Over the past week I've just about finished the mechanical portion of the build, or at least I hope so. One lesson I've learned from this is that if I were to start over from scratch, I'd begin with the gripper, and work backwards from there.
As I've begun final assembly of the gripper and wrist, the weight has really started to pile on faster than I expected. So that's one thing robots and humans have in common. I'm starting to think that the NEMA11 motor I spec'd for the wrist joint won't be enough, even though it's on a 2.5:1 reduction, as it only produces 13oz-in of torque. If so, it's straightforward enough to switch to a geared stepper, or a NEMA14 or even NEMA17 frame without adding a lot of cost. Because the motor is located behind the joint on the back of the second arm section, weight is less of a concern. I went with the existing motor because I was already using one on the wrist, and the frame size was easier to accommodate than a 14-frame. That may turn out to be a dead end. So it goes.
The gripper surprised me by actually more or less working the first time. The principle of operation is that there are two jaws and two screws, one with a right-hand thread, the other left-handed. A stepper drives one screw, while a belt turns the other screw in the same direction. Each jaw is tapped for one screw and has a clearance hole for the other one. Turn the drive shaft, and the jaws move either towards each other or apart, at the same rate. The nice thing about this (it's what's known as a "parallel gripper") is that the jaws only move in one axis. With other common gripper types like a 4-link or angular gripper, the jaws move not only back and forth as they open and close, but also forward and backward. So if you want to grip a 3-inch object, the arm position would be slightly different than if you were gripping a 1-inch object located in the same place (generally speaking--there are a ton of different designs).
Because the screws supply an 18:1 reduction, a relatively small motor can provide a lot of grip force. Pneumatics are another way to do this, and their force:weight ratio can be excellent, but I wanted an all-electric design for a lot of reasons. I also picked up a small force-sensitive resistor (FSR). This is a neat little component whose resistance changes depending on the amount of force placed on a very thin circular pad. My plan is to integrate a stepper drive, mini Arduino, and the FSR together so that I can send the gripper commands like "open all the way," "set jaws 2 inches apart," "close until light contact," or "close until heavy contact," etc.
The only rain on this parade is that after assembling the gripper, there's just enough drag in the whole assembly that the NEMA11 motor may not be quite enough. I'll just have to test it. If not, an upgrade will be in order. It may also be possible to get better results by improving the alignment and setup of the mechanics, or by using better bearings. As with some other parts, I'm using 608ZZ skate bearings because they're dirt-cheap, but these aren't the smoothest.