Calculate pitch diameter and angular backlash via expressions

Using expressions instead of filling these values in the `execute` method
has the advantage that the values can be used in other expressions on the
same object. First, the expressions are evaluated, then the feature is
"executed". Previously, the changed values are only available after the
execution and thus had no effect if e.g. used in calculating the
placement of the very same gear (until another recompute happens).

Author: jbaehr

freecad/gears/features.py

@@ -188,8 +188,13 @@ def add_computed_properties(self, obj):
                         "computed", "outside diameter", 1)
         obj.addProperty("App::PropertyLength", "df",
                         "computed", "root diameter", 1)
-        obj.addProperty("App::PropertyLength", "dw",
-                        "computed", "pitch diameter", 1)
+        obj.addProperty("App::PropertyLength", "dw", "computed", "The pitch diameter.")
+        obj.setExpression('dw', 'teeth * module') # calculate via expression to ease usage for placement
+        obj.setEditorMode('dw', 1) # set read-only after setting the expression, else it won't be visible. bug?
+        obj.addProperty("App::PropertyAngle", "angular_backlash", "computed",
+            "The angle by which this gear can turn without moving the mating gear.")
+        obj.setExpression('angular_backlash', 'backlash / dw * 360° / pi') # calculate via expression to ease usage for placement
+        obj.setEditorMode('angular_backlash', 1) # set read-only after setting the expression, else it won't be visible. bug?
         obj.addProperty("App::PropertyLength", "transverse_pitch",
                         "computed", "transverse_pitch", 1)
 
@@ -225,7 +230,6 @@ def generate_gear_shape(self, fp):
         fp.gear._update()
 
         # computed properties
-        fp.dw = "{}mm".format(fp.gear.dw)
         fp.transverse_pitch = "{}mm".format(fp.gear.pitch)
         fp.da = "{}mm".format(fp.gear.da)
         fp.df = "{}mm".format(fp.gear.df)
@@ -350,7 +354,13 @@ def add_limiting_diameter_properties(self, obj):
                         "computed", "root diameter", 1)
 
     def add_computed_properties(self, obj):
-        obj.addProperty("App::PropertyLength", "dw", "computed", "pitch diameter", 1)
+        obj.addProperty("App::PropertyLength", "dw", "computed", "The pitch diameter.")
+        obj.setExpression('dw', 'teeth * module') # calculate via expression to ease usage for placement
+        obj.setEditorMode('dw', 1) # set read-only after setting the expression, else it won't be visible. bug?
+        obj.addProperty("App::PropertyAngle", "angular_backlash", "computed",
+            "The angle by which this gear can turn without moving the mating gear.")
+        obj.setExpression('angular_backlash', 'backlash / dw * 360° / pi') # calculate via expression to ease usage for placement
+        obj.setEditorMode('angular_backlash', 1) # set read-only after setting the expression, else it won't be visible. bug?
         obj.addProperty("App::PropertyLength", "transverse_pitch", "computed", "transverse_pitch", 1)
         obj.addProperty("App::PropertyLength", "outside_diameter", "computed", "Outside diameter", 1)
 
@@ -394,7 +404,6 @@ def generate_gear_shape(self, fp):
         fp.gear._update()
 
         # computed properties
-        fp.dw = "{}mm".format(fp.gear.dw)
         fp.transverse_pitch = "{}mm".format(fp.gear.pitch)
         fp.outside_diameter = fp.dw + 2 * fp.thickness
         # checksbackwardcompatibility:
@@ -876,7 +885,6 @@ def __init__(self, obj):
         obj.head = 0
         obj.head_fillet = 0
         obj.root_fillet = 0
-        obj.dw = obj.module * obj.teeth
         obj.Proxy = self
 
     def add_helical_properties(self, obj):
@@ -898,7 +906,13 @@ def add_cycloid_properties(self, obj):
         obj.addProperty("App::PropertyFloat", "outer_diameter", "cycloid", "outer_diameter divided by module (epicycloid)")
 
     def add_computed_properties(self, obj):
-        obj.addProperty("App::PropertyLength", "dw", "computed", "pitch diameter", 1)
+        obj.addProperty("App::PropertyLength", "dw", "computed", "The pitch diameter.")
+        obj.setExpression('dw', 'teeth * module') # calculate via expression to ease usage for placement
+        obj.setEditorMode('dw', 1) # set read-only after setting the expression, else it won't be visible. bug?
+        obj.addProperty("App::PropertyAngle", "angular_backlash", "computed",
+            "The angle by which this gear can turn without moving the mating gear.")
+        obj.setExpression('angular_backlash', 'backlash / dw * 360° / pi') # calculate via expression to ease usage for placement
+        obj.setEditorMode('angular_backlash', 1) # set read-only after setting the expression, else it won't be visible. bug?
 
     def generate_gear_shape(self, fp):
         fp.gear.m = fp.module.Value
@@ -976,6 +990,13 @@ def __init__(self, obj):
             "The arc length on the pitch circle by which the tooth thicknes is reduced.")
         obj.addProperty("App::PropertyPythonObject", "gear", "base", "test")
         obj.addProperty("App::PropertyAngle", "beta","helical", "angle used for spiral bevel-gears")
+        obj.addProperty("App::PropertyLength", "dw", "computed", "The pitch diameter.")
+        obj.setExpression('dw', 'teeth * module') # calculate via expression to ease usage for placement
+        obj.setEditorMode('dw', 1) # set read-only after setting the expression, else it won't be visible. bug?
+        obj.addProperty("App::PropertyAngle", "angular_backlash", "computed",
+            "The angle by which this gear can turn without moving the mating gear.")
+        obj.setExpression('angular_backlash', 'backlash / dw * 360° / pi') # calculate via expression to ease usage for placement
+        obj.setEditorMode('angular_backlash', 1) # set read-only after setting the expression, else it won't be visible. bug?
         obj.gear = self.bevel_tooth
         obj.module = '1. mm'
         obj.teeth = 15