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M851 issue

Posted by MikeCL

MikeCL

M851 issue
October 26, 2019 06:52PM

Registered: 5 years ago
Posts: 45

After doing a G28 and then M851 Z0 should the Z value when doing M114 be Z0? Its not showing as 0 to me.. also then doing G0 Z0 my nozzle crashes into the bed hard with a Bltouch.. the nozzle hits first.

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MMcLure

Re: M851 issue
October 28, 2019 01:17PM

Registered: 6 years ago
Posts: 265

The M851 setting determines what Marlin will consider as the value for Z when your z-probe is triggered. So if you run "M851 Z0" and "G28" then it will home X and Y, and then move Z until the sensor triggers, and it will consider that to be Z=0. Assuming you're using your Z probe for homing:

Use "M851 Z0" to reset the Z offset.
Home the printer with "G28". Your LCD should show Z=0 at this point.
Execute "M121" to disable hardware endstops, and "M211 S0" to disable software endstops.
Jog the Z axis down with your LCD until the nozzle just grabs a piece of paper. The LCD should show Z as a negative number, for example Z=-2.1
Run "M851 Zwhatever you measured" - for the example above use "M851 Z-2.1"
Home the printer again with "G28". The nozzle should end up at the same location as before but the Z on the LCD should read the negative of whatever you set for M851 (for example Z=2.1)
Execute "G0 Z0" and the nozzle should just grab a piece of paper and the LCD should show Z=0
Save the value with M500, and copy it into your Configuration.h.

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bulldogbob

Re: M851 issue
February 23, 2020 12:12PM

Registered: 7 years ago
Posts: 8

Hey this could be my problem too , i have setup the Z offset by pronterface like on youtube . I did M851 Z0 first then M500 , M501 and M503 . M211 S0 and set the offset , did M211 S1 , G1 F60 Z0 and it was spot on . But after bedlevel the nozzle went to low . I wil try your advice.