> OK, here it is again with more pictures and explanations. Sorry for
> starting a new thread, but it seemed reasonable given that I did such a
> poor job of getting the design problem stated in the last one.
> 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
> 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
> 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
> 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
that is causing the failure... it gets pushed off the PCB. (lets call
which would push the pot in the plane of the pcb. So something that
allowed motion in Z-axis but coupled rotation... Will a spline cause
togehter... Something like the spring used in a retractable pen?