On Thu, 01 May 2014 17:31:27 -0400, wrote:

On Thu, 01 May 2014 11:12:52 -0500, 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?
Easiest way is slip fit and epoxy. Best way is 2 peices, with relief
machined in both halves, with the magnet sandwiched between, held in
the hole with epoxy or silicone or polyurethane glue,

+1


Or better yet, use radially mounted magnet rod, in hole drilled across
the diameter of the spinner, but not all the way through, so the
magnet is slung to the outside by cent force, and can NOT get out.

The problem with either solution is balanace - the magnet is heavier
than the aluminum removed to put it in.

Adding mass to a rotating crank can be iffy, adding considerable
vibration and shorten crank life. (Ask Ford with its cut-down-V-8 V-6
diesel. A CA fleet truck dealer told me in 2007 that only two of the
hundreds they sold had NOT come back for major engine repairs within a
year. I ended up with the Tundra.) Balancing with an equal weight of
steel rod 180-degrees out would double that mass and any problem
associated with it.


LJ's 2-cents: Optical seems the best route. If it's a dirty enviro,
shield it, wot?

--
If government were a product,
selling it would be illegal.
--P.J. O'Rourke