diff --git a/src/main/mesh.js b/src/main/mesh.js
index eeac19a4..21ee2ba8 100644
--- a/src/main/mesh.js
+++ b/src/main/mesh.js
@@ -276,6 +276,7 @@ function space_init(data) {
                 case 'KeyF':
                     return selection.floor().focus();
                 case 'KeyM':
+                    if (shiftKey) return api.tool.mirror();
                     return api.tool.merge();
                 case 'KeyA':
                     return api.tool.analyze();
diff --git a/src/mesh/api.js b/src/mesh/api.js
index 8d988ba8..38a13eca 100644
--- a/src/mesh/api.js
+++ b/src/mesh/api.js
@@ -140,6 +140,8 @@ let selection = {
         }
         // update saved selection id list
         prefs.save( prefs.map.space.select = selected.map(s => s.id) );
+        // force repaint in case of idle
+        space.update();
         return selection;
     },
 
@@ -293,6 +295,9 @@ function fallback(models, strict) {
 let tool = {
     merge(models) {
         models = fallback(models);
+        if (models.length <= 1) {
+            return api.log.emit('nothing to merge');
+        }
         api.log.emit(`merging ${models.length} models`).pin();
         worker.model_merge(models.map(m => {
             return { id: m.id, matrix: m.matrix }
@@ -313,6 +318,12 @@ let tool = {
         }
     },
 
+    mirror() {
+        for (let m of selection.models()) {
+            m.mirror();
+        }
+    },
+
     invert() {
         for (let m of selection.models()) {
             m.invert(api.mode.get());
diff --git a/src/mesh/build.js b/src/mesh/build.js
index 704f2378..6502fb71 100644
--- a/src/mesh/build.js
+++ b/src/mesh/build.js
@@ -267,6 +267,7 @@ function ui_build() {
             // h.div({ class: "vsep" }),
             h.button({ _: 'duplicate', onclick: tool.duplicate }),
             h.button({ _: 'merge', onclick: tool.merge }),
+            h.button({ _: 'mirror', onclick: tool.mirror }),
             h.button({ _: 'split', onclick: call.edit_split }),
             h.button({ _: 'invert', onclick: tool.invert }),
         ]),
diff --git a/src/mesh/group.js b/src/mesh/group.js
index 5e4fbb99..f8a851c9 100644
--- a/src/mesh/group.js
+++ b/src/mesh/group.js
@@ -108,6 +108,13 @@ mesh.group = class MeshGroup extends mesh.object {
         return this.centerXY().floor();
     }
 
+    scale(x = 1, y = 1, z = 1) {
+        for (let m of this.models) {
+            m.scale(x, y, z);
+        }
+        return this;
+    }
+
     // center objects to group bounds
     // dependent on first being added to world/scene
     centerModels() {
@@ -141,6 +148,11 @@ mesh.group = class MeshGroup extends mesh.object {
             model.select(...arguments);
         }
     }
+
+    // ok, this is confusing, but necessary for now :/
+    setSelected() {
+        super.select();
+    }
 };
 
 })();
diff --git a/src/mesh/model.js b/src/mesh/model.js
index f81dfc3f..080192a2 100644
--- a/src/mesh/model.js
+++ b/src/mesh/model.js
@@ -108,16 +108,45 @@ mesh.model = class MeshModel extends mesh.object {
         return this.mesh.matrixWorld.elements;
     }
 
+    // override and translate mesh
+    move(x = 0, y = 0, z = 0) {
+        let arr = this.attributes.position.array;
+        for (let i=0, l=arr.length; i<l; ) {
+            arr[i] = arr[i++] + x;
+            arr[i] = arr[i++] + y;
+            arr[i] = arr[i++] + z;
+        }
+        this.reload(arr);
+        return this;
+    }
+
+    // override and translate mesh
+    scale(x = 1, y = 1, z = 1) {
+        let arr = this.attributes.position.array;
+        for (let i=0, l=arr.length; i<l; ) {
+            arr[i] = arr[i++] *= x;
+            arr[i] = arr[i++] *= y;
+            arr[i] = arr[i++] *= z;
+        }
+        this.reload(arr);
+        return this;
+    }
+
+    mirror() {
+        return this.duplicate({ mirror: true });
+    }
+
     // return new group containing just this model in world coordinates
-    duplicate() {
+    duplicate(opt = {}) {
         worker.model_duplicate({
             matrix: this.matrix,
-            id: this.id
+            id: this.id,
+            opt
         }).then(data => {
             mesh.api.group.new([new mesh.model({
-                file: `${this.file}-dup`,
+                file: `${this.file}`,
                 mesh: data
-            })]).select();
+            })]).setSelected();
         });
     }
 
@@ -357,7 +386,7 @@ mesh.model = class MeshModel extends mesh.object {
                 this.group.add(model = new mesh.model({
                     file: `${this.file}`,
                     mesh: o2
-                }).applyMatrix4(m4)).select();
+                }).applyMatrix4(m4)).setSelected();
                 if (o1.length) {
                     // this becomes bottom
                     this.reload(o1);
diff --git a/src/mesh/util.js b/src/mesh/util.js
index 177097a3..9403f197 100644
--- a/src/mesh/util.js
+++ b/src/mesh/util.js
@@ -103,6 +103,10 @@ let util = mesh.util = {
     // https://discourse.threejs.org/t/bounding-box-bigger-than-concave-object-extrudegeometry/26073/2
     // https://discourse.threejs.org/t/invalid-bounds-generated-for-some-orientations/33205
     box3expand(box3, object) {
+        if (object._no_bounds) {
+            return;
+        }
+
         let geometry = object.geometry;
         object.updateWorldMatrix(geometry ? true : false, false);
 
@@ -183,6 +187,7 @@ let util = mesh.util = {
                 this.size = size;
                 this.type = 'FaceNormalsHelper';
                 this.matrixAutoUpdate = false;
+                this._no_bounds = true;
                 this.update();
             }
 
diff --git a/src/mesh/work.js b/src/mesh/work.js
index 97601f69..8158f332 100644
--- a/src/mesh/work.js
+++ b/src/mesh/work.js
@@ -72,8 +72,30 @@ let model = {
 
     // return new vertices in world coordinates
     duplicate(data) {
-        let { id, matrix } = data;
-        return translate_encode(id, matrix);
+        let { id, matrix, opt } = data;
+        let array = translate_encode(id, matrix);
+        if (opt.mirror) {
+            // find max z and invert z
+            let maxz = -Infinity;
+            for (let i=2, l=array.length; i<l; i += 3) {
+                maxz = Math.max(maxz, array[i]);
+                array[i] = -array[i];
+            }
+            maxz *= 2;
+            for (let i=0, l=array.length; i<l; i += 9) {
+                // swap first two vertices in face to invert normals
+                let v1 = array.slice(i, i+3);
+                for (let j=0; j<3; j++) {
+                    array[i+j] = array[i+j+3];
+                    array[i+j+3] = v1[j];
+                }
+                // move part up in Z
+                array[i+2] += maxz;
+                array[i+5] += maxz;
+                array[i+8] += maxz;
+            }
+        }
+        return array;
     },
 
     // merge several model vertices into a single array