Use dihedral angle to calculate sanding block

nhf/touhou/yasaka_kanako/onbashira.py

@@ -149,13 +149,16 @@ class Onbashira(Model):
         dx = self.bearing_gap
         return math.sqrt(diag ** 2 - dx ** 2)
 
-    @target(name="sanding_block")
+    @target(name="sanding-block")
     def sanding_block(self) -> Cq.Workplane:
+        # Dihedral angle / 2
+        angle = math.radians(180 / self.n_side)
+        r = math.sin(angle)
         x = 50.0
         return (
             Cq.Workplane()
             .sketch()
-            .polygon([(0,0), (0, x), (x, x/2), (x, 0)])
+            .polygon([(0,0), (0, x), (x, (1-r) * x), (x, 0)])
             .finalize()
             .extrude(self.side_width * 1.5)
         )