Gears: Use proper metric module

with diameter / #teeth instead of circumference / #teeth.

Thanks to nialabert (https://github.com/nialabert) for pointing this
out!

Resolves: #260

boxes/gears.py

@@ -417,7 +417,7 @@ class Gears():
         elif self.options.system == 'DP': # diametral pitch 
             circular_pitch = pi * 25.4 / dimension
         elif self.options.system == 'MM': # module (metric)
-            circular_pitch = dimension
+            circular_pitch = pi * dimension
         else:
             raise ValueError("unknown system '%s', try CP, DP, MM" % self.options.system)
 

boxes/generators/gear.py

@@ -44,8 +44,8 @@ class Gears(Boxes):
             help="percent of the D section of shaft 1 (0 for same as shaft 1)")
 
         self.argparser.add_argument(
-            "--modulus",  action="store", type=float, default=5,
-            help="width of teeth in mm")
+            "--modulus",  action="store", type=float, default=2,
+            help="size of teeth (diameter / #teeth) in mm")
         self.argparser.add_argument(
             "--pressure_angle",  action="store", type=float, default=20,
             help="angle of the teeth touching (in degrees)")