Summary

A little on my take on the whole bed gap (bed leveling).

It can also be done hot but its just safer and easier while its cold. There is really not going to be a "set" standard on what size feeler gauge to use and even with paper everyone has a different "tension" they feel and gotten used to. You can use another's as a starting point and work from there. When the machine is cold you place a gap between your bed and the nozzle. Heat makes things expand! So your print bed and the glass will expand upwards as well as your extruder and nozzle will expand downwards. So once things are properly preheated they "should" touch. Which is what should happen because that's where the printer thinks zero is. On the bed. So when your printer moves up to say the .2mm of your initial layer height, the print head will physically be .2mm above your bed.

If you gap your bed wrong - say for a second you used .1mm to thick of a method - Now when everything preheats and expands, the print head is no longer on the bed, its now .1mm above the bed. Now when it moves up to your initial layer height of .2mm your first layer will actually be .3mm The printer will still print this but it will only extrude .2mm worth of material for the .2mm gap that's programmed. This will result in a bad layer contact with the bed which equals poor adhesion, prints lifting off the bed, warping, prints completely popping off mid print.

If the bed is gapped too close to the bed the nozzle can scrape the bed on the first layer and damage the print bed or the nozzle. Issues with back pressure as its trying to push out more filament than the gap that's available - This can also result in filament grinding which causes the extruder gear to fill up with filament - become smooth and cause print issues by slipping. Also excessive clogging

I added a few different layer thicknesses just in case you cant scale them.

As a first layer, It should make only one pass. It should also measure the same thickness as the initial layer height you have set in your slice. If they do not measure the same you need to adjust how you gap your bed.

If you really want to get crazy, scale it to the width and depth of your entire print surface! Measure to know how far off your first layer is to the actual settings are to your slicer settings.

You can also do a thicker test, in witch you could see if there's a difference between the layers as well as a how your top layer looks.

Layers should be smooth and all lines connected to appear to be a completely smooth skin. As you try printing faster, you might notice like its not laying enough material/ lines are thinning, so increase temp to properly melt the filament.

Or the lines are overlapping on one side and have gaps on the other, then you probably have mechanical issues within the printer, belt or alignments.

How to gap the bed with my Monoprice Maker Select v2.1. While the machine is cold and the tip of the nozzle is clean - just metal, no plastic hanging down. I use a piece of paper between the nozzle and check the 4 corners of the bed. Once I can feel a little drag from the nozzle pressing on the paper between the bed, I continue around the bed. I personally find it easiest to get 1 edge first, then move around the bed. The first time may take a bit as when you adjust one corner then another, the previous will become loose or tighter. This will become second nature and much faster the more you do it. Repeat this process until all corners are the same and no longer need adjustment.

Why is there is no set standard in what gauge thickness to use?
Because nobody is really using the same setup. Stainless nozzles, borosilicate glass, window glass, buildtack, pei, ect, ect, ect. These all have different thermal expansions and in so will require a different gauge thickness. On top of the temp used which will cause more/less expansion.

Granted - most of the time, close is good enough. But if you have ever wondered why someone else's thickness doesn't work for you. Now you know.