In our last episode, the titanium block was being unevenly sanded because the top-mounted arm caused it to buck up on edge during the return stroke. This was remedied by attaching the arm to the bottom edge of the block.



To prevent the little motor from overheating, a timer was added which cycled the system on and off each hour. But repeated overnight trials demonstrated that the rubber band could only sustain an hour or two before breaking. Incriminating eraser-type dust suggest that the rubber band abrades against the edges of the wheel, rather than failing due to heat.

A cardboard sleeve was added to prevent the wheel from sliding down the axle.

With the major problems taken care of, the motor was run one or two hours a night for about a week before the power supply gave out. It was a 14V DC adapter harvested from an answering machine, and should have been more than sufficient to power the 12V motor.

There is noticeable progress in the polishing.

Microsaccades are tiny, involuntary quivering movements of the eyes. The constant motion tends to vary the stimuli to the retina so the rods and cones don't tire out and stop firing (known as Troxler fading).

To observe them, stare at the center of the red dot for 15 seconds, then stare at the blue dot. You should see a jiggling afterimage.



Saccades (from the French word for "twitch") are the larger movements your eyes make as you read or take in a scene, jumping from point to point. Vision is far better in a small region in the middle of the retina called the fovea, so the eye scans around to take it all in. These are apparently the quickest movements in the body.

There are plenty of other fun facts about eye movement out there and lots of cool words. Extorsions and intorsions are rotations around the axis of vision, something I did not know the eye was capable of. (Sure enough, if I look at my eye in the mirror and rotate my head to one side, the eye rotates toward level.) Optokinetic nystagmus is the tendency to track individual items in a moving stream, like a stock ticker, and jump from one to another. Smooth pursuit allows you to lock on and follow an object moving at constant speed. Easy enough, but it can't be done without the object to track. Try sweeping your eyes in a smooth circle while looking at a blank wall.

One more image - the scan pattern of a person looking at a face.



Further reading:
Types of eye movements (try the demonstration of Troxler fading with the orange circles)

Gaze control in face perception

Short list of eye movements with graphs of movement speeds and the moment of blindness that occurs during a saccade

So, I've had this block of titanium sitting around for a few years, after buying it on eBay with my PayPal signup bonus. It's kind of cool and elemental, but a bit rough around the edges. I did some research and found that titanium polishes up nice, but because of the hardness it takes some work. It keeps a nice finish because a thin layer of titanium oxide forms on the surface, which resists corrosion. I got some fine grit sandpaper and quickly found that it was going nowhere fast. At some point I got the idea to automate the process.



This was my first setup. From the left, there is a motor harvested from a printer or fax machine, a rubber band drive belt, a CD sandwich with a thick cardboard disk on a big screw axle, and a dowel arm connected to a wood block taped to the titanium, which is sitting on a piece of sandpaper.

Problems with this design:
1. The motor gets too hot and the rubber band breaks.
2. The Ti block bucks up on its edge on the return stroke.
3. The CD wheel, held up with an alligator clip, comes loose and slides down the screw. The brad holding the arm to the CD then slips loose.