feat: Torsion joint

nhf/joints.py

@@ -1,5 +1,7 @@
-import cadquery as Cq
+from dataclasses import dataclass
 import math
+import cadquery as Cq
+import nhf.springs as NS
 
 def hirth_joint(radius=60,
                 radius_inner=40,
@@ -174,54 +176,205 @@ def comma_joint(radius=30,
     result.faces('>X').tag("tail_end")
     return result
 
-def torsion_spring(radius=12,
-                   height=20,
-                   thickness=2,
-                   omega=90,
-                   tail_length=25):
-    """
-    Produces a torsion spring with abridged geometry since sweep is very slow in
-    cq-editor.
-    """
-    base = (
-        Cq.Workplane('XY')
-        .cylinder(height=height, radius=radius,
-                  centered=(True, True, False))
-    )
-    base.faces(">Z").tag("mate_top")
-    base.faces("<Z").tag("mate_bottom")
-    result = (
-        base
-        .cylinder(height=height, radius=radius - thickness, combine='s',
-                  centered=(True, True, True))
-        .transformed(
-            offset=(0, radius-thickness),
-            rotate=(0, 0, 0))
-        .box(length=tail_length, width=thickness, height=thickness, centered=False)
-        .copyWorkplane(Cq.Workplane('XY'))
-        .transformed(
-            offset=(0, 0, height - thickness),
-            rotate=(0, 0, omega))
-        .center(-tail_length, radius-thickness)
-        .box(length=tail_length, width=thickness, height=thickness, centered=False)
-    )
-
-    return result
-
 def comma_assembly():
     joint1 = comma_joint()
     joint2 = comma_joint()
-    spring = torsion_spring()
+    spring = NS.torsion_spring()
     result = (
         Cq.Assembly()
         .add(joint1, name="joint1", color=Cq.Color(0.8,0.8,0.5,0.3))
         .add(joint2, name="joint2", color=Cq.Color(0.8,0.8,0.5,0.3))
         .add(spring, name="spring", color=Cq.Color(0.5,0.5,0.5,1))
-        .constrain("joint1?serrated", "spring?mate_bottom", "Plane")
+        .constrain("joint1?serrated", "spring?bot", "Plane")
-        .constrain("joint2?serrated", "spring?mate_top", "Plane")
+        .constrain("joint2?serrated", "spring?top", "Plane")
         .constrain("joint1?tail", "FixedAxis", (1, 0, 0))
         .constrain("joint2?tail", "FixedAxis", (-1, 0, 0))
         .solve()
     )
     return result
 
