Yesterday I got to the final assembly stage of the first batch of product. Everything was going well. I was installing the first one on a bike for pictures when I hit a problem. A big embarrassing problem that I hesitate to disclose, but then what are blogs for? Besides, people want to know where their kits are, and I can either stonewall, lie, or make a pathetic bid for sympathy here.
One design problem of Stokemonkey has been clearance, specifically clearance of the rider’s legs with the motor mount. I’ve gone to great lengths to eliminate the likelihood of the mount grazing the rider’s ankle. This means, among other measures, that I have the driving sprocket passing less than a millimeter from the mount arm.
Well, it turns out that the motor axles of this batch vary from the specification in a critical dimension by as much as 1.7mm, generally in the direction that causes the drive sprocket to hit the mount arm, and thereby not be able to move. This discovery ruined my whole day, to say the least.
The problem has been made worse by the hand filing I’ve had to do on the axles to accommodate the thickness of the plating of the mount. The filing causes the mount arm to seat itself a little further on the axle than before, inducing a collision even in the few cases where the axles were shipped within tolerance. I didn’t see this problem on the “production spec” prototype, because (a) it was dumb luck that the production sample I received in advance was within tolerance, and (b) because the mount wasn’t plated, but simply painted as I was unhappy with the quality of the first plating run.
I sacrificed some parts trying to file my way out of the mess I’d (partially) filed my way into, but I’m not going to ship a product that depends on hand-filed moving parts (the sprocket) to work. The sprocket is one of few parts subject to wear, so it’s important that it be replaceable with off-the-shelf parts.
After sleeping on it (badly), and processing some tense correspondence with my supplier, I’m going to rescue this batch by fabricating a special spacer. Laser cut. I’ll stock replacements. Would I be the only one to notice that the spacer wouldn’t be necessary if the axles were right in the first place, and disapprove? I mean without me having blogged the issue?
This will delay shipment of the first kits by about ten days in the best case, I think. Meanwhile, I can proceed to get the rest ready, finish the manual, update the site, and so on. That’s the best spin I can put on it — I should really do these things before shipping anyway.
In brighter news, the battery packs are coming along nicely. I’ll sell these independently of Stokemonkey to anybody wanting ruggedized high-capacity 36V NiMH batteries with quality connectors: