diff --git a/README.md b/README.md
index c813056..5f45975 100644
--- a/README.md
+++ b/README.md
@@ -17,9 +17,8 @@ const geometry = new THREE.TorusGeometry(20, 10, 30, 30);
 
 const slicer = new SLICER.Slicer();
 slicer.setGeometry(geometry);
-const gcode = await slicer.slice(settings)
-  .progress(({ done, total, action }) => {
-    const percentage = `${(done / total * 100).toFixed()}%`
-    console.log(action, percentage);
-  });
+const gcode = await slicer.slice(settings, ({ progress: { done, total, action } }) => {
+  const percentage = `${(done / total * 100).toFixed()}%`
+  console.log(action, percentage);
+}));
 ```
diff --git a/example/save.js b/example/save.js
index 353ef0d..9524c6c 100644
--- a/example/save.js
+++ b/example/save.js
@@ -17,7 +17,7 @@ jsonLoader.load('models/airplane.json', async geometry => {
 
   const slicer = new Slicer().setGeometry(geometry);
   const gcode = await slicer.slice(settings)
-    .progress(({ done, total, action }) => {
+    .progress(({ progress: { done, total, action } }) => {
       const percentage = `${(done / total * 100).toFixed()}%`
       document.write(`<p>${action}, ${percentage}</p>`);
     });
diff --git a/jspm.config.js b/jspm.config.js
index 39be6b7..fccd3c9 100644
--- a/jspm.config.js
+++ b/jspm.config.js
@@ -151,7 +151,6 @@ SystemJS.config({
   ],
   map: {
     "three": "npm:three@0.83.0",
-    "progress-promise": "npm:progress-promise@0.0.6",
     "text": "github:systemjs/plugin-text@0.0.11",
     "js-yaml": "npm:js-yaml@3.9.0",
     "clipper-js": "github:Doodle3D/clipper-js@1.0.2",
diff --git a/package.json b/package.json
index 45167f8..c46dbe3 100644
--- a/package.json
+++ b/package.json
@@ -10,7 +10,6 @@
       "clipper-js": "github:Doodle3D/clipper-js@1.0.2",
       "js-yaml": "npm:js-yaml@^3.9.0",
       "json": "github:systemjs/plugin-json@^0.1.2",
-      "progress-promise": "npm:progress-promise@^0.0.6",
       "text": "github:systemjs/plugin-text@^0.0.11",
       "three": "npm:three@0.83.0",
       "worker": "github:casperlamboo/plugin-worker@master"
diff --git a/src/Slicer.js b/src/Slicer.js
index 280a6df..c80959b 100644
--- a/src/Slicer.js
+++ b/src/Slicer.js
@@ -1,15 +1,12 @@
 import * as THREE from 'three';
 import slice from './sliceActions/slice.js';
 import SlicerWorker from './slicerWorker.js!worker';
-import ProgressPromise from 'progress-promise';
 
 export default class {
   setMesh(mesh) {
     mesh.updateMatrix();
 
-    this.setGeometry(mesh.geometry, mesh.matrix);
-
-    return this;
+    return this.setGeometry(mesh.geometry, mesh.matrix);
   }
   setGeometry(geometry, matrix) {
     if (geometry.isBufferGeometry) {
@@ -20,7 +17,7 @@ export default class {
       throw new Error('Geometry is not an instance of BufferGeometry or Geometry');
     }
 
-    if (matrix) {
+    if (typeof matrix !== 'undefined') {
       geometry.applyMatrix(matrix);
     }
 
@@ -28,17 +25,20 @@ export default class {
 
     return this;
   }
-  sliceSync(settings, onprogress) {
-    return slice(this.geometry, settings, onprogress);
+  sliceSync(settings, onProgress) {
+    return slice(this.geometry, settings, onProgress);
   }
-  slice(settings) {
-    const slicerWorker = new SlicerWorker();
+  slice(settings, onProgress) {
+    if (!this.geometry) {
+      throw new Error('Geometry is not set, use Slicer.setGeometry or Slicer.setMesh first');
+    }
 
-    const geometry = this.geometry.toJSON();
-
-    return new ProgressPromise((resolve, reject, progress) => {
+    return new Promise((resolve, reject) => {
+      // create the slicer worker
+      const slicerWorker = new SlicerWorker();
       slicerWorker.onerror = reject;
 
+      // listen to messages send from worker
       slicerWorker.addEventListener('message', (event) => {
         const { message, data } = event.data;
         switch (message) {
@@ -48,12 +48,14 @@ export default class {
             break;
           }
           case 'PROGRESS': {
-            progress(data);
+            onProgress(data);
             break;
           }
         }
       });
 
+      // send geometry and settings to worker to start the slicing progress
+      const geometry = this.geometry.toJSON();
       slicerWorker.postMessage({
         message: 'SLICE',
         data: { geometry, settings }
diff --git a/src/sliceActions/addBrim.js b/src/sliceActions/addBrim.js
index 33d2475..9812598 100644
--- a/src/sliceActions/addBrim.js
+++ b/src/sliceActions/addBrim.js
@@ -8,7 +8,9 @@ const offsetOptions = {
 };
 
 export default function addBrim(slices, settings) {
-  let { brim: { offset: brimOffset } } = settings;
+  let {
+    brim: { offset: brimOffset }
+  } = settings;
   brimOffset /= PRECISION;
 
   const [firstLayer] = slices;
diff --git a/src/sliceActions/calculateLayersIntersections.js b/src/sliceActions/calculateLayersIntersections.js
index 76b0e9f..78c2f21 100644
--- a/src/sliceActions/calculateLayersIntersections.js
+++ b/src/sliceActions/calculateLayersIntersections.js
@@ -1,7 +1,10 @@
 import * as THREE from 'three';
 
 export default function calculateLayersIntersections(lines, settings) {
-  const { layerHeight, dimensions: { z: dimensionsZ } } = settings;
+  const {
+    layerHeight,
+    dimensions: { z: dimensionsZ }
+  } = settings;
 
   const numLayers = Math.floor(dimensionsZ / layerHeight);
 
@@ -9,9 +12,9 @@ export default function calculateLayersIntersections(lines, settings) {
   const layerIntersectionPoints = Array.from(Array(numLayers)).map(() => []);
 
   for (let lineIndex = 0; lineIndex < lines.length; lineIndex ++) {
-    const line = lines[lineIndex].line;
+    const { line, isFlat } = lines[lineIndex];
 
-    if (line.isFlat) continue;
+    if (isFlat) continue;
 
     const min = Math.ceil(Math.min(line.start.y, line.end.y) / layerHeight);
     const max = Math.floor(Math.max(line.start.y, line.end.y) / layerHeight);
diff --git a/src/sliceActions/createLines.js b/src/sliceActions/createLines.js
index 188f1c3..5a6e3fc 100644
--- a/src/sliceActions/createLines.js
+++ b/src/sliceActions/createLines.js
@@ -25,7 +25,7 @@ export default function createLines(geometry, settings) {
     const lookupB = lineLookup[`${face.c}_${face.b}`];
     const lookupC = lineLookup[`${face.a}_${face.c}`];
 
-    const isFlat = face.normal.y !== 1 && face.normal.y !== -1;
+    const isFlat = face.normal.y > 0.999 || face.normal.y < -0.999;
 
     // only add unique lines
     // returns index of said line
diff --git a/src/sliceActions/generateInfills.js b/src/sliceActions/generateInfills.js
index f9ec418..e959475 100644
--- a/src/sliceActions/generateInfills.js
+++ b/src/sliceActions/generateInfills.js
@@ -26,8 +26,8 @@ export default function generateInfills(slices, settings) {
 
     let surroundingLayer;
     if (layer - bottomSkinCount >= 0 && layer + topSkinCount < slices.length) {
-      const downSkin = slices[layer - bottomSkinCount].getOutline();
-      const upSkin = slices[layer + topSkinCount].getOutline();
+      const downSkin = slices[layer - bottomSkinCount].outline;
