Piezo bed leveling for Kossel mini with 220mm bed

thingiverse

Hi, This is a mod of the Auto Bed Leveling using Piezo Discs by elenhinan for the Kossel Mini. Having a rigid printer with zero backlash and all metal components can make very fine prints. I haven't needed auto bed leveling in a long time because my printer is very precise. However, recently I had to print small figurines with a 0.2 nozzle and 0.1 layer height. Every time I change the nozzle, I had to recalibrate the printer. So, having any other kind of bed levelling on the effector would be useless for this purpose. The Piezo Bed Leveling by elenhinan is excellent in this case. The sensors are in the bed mounts and the measurement is done exactly from the nozzle tip with zero offset. So, as my printer is a little unusual, I had to make my own mounts. I made a simple stick with mounts that can deform a little but rigid enough to keep the bed in place. This is done in ABS (not PLA!). The bed can be connected in one of three holes for mounting flexibility using an M3x10mm screw, leaving the other two for the piezo actuator screw also m3x10mm. I am using different screws for the bed and piezo to reduce the piezo disk heating. You can put a small piece of FR4 between the disk and the screw as a heat break. Also, I raised the bed from the mount with some washers and an M4 nut so it won't touch the piezo screw. The Piezo disk (20mm diameter) is stuck to the 20x20 aluminum rail with Kapton tape (one layer as isolation on the rail and one layer over the piezo disk). The mount sits over it with the actuator screw touching it until it puts some tension on the sensor. For the electronics, I used the first schematic uploaded by elenhinan. It works very well for me. You can adjust the sensitivity of the bed sensor with the R8 resistor. I set mine to trigger just by lightly tapping on the bed. I used simple electrical wire for the sensors (piezo disks). Twisting them gives some electromagnetic rejection. I will upgrade the wiring to coaxial microphone cable if needed. To minimize false triggering, I routed the wires away from the stepper motors. A note on the piezo disks: Use simple no-integrated electronic amp, no plastic case disks. Buy more than three (they are very easily damaged during soldering). Test each one with a multimeter by gently bending it in one direction and note the polarity. I found reversed polarity disks in the same batch. They can be used by connecting the wires in reverse. Test each disk by itself on the schematic. The sensor needs to trigger when the bed is pushed down, not when it springs back up. Slowly push the bed with your finger. If everything works fine, the LED will light up when you push on the bed and stay dark when you let go. If it's dark when you push and lights up when you let go, connect that piezo in the other way around. Note that the piezo generates a voltage (negative or positive depending on the direction) when it actually bends, NOT when it stays bent under load. It is not a button/switch. For three mounts, you need: six M3 nuts, six M3x10mm screws, and six T-slot/V-slot nuts and screws (in my case, I use M4).