Strav's blog

The bed on my CR-10 printer is definitely not flat.  It seems to be higher around the edges then dip down in the middle.  For small prints, it's OK but the larger ones are more troublesome since the level changes enough to mess up the first layer.  

I found out that there is a bed leveling sensor that Creality makes for the CR-10 called the BLTouch and they only run about $40.  I bought one and tried to follow the included instructions to install it.  Unfortunately, all the website links in the documentation resulted in 404 errors.  I wrote to Creality's support departmant and got a response back but it was very generic and still did not have include necessary information such as where to download the firmware or the progisp program used to flash the firmware to the memory of the CR-10.  

I used a mix of information from a number of sites to get it installed, install new firmware to work with the BLTouch and figure out how to configure it properly.  

These are the links that Creality support sent:

CR-10 ISP Wire brush firmware tutorial
Service tutorial CR 10 online print
Service tutorial CR 10 the BL Touch installation

And here are a number of other relevant links.  The first two were the most helpful.

Installing BLTouch Auto Bed Leveling On The Creality CR-10 3D Printer
BLtouch for any 3D printer - Comprehensive step by step guide
Creality BL Touch Auto Bed Leveling Sensor Installation Tutorial
Marlin 2.0.x firmware - Add preheat profile?
Help! After installing BLTouch, cannot get nozzle close enough to bed to print.

I ended up using the TH3D Unified 2 firmware and compiling my own firmware.  

TH3D CR-10 Firmware
Creality BL Touch Probe Mount Offsets (Metal Bracket)

When all is said and done, these are the steps I used to calibrate the Z-offset:

Use the terminal window in Octoprint.

Send the following commands:

M502 ;reset printer settings to default
M500 ;save default settings
G28 ;home head to middle of bed
G1 F60 Z0 ;move printer head to absolute zero position (defined by EEPROM settings)

Lower Z axis in .1 increments until print head lightly touches piece of paper between print head and bed.  Go back to home screen and note the position of the Z axis.  Enter that number in the M851 command below for +/-x.x

M851 Zx.x ;Z offset - Note that for negative numbers it will be Z-x.x
M500 ;save default settings