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The stock Ender 3 lead screw is a standard TR8*8 which is a 4 start screw with a 8mm lead. Meaning that every rotation produces 8mm of travel. When coupled with a 1.8 degree/200 step motor, the full step resolution is 0.04mm, which is not ideal for layer height selection since the layer height must be a multiple of 0.04mm.

The simplest way to overcome this limitation is to swap the lead screw and nut for one with a finer pitch. In this case, a single start 2mm pitch/lead. Meaning that every rotation produces 2mm of travel. Using the same stock motor we now get 0.01mm full step resolution, and layer height selection is virtually unlimited.

All you need is a 400mm long single start TR8*2 lead screw with 2mm lead and 2mm pitch plus a matching 2mm pitch nut. These can be easily found online at the usual places for very little money.

The second photo shows the difference in pitch angle between the 8mm and 2mm. The flatter angle results in much lower starting resistance, and much greater resistance to falling when the motor is deenergized.

The final photo is a screen capture from the Prusa Calculators page showing the step size for a TR8*8 lead screw with 1.8 degree motor. Note the incompatible layer height.

The next few steps will cover installation.

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