Neil's News

Wrist Gears

16 January 2018

The Robotic Arm Edge is a fantastic educational kit. It comes as a box of about a hundred parts, and assembles to form a four-axis robot (plus grip, and light). There are five small DC motors, each controlled electrically by switches on the wired controller -- there are no electronics.

With one exception, the robot is very well-designed. The exception is the wrist joint. This joint only has ±60° of movement, meaning that when the arm is horizontal, the gripper can't point straight down. The reason for this limitation is a pair of linkages that transmit the motor power to the joint. Any more than 60° would result in binding as the linkages intersect.

I used Inkscape to design some gears to replace these linkages (SVG source file). I fed some scrap 3mm plywood into Google's laser cutter, and about 30 seconds later the gears were done. Much faster than 3D printing, which was the other viable option.

One of the gears needed a wee bit of filing in a spot where the laser didn't quite make it through the wood, but otherwise the run was perfect. Here are the finished gears (along with a spacer and some washers) with a dollar coin for scale.

Before installing the gears, some plastic around the wrist joint needed filing to increase clearances for the newly extended range of motion. The motor also needed its polarity reversed, since the addition of gears reverses the rotational direction. And with that, the joint now has ±90° of movement.

Next up, computer control...

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