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1TR, TTR, 3YR, 4TR - Travel per Revolution
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TR needs to be set to the distance the mapped axis will move for one revolution of the
motor. e.g. if motor 1 is mapped to the X axis, then $1tr applies to the Xaxis. If the
machine is in mm mode (G21) the TR value for XYZ axes should be entered in mm. If in 
inches mode (G20) XYZ should be entered in inches. ABC axes are always entered in 
degrees. See examples below.

For XYZ the travel-per-revolution value is usually the result of the lead screw pitch
or pulley circumference.

A 10 thread-per-inch (TPI) leadscrew moves 0.100" per revolution. TR in inches would
be 0.100, or 2.54 in mm mode.
 
A 0.500" radius pulley will travel 3.14159" per revolution, absent any other gearing.
A typical value for a Shapeoko or Reprap belt driven machine is on the order of 36.540
mm per revolution. Don't take this as exact - you will need to do your own calibration
on your machine to get this setting exact.

For ABC the travel-per-revolution value is entered in degrees. This value will be 360
degrees for an axis that is not geared down - one revolution = 360 degrees. The value
for a geared rotary axis is 360 divided by the gear ratio. For example, a motor-driven
rotary table with 4 degrees of table movement per handle rotation has a gear ratio of
90:1. The Travel per Revolution value should be set to 4.

Note that the travel-per-revolution is independent of the radius setting in the rotary
axis settings. Set TR first to reflect the gearing, then set any Radius values if that
is needed.

Note that Travel per Revolution is a motor parameter, not an axis parameter as one 
might think. Consider the case of a dual Y gantry with lead screws of different pitch
(how weird). The travel per revolution would be different for each motor.

 11tr = 2.54  Sets motor 1 to a 10 TPI travel from millimeters (2.54 mm per revolution)