+      const upSkin = slices[layer + topSkinCount].outline;
       surroundingLayer = upSkin.intersect(downSkin);
     }
 
diff --git a/src/sliceActions/generateInnerLines.js b/src/sliceActions/generateInnerLines.js
index a25550d..a829a43 100644
--- a/src/sliceActions/generateInnerLines.js
+++ b/src/sliceActions/generateInnerLines.js
@@ -9,7 +9,11 @@ const offsetOptions = {
 
 export default function generateInnerLines(slices, settings) {
   // need to scale up everything because of clipper rounding errors
-  let { layerHeight, nozzleDiameter, shell: { thickness: shellThickness } } = settings;
+  let {
+    layerHeight,
+    nozzleDiameter,
+    shell: { thickness: shellThickness }
+  } = settings;
   nozzleDiameter /= PRECISION;
   shellThickness /= PRECISION;
   const nozzleRadius = nozzleDiameter / 2;
diff --git a/src/sliceActions/generateOutlines.js b/src/sliceActions/generateOutlines.js
new file mode 100644
index 0000000..d98ccdf
--- /dev/null
+++ b/src/sliceActions/generateOutlines.js
@@ -0,0 +1,12 @@
+import Shape from 'Doodle3D/clipper-js';
+
+export default function calculateOutlines(slices, settings) {
+  for (let layer = 0; layer < slices.length; layer ++) {
+    const slice = slices[layer];
+
+    slice.outline = slice.parts.reduce((shape, part) => {
+      if (part.outerLine) shape.join(part.outerLine);
+      return shape;
+    }, new Shape([], true));
+  }
+}
diff --git a/src/sliceActions/generateSupport.js b/src/sliceActions/generateSupport.js
index 2a1a01e..d8d2e37 100644
--- a/src/sliceActions/generateSupport.js
+++ b/src/sliceActions/generateSupport.js
@@ -9,9 +9,9 @@ export default function generateSupport(slices, settings) {
     layerHeight,
     support: {
       gridSize: supportGridSize,
-      margin: AcceptanceMargin,
+      margin: supportMargin,
       plateSize: plateSize,
-      distanceY: DistanceY
+      distanceY: supportDistanceY
     },
     nozzleDiameter
   } = settings;
@@ -30,7 +30,7 @@ export default function generateSupport(slices, settings) {
     if (supportAreas.length > 0) {
 
       if (layer >= supportDistanceLayers) {
-        var sliceSkin = slices[layer - supportDistanceLayers].getOutline();
+        var sliceSkin = slices[layer - supportDistanceLayers].outline;
         sliceSkin = sliceSkin;
 
         var supportAreasSlimmed = supportAreas.difference(sliceSkin.offset(supportMargin));
@@ -52,7 +52,7 @@ export default function generateSupport(slices, settings) {
       }
     }
 
-    var supportSkin = slices[layer + supportDistanceLayers - 1].getOutline();
+    var supportSkin = slices[layer + supportDistanceLayers - 1].outline;
 
     var slice = slices[layer + supportDistanceLayers];
     for (var i = 0; i < slice.parts.length; i ++) {
@@ -62,10 +62,10 @@ export default function generateSupport(slices, settings) {
         var outerLine = slicePart.outerLine;
       }
       else {
-        var outerLine = slicePart.intersect.offset(supportAcceptanceMargin);
+        var outerLine = slicePart.intersect.offset(supportMargin);
       }
 
-      var overlap = supportSkin.offset(supportAcceptanceMargin).intersect(outerLine);
+      var overlap = supportSkin.offset(supportMargin).intersect(outerLine);
       var overhang = outerLine.difference(overlap);
 
       if (overlap.length === 0 || overhang.length > 0) {
diff --git a/src/GCode.js b/src/sliceActions/helpers/GCode.js
similarity index 100%
rename from src/GCode.js
rename to src/sliceActions/helpers/GCode.js
diff --git a/src/Slice.js b/src/sliceActions/helpers/Slice.js
similarity index 65%
