diff --git a/src/lib.rs b/src/lib.rs
index 66a39ae..643b3ad 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,6 +1,9 @@
-use std::{error::Error, f32::consts::FRAC_PI_2};
+use std::{
+	error::Error,
+	f32::consts::{FRAC_PI_2, PI},
+};
 
-use glam::{Affine3A, Mat4, Quat, UVec2, Vec3, Vec4, vec3, vec4};
+use glam::{Affine3A, FloatExt, Mat4, Quat, UVec2, Vec3, Vec4, vec3, vec4};
 
 use crate::render::mesh::GpuParams;
 
@@ -71,6 +74,7 @@ pub struct Core {
 	bevel_gear_desc: bevel_gear::BevelGearDesc,
 	bevel_gear_shape: bevel_gear::BevelGearShape,
 	crown_gear_desc: crown_gear::CrownGearDesc,
+	t: f32,
 }
 
 fn srgb_to_linear(color: Vec3) -> Vec3 {
@@ -162,6 +166,7 @@ impl Core {
 			bevel_gear_desc,
 			bevel_gear_shape,
 			crown_gear_desc,
+			t: 0.,
 		}
 	}
 
@@ -195,6 +200,12 @@ impl Core {
 			);
 		};
 
+		let turn_teeth = self.t;
+		let bevel_turns = turn_teeth / self.bevel_gear_desc.teeth as f32;
+		let crown_skip_teeth = self.crown_gear_desc.pitch_radius.floor();
+		let crown_turns =
+			(turn_teeth.fract_gl() + crown_skip_teeth) / (2. * self.crown_gear_desc.pitch_radius);
+
 		set_params(
 			&self.params_gear,
 			vec4(0.2, 0.8, 0.1, 1.0),
@@ -204,7 +215,8 @@ impl Core {
 			&self.params_cutter,
 			vec4(0.8, 0.2, 0.1, 1.0),
 			Affine3A::from_rotation_translation(
-				Quat::from_rotation_y(FRAC_PI_2 + self.bevel_gear_desc.cone_halfangle),
+				Quat::from_rotation_y(FRAC_PI_2 + self.bevel_gear_desc.cone_halfangle)
+					* Quat::from_rotation_z(2. * PI * crown_turns),
 				vec3(0., 0., self.bevel_gear_desc.pitch_distance()),
 			),
 		);
@@ -290,6 +302,7 @@ impl Core {
 		let output = self.surface.get_current_texture().unwrap();
 		self.render(&output.texture, args);
 		output.present();
+		self.t += 1. / 60.;
 	}
 }
 
diff --git a/ui/src/viewport.cxx b/ui/src/viewport.cxx
index 833cd83..14ca272 100644
--- a/ui/src/viewport.cxx
+++ b/ui/src/viewport.cxx
@@ -44,6 +44,7 @@ void Viewport::paintEvent(QPaintEvent* event) {
 	if (!core)
 		recreate();
 	core->redraw(args);
+	update();
 }
 
 void Viewport::resizeEvent(QResizeEvent* event) {