Mounting a rare earth magnet in a thin plate

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?
Reply to
Tim Wescott
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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.
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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.
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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.

Reply to
Bob La Londe
I would point out that a reed switch is far to slow to be an adequate sensor for your application. I just used them as a reference item in the uses of which I am accustomed.

Reply to
Bob La Londe
Tim Wescott fired this volley in news:YfWdnWNpIY4Z7__OnZ2dnUVZ snipped-for-privacy@giganews.com:
Yup. I use them all the time for Hall-Sensor magnets on my machines. Drilling slightly over-size and potting them completely works sometimes. For more rigorous environments, consider sandwiching them between two plates, each drilled half-way, and with the magnet potted and the plates epoxied together.
LLoyd
Reply to
Lloyd E. Sponenburgh
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?
George H.
Reply to
ggherold
I use silicone, it's more resistant to vibration that epoxy.
Reply to
Tom Gardner
Maka a steel button slightly larger in dia than your magnet. Press the steel button into a hole and let the magnet field hold the magnet to the button.
While you are doing this make another steel button so that it weighs as much as the first steel button plus the magnet. Press fit this button in a hole to balance the weight of magnet, etc.
Dan
Reply to
dcaster
How big is the spinner, Tim?
Is there room to mount a magnet with a center hole - AND a short steel cross bar (arms) inside the spinner?
Getting the magnet located at the center of the shaft would reduce the effects of vibration (impact impulses) and G loads on the magnet itself.
That's going to double the pulse rate, but it would fer-sure be easier to balance!
IF the polarity will allow it, anyway...
What RPM are you talking about?
Reply to
Richard
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,
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.
Reply to
clare
Works good with a reflective spot on the back of the spinner and a photo-tach. Or like a lot of the model plane guys do, a shiny tip on the prop, and a phototach reading out at the tip instrad of at the hub - better chance of getting accuracy at speed.
Reply to
clare
Why not sense it optically? jsw
Reply to
Jim Wilkins
On a bicycle crank (max way less than 200 rpm) I sandpaper one side of the magnet and use epoxy glue to stick it to the aluminum crank arm. It's held for several years now.
Reply to
John B.
Tear apart an old computer hard drive. There is a supermagnet in there (apparently) epoxied to a steel backing plate.
The magnet is brittle, you should be able to get a size small enough to fit the spinner but strong enough to trigger your sensor.
Perhaps mount two magnets, diametrically opposed on the spinner, and a half divider circuit for the tach?
technomaNge
Reply to
technomaNge
A tach for model engine speed? If your sensor is a reed switch, it almost certainly won't open and close fast enough. (As another poster has observed already.)
I did this with the exercise wheel for my pet deer mouse that works like a charm but flat out he's only running 4 mph max. (You do the conversion to RPM of a 6" wheel. :-)
Take apart an old (computer) mouse and get the LED and sensor. Should be far more responsive than a reed switch. (But I'm only guessing. Electronics weenies to the bridge, please. Is there such a thing as a high-speed, solid state magnetic switch?)
Reply to
Mike Spencer
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.
Reply to
Tim Wescott
Yes there is -- they're called Hall switches, or Hall sensors (they work by the Hall effect. You get one guess as to the last name of the guy who wrote it up first.)
I just don't feel that doing this optically will be robust.
Reply to
Tim Wescott
Tim Wescott on Thu, 01 May 2014 11:12:52 -0500 typed in rec.crafts.metalworking the following:
Simplest means to accomplish the task. Although I would personally consider "Barge Cement" over epoxy, for purely personal reasons.
Don't try to press fit something which crumbles. -- pyotr filipivich "With Age comes Wisdom. Although more often, Age travels alone."
Reply to
pyotr filipivich
If drilling some information I have suggests using diamond tooling and coolant as the dust produced is apparently extremely flammable.
Reply to
David Billington
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..
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Reply to
Steve W.
Should be fine if you add a recess in the spinner to protect the sensor.
Does this mount on a shaft with a through hole? Could you drill diagonal from inside that hole and mount the magnet in a pocket with some epoxy?
Reply to
Steve W.

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