diff --git a/notes.md b/notes.md
index 4fd36500..e030670e 100644
--- a/notes.md
+++ b/notes.md
@@ -86,3 +86,7 @@
 * http://wiki.imal.org/howto/cnc-milling-introduction-cutting-tools
 * http://www.twak.co.uk/2011/01/degeneracy-in-weighted-straight.html
 * http://photon.chrisgraf.de/
+
+# More CNC
+
+* https://www.researchgate.net/publication/250328721_Toolpath_Optimization_on_Automatic_Removal_Uncut_and_Application
diff --git a/src/geo/polygons.js b/src/geo/polygons.js
index 32bf07b6..2c6012fb 100644
--- a/src/geo/polygons.js
+++ b/src/geo/polygons.js
@@ -531,7 +531,7 @@
             let circ1 = sumCirc(orig),
                 circ2 = sumCirc(polys),
                 diff = Math.abs(1 - (circ1 / circ2));
-
+            // if circularity of original and offset differ, possible wall gaps
             if (diff > 0.2) {
                 thins(orig, out.length ? polys : null, diff, -over);
             }
diff --git a/src/kiri/init.js b/src/kiri/init.js
index 96177fc6..75709b76 100644
--- a/src/kiri/init.js
+++ b/src/kiri/init.js
@@ -1512,8 +1512,8 @@
             zHopDistance:        UC.newInput(LANG.ad_zhop_s, {title:LANG.ad_zhop_l, bound:UC.bound(0,3.0), convert:UC.toFloat, modes:FDM, expert:true}),
             antiBacklash:        UC.newInput(LANG.ad_abkl_s, {title:LANG.ad_abkl_l, bound:UC.bound(0,3), convert:UC.toInt, modes:FDM, expert:true}),
             sliceRotation:       LOCAL ? UC.newInput(LANG.ad_slrt_s, {title:LANG.ad_slrt_l, bound:UC.bound(-45,45), convert:UC.toFloat, modes:FDM, expert:true}) : null,
-            // detectThinWalls: UC.newBoolean("thin wall fill", onBooleanClick, {title: "detect and fill thin openings\nbetween shells walls", modes:FDM, expert:true})
             gcodePauseLayers:    UC.newInput(LANG.ag_paws_s, {title:LANG.ag_paws_l, modes:FDM, expert:true, comma:true}),
+            detectThinWalls:     LOCAL ? UC.newBoolean(LANG.ad_thin_s, onBooleanClick, {title: LANG.ad_thin_l, modes:FDM, expert:true}) : null,
             outputLayerRetract:  UC.newBoolean(LANG.ad_lret_s, onBooleanClick, {title:LANG.ad_lret_l, modes:FDM, expert:true}),
 
             // SLA
diff --git a/src/kiri/slice.js b/src/kiri/slice.js
index afaa8b4f..b3f00249 100644
--- a/src/kiri/slice.js
+++ b/src/kiri/slice.js
@@ -358,20 +358,21 @@
             }
 
             if (count) {
-                // permit offset of 0 for laser
+                // permit offset of 0 for laser and drag knife
                 if (offset1 === 0 && count === 1) {
                     last = top_poly.clone(true);
                     top.traces = last;
                 } else {
                     if (opt.thin) {
+                        // thin wall offset strategy
                         let on1s2 = offset1 * 2,
                             on2s2 = offsetN * 2;
                         POLY.expand2(
-                            top_poly,
-                            -offset1,
-                            -offsetN,
-                            top.traces,
-                            count,
+                            top_poly,   // reference polygon(s)
+                            -offset1,   // first inset distance
+                            -offsetN,   // subsequent inset distnace
+                            top.traces, // accumulator array
+                            count,      // numberof insets to perform
                             // on each new offset trace ...
                             function(polys, countNow) {
                                 last = polys;
@@ -385,6 +386,10 @@
                                 });
                             },
                             // thin wall probe
+                            // p1 = original polys
+                            // p2 = collected polys
+                            // diff = circularity difference
+                            // dist = over-expansion distance
                             function(p1, p2, diff, dist) {
                                 if (p2) {
                                     // nth offset
@@ -405,13 +410,14 @@
                             },
                             z);
                     } else {
+                        // standard wall offsetting strategy
                         POLY.expand(
-                            top_poly,
-                            -offset1,
-                            z,
-                            top.traces,
-                            count,
-                            -offsetN,
+                            top_poly,   // reference polygon(s)
+                            -offset1,   // first inset distance
+                            z,          // set new polys to this z
+                            top.traces, // accumulator array
+                            count,      // number of insets to perform
+                            -offsetN,   // subsequent inset distance
                             // on each new offset trace ...
                             function(polys, countNow) {
                                 last = polys;
@@ -427,6 +433,7 @@
                     }
                 }
             } else {
+                // no shells, just infill, is permitted
                 last = [top.poly];
             }
 
diff --git a/src/mode/fdm/driver.js b/src/mode/fdm/driver.js
index 8ce1e85d..0897acf2 100644
--- a/src/mode/fdm/driver.js
+++ b/src/mode/fdm/driver.js
@@ -172,7 +172,7 @@
                     ) && !vaseMode;
                 slice.doShells(spro.sliceShells, firstOffset, shellOffset, fillOffset, {
                     vase: vaseMode,
-                    thin: false && spro.detectThinWalls
+                    thin: spro.detectThinWalls
                 });
                 if (solid) slice.doSolidLayerFill(fillSpacing, sliceFillAngle);
                 sliceFillAngle += 90.0;
@@ -192,6 +192,11 @@
                     slice.doThinFill(fillSpacing, sliceFillAngle);
                     sliceFillAngle += 90.0;
                 }, "solids");
+            } else if (spro.detectThinWalls) {
+                forSlices(0.35, 0.5, function(slice) {
+                    slice.doThinFill(fillSpacing, sliceFillAngle);
+                    sliceFillAngle += 90.0;
+                }, "thins");
             }
 
             // calculations only relevant when supports are enabled
diff --git a/web/kiri/lang/en.js b/web/kiri/lang/en.js
index fc207937..420b2876 100644
--- a/web/kiri/lang/en.js
+++ b/web/kiri/lang/en.js
@@ -427,6 +427,8 @@ kiri.lang['en-us'] = {
     ad_zhop_l:      "amount to raise z\non retraction moves\nin millimeters\n0 to disable",
     ad_abkl_s:      "anti-backlash",
     ad_abkl_l:      "use micro-movements to cancel\nbacklash during fills\nin millimeters",
+    ad_thin_s:      "thin walls",
+    ad_thin_l:      "detect and fill gaps\nbetween shell walls",
     ad_slrt_s:      "slice rotation",
     ad_slrt_l:      "slice object on a bias\nangle in the y axis.\nuseful for extremely\nlarge format prints",
     ad_lret_s:      "layer retract",