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Posted by keen101 
June 28, 2020 11:58AM
Could someone clarify the difference between Z_MIN_POS and MANUAL_Z_HOME_POS?

I am currently modding a Lulzbot mini to have a prusa mini bed. As such i need to write custom marlin firmware for the new bed height not to crash. I am confused at how to use each setting properly.
June 30, 2020 10:05AM
Disclaimer: I am not an expert.
There must be something about a Luzabot printer that I don't know.
My home built CoreXY, and every other printer I have seen, has an adjustable Z-endstop.
Therefore, you would just set that and nothing else changes. I used to swap my bed by laying a piece of glass over a small hotbed. I had to readjust the endstop each time. I now have a nozzle endstop/probe and it is automatic.

Unfortunately, a developer's ability to define things for users is limited.

// Travel limits (mm) after homing, corresponding to endstop positions.
#define Z_MIN_POS 0

Doing a search of all the files, it appears that Z_MIN_POS sets software endstops
You might want this negative if you have a dump area or a nozzle wipe area that is below the bed.
I can't rationalize a positive number for it.

If you define this, Marlin sets Z_HOME_POS to this. That is the only use I find in the search.

// Manually set the home position. Leave these undefined for automatic settings.
// For DELTA this is the top-center of the Cartesian print volume.
//#define MANUAL_Z_HOME_POS 0

Why you would want this anything other than 0, I have no idea.
Changing it would appear to have the same affect as Z offset and would add to Z offset.

I think you just need to experiment if you think you need to change these. But do so carefully. An easily accessible reset button is convenient.
June 30, 2020 10:44AM
Thanks for the reply. I think i figured this one out. Yeah I've been messing with it this week.

The lulzbot default firmware had a z-offset but that was tied to the automatic bed levelling which I temporarily disabled since I no longer have bed levelling washers.

So in essence I am trying to do this on a printer with no bed levelling and therefore no M581 z height offset anymore.

As of now I have Z_MIN_POS 0 and i set #define Z_MAX_POS 149. Z_MAX_POS was originally something like 159. Since the lulzbot mini homes z up at the top and at the max height making the z-axis value smaller works. So in essence i figured it out.

I now have encountered a weird issue when I'm trying to center the x and y values to the center of the bed using #define MANUAL_X_HOME_POS and #define MANUAL_Y_HOME_POS. Setting the define MANUAL_X_HOME_POS to something like -8.5 works perfectly. When i enable the manual y though things go haywire and sometimes a G00 X90 will result in the Y-axis stepper motor also moving! I'm having this issue in both Marlin 2.0 and Marlin 1.9 or whatever the latest version was. However this does not happen with the original lulzbot version of marlin that is much older. I opened a new thread for this issue here: https://reprap.org/forum/read.php?415,874363

So i either encountered a weird bug that no one caught (perhaps because i am the only one not using bed levelling?) or i missed a critical setting somewhere. I will keep messing with it to see what i can figure out.

eventually i plan on adding a pinda probe so bed levelling will come back at some point.
June 30, 2020 11:34AM
The Z-OFFSET is not a function associated with bed leveling directly.
It is to define the difference between where your endstop says Z0 is and where it actually is. This is typically used when using a bed leveling probe as an endstop also. I guess it might be used for a mechanical endstop but usually not.
June 30, 2020 12:25PM
On the lulzbot mini the hotend probe is directly connected to where the mechanical end stop would normally go. Bad engineering.

Edited 1 time(s). Last edit at 06/30/2020 12:25PM by keen101.
June 30, 2020 06:35PM
I don't think that is bad. My probe is a piezo probe activated by the nozzle and connected to the Z-min port.

There are a few things in Marlin to configure for it, but it works well.
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