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Setting up automatic probing of the print bed

The first layer of a print must be printed at the right height, to within perhaps 50 microns, everywhere across the print bed, to ensure that the print sticks to the bed and the nozzle doesn't crash into it. For a Cartesian printer this may be as simple as ensuring that the bed is level, but a delta printer has a number of additional parameters to adjust. Even if your printer is well calibrated and unchanging, a high-quality Z probe may be more repeatable than your endstops. It is often useful to run a quick autoprobe before every print (and not re-home afterwards). Setting up such an autoprobe requires a certain amount of configuration.

See Connecting a Z probe for details on the electrical connections.

Instructions on setting up the mini differential IR probe, also from dc42, are available.

Choosing which types(s) of probing to use

RepRapFirmware supports two different types of Z probing:

  • G32 probing uses a bed.g file to define the probe points individually. On a Delta printer it is used for auto calibration. On a Cartesian or CoreXY printer it can be used for bed compensation, however the number of probe points is limited to 5.
  • G29 probing uses a grid of points defined by its size and the spacing between points. You can probe a large number of points without having to define their coordinates individually. It is only used for bed compensation.

On a Delta printer, you can if you wish use G32 to perform auto calibration followed by G29 to compensate for any residual bed height errors. On a Cartesian or CoreXY printer you can use either G32 bed probing or G29 probing to do bed compensation, but not both at the same time. If your Z probe has a constant trigger height regardless of XY position, then G29 bed probing is recommended.

The remainder of this page describes G32 bed probing. For G29 bed probing, see Using mesh bed compensation.

Setting up the probe points for G32 probing

The autoprobing command, G32, probes the bed at some number of points and then tells the printer to do a least-squares fit for some of the parameters. The set of points and number of parameters is set in the file bed.g. The easiest way to make such a file is with the online calculator.

Because the fitting process is a least-squares process, measurement errors from different points will average out. If your surface is somewhat rough, or if your Z probe is somewhat "noisy", increasing the number of points will have a similar effect to repeatedly probing the same points: the averaging process will improve the fitting process as the square root of the number of points. Currently the firmware limit is something like 16, but it may be as high as 64. (Obviously if you can it is better to use a smoother surface and a better sensor.)

Some sensors produce height errors that depend on the (X,Y) position. For example, a slightly misaligned delta robot will have an effector platform that tilts as it moves around the bed; if the sensor is not exactly at the same place as the nozzle, this will slightly change the height readings. If this is a problem with your printer, you can provide a height correction to each probe point with the H parameter. These would be determined by manually measuring the right height (perhaps with the paper test), probing at the same position, and recording the difference.

Setting probe parameters

The most important probe parameter is the height of the nozzle when the probe triggers. This will determine the height of the first layer. It is set with the G31 command, usually placed in your config.g. The process of determining this value is fairly straightforward:

  • Move the nozzle until it is at a good Z=0 height (for example a height that will touch but not pin a piece of paper against the bed).
  • Set this height as Z=0 with G92 Z0
  • Move the print head up perhaps a centimeter
  • Run a single probe without updating the printer's coordinates: G30 S-1
  • Use M114 to find the Z height. This is the value to put in your G31 command
  • If, in later prints, you find the first layer is too thick or too thin, adjust the value in your G31 command

Sensor offset adjustment (H parameter in bed and bed_report macro)

Mechanical sensors (FSR and other switches types) have offsets which may vary with the head position, especially on delta printers. The offset shall be adjusted for each probe point.

For this purpose, it is practical to create a macro for each probe point , moving to this point: p0.g, p1.g, etc.

On delta printers, depending the sensor type, you may have more offset on the side than in center or the reverse (bed sensors like FSR have more offset in the center while effector sensors have more offset on the sides). Bed, bed_report and p0.g to p6.g (or more) macros shall have same coordinates and be adjusted while you displace probing points, which may notably be needed if your bed surface is weared (as could occurred easily with BuildTak) or for a different usable diameter.

To test the offset, you shall :

  • Do at least 3 calibrations till you get consistent results.
  • Execute M665 and note the reported parameters to modify your configuration file.

Please note that if you use the homing macro without end switches, the height in M665 will not be used and the height shall be modified in the homedelta.g

Move to the calibration points with the P0 to P6 macros and lower the head manually atop a paper sheet till the paper sheet began to have difficulties to slide. Repeat for all the probe points, note the values on a paper. You shall then modify the H parameters in the bed.g file and reload this file on the SD card. Use web interface system panel (not macro panel) for that.

Values found shall be substracted to the already defined offset, meaning that if you find +0,1 and the offset is already -0,22, you shall do -0,22-(+0,1) = -0,32

If using a ‘BuildTak’ surface, you shall not ‘crush’ the filament on the surface and the resulting value while paper began not to slide shall be within – 0.1~0,12 mm , so you shall substract 0,12 to lift up the hotend, say if offset is -0,25 while paper is sliding at exactly 0, you shall set -0,36.

After adjustment of H parameters in bed.g, modify the bed_report.g with same values, then reload bed_report.g on the SD card and run the macro.

If your paper report is neat, but the results of the calibration and bed_reports shows differences, that means that your bed is not flat. What count is the paper measure, not the calibration result, so do not cheat by modifying the H parameter to obtain a very low calibration value.

Note that all macros could be run after being loaded on the SD card, without rebooting or restarting.

Probing-related G-code

  • G30 - run a single probe. Leaves the probe at its trigger height; by default sets Z=0 based on this, but parameters can change what to do with the probe results. This defaults to setting Z=0 but it can store the value or run least-squares fitting on the stored values.
  • G31 - set up probe, in particular the Z height at which the probe triggers; note the firmware supports multiple probes, so you must run M558 first to specify which one.
  • G32 - run a full autoprobe (that is, execute bed.g)
  • G92 - redefine coordinates so that current position is zero (or values specified)
  • M114 - report current position; this is one way to get the results from G30 S-1 (probe but don't change anything).
  • M558 - set probe type (e.g. analog); also select which probe to use, so run this early in the setup process
  • M561 - clear all autocalibration or G92 settings
  • M665 - set delta configuration; this includes the manual calibration values that are the starting point for autocalibration. You can also use this command after an autoprobe to see the fitted values
  • M666 - set delta endstop adjustment; this includes the manual calibration values that are the starting point for autocalibration. You can also use this command after an autoprobe to see the fitted values
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