Mars Pro WILL NOT ZERO - HELP !

[cliffyk]

The firmware has only one absolute reference for a "zero" position--that is the optical limit switch at the base of the Z-axis mast. It calculates all other Z-axis positions logically based upon the number of "pulses" (steps) it has submitted to the stepper motor since determining the physical lower limit.

Without first dropping to the hardware limited empirical "zero" position it has no idea where the trolley is actually positioned, other than it's accumulated history of stepper motor commands. This is why in initial startup of a print it doesn't matter where the logical "Z=0" position has been set--the trolley nust drop to the hard limit to get "it's bearings". It is not ignoring the logical "Z=0" setting, it's just that that setting is meaningless without knowing where the real bottom is.

The "Z=0" function only sets a "logical" "zero" (home) position, an offset from the physical zero point--it has nothing to do with vertical calibration and planar/axial  alignment¹ of the build plate when the Z-axis trolley is physically positioned at it's lowest point (as moved to by the "Home" button alignment function, and as determined by the printer's firmware sensing the lower limit switch's state change). Read more about "Z-0" here.

If at the bottom of the Z-axis travel your printer is "grinding" (I.e. the stepper motor is not turning and "cogging") the possible problems are: a) miscalibration of the build plate; b) the trolley is jamming up in the rails; or c) the optical limit switch is faulty.

The limit switch can be tested as follows:

• remove the build plate from the trolley;
  
• press the "Home" button;
  
• the trolley should fall, stop, then rise 4-5 mm and fall again, then finally rise just a tiny bit;
  

If it does this the limit switch is OK and the problem is most likely calibration/misalignment of the build plate.

To calibrate it properly do this:

• place 2 layers of common (20 lb) printer paper on the LCD screen;
  
• loosen the build plate lock screws;
  
• press the "Home" button;
  
• when the trolley comes to rest adjust the build plate so it's edges are squarely aligned with the LCD's edges;
  
• press down LIGHTLY (you're not cooking a Cuban sandwich) on the build plate;
  
• snug up the front locking screw, the the side screw; 
  

I have never used, or found any need for the "Z=0" function, which Elegoo describes as a way to adjust the vertical calibration "on the fly" as opposed to performing the complete physical calibration and alignment (as might be done in production runs when alignment of the build plate was not needed).

Note that I said "snug up", not tighten. You are not replacing a wheel on an F-250 "dually". Many users stripped the socket set screws supplied with theearly MARS units (which were not a poor design choice, but chosen with a missing knowledge of what "tighten" means to the average American) because of overzealous tightening of same.

I would also strongly recommend removing the lock screws and filing/grinding off the ridges that are artifacts of the thread forming process--a ball end would be best:



They will almost certainly cause the split sleeve that clamps the build plate mounting ball to shift when tightened (or even "snugged"). I do this also to the M8 screw that clamps the build plate assembly to the trolley, and the small thumbscrews that clamp the platen in place. It prevents those ugly ridges from chewing up the aluminum on the trolley bracket and resin vat frame.

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¹- I do wish the misleading and just plain wrong term leveling could go away--with our printers there is no act of "leveling" involved in the vertical calibration and alignment of the build plate--unlike with filament deposition printers where the build plate is actually leveled.