+@dataclass(frozen=True)
+class TorsionJoint:
+    """
+    This jonit consists of a rider puck on a track puck. IT is best suited if
+    the radius has to be small and vertical space is abundant.
+    """
+
+    # Radius limit for rotating components
+    radius = 40
+    disk_height = 10
+
+    radius_spring = 15
+    radius_axle = 10
+
+    # Offset of the spring hole w.r.t. surface
+    spring_hole_depth = 4
+
+    # Also used for the height of the hole for the spring
+    spring_thickness = 2
+    spring_height = 15
+
+    spring_tail_length = 40
+
+    groove_radius_outer = 35
+    groove_radius_inner = 20
+    groove_depth = 5
+    rider_gap = 2
+    n_slots = 8
+
+    right_handed: bool = False
+
+    def __post_init__(self):
+        assert self.disk_height > self.spring_hole_depth
+        assert self.radius > self.groove_radius_outer
+        assert self.groove_radius_outer > self.groove_radius_inner
+        assert self.groove_radius_inner > self.radius_spring
+        assert self.spring_height > self.groove_depth, "Groove is too deep"
+
+    @property
+    def total_height(self):
+        return 2 * self.disk_height + self.spring_height
+
+    @property
+    def _radius_spring_internal(self):
+        return self.radius_spring - self.spring_thickness
+
+    def _slot_polygon(self, flip: bool=False):
+        r1 = self.radius_spring - self.spring_thickness
+        r2 = self.radius_spring
+        flip = flip != self.right_handed
+        if flip:
+            r1 = -r1
+            r2 = -r2
+        return [
+            (0, r2),
+            (self.spring_tail_length, r2),
+            (self.spring_tail_length, r1),
+            (0, r1),
+        ]
+    def _directrix(self, height, theta=0):
+        c, s = math.cos(theta), math.sin(theta)
+        r2 = self.radius_spring
+        l = self.spring_tail_length
+        if self.right_handed:
+            r2 = -r2
+        # This is (0, r2) and (l, r2) transformed by rotation matrix
+        # [[c, s], [-s, c]]
+        return [
+            (s * r2,         -s * l + c * r2, height),
+            (c * l + s * r2, -s * l + c * r2, height),
+        ]
+
+
+    def spring(self):
+        return NS.torsion_spring(
+            radius=self.radius_spring,
+            height=self.spring_height,
+            thickness=self.spring_thickness,
+            tail_length=self.spring_tail_length,
+        )
+
+    def track(self):
+        groove_profile = (
+            Cq.Sketch()
+            .circle(self.radius)
+            .circle(self.groove_radius_outer, mode='s')
+            .circle(self.groove_radius_inner, mode='a')
+            .circle(self.radius_spring, mode='s')
+        )
+        spring_hole_profile = (
+            Cq.Sketch()
+            .circle(self.radius)
+            .polygon(self._slot_polygon(flip=False), mode='s')
+            .circle(self.radius_spring, mode='s')
+        )
+        result = (
+            Cq.Workplane('XY')
+            .cylinder(radius=self.radius, height=self.disk_height)
+            .faces('>Z')
+            .tag("spring")
+            .placeSketch(spring_hole_profile)
+            .extrude(self.spring_thickness)
+            # If the spring hole profile is not simply connected, this workplane
+            # will have to be created from the `spring-mate` face.
+            .faces('>Z')
+            .placeSketch(groove_profile)
+            .extrude(self.groove_depth)
+            .faces('>Z')
+            .hole(self.radius_axle)
+        )
+        # Insert directrix`
+        result.polyline(self._directrix(self.disk_height),
+                        forConstruction=True).tag("directrix")
+        return result
+
+    def rider(self):
+        def slot(loc):
+            wire = Cq.Wire.makePolygon(self._slot_polygon(flip=False))
+            face = Cq.Face.makeFromWires(wire)
+            return face.located(loc)
+        wall_profile = (
+            Cq.Sketch()
+            .circle(self.radius, mode='a')
+            .circle(self.radius_spring, mode='s')
+            .parray(
+                r=0,
+                a1=0,
+                da=360,
+                n=self.n_slots)
+            .each(slot, mode='s')
+            #.circle(self._radius_wall, mode='a')
+        )
+        contact_profile = (
+            Cq.Sketch()
+            .circle(self.groove_radius_outer, mode='a')
+            .circle(self.groove_radius_inner, mode='s')
+            #.circle(self._radius_wall, mode='a')
+            .parray(
+                r=0,
+                a1=0,
+                da=360,
+                n=self.n_slots)
+            .each(slot, mode='s')
+        )
+        middle_height = self.spring_height - self.groove_depth - self.rider_gap
+        result = (
+            Cq.Workplane('XY')
+            .cylinder(radius=self.radius, height=self.disk_height)
+            .faces('>Z')
+            .tag("spring")
+            .placeSketch(wall_profile)
+            .extrude(middle_height)
+            # The top face might not be in one piece.
+            #.faces('>Z')
+            .workplane(offset=middle_height)
+            .placeSketch(contact_profile)
+            .extrude(self.groove_depth + self.rider_gap)
+            .faces(tag="spring")
+            .circle(self._radius_spring_internal)
+            .extrude(self.spring_height)
+            .faces('>Z')
+            .hole(self.radius_axle)
+        )
+        for i in range(self.n_slots):
+            theta = 2 * math.pi * i / self.n_slots
+            result.polyline(self._directrix(self.disk_height, theta),
+                            forConstruction=True).tag(f"directrix{i}")
+        return result
+
+    def rider_track_assembly(self):
+        rider = self.rider()
+        track = self.track()
+        spring = self.spring()
+        result = (
+            Cq.Assembly()
+            .add(spring, name="spring", color=Cq.Color(0.5,0.5,0.5,1))
+            .add(track, name="track", color=Cq.Color(0.5,0.5,0.8,0.3))
+            .constrain("track?spring", "spring?top", "Plane")
+            .add(rider, name="rider", color=Cq.Color(0.8,0.8,0.5,0.3))
+            .constrain("rider?spring", "spring?bot", "Plane")
+            .constrain("track?directrix", "spring?directrix_bot", "Axis")
+            .constrain("rider?directrix0", "spring?directrix_top", "Axis")
+            .solve()
+        )
+        return result

nhf/springs.py

@@ -0,0 +1,42 @@
+import math
+import cadquery as Cq
+
+def torsion_spring(radius=12,
+                   height=20,
+                   thickness=2,
+                   omega=90,
+                   tail_length=25):
+    """
+    Produces a torsion spring with abridged geometry since sweep is very slow in
+    cq-editor.
+    """
+    base = (
+        Cq.Workplane('XY')
+        .cylinder(height=height, radius=radius,
+                  centered=(True, True, False))
+    )
+    base.faces(">Z").tag("top")
+    base.faces("<Z").tag("bot")
+    result = (
+        base
+        .cylinder(height=height, radius=radius - thickness, combine='s',
+                  centered=(True, True, True))
+        .transformed(
+            offset=(0, radius-thickness),
+            rotate=(0, 0, 0))
+        .box(length=tail_length, width=thickness, height=thickness, centered=False)
+        .copyWorkplane(Cq.Workplane('XY'))
+        .transformed(
+            offset=(0, 0, height - thickness),
+            rotate=(0, 0, omega))
+        .center(-tail_length, radius-thickness)
+        .box(length=tail_length, width=thickness, height=thickness, centered=False)
+    )
+    result.polyline([(0, radius, 0), (tail_length, radius, 0)],
+                    forConstruction=True).tag("directrix_bot")
+    c, s = math.cos(omega * math.pi / 180), math.sin(omega * math.pi / 180)
+    result.polyline([
+        (s * tail_length, c * radius - s * tail_length, height),
+        (c * tail_length + s * radius, c * radius - s * tail_length, height)],
+        forConstruction=True).tag("directrix_top")
+    return result

nhf/test.py

@@ -15,6 +15,11 @@ class TestJoints(unittest.TestCase):
         nhf.joints.hirth_assembly()
     def test_joints_comma_assembly(self):
         nhf.joints.comma_assembly()
+    def test_torsion_joint(self):
+        j = nhf.joints.TorsionJoint()
+        assembly = j.rider_track_assembly()
+        bbox = assembly.toCompound().BoundingBox()
+        self.assertAlmostEqual(bbox.zlen, j.total_height)
 
 
 class TestHandle(unittest.TestCase):