diff --git a/src/csg/levelset_3d.cpp b/src/csg/levelset_3d.cpp
index 1f3756c7f26710bd64f0b1dba1ce2a9d6834f3e1..61701ce6e7e88401e97d4a171fbb74e61f8ede11 100644
--- a/src/csg/levelset_3d.cpp
+++ b/src/csg/levelset_3d.cpp
@@ -146,14 +146,13 @@ double signed_distance_3d(const LinearExtrude &lin_ext, double xx, double yy,
 
 double signed_distance_3d(const RotateExtrude &rot_ext, double xx, double yy,
                           double zz) {
-  double tt = atan(yy / xx) * DEGREES_PER_RADIAN;
-  if ((xx < 0 && yy >= 0) || (xx < 0 && yy < 0))
-    tt += 180; // Quadrants II & III
-  else if (xx >= 0 && yy < 0)
-    tt += 360; // Quadrant IV
-
-  double sign_t = (tt >= 0 && tt <= rot_ext.angle) ? -1.0 : 1.0;
-  double dist_t = std::min(std::fabs(rot_ext.angle - tt), std::fabs(360 - tt));
+  double tt = std::atan2(yy, xx) * DEGREES_PER_RADIAN;
+  tt = tt >= 0 ? tt : 360 + tt;
+
+  assert(tt >= 0 && tt <= 360); //tt must be between 0 and 360
+
+  double sign_t = (tt <= rot_ext.angle) ? -1.0 : 1.0;
+  double dist_t = std::min(std::fabs(rot_ext.angle - tt), (360 - tt));
 
   auto XX = std::hypot(xx, yy);
   auto YY = zz;