diff --git a/screw_drive.scad b/screw_drive.scad
index 40c1a90..179d46c 100644
--- a/screw_drive.scad
+++ b/screw_drive.scad
@@ -330,10 +330,14 @@ module torx_mask(size, l=5, center, anchor, spin=0, orient=UP) {
 //   robertson_mask(size, [extra]);
 // Description:
 //   Creates a mask for creating a Robertson/Square drive recess given the drive size as an integer.
-//   The width of the recess will be oversized by `2 * $slop`.  
+//   The width of the recess will be oversized by `2 * $slop`.  Note that this model is based
+//   on an incomplete spec.   https://www.aspenfasteners.com/content/pdf/square_drive_specification.pdf
+//   We determined the angle by doing print tests on a Prusa MK3S with $slop set to 0.05.
 // Arguments:
 //   size = The size of the square drive, as an integer from 0 to 4.
 //   extra = Extra length of drive mask to create.
+//   ang = taper angle of each face.  Default: 2.5
+//   $slop = enlarge recess by this twice amount.  Default: 0
 // Example:
 //   robertson_mask(size=2);
 // Example:
@@ -341,7 +345,7 @@ module torx_mask(size, l=5, center, anchor, spin=0, orient=UP) {
 //       cyl(d1=2, d2=8, h=4, anchor=TOP);
 //       robertson_mask(size=2);
 //   }
-module robertson_mask(size, extra=1) {
+module robertson_mask(size, extra=1, ang=2.5) {
     assert(is_int(size) && size>=0 && size<=4);
     Mmin = [0.0696, 0.0900, 0.1110, 0.1315, 0.1895][size];
     Mmax = [0.0710, 0.0910, 0.1126, 0.1330, 0.1910][size];
@@ -352,7 +356,6 @@ module robertson_mask(size, extra=1) {
     Fmin = [0.032, 0.057, 0.065, 0.085, 0.090][size];
     Fmax = [0.038, 0.065, 0.075, 0.095, 0.100][size];
     F = (Fmin + Fmax) / 2 * INCH;
-    ang = 4;
     h = T + extra;
     Mslop=M+2*$slop;
     down(T) {
diff --git a/shapes3d.scad b/shapes3d.scad
index 16240cc..ba5bf64 100644
--- a/shapes3d.scad
+++ b/shapes3d.scad
@@ -1696,7 +1696,7 @@ function spheroid(r, style="aligned", d, circum=false, anchor=CENTER, spin=0, or
         rr = circum? (r / cos(90/vsides) / cos(180/hsides)) : r,
         stagger = style=="stagger"
      )
-     style=="icosa2" ?    // subdivide faces of an icosahedron and project them onto a sphere
+     style=="icosa" ?    // subdivide faces of an icosahedron and project them onto a sphere
          let(  
              N = icosa_steps-1,
              // construct an icosahedron
@@ -1744,32 +1744,6 @@ function spheroid(r, style="aligned", d, circum=false, anchor=CENTER, spin=0, or
         ) [
             for (i=idx(meridians), j=[0:1:meridians[i]-1])
             spherical_to_xyz(rr, j*360/meridians[i], i*180/(len(meridians)-1))
-        ] : style=="icosa"? [
-            for (tb=[0,1], j=[0,2], i = [0:1:4]) let(
-                theta0 = i*360/5,
-                theta1 = (i-0.5)*360/5,
-                theta2 = (i+0.5)*360/5,
-                phi0 = 180/3 * j,
-                phi1 = 180/3,
-                v0 = spherical_to_xyz(1,theta0,phi0),
-                v1 = spherical_to_xyz(1,theta1,phi1),
-                v2 = spherical_to_xyz(1,theta2,phi1),
-                ax0 = vector_axis(v0, v1),
-                ang0 = vector_angle(v0, v1),
-                ax1 = vector_axis(v0, v2),
-                ang1 = vector_angle(v0, v2)
-            )
-            for (k = [0:1:icosa_steps]) let(
-                u = k/icosa_steps,
-                vv0 = rot(ang0*u, ax0, p=v0),
-                vv1 = rot(ang1*u, ax1, p=v0),
-                ax2 = vector_axis(vv0, vv1),
-                ang2 = vector_angle(vv0, vv1)
-            )
-            for (l = [0:1:k]) let(
-                v = k? l/k : 0,
-                pt = rot(ang2*v, v=ax2, p=vv0) * rr * (tb? -1 : 1)
-            ) pt
         ] : assert(in_list(style,["orig","aligned","stagger","octa","icosa"])),
         lv = len(verts),
         faces = style=="orig"? [
@@ -1830,25 +1804,6 @@ function spheroid(r, style="aligned", d, circum=false, anchor=CENTER, spin=0, or
                 [p5, p7, p8],
                 if (k<m-1) [p5, p8, p6],
             ],
-        ] : style=="icosa"? let(
-            pyr = [for (x=[0:1:icosa_steps+1]) x],
-            tri = sum(pyr),
-            soff = cumsum(pyr)
-        ) [
-            for (tb=[0,1], j=[0,1], i = [0:1:4]) let(
-                base = ((((tb*2) + j) * 5) + i) * tri
-            )
-            for (k = [0:1:icosa_steps-1])
-            for (l = [0:1:k]) let(
-                v1 = base + soff[k] + l,
-                v2 = base + soff[k+1] + l,
-                v3 = base + soff[k+1] + (l + 1),
-                faces = [
-                    if(l>0) [v1-1,v1,v2],
-                    [v1,v3,v2],
-                ],
-                faces2 = (tb+j)%2? [for (f=faces) reverse(f)] : faces
-            ) each faces2
         ] : []
     ) [reorient(anchor,spin,orient, r=r, p=verts), faces];