On Thu, 1 May 2014 09:28:19 -0700, "Bob La Londe"
wrote:

"Tim Wescott" wrote in message
m...
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?

In the alarm industry rare earth magnets are often used for problem or
difficult installation situations. Perhaps a magnet like this with a screw
mounting hole might be of some use to you.

http://grisk.com/recessed/mighty_mag.html

I would think even using epoxy and a light interference fit with the resin
squeezing up through the hole, or the use of a screw would work fairly well.
To be fair, we do not typically use these in a high vibration environment in
the alarm industry, but magnetic reed switches and magnets are often used on
machinery where abrasive grit will work into most other types of switches
and destroy them. I would expect there is some vibration in those
environments.

Anyway, the rare earth magnets pictured on that page might give you other
options.

Drop me a line if you like.

alarm(underscore)wizard
dfkgjs-to-confuse-email-harvesters-asljdprhrtph
(at)hotmail(dot)com

I have found when using a raw bare metal rare earth magnet they will not
hold up to more than the lightest interference fit. In practice we often
get them already glued into a plastic sleeve which will take a rather firm
interference fit, and is how they are typically installed such as the
magnets on this page.

http://grisk.com/recessed/20rs.html

I would point out that a rare earth magnet may have to wide of a magnetic
field for optimum detection by your sensor. As it spins past it will be
detected for a wider range of travel. Depending on the sensing speed of
your sensor, its reset time, and the distance from the axis of rotation you
may wind up with a continuous or nearly continuous activation.

Well stated.











--

"
I was once told by a “gun safety” advocate back in the Nineties
that he favored total civilian firearms confiscation.
Only the military and police should have weapons he averred and what did I think about that?

I began to give him a reasoned answer and he
cut me off with an abrupt, “Give me the short answer.”

I thought for a moment and said, “If you try to take our firearms we will kill you.”"