Add chain sprocket primitive and example

.gitignore

@@ -1,3 +1,4 @@
 __pycache__/
 *.pyc
 output/
+*.log

parametric_cad/__init__.py

@@ -3,7 +3,16 @@
 from .core import tm, safe_difference
 from .primitives.box import Box
 from .primitives.gear import SpurGear
+from .primitives.sprocket import ChainSprocket
 from .mechanisms.butthinge import ButtHinge
 from .export.stl import STLExporter
 
-__all__ = ["tm", "safe_difference", "Box", "SpurGear", "ButtHinge", "STLExporter"]
+__all__ = [
+    "tm",
+    "safe_difference",
+    "Box",
+    "SpurGear",
+    "ChainSprocket",
+    "ButtHinge",
+    "STLExporter",
+]

parametric_cad/examples/sprocket_example.py

@@ -0,0 +1,10 @@
+from parametric_cad.primitives.sprocket import ChainSprocket
+from parametric_cad.export.stl import STLExporter
+
+# Example sprocket for #420 chain (pitch 12.7 mm, roller dia ~7.75 mm)
+
+sprocket = ChainSprocket(pitch=12.7, roller_diameter=7.75, teeth=14,
+                         thickness=6.0, bore_diameter=25.0)
+
+exporter = STLExporter(output_dir="output/sprocket_example_output")
+exporter.export_mesh(sprocket.mesh(), "sprocket")

parametric_cad/primitives/sprocket.py

@@ -0,0 +1,56 @@
+from math import cos, sin, pi
+from parametric_cad.core import tm, safe_difference
+
+class ChainSprocket:
+    """Simple chain sprocket for roller chain."""
+
+    def __init__(self, pitch=12.7, roller_diameter=7.75, teeth=16,
+                 thickness=5.0, bore_diameter=10.0, clearance=0.5):
+        self.pitch = float(pitch)
+        self.roller_diameter = float(roller_diameter)
+        self.teeth = int(teeth)
+        self.thickness = float(thickness)
+        self.bore_diameter = float(bore_diameter)
+        self.clearance = float(clearance)
+
+    @property
+    def pitch_radius(self):
+        return self.pitch / (2 * sin(pi / self.teeth))
+
+    @property
+    def pitch_diameter(self):
+        return self.pitch_radius * 2
+
+    def mesh(self):
+        # Base disc sized so pockets can be subtracted
+        outer_radius = self.pitch_radius + self.roller_diameter / 2 + self.clearance
+        disc = tm.creation.cylinder(radius=outer_radius, height=self.thickness,
+                                    sections=self.teeth * 4)
+
+        disc.apply_translation([0, 0, self.thickness / 2])
+
+        bore = tm.creation.cylinder(radius=self.bore_diameter / 2,
+                                    height=self.thickness + 0.1)
+        bore.apply_translation([0, 0, self.thickness / 2])
+        sprocket = safe_difference(disc, bore)
+
+        pocket_radius = self.roller_diameter / 2 + self.clearance
+        pockets = []
+        for i in range(self.teeth):
+            angle = 2 * pi * i / self.teeth
+            x = cos(angle) * self.pitch_radius
+            y = sin(angle) * self.pitch_radius
+            pocket = tm.creation.cylinder(radius=pocket_radius,
+                                          height=self.thickness + 0.1,
+                                          sections=16)
+            pocket.apply_translation([x, y, self.thickness / 2])
+            pockets.append(pocket)
+
+        sprocket = safe_difference(sprocket, pockets)
+        if not sprocket.is_watertight:
+            repaired = sprocket.fill_holes()
+            if repaired is not False:
+                sprocket = repaired
+            else:
+                sprocket = sprocket.convex_hull
+        return sprocket

run_examples.py

@@ -49,11 +49,12 @@ if __name__ == "__main__":
     logging.debug("Starting run_examples.py execution")
     set_environment()
     
-    examples = [
-        "box_with_door.py",
-        "hollow_box.py",
-        "spur_gear_example.py"
-    ]
+    examples = [
+        "box_with_door.py",
+        "hollow_box.py",
+        "spur_gear_example.py",
+        "sprocket_example.py",
+    ]
     
     for example in examples:
         run_example_script(example)

tests/test_primitives.py

@@ -1,12 +1,13 @@
 import pytest
 import numpy as np
 from parametric_cad.core import tm, safe_difference
-from math import cos, pi
+from math import cos, sin, pi
 
 from parametric_cad.primitives.box import Box
 from parametric_cad.primitives.gear import SpurGear
 from parametric_cad.primitives.cylinder import Cylinder
 from parametric_cad.primitives.sphere import Sphere
+from parametric_cad.primitives.sprocket import ChainSprocket
 
 
 def test_box_mesh_extents_and_position():
@@ -41,3 +42,12 @@ def test_safe_difference_returns_mesh():
     inner = Box(0.5, 0.5, 0.5).at(0.25, 0.25, 0.25)
     result = safe_difference(outer.mesh(), inner.mesh(), engine="invalid")
     assert isinstance(result, tm.Trimesh)
+
+
+def test_chain_sprocket_properties_and_mesh():
+    sprocket = ChainSprocket(pitch=12.7, roller_diameter=7.75, teeth=10)
+    expected_pitch_dia = 2 * sprocket.pitch / (2 * sin(pi / sprocket.teeth))
+    assert sprocket.pitch_diameter == pytest.approx(expected_pitch_dia)
+    mesh = sprocket.mesh()
+    assert isinstance(mesh, tm.Trimesh)
+    assert mesh.is_watertight