add mid-part from z-holder

helpers/Holes.scad

@@ -1,9 +1,13 @@
+//https://de.wikipedia.org/wiki/Durchgangsbohrung
+
 M3HoleDiam = 3.4;
 M4HoleDiam = 4.4;
 
 M3NutDiam=6.01+.4;
 
-M3HeadDiam = 5.8;
+_printoffset=.3;
+M3HeadDiam = 5.5+_printoffset;
+M4HeadDiam = 7+_printoffset;
 
 module M3Hole(thickness) {
     rotate([0, 90, 0]) cylinder(h = thickness, d = M3HoleDiam);
@@ -19,4 +23,8 @@ module M3Nut(thickness) {
 
 module M3Head(thickness) {
     rotate([0, 90, 0]) cylinder(h = thickness, d = M3HeadDiam);
+}
+
+module M4Head(thickness) {
+    rotate([0, 90, 0]) cylinder(h = thickness, d = M4HeadDiam);
 }