Notes on Micro Servo Output Shaft

Mounting a micro servo in the chassis is only half the battle, we also need to attach something useful to its output shaft. Historically servo horn is one of those things that aren’t practical for FDM 3D printing due to the precision fine teeth required to engage output shaft splines. I had thought it would be an interesting thing to try if I ever have access to a high resolution resin 3D printer, but after this study session I’ve changed my mind: there’s too much variation between servos and I’ve come to accept that I’ll need to use whatever horn that came bundled with the servo.

Or at least that’s true at this low-cost end of the generic micro-servo market. My previous knowledge came from the market of standard sized servos, where there are two fairly established brands (JR and Futaba) each with their de-facto standards that other servo makers typically emulate. That might be worth investigating for later for a larger rover, but we’ll set that aside and stay focused on the current micro servo rover project.

SG90 servos have a plastic output shaft and MG90S has a metal shaft, but they differ by more than the type of material. The servo horn for a SG90 is fastened with self-tapping screws that cut their own threads into the smooth cylindrical cavity molded into the center of the plastic output shaft. In contrast, the MG90S output shaft has thread already tapped from factory, and we fasten horns to them with a machine screw. Machine screws typically have a stronger holding force, and they will also remain strong through multiple fasten cycles. Unlike self-tapping screws, which tend to wear down their plastic threads more quickly over time and, if reinstalled incorrectly, can destroy the plastic thread outright.

With all the differences above, it was not a surprise that the servo horns are not interchangeable between SG90 and MG90S servos. What was a surprise was that the output splines can differ between different batches. Two of the MG90S batches had servo horns that were similar but not identical. Trying to swap them would result in one arrangement that is far too loose, and the reverse does not fit at all: it split open when I tried to force it on. This discovery of incompatible variations in servo horns is what torpedoed the resin printer idea or anything that deviates from using the bundled servo horns.

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