Clearance

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.clearance

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.

spacerAfter 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: batts

10 thoughts on “Clearance”

  • John

    You are doing an amazing amount of work – it’s a huge undertaking to engineer and build something that works all the time, everytime. Every bump in the road you have means a smoother road for your customers. I am watching VERY close – a Stokemonkey equipped Xtracycle is likely to be my next car. Keep it up… jj

    Reply
  • Bill Manewal

    1.7 mm out of spec on a machined axle seems absurdly excessive to me. What were they smoking? Guess we can relax about Chinese missiles hitting us!

    IMO, the spacers are no big deal. Bikes use them all the time; they won’t negatively impact functionality; they’ll be cheap to stock a few spares; and nobody will care, even we blog readers, probably.

    Perhaps another fix would be to have a few axles machined locally. Shouldn’t be too expensive, but probably a hassle replacing them in the motors. I’d go with the spacers and get some sleep.


    The batteries look slick

    Reply
  • Todd

    Thanks John. I’m pretty bad at accepting kind words generally, but now’s a good time.

    Bill, the axles themselves are machined identically, more or less. The steel axles are pressed into the stator frame with huge torque, and this is where the variation occurred: some got pushed more than others. With my very small runs and unusual specs over the years, I suspect some hasty tooling setup is to blame. Eyeballing it? I’ve already asked about pushing and pulling to correct: no. But yeah, it’s not something I’ll allow to happen again, with a larger batch. A trip to China might be in order. My supplier has been responsive and quite accommodating of my many, many change requests: it’s the execution that has varied, and at least part of it is his/their imperfect grasp of the application, depriving him/them of the ability to identify the critical tolerances. They need a test ride.

    Reply
  • Andrew

    Todd -

    I second John’s praises. This sort of minor setback is to be expected, and anyone who is willing to invest in a bleeding edge product like the SM is likely to be tolerant of these delays. I much prefer to be able to look over your shoulder as you work out the kinks. I have developed a much greater appreciation for just how difficult it can be to get this kind of venture off the ground.

    I doubt any early adopters would mind working with spacers or similar hacks (so long as they don’t need to invest in laser cutters to fashion needed replacement parts themselves). I imagine that by now you are aware that the great majority of your customers will be tinkerers and similar handy types who are already comfortable with the DIY lifestyle (as is probably the case with most Xtracycle users in general).

    Good luck, and keep keeping us posted,
    Andrew

    Reply
  • Bill Manewal

    Thanks for clarifying the specifics of the foul-up. Yet 1.7 mm is still a lot of slop for a press-fit job. That’s why they God invented jigs!

    I suspect you are right on the money saying they don’t adequately visualize the application. And while a test ride would do the trick, your going to China would probably not promote domestic harmony! Just guessing.

    But now that they know you really do care about how far they mash the axle into the stator, it should be a simple matter of your specifying the exact tolerance of the axle position and have your supplier reject any that are out of spec. But maybe my grasp of Chinese industry is grossly naive?

    In the final analysis, this is a glitch, highly irritating but not a deal killer… for anybody. And as Captain Kirk said, “Spacers rule!”. OK, he didn’t really say that, but Scotty might have.

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  • Kipchoge

    Seriously, Todd, I wouldn’t worry about this more than 10 minutes if it happened to us, now. Used to cause lots of lost sleep, but after several containers of FreeRadicals that need a little fix in some way or another, it’s just par for the course. the way to cut it down is to go over there during manufacture, right before shipping. even then things can slip. but preferring to minimize fly-time, we don’t do that, either.

    IMO, if you don’t need to send ‘em all back, it’s ok! Spacer? who cares! what bike axle don’t got a spacer? and another thing, though details are important to many people within and without, no one has ever once commented on one of the fixes that we’ve engineered into an out-of-spec part…

    one more thing: we haven’t had success in the “hey, manufacturer, install it on a bike and use it so you can see what the important details are and if anything is wrong” department.


    worry not! no fun! no benefit!

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  • Mike Leger

    I’m cool with the spacer also, and fall into the DIY category mentioned above. However, 1.7mm out-of-spec sounds excessive to me also.

    Getting a business off the ground–in this case getting your first product out the door–is very much a birthing process. Once you’ve done a few it will become more routine.

    I too like watching over your shoulder. It’s very educational!

    Reply
  • Todd

    I now have the spacers, people. In 304 stainless. I’ve tried one out: yup! They need deburring, and now the other side of the axles need filing to account for the spacer’s width, but things are looking up.

    SpacersI suspect that running a business doesn’t become empirically easier overall at any point, it’s just that business-runners eventually develop a certain equanimity about the usual problems. I still feel completely vulnerable to hundreds of tiny problems, like sand in gears.

    Reply
  • Bill Manewal

    I once heard a business coach advise owners to welcome problems, for it is the solving of problems that generates cash flow.

    He also said that most millionaires say that, having made it, it is not the money they value but who they have become in the process of generating it. I think this speaks to your point about developing a certain equanimity. Congratulations!

    Spacers look good too.

    Reply
  • Mike Leger

    Agreed, the spacers look good.

    Re: problems for business, as Bill notes, welcoming problems is a way to generate cash flow. I would add that you want ‘good’ problems, which are problems you want to solve. If the time comes when bad problems, and there will be bad problems, consistently overwhelm the good ones, it might be time to do something else.

    Reply
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