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xJM - Jerk Maximum
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Sets the maximum jerk value for that axis. Jerk is settable independently for each
axis to support machines with different dynamics per axis - such as Shapeoko with 
belts for X and Y, screws for Z, Probotix with 5 pitch X and Y screws and 12 pitch 
Z screws, and any machine with both linear and rotary axes.

Jerk is in units per minutes^3, so the numbers are quite large (but see note below). 
Some common values are shown in millimeters in the examples below

 XJM = 50,000,000     Set X jerk to 50 million MM per min^3. This is a good value for a 
                      moderate speed machine
 ZJM = 25,000,000     A reasonable setting for a slower Z axis
 XJM = 5,000,000,000  X jerk for Shapeoko. Yes, that's 5 billion

The jerk term in mm is measured in mm/min^3. In inches mode it's units are inches/min^3. 
So the conversion from mm to inches is 1/(25.4). The same values as above are shown in inches are:

50,000,000 mm/min^3      is 1,968,504 in/min^3 2,000,000 would suffice
25,000,000 mm/min^3      is 984,251 in/min^3 1,000,000 would suffice
5,000,000,000 mm/min^3   is 196,850,400 in/min^3 200,000,000 would suffice

Note: Jerk values that are less than 1,000,000 are assumed to be multiplied by 1 million.
This keeps from having to keep track of all those zeros. For example, to enter 5 billion 
the value '5000' can be entered.