diff --git a/src/geo/polygons.js b/src/geo/polygons.js
index 3466ca30..61d0dffa 100644
--- a/src/geo/polygons.js
+++ b/src/geo/polygons.js
@@ -40,6 +40,8 @@ const POLYS = base.polygons = {
     route,
     union,
     inset,
+    outer,
+    inner,
     nest,
     diff,
     setZ,
@@ -53,6 +55,23 @@ const POLYS = base.polygons = {
     fingerprint
 };
 
+function outer(polys) {
+    for (let p of polys) {
+        p.inner = undefined;
+    }
+    return polys;
+}
+
+function inner(polys) {
+    const ret = [];
+    for (let p of polys) {
+        if (p.inner) {
+            ret.push(p.inner);
+        }
+    }
+    return ret;
+}
+
 function length(polys) {
     let length = 0;
     for (let p of polys) {
diff --git a/src/kiri-mode/cam/ops.js b/src/kiri-mode/cam/ops.js
index 06a3ffce..145a2359 100644
--- a/src/kiri-mode/cam/ops.js
+++ b/src/kiri-mode/cam/ops.js
@@ -117,7 +117,7 @@ class OpLevel extends CamOp {
         updateToolDiams(toolDiam);
 
         let points = [];
-        let clear = POLY.offset(tshadow, toolDiam * (op.over || 0));
+        let clear = POLY.outer(POLY.offset(tshadow, toolDiam * (op.over || 0)));
         // let tabsub = [];
         // POLY.subtract(clear, POLY.offset(tabs.map(t => t.poly), toolDiam/2), tabsub, []);
         POLY.fillArea(clear, 90, stepOver, points);