diff --git a/barmesh/draw.gd b/barmesh/draw.gd
index f249d51..4a6e20d 100644
--- a/barmesh/draw.gd
+++ b/barmesh/draw.gd
@@ -182,16 +182,13 @@ func _draw_barmesh(bm: BarMesh) -> void:
 	if show_normals:
 		var nim := ImmediateMesh.new()
 		nim.surface_begin(Mesh.PRIMITIVE_LINES)
-		var tick: float = 0.003
 		for node in bm.nodes:
 			var nd: BarMesh.BMNode = node
 			if nd.contact_kind == BarMesh.BMNode.ContactFeature.NONE:
 				continue
-			if nd.contact_normal.length_squared() < 1e-12:
-				continue
-			# Normals start at mesh contact; bars use cutter CL (fixed XY).
+			# Radius-length: mesh contact ↔ tool centre (CL). Length should be R when tangent.
 			var p0: Vector3 = nd.contact_point
-			var p1: Vector3 = p0 + nd.contact_normal * tick
+			var p1: Vector3 = nd.p
 			nim.surface_add_vertex(cad_to_godot(p0))
 			nim.surface_add_vertex(cad_to_godot(p1))
 		nim.surface_end()
diff --git a/barmesh/tool_contact.gd b/barmesh/tool_contact.gd
index 045d69a..6147558 100644
--- a/barmesh/tool_contact.gd
+++ b/barmesh/tool_contact.gd
@@ -160,22 +160,65 @@ static func _ball_edge_z(x: float, y: float, R: float, p0: Vector3, p1: Vector3)
 
 
 static func _ball_edge_hit(x: float, y: float, R: float, p0: Vector3, p1: Vector3) -> Dictionary:
-	var ex := p1.x - p0.x
-	var ey := p1.y - p0.y
-	var ez := p1.z - p0.z
-	var len2 := ex * ex + ey * ey
-	if len2 < 1e-18:
+	## True ball↔segment contact: (CL − pt) ⊥ edge and |CL − pt| = R.
+	## (Old XY-only projection is wrong for tilted edges and causes fall-through.)
+	var dx := p1.x - p0.x
+	var dy := p1.y - p0.y
+	var dz := p1.z - p0.z
+	var L2 := dx * dx + dy * dy + dz * dz
+	if L2 < 1e-18:
 		return {}
-	var t := ((x - p0.x) * ex + (y - p0.y) * ey) / len2
-	t = clampf(t, 0.0, 1.0)
-	var pt := Vector3(p0.x + ex * t, p0.y + ey * t, p0.z + ez * t)
-	var dx: float = x - pt.x
-	var dy: float = y - pt.y
-	var d2: float = dx * dx + dy * dy
-	var R2: float = R * R
-	if d2 > R2:
+	var cx := x - p0.x
+	var cy := y - p0.y
+	var A := cx * dx + cy * dy
+	var H2 := dx * dx + dy * dy
+	var R2 := R * R
+	var best_z: float = -1e30
+	var best_pt := Vector3.ZERO
+	var found := false
+
+	if H2 < 1e-18:
+		# Nearly vertical edge: cylinder test in XY, z from segment extent.
+		var d2xy := cx * cx + cy * cy
+		if d2xy > R2:
+			return {}
+		var z_off := sqrt(R2 - d2xy)
+		for t in [0.0, 1.0]:
+			var pt_v := Vector3(p0.x + dx * t, p0.y + dy * t, p0.z + dz * t)
+			var z_cl: float = pt_v.z + z_off
+			if z_cl > best_z:
+				best_z = z_cl
+				best_pt = pt_v
+				found = true
+	else:
+		# Quadratic in cz = z - p0.z from |C-P|^2=R^2 and (C-P)·d=0.
+		var a := H2
+		var b := -2.0 * A * dz
+		var c := L2 * (cx * cx + cy * cy) - A * A - R2 * L2
+		var disc := b * b - 4.0 * a * c
+		if disc < 0.0:
+			return {}
+		var sdisc := sqrt(disc)
+		for sign in [-1.0, 1.0]:
+			var cz: float = (-b + sign * sdisc) / (2.0 * a)
+			var z: float = p0.z + cz
+			var t: float = (A + cz * dz) / L2
+			if t < -1e-6 or t > 1.0 + 1e-6:
+				continue
+			t = clampf(t, 0.0, 1.0)
+			var pt := Vector3(p0.x + dx * t, p0.y + dy * t, p0.z + dz * t)
+			var cl := Vector3(x, y, z)
+			if absf(cl.distance_to(pt) - R) > 1e-3:
+				continue
+			if z > best_z:
+				best_z = z
+				best_pt = pt
+				found = true
+
+	# Endpoints are also covered by vertex tests; keep segment-interior hits here.
+	if not found:
 		return {}
-	return {"z": pt.z + sqrt(R2 - d2), "point": pt}
+	return {"z": best_z, "point": best_pt}
 
 
 static func _ball_face_z(x: float, y: float, R: float, a: Vector3, b: Vector3, c: Vector3) -> float:
diff --git a/scenes/main.tscn b/scenes/main.tscn
index 74c7bd3..667aceb 100644
--- a/scenes/main.tscn
+++ b/scenes/main.tscn
@@ -148,6 +148,10 @@ layout_mode = 2
 button_pressed = true
 text = "Orthographic camera"
 
+[node name="DebugFlatMesh" type="CheckBox" parent="ToolpathUI/Panel/VBox"]
+layout_mode = 2
+text = "Debug flat mesh (2 tris)"
+
 [node name="Bake" type="Button" parent="ToolpathUI/Panel/VBox"]
 layout_mode = 2
 text = "Build tool surface"
diff --git a/scripts/load_obj_mesh.gd b/scripts/load_obj_mesh.gd
index f356580..5849acb 100644
--- a/scripts/load_obj_mesh.gd
+++ b/scripts/load_obj_mesh.gd
@@ -3,14 +3,44 @@ extends MeshInstance3D
 ## Adds collision for raycasts and applies flat (faceted) normals.
 ## Mesh is assigned in the editor. Units: Godot metres (STL mm × 0.001).
 
