Tim Wescott wrote:
On Thu, 01 May 2014 10:15:48 -0700, ggherold wrote:

On Thursday, May 1, 2014 12:12:52 PM UTC-4, Tim Wescott wrote:
Problem statement:

Mount a small (nominally 3.2 x 1.6mm) NdFeB magnet in an aluminum model
airplane spinner backplate (for a tachometer).

The thing needs to work with a model airplane engine, so it's going to
be a high vibration environment.

The easy to get magnets (which is what I have) are nickel plated, so
they're slippery as hell. They're also not machined to super-tight
tolerances, and NdFeB is brittle as hell (it's optimized for magnetic
hardness, not physical strength!).

The best I can think of is to drill the hole to a slip fit and then
epoxy. I have visions of actually achieving a press fit, then watching
magnet after magnet crumble into clinging dust trying to press them in.

Suggestions?
Maybe an over size hole, epoxy and then a cover plate/tab that's held
with a screw.

What's your detector?
Could you use an LED and photo diode?

It's in a very uncontrolled environment both for dirt and for light (it's
gotta work in full sun, in just about every conceivable orientation), and
there's no room to shoot light through an aperture -- it would have to be
reflective.


No aperture needed for a photodetector sensor. Go find an old VCR and
rip out one of the IR spindle sensors and its reflector. The sensor is
the size of a TO-92 case transistor. You can simply blacken the rear of
the spinner and polish 2 spots on it to act as the reflectors. Mill a
recess a couple thousands into the back of the spinner for the sensor to
tuck into and it should be fine.
something like this..
http://www.marktechopto.com/img/prod...-reflector.jpg


--
Steve W.