There are a few reasons for a hot end crashing into the bed, but it basically comes down to the Auto-Leveling Probe every time. The Auto-Leveling Probe is an induction sensor that looks for metal. Once the sensor finds metal, it sends a signal back to the electronics to say "Stop! I found the print bed." However, if the signal never reaches the electronics, the Z axis motor will continue to drive the extruder down into the print bed.
Before reading this article, be sure that you have already calibrated your Auto-Leveling Probe. This issue is specific to those who are only seeing crashing on the third probe point.
A failed sensor signal can be a result of wire fatigue. To test this, power ON and connect your Printrbot. First place something metal (screwdriver, pocket knife blade, pliers, etc) under the probe. You should see the red LED light up. This means that the probe is functional. Next, connect via USB and give the "Home All" command (G28). After the X and Y axes home, place your metal object (pliers) under the probe again. You should be able to stop the extruder from lowering by giving the Z axis a "false" home. If the LED comes on, but the Z axis continues to lower, you may need to replace the probe extension wire.
Set up to find metal?
Once you have verified that the probe and extension are functional, home all of the axes again (G28). Now, enter the command G29 into your send line. This gives the Printrbot the command to perform the auto-leveling probes. *NOTE: You must "Home all" (G28) before probing the bed (G29).
If you notice that the probe is not lowering down over metal, you have found your problem. The X offset needs to be adjusted. This problem is common for those who have recently upgraded their Printrbot or accidentally changed the wrong value when calibrating their probe.
In this picture, the probe will crash the bed because it is lowering down over the cut out hole in the print bed. Not to worry. This is a problem easily solved.
Enter M501 into the command line in your software. M501 requests the settings that are loaded onto the electronics on your Printrbot. Line M212 settings should read "M212 X30".
If the M212 line reads "M212 X0 Y0 Z-...", you can adjust it by entering the following lines of commands.
- M212 X30
..."M500" saves the command. "M501" is a readback to make sure that the setting was saved.
Now that you have corrected the X offset, enter the G28 and G29 commands to make sure that your Printrbot is probing properly. Happy printing!