skew_correction: add documentation

Signed-Off-By:  Eric Callahan <arksine.code@gmail.com>

1 files changed, 52 insertions, 0 deletions

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+Software based skew correction can help resolve dimensional inaccuracies
+resulting from a printer assembly that is not perfectly square.  Note
+that if your printer is significantly skewed it is strongy recommended to
+first use mechanical means to get your printer as square as possible prior
+to applying software based correction.
+
+# Print a Calibration Object
+The first step in correcting skew is to print a
+[calibration object](https://www.thingiverse.com/thing:2563185/files)
+along the plane you want to correct.  There is also a
+[calibration object](https://www.thingiverse.com/thing:2972743)
+that includes all planes in one model.  You want the object oriented
+so that corner A is toward the origin of the plane.
+
+Make sure that the [skew_correction] module is not enabled in printer.cfg
+prior to printing the calibration part.
+
+# Take your measurements
+The [skew_correcton] module requires 3 measurements for each plane you want
+to correct; the length from Corner A to Corner C, the length from Corner B
+to Corner D, and the length from corner A to corner D.  When measuring length
+AD do not include the flats on the corners that some test objects provide.
+
+![skew_lengths](img/skew_lengths.png)
+
+# Update printer.cfg
+Enter your lengths for the calibration print corresponding to the correct
+plane under the [skew_correction] section in printer.cfg, for example:
+```
+[skew_correction]
+xy_ac_length: 140.4
+xy_bd_length: 142.8
+xy_ad_length: 99.8
+```
+Restart your printer.  If desired, you can reprint the calibration part
+with skew correction enabled. The following gcode can be used to calculate
+skew of a single plane and output the results:
+
+```
+CALC_MEASURED_SKEW AC=<ac_length> BD=<bd_length> AD=<ad_length>
+```
+
+# Caveats
+
+When using the [skew_correction] module it is suggested to home all axes
+before attempting a move.  Homing a single axis to move could result in a
+correction along the x and/or y axis, potentially leading to a homing error.
+
+It is also important to keep in mind that it is possible for [skew_correction]
+to generate a correction that moves the tool beyond the printer's boundries
+on the X and/or Y axes.  It is recommended to arrange parts away from the
+edges when using [skew_correction].