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.
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.
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.
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.
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.
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.
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.
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
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.
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?)
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.
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.
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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