I have just built my first 3D printer, and I am having some problems. It is a Cartesian based 3D printer, using Marlin firmware and Pronterface software. My problem is homing the 3D printer. I do not have a probe to calibrate the z offset but I have all 6 endstops. The problem is that the "ZMIN" endstop isn't precise enough and my hotend is always either too far from or too close to the heatbed. Is there a way to manually set the home position, so when I start the printer, It just starts printing and it doesn't have to home again; Or maybe some other way to set the correct offset. It would also be helpful if I could use just the "ZMAX" plug, and then manually set the minimum Z position using a piece of paper.
It is possible to "home a printer" without having endstops, technically, you don't require endstops, but it makes your job a lot easier if you want to print something!
Basically, when you don't have endstops or limit switches, you need to define where the head of the printer is located. E.g. you can set the nozzle at [0, 0, 0] (origin in [x, y, z]) and add the command
G92 to your print G-code file that it is at that position using
G92 X0 Y0 Z0 (or any other location you use, e.g. you could engineer a parking position and refer to that position instead, note that you also need to write the movement commands to get out of that location safely). Don't forget to remove the homing command from your start code in your slicer, replace
G28 with the
G92 command with appropriate X, Y and Z values.
Regarding the inaccurate repetitive accuracy of your Z min endstop, it might be worth to find out why this is causing such a spread in triggering, maybe you need to invest in some new endstop switches or look more closely to the heated bed attachment to the frame.
Using Z max as a reference point is e.g. used by Ultimaker machines. The heated platform lowers to Z max; the printer knows from calibration and geometry how far it needs to rise to get to Z=0.
You can reconfigure marlin to use the MAXZ endstop instead of MINZ in Configuration.h, and then reflash it to your printer.
Now this is highly unusual, as most printers only have MIN endstops. If you post a pic of your faulty MINZ endstop, we can probably figure out how you could fix it. Zipties come to mind as a likely solution.