+var _original_mesh: Mesh
+var _debug_flat: bool = false
+
+
 func _ready() -> void:
 	if mesh == null:
 		push_error("Part mesh missing — assign meshes/eartip.obj in the editor")
 		return
+	_original_mesh = mesh
+	_apply_flat_shading()
+	_rebuild_collision()
+
+
+func set_debug_flat_mesh(enabled: bool) -> void:
+	## Same XY/Z AABB as the part, but only two triangles (flat top of the bbox).
+	if mesh == null and _original_mesh == null:
+		return
+	if _original_mesh == null:
+		_original_mesh = mesh
+	_debug_flat = enabled
+	if enabled:
+		mesh = _make_two_triangle_bbox(_original_mesh)
+	else:
+		mesh = _original_mesh.duplicate() if _original_mesh else null
 	_apply_flat_shading()
+	_rebuild_collision()
+
+
+func is_debug_flat_mesh() -> bool:
+	return _debug_flat
+
+
+func _rebuild_collision() -> void:
 	for c in get_children():
 		if c is StaticBody3D:
 			c.queue_free()
+	if mesh == null:
+		return
 	var body := StaticBody3D.new()
 	var col := CollisionShape3D.new()
 	var shape := ConcavePolygonShape3D.new()
@@ -19,8 +49,30 @@ func _ready() -> void:
 	body.add_child(col)
 	add_child(body)
 
+
+func _make_two_triangle_bbox(src: Mesh) -> ArrayMesh:
+	var aabb: AABB = src.get_aabb()
+	# Godot Y-up mesh space: flat plate at max Y (CAD Z-up max), covering XY footprint.
+	var x0 := aabb.position.x
+	var x1 := aabb.position.x + aabb.size.x
+	var z0 := aabb.position.z
+	var z1 := aabb.position.z + aabb.size.z
+	var y := aabb.position.y + aabb.size.y
+	var st := SurfaceTool.new()
+	st.begin(Mesh.PRIMITIVE_TRIANGLES)
+	# Two tris, CCW when viewed from above (+Y).
+	st.add_vertex(Vector3(x0, y, z0))
+	st.add_vertex(Vector3(x1, y, z0))
+	st.add_vertex(Vector3(x1, y, z1))
+	st.add_vertex(Vector3(x0, y, z0))
+	st.add_vertex(Vector3(x1, y, z1))
+	st.add_vertex(Vector3(x0, y, z1))
+	st.generate_normals()
+	return st.commit()
+
+
 func _apply_flat_shading() -> void:
-	if mesh.get_surface_count() < 1:
+	if mesh == null or mesh.get_surface_count() < 1:
 		return
 	var st := SurfaceTool.new()
 	st.create_from(mesh, 0)
diff --git a/scripts/toolpath_ui.gd b/scripts/toolpath_ui.gd
index f91a8f3..d72928a 100644
--- a/scripts/toolpath_ui.gd
+++ b/scripts/toolpath_ui.gd
@@ -70,6 +70,16 @@ func _ready() -> void:
 			if cam and cam.has_method("apply_orthogonal"):
 				cam.apply_orthogonal(v)
 		)
+	var flat := $Panel/VBox/DebugFlatMesh as CheckBox
+	if flat:
+		flat.button_pressed = false
+		flat.toggled.connect(func(v: bool) -> void:
+			var part := get_parent().get_node_or_null("MeshRoot/Part")
+			if part and part.has_method("set_debug_flat_mesh"):
+				part.set_debug_flat_mesh(v)
+			# Rebuild lattice on the substituted mesh.
+			_on_bake()
+		)
 
 
 func _preview() -> Node:
diff --git a/tools/test_contact.gd b/tools/test_contact.gd
index c799c77..d328ec8 100644
--- a/tools/test_contact.gd
+++ b/tools/test_contact.gd
@@ -93,5 +93,21 @@ func _run() -> void:
 		_fail("tilt face contact XY unexpected equal to CL")
 		return
 
-	print("CONTACT_OK face,vertex,edge,tilt_xy R=", R)
+	# Tilted edge: |CL−pt|=R and (CL−pt)⊥edge (XY-only drop was wrong here).
+	var e0 := Vector3(0, 0, 0)
+	var e1 := Vector3(0.02, 0, 0.01)
+	var skinny: Array = [[e0, e1, Vector3(0.01, 1e-4, 0.005)]]
+	var eh: Dictionary = Contact.drop_tool_contact(0.01, 0.0, R, skinny, -1.0)
+	_assert_cl_xy(eh, 0.01, 0.0)
+	var cl_e := Vector3(float(eh["x"]), float(eh["y"]), float(eh["z"]))
+	if absf(cl_e.distance_to(eh["point"]) - R) > 1e-4:
+		_fail("tilted edge |CL-pt| != R " + str(eh))
+		return
+	var edir: Vector3 = (e1 - e0).normalized()
+	var radial: Vector3 = cl_e - eh["point"]
+	if absf(radial.dot(edir)) > 1e-3:
+		_fail("tilted edge not perpendicular " + str(radial.dot(edir)))
+		return
+
+	print("CONTACT_OK face,vertex,edge,tilt_xy,tilt_edge R=", R)
 	quit(0)