docs: Add Bed_Level.md and Probe_Calibrate.md documents

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

docs/Bed_Level.md

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+Bed leveling (sometimes also referred to as "bed tramming") is
+critical to getting high quality prints. If a bed is not properly
+"leveled" it can lead to poor bed adhesion, "warping", and subtle
+problems throughout the print. This document serves as a guide to
+performing bed leveling in Klipper.
+
+It's important to understand the goal of bed leveling. If the printer
+is commanded to a position `X0 Y0 Z10` during a print, then the goal
+is for the printer's nozzle to be exactly 10mm from the printer's
+bed. Further, should the printer then be commanded to a position of
+`X50 Z10` the goal is for the nozzle to maintain an exact distance of
+10mm from the bed during that entire horizontal move.
+
+In order to get good quality prints the printer should be calibrated
+so that Z distances are accurate to within about 25 microns (.025mm).
+This is a small distance - significantly smaller than the width of a
+typical human hair. This scale can not be measured "by eye". Subtle
+effects (such as heat expansion) impact measurements at this scale.
+The secret to getting high accuracy is to use a repeatable process and
+to use a leveling method that leverages the high accuracy of the
+printer's own motion system.
+
+# Choose the appropriate calibration mechanism
+
+Different types of printers use different methods for performing bed
+leveling. All of them ultimately depend on the "paper test" (described
+below). However, the actual process for a particular type of printer
+is described in other documents.
+
+Prior to running any of these calibration tools, be sure to run the
+checks described in the [config check document](Config_checks.md). It
+is necessary to verify basic printer motion before performing bed
+leveling.
+
+For printers with an "automatic Z probe" be sure to calibrate the
+probe following the directions in the
+[Probe Calibrate](Probe_Calibrate.md) document. For delta printers,
+see the [Delta Calibrate](Delta_Calibrate.md) document.
+
+During calibration it may be necessary to set the printer's Z
+`position_min` to a negative number (eg, `position_min = -2`). The
+printer enforces boundary checks even during calibration
+routines. Setting a negative number allows the printer to move below
+the nominal position of the bed, which may help when trying to
+determine the actual bed position.
+
+# The "paper test"
+
+The primary bed calibration mechanism is the "paper test". It involves
+placing a regular piece of "copy machine paper" between the printer's
+bed and nozzle, and then commanding the nozzle to different Z heights
+until one feels a small amount of friction when pushing the paper back
+and forth.
+
+It is important to understand the "paper test" even if one has an
+"automatic Z probe". The probe itself often needs to be calibrated to
+get good results. That probe calibration is done using this "paper
+test".
+
+In order to perform the paper test, cut a small rectangular piece of
+paper using a pair of scissors (eg, 5x3 cm). The paper generally has a
+width of around 100 microns (0.100mm). (The exact width of the paper
+isn't crucial.)
+
+The first step of the paper test is to inspect the printer's nozzle
+and bed. Make sure there is no plastic (or other debris) on the nozzle
+or bed.
+
+**Inspect the nozzle and bed to ensure no plastic is present!**
+
+If one always prints on a particular tape or printing surface then one
+may perform the paper test with that tape/surface in place. However,
+note that tape itself has a width and different tapes (or any other
+printing surface) will impact Z measurements. Be sure to rerun the
+paper test to measure each type of surface that is in use.
+
+If there is plastic on the nozzle then heat up the extruder and use a
+metal tweezers to remove that plastic. Wait for the extruder to fully
+cool to room temperature before continuing with the paper test. While
+the nozzle is cooling, use the metal tweezers to remove any plastic
+that may ooze out.
+
+**Always perform the paper test when both nozzle and bed are at room
+temperature!**
+
+When the nozzle is heated, its position (relative to the bed) changes
+due to thermal expansion. This thermal expansion is typically around a
+100 microns, which is about the same width as a typical piece of
+printer paper. The exact amount of thermal expansion isn't crucial,
+just as the exact width of the paper isn't crucial. Start with the
+assumption that the two are equal (see below for a method of
+determining the difference between the two widths).
+
+It may seem odd to calibrate the distance at room temperature when the
+goal is to have a consistent distance when heated. However, if one
+calibrates when the nozzle is heated, it tends to impart small amounts
+of molten plastic on to the paper, which changes the amount of
+friction felt. That makes it harder to get a good calibration.
+Calibrating while the bed/nozzle is hot also greatly increases the