rename from src/Slice.js
rename to src/sliceActions/helpers/Slice.js
index c9a1ea3..4e1002e 100644
--- a/src/Slice.js
+++ b/src/sliceActions/helpers/Slice.js
@@ -4,12 +4,6 @@ export default class {
   constructor() {
     this.parts = [];
   }
-  getOutline() {
-    return this.parts.reduce((shape, part) => {
-      if (part.outerLine) shape.join(part.outerLine);
-      return shape;
-    }, new Shape([], true));
-  }
   add(shape) {
     const part = { shape };
 
diff --git a/src/sliceActions/shapesToSlices.js b/src/sliceActions/shapesToSlices.js
index 615d242..f77eaee 100644
--- a/src/sliceActions/shapesToSlices.js
+++ b/src/sliceActions/shapesToSlices.js
@@ -1,5 +1,5 @@
 import Shape from 'Doodle3D/clipper-js';
-import Slice from '../Slice.js';
+import Slice from './helpers/Slice.js';
 
 import { CLEAN_DELTA, PRECISION } from '../constants.js';
 
diff --git a/src/sliceActions/slice.js b/src/sliceActions/slice.js
index 2042875..e0d63e1 100644
--- a/src/sliceActions/slice.js
+++ b/src/sliceActions/slice.js
@@ -2,6 +2,7 @@ import calculateLayersIntersections from './calculateLayersIntersections.js';
 import createLines from './createLines.js';
 import generateInfills from './generateInfills.js';
 import generateInnerLines from './generateInnerLines.js';
+import generateOutlines from './generateOutlines.js';
 import generateSupport from './generateSupport.js';
 import intersectionsToShapes from './intersectionsToShapes.js';
 import addBrim from './addBrim.js';
@@ -13,7 +14,7 @@ import applyPrecision from './applyPrecision.js';
 import removePrecision from './removePrecision.js';
 
 export default function(geometry, settings, onProgress) {
-  const totalStages = 11;
+  const totalStages = 12;
   let current = -1;
   const updateProgress = (action) => {
     current ++;
@@ -27,7 +28,7 @@ export default function(geometry, settings, onProgress) {
   const lines = createLines(geometry, settings);
 
   updateProgress('Detecting open vs closed shapes');
-  const openClosed = detectOpenClosed(lines);
+  detectOpenClosed(lines);
 
   updateProgress('Calculating layer intersections');
   const {
@@ -45,6 +46,8 @@ export default function(geometry, settings, onProgress) {
 
   updateProgress('Generating inner lines');
   generateInnerLines(slices, settings);
+  updateProgress('Generating out lines');
+  generateOutlines(slices, settings);
   updateProgress('Generating infills');
   generateInfills(slices, settings);
   updateProgress('Generating support');
diff --git a/src/sliceActions/slicesToGCode.js b/src/sliceActions/slicesToGCode.js
index 7268ae3..407619a 100644
--- a/src/sliceActions/slicesToGCode.js
+++ b/src/sliceActions/slicesToGCode.js
@@ -1,4 +1,4 @@
-import GCode from '../GCode.js';
+import GCode from './helpers/GCode.js';
 
 export default function slicesToGCode(slices, settings) {
   const gcode = new GCode(settings);
diff --git a/src/slicerWorker.js b/src/slicerWorker.js
index a761bd4..ff1797e 100644
--- a/src/slicerWorker.js
+++ b/src/slicerWorker.js
@@ -6,7 +6,7 @@ const loader = new THREE.JSONLoader();
 const onProgress = progress => {
   self.postMessage({
     message: 'PROGRESS',
-    data: progress
+    data: { progress }
   });
 }
 
@@ -16,7 +16,7 @@ self.addEventListener('message', (event) => {
     case 'SLICE': {
       const { geometry: JSONGeometry, settings } = data;
 
-      const { geometry } = new loader.parse(JSONGeometry.data);
+      const { geometry } = loader.parse(JSONGeometry.data);
 
       const gcode = slice(geometry, settings, onProgress);