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?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

"Tim Wescott" wrote in message
...
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.

"Bob La Londe" wrote in message
...
"Tim Wescott" wrote in message
...
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.


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
:

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?


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

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?

George H.



--



Tim Wescott

Wescott Design Services

http://www.wescottdesign.com

On 5/1/2014 12:12 PM, 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?



I use silicone, it's more resistant to vibration that epoxy.

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).


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

On 5/1/2014 11:12 AM, 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?


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?

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,

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.

On Thu, 1 May 2014 10:15:48 -0700 (PDT), 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?

George H.



--



Tim Wescott

Wescott Design Services

http://www.wescottdesign.com
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.

"Tim Wescott" wrote in message
...
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?

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Why not sense it optically?
jsw

"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?

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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.
--
Cheers,

John B.

On 05/01/2014 11:12 AM, 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).


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
--

Tim Wescott writes:

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.

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?)

--
Mike Spencer Nova Scotia, Canada

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On Thu, 01 May 2014 23:55:27 -0300, Mike Spencer wrote:

snip

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?)

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Tim Wescott on Thu, 01 May 2014 11:12:52
-0500 typed in rec.crafts.metalworking the following:
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.

Simplest means to accomplish the task. Although I would
personally consider "Barge Cement" over epoxy, for purely personal
reasons.

I have visions of actually achieving a press fit, then watching
magnet after magnet crumble into clinging dust trying to press them in.

Suggestions?

Don't try to press fit something which crumbles.
--
pyotr filipivich
"With Age comes Wisdom. Although more often, Age travels alone."

On 01/05/14 17:12, 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?

If drilling some information I have suggests using diamond tooling and
coolant as the dust produced is apparently extremely flammable.

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.

Tim Wescott wrote:
On Thu, 01 May 2014 23:55:27 -0300, Mike Spencer wrote:

snip

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?)

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.


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?

--
Steve W.

Are you aware of slot sensors? come in a HUGE assortment
http://www.digikey.com/product-searc...=slot%20sensor

You'll need a gap, or an interuption in the slot once per rev.

Karl

Karl Townsend fired this volley in
:

You'll need a gap, or an interuption in the slot once per rev.

A drilled hole works fine for a chopper. You can also balance your
wheel/spinner backplate easily using drilled holes.

I personally think the whole idea of direct-sensing of the motor shaft on a
model airplane is a fool's errand.

Why not just use a piezo-electric vibration pickup anywhere _near_ the
engine, and be done with it?

LLoyd

"Mike Spencer" wrote in message
...
Tim Wescott writes:
...
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.)

The fastest I could make a reed relay cycle was around 2 KHz, by
overdriving it. I've operated laser diode switches above 3 GHz.
jsw

"Tim Wescott" wrote in message
...

I just don't feel that doing this optically will be robust.

Tim Wescott

Will it fall back to open-loop if the RPM sensor fails or goes out of
range? I'd be much more concerned with the magnet or counterweight
flying out and the aircraft self-destructing.

You could mount the magnet stationary on one side of the shaft and
couple it to the sensor on the other side with a symmetrical steel bar
on the shaft.

On Thursday, May 1, 2014 11:12:52 AM UTC-5, 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?



--



Tim Wescott

Wescott Design Services

http://www.wescottdesign.com

Might be a good application for J B Weld.

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

Larry Jaques fired this volley in
:

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


Still... why not a vibrational pickup? Then there's _nothing_ to get
dirty, no imbalancing of the engine, and no 'attachments'. Just mount the
pickup _near_ the motor mounts and be done with it.

They've got them in teesy sizes that will handle up to a couple of hundred
KHz, can consume NO power (some active ones will pull a couple of mA.)

Lloyd

On Thursday, May 1, 2014 5:35:18 PM UTC-4, Clare wrote:
On Thu, 1 May 2014 10:15:48 -0700 (PDT), wrote:
snip
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.







Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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.

Maybe some of the nice retro reflective tape that 3M makes?
George H.

On Thursday, May 1, 2014 11:20:44 PM UTC-4, Tim Wescott wrote:
On Thu, 01 May 2014 10:15:48 -0700, ggherold wrote:
snip
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.

Hi Tim, I've never done light detection in full sunlight,
so I may be full of ****. But there is some nice 3M retro reflective tape
that Phil H. (of SED) turned me on to. As long as you have enough dynamic range in the detector (to take care of full sun) cna't you you then AC couple the signal? (White leds are blinding these days.)
(Will it kill you if you get a few false counts as full sun blinks through the spinner onto the detector. That's going to be a very small solid angle)
I've no answer for dirt and grease.

George H.



--



Tim Wescott

Wescott Design Services

http://www.wescottdesign.com

On Fri, 02 May 2014 08:53:21 -0700, ggherold wrote:

On Thursday, May 1, 2014 11:20:44 PM UTC-4, Tim Wescott wrote:
On Thu, 01 May 2014 10:15:48 -0700, ggherold wrote:
snip
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.

Hi Tim, I've never done light detection in full sunlight,
so I may be full of ****. But there is some nice 3M retro reflective
tape that Phil H. (of SED) turned me on to. As long as you have enough
dynamic range in the detector (to take care of full sun) cna't you you
then AC couple the signal? (White leds are blinding these days.)
(Will it kill you if you get a few false counts as full sun blinks
through the spinner onto the detector. That's going to be a very small
solid angle)
I've no answer for dirt and grease.

I am starting to think in terms of opto -- but usually the decision tree
for industrial equipment for optical vs. magnetic starts with "will there
be dirt on the surfaces?" -- if the answer is "yes", then the following
box says "use magnetic sensing".

The spinner comes with big lightening holes, much larger than the magnet
I'm thinking of using. I'm considering making up some high strength
epoxy putty with some chopped carbon fiber strand and microballoons, and
holding the magnet in with that. It's kind of a cro-magnon solution, but
it may work.

Now, if only someone made rare-earth magnets in the form of little
screws, I'd be all set!

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Tim Wescott writes:

On Thu, 01 May 2014 23:55:27 -0300, Mike Spencer wrote:

[snip]

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?)

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.)

Ah, tnx for that. Heard of "Hall effect", didn't know that commonplace
articles implemented it. Where would I look in old junk to find one?

I just don't feel that doing this optically will be robust.

Someone gave me a DecWriter II in 1990, when it was at
archaeological-artifact stage of life, which I played with for a
while. The opto-mechanical unit for spacing eventually failed and I
was never able to get it working again. I suppose the similar little
wheels in a mouse would have the same potential for refractory
NFG-ness, which is why I thought of but didn't try that for my (real)
mouse tach.

--
Mike Spencer Nova Scotia, Canada

On Fri, 02 May 2014 14:54:05 -0300, Mike Spencer wrote:

Tim Wescott writes:

On Thu, 01 May 2014 23:55:27 -0300, Mike Spencer wrote:

[snip]

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?)

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.)

Ah, tnx for that. Heard of "Hall effect", didn't know that commonplace
articles implemented it. Where would I look in old junk to find one?

Crank position sensors (they'll have a hall sensor and a magnet).

Laptops and flip-phones often use them to sense that the thing is closed.

Be warned that the laptop and flip-phone ones achieve very low power
consumption by only turning on about ten times a second -- at all other
times they pretty much ignore the world. Also be warned that this is the
biggest market share by far -- finding a Hall sensor that'll work on a
12000 RPM engine takes some digging (I've got part numbers, if you want
to order some from DigiKey).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On 01 May 2014 23:55:27 -0300, Mike Spencer
wrote:


Tim Wescott writes:

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.

