diff --git a/printed/butt_box.scad b/printed/butt_box.scad
index 1dc79cc..b2fb650 100644
--- a/printed/butt_box.scad
+++ b/printed/butt_box.scad
@@ -110,12 +110,9 @@ function fixing_block_positions(type) = let(
 
 function side_holes(type) = [for(p = fixing_block_positions(type), q = fixing_block_holes(bbox_screw(type))) p * q];
 
-module drill_holes(type, t)
-    for(list = [corner_holes(type), side_holes(type)], p = list)
-        let(q = t * p)
-            if(abs(transform([0, 0, 0], q).z) < eps)
-                multmatrix(q)
-                    drill(screw_clearance_radius(bbox_screw(type)), 0);
+module bbox_drill_holes(type, t)
+    position_children(concat(corner_holes(type), side_holes(type)), t)
+        drill(screw_clearance_radius(bbox_screw(type)), 0);
 
 module bbox_base_blank(type) { //! 2D template for the base
     dxf(str(bbox_name(type), "_base"));
@@ -123,7 +120,7 @@ module bbox_base_blank(type) { //! 2D template for the base
     difference() {
         sheet_2D(bbox_base_sheet(type), bbox_width(type), bbox_depth(type), 1);
 
-        drill_holes(type, translate(bbox_height(type) / 2));
+        bbox_drill_holes(type, translate(bbox_height(type) / 2));
     }
 }
 
@@ -136,7 +133,7 @@ module bbox_top_blank(type) { //! 2D template for the top
         translate([0, t / 2])
             sheet_2D(bbox_top_sheet(type), bbox_width(type) + 2 * t, bbox_depth(type) + t);
 
-        drill_holes(type, translate(-bbox_height(type) / 2));
+        bbox_drill_holes(type, translate(-bbox_height(type) / 2));
     }
 }
 
@@ -154,7 +151,7 @@ module bbox_left_blank(type, sheet = false) { //! 2D template for the left side
         translate([-t / 2, -bb / 2])
             sheet_2D(subst_sheet(type, sheet), bbox_depth(type) + t, bbox_height(type) + bb);
 
-        drill_holes(type, rotate([0, 90, 90]) * translate([bbox_width(type) / 2, 0]));
+        bbox_drill_holes(type, rotate([0, 90, 90]) * translate([bbox_width(type) / 2, 0]));
     }
 }
 
@@ -168,7 +165,7 @@ module bbox_right_blank(type, sheet = false) { //! 2D template for the right sid
         translate([t / 2, -bb / 2])
             sheet_2D(subst_sheet(type, sheet), bbox_depth(type) + t, bbox_height(type) + bb);
 
-        drill_holes(type, rotate([0, 90, 90]) * translate([-bbox_width(type) / 2, 0]));
+        bbox_drill_holes(type, rotate([0, 90, 90]) * translate([-bbox_width(type) / 2, 0]));
     }
 }
 
@@ -183,7 +180,7 @@ module bbox_front_blank(type, sheet = false, width = 0) { //! 2D template for th
         translate([0, (bt - bb) / 2])
             sheet_2D(subst_sheet(type, sheet), max(bbox_width(type) + 2 * t, width), bbox_height(type) + bb + bt);
 
-        drill_holes(type, rotate([-90, 0, 0]) * translate([0, bbox_depth(type) / 2]));
+        bbox_drill_holes(type, rotate([-90, 0, 0]) * translate([0, bbox_depth(type) / 2]));
     }
 }
 
@@ -197,7 +194,7 @@ module bbox_back_blank(type, sheet = false) { //! 2D template for the back
         translate([0, -bb / 2])
             sheet_2D(subst_sheet(type, sheet), bbox_width(type), bbox_height(type) + bb);
 
-        drill_holes(type, rotate([-90, 0, 0]) * translate([0, -bbox_depth(type) / 2]));
+        bbox_drill_holes(type, rotate([-90, 0, 0]) * translate([0, -bbox_depth(type) / 2]));
     }
 }
 
diff --git a/readme.md b/readme.md
index f89ec4c..e3c18dd 100644
--- a/readme.md
+++ b/readme.md
@@ -5231,6 +5231,11 @@ Maths utilities for manipulating vectors and matrices.
 | ```vec3(v)``` | Return a 3 vector with the first three elements of ```v``` |
 | ```vec4(v)``` | Return a 4 vector with the first three elements of ```v``` |
 
+### Modules
+| Module | Description |
+|:--- |:--- |
+| ```position_children(list, t)``` | Position children if they are on the Z = 0 plane when transformed by t |
+
 ![maths](tests/png/maths.png)
 
 
diff --git a/utils/maths.scad b/utils/maths.scad
index e83ac73..e8db429 100644
--- a/utils/maths.scad
+++ b/utils/maths.scad
@@ -90,6 +90,14 @@ function euler(R) = let(ay = asin(-R[2][0]), cy = cos(ay)) //! Convert a rotatio
     cy ? [ atan2(R[2][1] / cy, R[2][2] / cy), ay, atan2(R[1][0] / cy, R[0][0] / cy) ]
        : R[2][0] < 0 ? [atan2( R[0][1],  R[0][2]),  180, 0]
                      : [atan2(-R[0][1], -R[0][2]), -180, 0];
+
+module position_children(list, t) //! Position children if they are on the Z = 0 plane when transformed by t
+    for(p = list)
+        let(q = t * p)
+            if(abs(transform([0, 0, 0], q).z) < 0.01)
+                multmatrix(q)
+                    children();
+
 // Matrix inversion: https://www.mathsisfun.com/algebra/matrix-inverse-row-operations-gauss-jordan.html
 
 function augment(m) = let(l = len(m), n = identity(l)) [ //! Augment a matrix by adding an identity matrix to the right