> I'm building a gizmo, which, for lack of a better name I'm calling a "fan
> trainer".  It consists of an arm about 30" long with a small DC motor and
> propeller on one end, and a pivot and counterweight on the other.  The
> thing is pivoted on a frame which has a controller circuit board.  The
> controller monitors the motor current and the arm position, and drives
> the motor voltage.

> This isn't an executive toy that you sit back and watch move (that would
> get old very fast).  Rather, it is a platform to provide a series of
> exercises for a student to tune the control system, and to do it in a way
> that you can see and feel it working.  I'd use it in conjunction with
> seminars, and perhaps make it part of a "seminar at home" package.

> So my larger problem is to manufacture this in small quantities (20 to
> 100 a year), and sell them at a reasonable price without losing my ass.
> "Not losing my ass" translates to a total bill-of-materials cost of $50
> or so, and if I could get it way lower then I'd just sell it for less and
> increase my potential market.

> My immediate problem is that the potentiometer that senses the arm
> position is getting punched off of the board in shipping.  The
> potentiometer has a hole, through which you pass a 4mm shaft with a flat
> milled into it.  If you mill the flat so that you can easily pass the
> shaft through the pot then the control is not smooth -- the arm hunts
> across the rotational slop caused by the shaft rotating within the pot.
> If you shim the flat for a snug fit (very very light press fit?) then
> when you chuck the thing into a box and fly with it in checked baggage,
> the pot gets punched off of the board, apparently by getting hit from
> behind.

> The shaft on which the arm pivots is restrained in the housing by a
> couple of model airplane wheel collars.  On the trip out, both of these
> collars, and the arm, loosened on the shaft, and the pot was punched
> out.  On the return trip I loosened the arm and tied it to the back plate
> of the frame -- the wheel collars were fine then, but the pot was still
> punched out.

> I really like that pot: it costs less than $2.00 for onsies (less,
> obviously, in higher quantities), it has undetectably small friction, it
> isn't noisy, and because it's board mounted it saves me from needing a
> bunch of brackets which would just drive up my BOM cost.  So any
> alternative that involves not using the pot has to compete with that
> price, and being practical to do in small quantities in an environment
> where labor is not free.

> So I'm thinking at this point that perhaps I just need to be happy with
> what I have, and to warn people not to drop the thing off of a table or
> to ship it without disassembling it first.  But it would be nice if there
> was a way to make it more robust (by isolating the pot from the shaft
> somehow).

> The best suggestion I got from the other thread, assuming that I can do
> it cheaply, is to put a slit in the end of the shaft, so I basically have
> a D-shaped shaft with some spring.  I would like that idea a lot if I
> knew what it would cost to have a batch of 20 shafts made with the slit,
> vs. without, and if that cost wasn't much greater than just making the
> shafts.

> The second-best suggestion is to use a flat coupling.  This would require
> (essentially) two shafts and the coupling, which is clearly going to
> drive up the BOM cost, but I'm still toying with how to make it cheap.

> Several people suggested reinforcing the mounting of the pot to the
> board: it probably doesn't show in the pictures, but it is clear from the
> construction of the thing that this would just result in _part_ of the
> now-destroyed pot remaining on the board.

> Here's a general arrangement shot:https://docs.google.com/open?id=0B5lSHlBBxGvjX0MxMEM3b1l2c0k

> And the thing in action:https://docs.google.com/file/d/0B5lSHlBBxGvjX0MxMEM3b1l2c0k

> And, finally, a close-up of the potentiometer in question:https://docs.google.com/open?id=0B5lSHlBBxGvjcTBCc3VzWDYxZ0E

> --
> My liberal friends think I'm a conservative kook.
> My conservative friends think I'm a liberal kook.
> Why am I not happy that they have found common ground?

> Tim Wescott, Communications, Control, Circuits & Softwarehttp://www.wescottdesign.com