    }
+  }
+  // TODO: add posts for the idlers
+  
+  translate([0, -sled_len/2, 0])
+  rotate([-90, 0, 0])
+  if (associated_parts) {
+    bushing();
+    translate([0,0,sled_len - bushing_len]) bushing();
+  }
+}
+
+module flcorner() {
+  translate([0, 0, 0])
+  rotate([0, 0, 0]) {
+    corner_base();
+    rail_holder(false);
+  }
+}
+
+module frcorner() {
+  rotate([0, 0, 90]) {
+    corner_base();
+    rail_holder(true);
+  }
+}
+
+module blcorner() {
+  rotate([0, 0, 270]) {
+    corner_base();
+    rail_holder(true);
+  }
+}
+
+module brcorner() {
+  rotate([0, 0, 180]) {
+    corner_base();
+    rail_holder(false);
+  }
+}

stock_parts.scad

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+stock_color = "lightgray";
+
+bushing_od = 12;
+bushing_id = 8;
+bushing_len = 8;
+
+module bushing() {
+  colorize(stock_color) {
+    difference() {
+      cylinder(d = bushing_od, h = bushing_len);
+      translate([0,0,-1])
+	cylinder(d = bushing_id, h = bushing_len + 2);
+    }
+  }
+}
+
+module colorize(colorname) {
+  if (use_colors) {
+    color(colorname) union() { children(); }
+  } else {
+    children();
+  }
+}

units.inc

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+inch = 25.4;
+cm = 10;