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