SpryDrive is an affordable actuator based on the quasi-direct drive (QDD) scheme. It is composed of a brushless outrunner motor, a low-cost metal gearbox, and a 3D-printed housing. Inspired by Gabrael Levine's OpenTorque actuator, but takes up less volume. The lower cost of the smaller bearing makes up for the added cost of the steel gears.
Output Backlash: ~0.13 degree
Gear Ratio: 7:1 (Can easily be modified to 3:1, 4:1, 5:1, 9:1, or 10:1).
I'm now using aluminum adapters to transfer torque between the motor and sun gear as well as the carrier and output plate. This will allow the actuator to bear the stress of high-torque applications.
As far as custom metal parts go, these are dirt cheap. I purchased them from SendCutSend for a total of $29 with free shipping. I did have to tap the output plate adapters, though. Despite the low precision of laser cutting, concentricity tolerance isn't an issue as the sun gears are already mounted and centered on the 3 mm motor shaft tip. They also don't add backlash, as they're press fitted.
Also, vent holes for better cooling, especially if paired with a fan.
After finally getting around to running a few more tests (I've been busy with other life stuff), I'm now satisfied enough with the design to release it:
New and improved design. This iteration is 3 mm shorter than the previous.
I'm still getting some slight, unbalanced stiction when playing with the mechanism by hand, until it's connected to the motor. Once it's all fastened together the motion feels perfectly smooth, which is odd to me. My only guess is that the bearing support the motor provides to the sun gear is enough to fix whatever axial misalignment there is, so it's a non-issue...?