.. _modeling-tips:

*************
Modeling Tips
*************

.. index:: Working in millimeters
.. index:: Modeling setup

.. _modeling-setup:

Modeling Setup
==============

Gears are sized in millimeters. You can save yourself lots of sizing
pain by setting units to millimeters in your scene properties.
All the node images in this reference document use these
`Units` changes so the distances are displayed in millimeters (`3mm`
instead of `0.003`).

.. figure:: /images/setup-scene_properties.png
   :align: center

   The `Scene Properties` panel with `Unit Scale` and `Length` adjusted to
   millimeters.


Aligning Gears
==============

All of the node groups in the gearbox present mechanisms for aligning
meshed gears.

.. glossary::

   Ref. Pitch Radius (output float)
     For circular gears this is the distance from the origin of the
     gear to the pitch circle.

   Pitch height (output float)
     For rack gears this is the distance from the origin to the pitch
     line.

   Half pitch rotation (input boolean)
     If true, will rotate the gear :math:`1/2` a gear tooth.

   Helix / Angle (input angle)
     For any gear with angled teeth the angle must match to mesh with
     the opposing gear.

   Helix / Left twist (input boolean)
     For gears with angled teeth the direction of the helix angle of
     one gear must oppose the other gear.

All circular gears are created with the origin in the center and a
tooth aligned on the X-axis. This is apparent on straight spur gears,
on helical and herringbone gears this tooth is at the lowest Z-axis of
the gear. I've tried to be as consistent as possible so that all gears
can be meshed using the same method.

 1. Join both gears in the geometry.
 2. Add the `Ref. Pitch Radius` from each gear with a math node.
 3. Use a `Translate Geometry` node to move one gear that
    distance. This will align the two gears along their pitch circles.
 4. Use the `Half pitch rotation` option if necessary to align cogs.


Smoothing
=========

My personal preference is to use normal smoothing and tweak the angle
to my liking. In most cases this seems to be :math:`50\%` but
experimentation is advised. This simple smoothing will leave the gears
mathematically correct and are most likely to work if you are printing
gears.

I have experimented with additional subsurface smoothing to good
effect. Due to the nature of the
`Catmull-Clark algorithm
<https://en.wikipedia.org/wiki/Catmull%E2%80%93Clark_subdivision_surface>`_,
it is wise to preface the subsurf with a bevel moderator.
The following example illustrates this (the :ref:`smoothing example
node group <smoothing-node-group>` is included below).

.. figure:: /images/tips-smoothing_no_mod.png
   :align: center
   :width: 300

   Closeup viewport image of two spur gears meshing.

The pitch radius output was used to precisely set the gears. Note the
tightness with this simple setup.

.. figure:: /images/tips-smoothing_with_bevel.png
   :align: center
   :width: 300

   After adding a bevel (Offset, 0.2mm, 1 segment, :math:`30^\circ` angle).

The tightness is maintained, even with a simple bevel. The bevel pulls
the surface away at its specified offset but keeps the meshing intact.

.. figure:: /images/tips-smoothing_with_bevel_subsurf.png
   :align: center

   After adding a subsurf modifier with 2 levels.

You can see the effects of the Catmull-Clark mechanism which has
pulled more of the gear surface away. Contact between gears is
probably fine but you would not want to do this with thin gears. Also
take note how subsurf has *stretched* the edges that form the flank
resolution of the gear.

.. _smoothing-node-group:

.. figure:: /images/tips-smoothing_gn.png
   :align: center
   :width: 600

   Node group used for this example

Experiment with *Z Faces*, which adds faces along the depth to help
control the effects of smoothing.