+risk of burning oneself. The amount of thermal expansion is stable, so
+it is easily accounted for later in the calibration process.
+
+**Use an automated tool to determine precise Z heights!**
+
+Klipper has several helper scripts available (eg, MANUAL_PROBE,
+Z_ENDSTOP_CALIBRATE, PROBE_CALIBRATE, DELTA_CALIBRATE). Choose one of
+them and follow the directions in the documents described above.
+
+Run the appropriate command in the OctoPrint terminal window. The
+script will prompt for user interaction in the OctoPrint terminal
+output. It will look something like:
+```
+Recv: // Starting manual Z probe. Use TESTZ to adjust position.
+Recv: // Finish with ACCEPT or ABORT command.
+Recv: // Z position: ?????? --> 5.000 <-- ??????
+```
+
+The current height of the nozzle (as the printer currently understands
+it) is shown between the "--> <--". The number to the right is the
+height of the last probe attempt just greater than the current height,
+and to the left is the last probe attempt less than the current height
+(or ?????? if no attempt has been made).
+
+Place the paper between the nozzle and bed. It can be useful to fold a
+corner of the paper so that it is easier to grab. (Try not to push
+down on the bed when moving the paper back and forth.)
+
+![paper-test](img/paper-test.jpg)
+
+Use the TESTZ command to request the nozzle to move closer to the
+paper. For example:
+```
+TESTZ Z=-.1
+```
+
+The TESTZ command will move the nozzle a relative distance from the
+nozzle's current position. (So, `Z=-.1` requests the nozzle to move
+closer to the bed by .1mm.) After the nozzle stops moving, push the
+paper back and forth to check if the nozzle is in contact with the
+paper and to feel the amount of friction. Continue issuing TESTZ
+commands until one feels a small amount of friction when testing with
+the paper.
+
+If too much friction is found then one can use a positive Z value to
+move the nozzle up. It is also possible to use `TESTZ Z=+` or `TESTZ
+Z=-` to "bisect" the last position - that is to move to a position
+half way between two positions. For example, if one received the
+following prompt from a TESTZ command:
+```
+Recv: // Z position: 0.130 --> 0.230 <-- 0.280
+```
+Then a `TESTZ Z=-` would move the nozzle to a Z position of 0.180
+(half way between 0.130 and 0.230). One can use this feature to help
+rapidly narrow down to a consistent friction. It is also possible to
+use `Z=++` and `Z=--` to return directly to a past measurement - for
+example, after the above prompt a `TESTZ Z=--` command would move the
+nozzle to a Z position of 0.130.
+
+After finding a small amount of friction run the ACCEPT command:
+```
+ACCEPT
+```
+This will accept the given Z height and proceed with the given
+calibration tool.
+
+The exact amount of friction felt isn't crucial, just as the amount of
+thermal expansion and exact width of the paper isn't crucial. Just try
+to obtain the same amount of friction each time one runs the test.
+
+If something goes wrong during the test, one can use the `ABORT`
+command to exit the calibration tool.
+
+# Determining Thermal Expansion
+
+After successfully performing bed leveling, one may go on to calculate
+a more precise value for the combined impact of "thermal expansion",
+"width of the paper", and "amount of friction felt during the paper
+test".
+
+This type of calculation is generally not needed as most users find
+the simple "paper test" provides good results.
+
+The easiest way to make this calculation is to print a test object
+that has straight walls on all sides. The large hollow square found in
+[docs/prints/square.stl](prints/square.stl) can be used for this.
+When slicing the object, make sure the slicer uses the same layer
+height and extrusion widths for the first level that it does for all
+subsequent layers. Use a coarse layer height (the layer height should
+be around 75% of the nozzle diameter) and do not use a brim or raft.
+
+Print the test object, wait for it to cool, and remove it from the
+bed. Inspect the lowest layer of the object. (It may also be useful to
+run a finger or nail along the bottom edge.) If one finds the bottom
+layer bulges out slightly along all sides of the object then it
+indicates the nozzle was slightly closer to the bed then it should
+be. One can issue a `SET_GCODE_OFFSET Z=+.010` command to increase the
+height. In subsequent prints one can inspect for this behavior and
+make further adjustment as needed. Adjustments of this type are
+typically in 10s of microns (.010mm).
+
+If the bottom layer consistently appears narrower than subsequent
+layers then one can use the SET_GCODE_OFFSET command to make a
+negative Z adjustment. If one is unsure, then one can decrease the Z
+adjustment until the bottom layer of prints exhibit a small bulge, and
+then back-off until it disappears.
+
+The easiest way to apply the desired Z adjustment is to create a
+START_PRINT g-code macro, arrange for the slicer to call that macro
+during the start of each print, and add a SET_GCODE_OFFSET command to
+that macro. See the [slicers](Slicers.md) document for further
+details.