i think your mockup doesn’t leave as much of the ball exposed as I’d like. For a 2-d trackball it’s probably fine, but for a 3-d trackball it’s a huge advantage to be able to grip as much of the ball as possible.
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Anyway, I’ve been playing with putting the ball transfer units into little pieces of cardboard and spinning them, and I have to say I’m very impressed.
The trackball spins in pretty much any direction as well as (or perhaps slightly better than) the best axis on the Penny & Giles. If I spin it fast, it will spin for several seconds before stopping (for comparison, a logitech trackball I have stops spinning almost instantly after I remove my finger). Very fine movements do seem to be just slightly “scratchier” (scratchy like a modern Cherry MX linear keyswitch), but I think it’ll be a bit smoother if the ball transfer units are mounted in a rigid housing with a very precise shape – currently they support the big ball slightly off axis, and cardboard is not the stiffest material out there. :-) The sound produced by this scratchiness is transmitted pretty well through cardboard, but I think in a housing made of a different material, possibly with a bit of dampening rubber carefully placed to prevent the vibration from traveling, they can be made quieter than a standard roller trackball.
There’s basically no scraping at all in any direction between the main trackball and the ball transfer units. I don’t know what kind of wear the ball units get between their main balls and the little helper ball bearings, but I think the wear between the ball units and the trackball ball is going to be negligible, regardless of the material used for the trackball or ball unit ball. This potentially opens up a lot of other trackball materials to be used, though I think whatever this P&G trackball is made of, or any random billiards/snooker/whatever ball, or whatever, is going to be great in practice. Also, I’m curious whether we can try some of the plastic (Delrin?) ball units, to see how they compare friction-wise to the stainless steel type.
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Along with maybe being able to use larger ball units and/or plastic ones, I also think we should try the other model type of Alwayse ball units. That is, try 11-MI-05-13 or 11-MI-05-15 (is carbon steel vs stainless steel going to matter for us?) instead of the 11-MI-05-17. The few millimeters of extra overall size aren’t going to be a dealbreaker for us, and because they seem to use larger helper bearings, I suspect they might spin even smoother. Also, the longer metal bolt might come in really handy. I don’t think there’s any disadvantage for us in picking the flat top instead of chamfered-top kind.
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After KeyCon I’m going to try to get the laser sensors hooked up to a Teensy 3.0 and see if they will usefully track the surface of the ball (it’s a smooth glossy black ball; not sure if that’s a good surface to track on or not). If I can get that working okay with one sensor, then I’ll start trying to design a housing I like. I’ll probably try to CNC cut it out of hardwood, and at some point integrate it with a keyboard.