Updated: Easier Leveling the Build Plate!

[shdwsng]

I wrote a thread awhile back about how to make leveling the build plate that first time easier, because it drove me nuts. I kept running into the same problem, and managed to put my machine out of order for a couple of months thanks to user error never being able to level it right. Today, I got in and managed to fix it and thought I'd share the updated flash of insight with actual numbers. 

The instructions about how to level with a piece of paper are kind of garbage, and prone to all sorts of adjustments and error. However, there's an easier way to do this. Loosen the 2 set screws on the build plate to allow it to move freely. Hit the "Home" button to bring the build plate down as low as it'll go, so that the springs are compressed against the LCD screen without the resin tank in place. Square the build plate up so that it's oriented correctly with the LCD screen, and while it is still sitting on the bottom of the tank tighten the set screws on the side. Tighten the screw on the right first, and when that's in as far as it'll go then tighten the front screw.

This is where it gets interesting. The official FEP according to the packaging is .15 mm, and most slices are .02 to .05 mm. So while it's down all the way set the lift height to 0.1 mm and hit the button to go up twice. This will clear the FEP and give you approximately .5 mm for the resin to flow. Back out of that menu and then hit Set Z = 0.  That'll ensure that the build plate is as level as the machine can make it, and it takes the guesswork out of "is the paper sliding freely enough".

These instructions work if you're using the official Elegoo Mars FEP. If you are using an aftermarket one that is thicker, use the manufacturer's instructions to sort out exactly how thick it is, add .05 mm to that, and use that to sort out how high you need to raise the build plate before setting Z to 0. Also, you can get a set of feeler gauges from any auto parts store or Harbor Freight here in the USA. Take the .2 mm feeler gauge and slide it around all sides to make sure things slip freely. It's okay if it's a tight fit, because it SHOULD be a tight fit. You just want to ensure that it doesn't bind in spots. If that happens, then you probably didn't tighten the screws enough and it's not level. Loosen everything, hit the home button, and tighten them again more securely once the build plate is tight on the LCD. If you start a dry run with a print and it heads to the bottom and just grinds then your FEP isn't flush with the LCD screen and you need to repeat these instructions except set Z=0 at .3 mm instead.

[cliffyk]

There is a potential conflict between the alignment and calibration procedure you describe, and the printer's fundamental operation.

The sequence of the first downward motion of the trolley at the start of a print job is as follows:

• the firmware commands the stepper motor to turn clockwise (facing the shaft) to lower the trolley;
  
• the trolley falls until it triggers the optical lower limit switch, causing firmware to stop the stepper motor;
  
• the firmware commands the stepper motor to turn anti-clockwise, raising the trolley ≈ 5-6 mm;
  
• as a validation the firmware again commands the motor to turn clockwise lowering the trolley 'til the limit switch once more triggers;
  
• the firmware resets it's internal "turn counter" variable; (a value of 0 = the physical zero trolley position;
  
• if a logical "zero" position has been set (via the "Z=0" GUI function), and the "print" file contains a "go to 0" G-Code command the motor is commanded to turn anti-clockwise the required number of steps¹ to relocate the platen to the set logical "zero" + one layer thickness (25 steps using your +0.2 mm logical "zero" setting + one 0.05 mm layer)--else it just moves up by one layer thickness (5 steps for a 50 micron layer)
  

It is vital to understand that in printing, in response to a G-Code "goto zero" command, the Z-axis ALWAYS first moves to the physical zero position (as determined by the limit switch) to establish the actual zero starting point--it has to to determine the trolley's actual zero reference. It and then rises to the logical "zero" position if set. Also, the "Home" function will always move the trolley to thee physical zero position regardless of how the logical "Z=0" position may have been set.

The only way to position the trolley to the logical "zero" position--other than manual re-positioning after the "Home" button has been pressed--is via G-Code commands ("G28 Z", "G 90 G1 Z0", etc.) in a "print" file.

I have validated this behaviour numerous time with a dial indicator.

After attaining the physical zero position the firmware maintains a running tally of the number of "up" and "down' motor steps it has commanded to maintain an accumulated value indicating the Z-axis current position I.e. where it is "supposed to/should be". If the trolley jams in the vertical rails, from being too tight in the rails or physically blocked, the actual position vs. the calculated "should be" position will be incorrect.

I have observed this in two MARS units I have repaired for others. In both the trolley was so tight in the rails that when the stepper motor attempted to raise it it could not, or "missed a step", causing the motor to "cog"² and not actually move one or more steps. When that happens the firmware thinks the Z-axis has moved but it has not; this fouls up the internal count and the firmware now has absolutely no idea of where the build-plate really is. Missing one or two steps may not be immediately audible as is the "grinding" of a complete jam. In both of the printers I repaired the Z-axis trolley was incorrectly adjusted so as to be absurdly tight within the rails, requiring 4 or 5 lbf to be moved. This is a pretty good YouTube video describing proper adjustment of the Z-axis trolley. The fellow calls it "calibration", which of course it is not as nothing is "calibrated" against any standard--it is simply an adjustment.

If I got a new MARS with the dual rail Z-axis this would be the first thing I'd check.

The purpose of using paper as a spacer (I use two sheets of common 20# copy paper, about 0.18 mm) is to establish the platen's position relative to the trolley's lowermost position--as determined by the limit switch. The paper is (as you have stated) intended as a stand-in for the FEP. When adjusted in this manner the build surface will be "one FEP thickness" above the LCD when the trolley is at the hardware determined absolute zero position. This is a physical calibration separate and distinct from setting a logical "zero" position using the "Z=0" function.

Not using the paper (or FEP) as a spacer will result in the build-plate being in contact with the LCD when the limit switch is triggered. With the resin vat in place the very first downward motion of the trolley--to the limit switch "physical zero" position-- will attempt to compress the FEP between the platen and LCD, and most likely flex the LCD as it is the more compliant of the two. The LCD is actually quite resilient for what it is however such repetitive flexing cannot do it any good. 


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¹ -The stepper appears to be a pretty much standard 1.8° per step (200 steps/turn) unit. the lead screw ha a 2 mm pitch, so one step = 1/200th of 2 mm = .01 mm (10 microns); your .2 mm ("{zero" offset) + one .05 mm layer = 0.25 mm = 25 steps

² - "Cogging" is a condition common to stepper motors. Often described as "grinding", it occurs when a stepper is told to turn but does not have sufficient torque to overcome the load. In open-loop control systems that's when the motor cogs, the controller thinks the motor has turned, but in reality it has not.