Calibrating z-offset after magnetic build plate upgrade

I read the manual and adjusted the value on the system by 1.5-mm. However, I’d like to double-check that the resulting value is accurate.

Just so I understand what the system is doing: With the auto-leveling feature the system uses, it essentially finds the bed using the washers on the side and that z-offset value on the system is the offset between the top of the washer and the top of the build plate. Is this correct?

When this value is calculated in the factory, is it just taking this measurement with some precision instruments at all four corners and using the average or min value? Or is there a routine to do a spot check in the corners and confirm with a feeler gauge?

The final test for accuracy is the first layer of each print. The starting value needs to put the nozzle at a positive distance above the bed surface (because a negative distance is destructive).

It is fairly easy to pick a z-offset value and then manually spot check with a feeler gauge but the final value is best determined by observing the first layer of a print.

Is there an optimum target for the feeler gauge? 1mm?

The z-offset is needed because the G28 zeroing process and the G29 auto level process don’t measure the bed, they measure limit switches and corner washers. In an ideal world, the z-offset would be set so that when the firmware sets Z = 0 it puts the tip of the nozzle at the bed surface. The assumption here is that the bed is a perfect plane and the corner washers are perfectly identical.

Since there is no margin for error (i.e. Z = 0 can’t be below the surface of the bed), any tests for correct setting of the z-offset should use a positive Z value and then measure the gap between the nozzle and the bed.

Pick any positive value for which you have a tool to measure. The firmware default is G21, mm units, so setting Z = 1.0 and using a 1mm feeler gauge will work if you are spot on with your first z-offset value. You should measure at multiple points on the bed including the center.

Most feeler gauge sets don’t have a lot of gauges on both sides of the 1mm so maybe Z = 0.8 might give you a couple of steps to either side. The metric set I have is .05 - 1.0 mm in .05 mm increments.

The goal of this exercise is to get the z-offset value “in the ball park”. Your final “measurement” should be with filament laid down in the first layer of a print. Typical first layer heights with a .5mm nozzle might range from .18mm to .425mm. You should be able to find articles and videos on “the perfect first layer” on the internet as well as numerous test prints.

What I would do is a G28 and G29 and then visually examine the nozzle gap at X, Y, Z of 20, 20, 2; 20, 260, 2; 260, 260, 2; 260, 20, 2; and 140, 140, 2. If the gap appears to be the same at all spots (and appears to be approximately 2mm) then test those five points with Z = 1 using your 1mm feeler gauge.