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diff --git a/docs/Bed_Level.md b/docs/Bed_Level.md new file mode 100644 index 00000000..4fd28f23 --- /dev/null +++ b/docs/Bed_Level.md @@ -0,0 +1,211 @@ +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.) + + + +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. |