docs: Add a FAQ document

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/FAQ.md

@@ -0,0 +1,103 @@
+Frequently asked questions
+==========================
+
+### How do I calculate the step_distance parameter in the printer config file?
+
+If you know the steps per millimeter for the axis then use a
+calculator to divide 1.0 by steps_per_mm. Then round this number to
+six decimal places and place it in the config (six decimal places is
+nano-meter precision).
+
+The step_distance defines the distance that the axis will travel on
+each motor driver pulse. It can also be calculated from the axis
+pitch, motor step angle, and driver microstepping. If unsure, do a web
+search for "calculate steps per mm" to find an online calculator.
+
+### When I restart my micro-controller the device changes to /dev/ttyACM1
+
+If the device sometimes changes from /dev/ttyACM0 to /dev/ttyACM1 then
+ssh into the host computer and run: ```ls -l /dev/serial/by-id/```
+
+The name found in the above command is stable and it is possible to
+use it in the config file. For example, an updated config might look
+like:
+```
+[mcu]
+serial: /dev/serial/by-id/usb-UltiMachine__ultimachine.com__RAMBo_12345678912345678912-if00
+```
+
+Be sure to copy-and-paste the name from the "ls" command above as the
+name will be different on each printer.
+
+### Can I run Klipper on something other than a Raspberry Pi?
+
+Klipper only requires Python running on a Linux (or similar)
+computer. However, if you wish to run it on a different machine you
+will need Linux admin knowledge to install the system prerequisites
+for the Linux distribution running on that particular machine. See the
+[install-octopi.sh](../scripts/install-octopi.sh) script for further
+information on the necessary Linux admin steps.
+
+### Why can't I move the stepper before homing the printer?
+
+The code does this to reduce the chance of accidentally commanding the
+head into the bed or a wall. Once the printer is homed the software
+attempts to verify each move is within the position_min/max defined in
+the config file. If the motors are disabled (via an M84 or M18
+command) then the motors will need to be homed again prior to
+movement.
+
+If you want to move the head after canceling a print via Octoprint,
+consider changing the Octoprint cancel sequence to do that for
+you. It's configured in Octoprint via a web browser under:
+Settings->GCODE Scripts
+
+If you want to move the head after a print finishes, consider adding
+the desired movement to the "custom g-code" section of your slicer.
+
+### Why is the Z position_endstop set to 0.5 in the default configs?
+
+For cartesian style printers the Z position_endstop specifies how far
+the nozzle is from the bed when the endstop triggers. If possible, it
+is recommended to use a Z-max endstop and home away from the bed (as
+this reduces the potential for bed collisions). However, if one must
+home towards the bed then it is recommended to position the endstop so
+it triggers when the nozzle is still a small distance away from the
+bed. This way, when homing the axis, it will stop before the nozzle
+touches the bed.
+
+Almost all mechanical switches can still move a small distance
+(eg, 0.5mm) after they are triggered. So, for example, if the
+position_endstop is set to 0.5mm then one may still command the
+printer to move to Z0.2. The position_min config setting (which
+defaults to 0) is used to specify the minimum Z position one may
+command the printer to move to.
+
+Note, the Z position_endstop specifies the distance from the nozzle to
+the bed when the nozzle and bed (if applicable) are hot. It is typical
+for thermal expansion to cause nozzle expansion of around .1mm, which
+is also the typical thickness of a sheet of printer paper. Thus, it is
+common to use the "paper test" to confirm calibration of the Z
+height - check that the bed and nozzle are at room temperature, check
+that there is no plastic on the head or bed, home the printer, place a
+piece of paper between the nozzle and bed, and repeatedly command the
+head to move closer to the bed checking each time if you feel a small
+amount of friction when sliding the paper between bed and nozzle - if
+all is calibrated well a small amount of friction would be felt when
+the height is at Z0.
+
+### I converted my config from Marlin and the X/Y axes work fine, but I just get a screeching noise when homing the Z axis
+
+Short answer: Try reducing the max_z_velocity setting in the printer
+config. Also, if the Z stepper is moving in the wrong direction, try
+inverting the dir_pin setting in the config (eg, "dir_pin: !xyz"
+instead of "dir_pin: xyz").
+
+Long answer: In practice Marlin can typically only step at a rate of
+around 10000 steps per second. If it is requested to move at a speed
+that would require a higher step rate then Marlin will generally just
+step as fast as it can. Klipper is able to achieve much higher step
+rates, but the stepper motor may not have sufficient torque to move at
+a higher speed. So, for a Z axis with a very precise step_distance the
+actual obtainable max_z_velocity may be smaller than what is
+configured in Marlin.

docs/Installation.md

@@ -125,4 +125,6 @@ the "help" command to get a list of other extended commands.
 Contacting the developers
 =========================
 
-See the [contact page](Contact.md) to ask questions or report a bug.
+Be sure to see the [FAQ](FAQ.md) for answers to some common questions.
+See the [contact page](Contact.md) to report a bug or to contact the
+developers.

docs/Overview.md

@@ -13,7 +13,8 @@ as a reference for the config file. The
 on tuning the pressure advance config.
 
 The [kinematics](Kinematics.md) document provides some technical
-details on how Klipper implements motion.
+details on how Klipper implements motion. The [FAQ](FAQ.md) answers
+some common questions.
 
 The history of Klipper releases is available at
 [releases](Releases.md). See [contact](Contact.md) for information on