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?)
Yes. It's called a hall efect sensor

On 02 May 2014 14:54:05 -0300, Mike Spencer
wrote:


Tim Wescott writes:

On Thu, 01 May 2014 23:55:27 -0300, Mike Spencer wrote:

[snip]

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?)

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.)

Ah, tnx for that. Heard of "Hall effect", didn't know that commonplace
articles implemented it. Where would I look in old junk to find one?

I just don't feel that doing this optically will be robust.

Someone gave me a DecWriter II in 1990, when it was at
archaeological-artifact stage of life, which I played with for a
while. The opto-mechanical unit for spacing eventually failed and I
was never able to get it working again. I suppose the similar little
wheels in a mouse would have the same potential for refractory
NFG-ness, which is why I thought of but didn't try that for my (real)
mouse tach.
An old Omni distributor generally had 2 of them. (Aries and reliant
too)

On Fri, 02 May 2014 08:08:27 -0700, Larry Jaques
wrote:

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.

That is not necessarally true when it comes to model airplanes. I once
saw a "speed job" a small controll line model designed to go fast,
that used a single bladed prop, with a conterweight with an engine
speed in excess of 10,000 rpm.
--
Cheers,

John B.
(invalid to gmail)

On Fri, 02 May 2014 12:53:09 -0500, Tim Wescott
wrote:

On Fri, 02 May 2014 08:53:21 -0700, ggherold wrote:

On Thursday, May 1, 2014 11:20:44 PM UTC-4, Tim Wescott wrote:
On Thu, 01 May 2014 10:15:48 -0700, ggherold wrote:
snip
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.

Hi Tim, I've never done light detection in full sunlight,
so I may be full of ****. But there is some nice 3M retro reflective
tape that Phil H. (of SED) turned me on to. As long as you have enough
dynamic range in the detector (to take care of full sun) cna't you you
then AC couple the signal? (White leds are blinding these days.)
(Will it kill you if you get a few false counts as full sun blinks
through the spinner onto the detector. That's going to be a very small
solid angle)
I've no answer for dirt and grease.

I am starting to think in terms of opto -- but usually the decision tree
for industrial equipment for optical vs. magnetic starts with "will there
be dirt on the surfaces?" -- if the answer is "yes", then the following
box says "use magnetic sensing".

The spinner comes with big lightening holes, much larger than the magnet
I'm thinking of using. I'm considering making up some high strength
epoxy putty with some chopped carbon fiber strand and microballoons, and
holding the magnet in with that. It's kind of a cro-magnon solution, but
it may work.

Now, if only someone made rare-earth magnets in the form of little
screws, I'd be all set!

A sort of left hand turn off the topic but why not just use an optical
tach for setting static speeds and (perhaps) and acustic tach to
calculate the increase in rpm when flying?

One example:
https://play.google.com/store/apps/d...viery.rpmgauge
--
Cheers,

John B.
(invalid to gmail)

On Fri, 02 May 2014 10:38:13 -0500, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote:

Larry Jaques fired this volley in
:

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


Still... why not a vibrational pickup? Then there's _nothing_ to get
dirty, no imbalancing of the engine, and no 'attachments'. Just mount the
pickup _near_ the motor mounts and be done with it.

They've got them in teesy sizes that will handle up to a couple of hundred
KHz, can consume NO power (some active ones will pull a couple of mA.)

Vibe tach? Nevahoiduvit. Are they available?

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

John B. fired this volley in
:

That is not necessarally true when it comes to model airplanes. I once
saw a "speed job" a small controll line model designed to go fast,
that used a single bladed prop, with a conterweight with an engine
speed in excess of 10,000 rpm.

What kind of chugging hunk of a model airplane engine goes 10K? (except at
a high idle?) There are lots of them out there that consistently tach
29Krpm.

Lloyd

Larry Jaques fired this volley in
:
Vibe tach? Nevahoiduvit. Are they available?


All day long. They're almost a commodity item. Anything with a reliably
repeatable cyclic vibration is a candidate for the method.

They even sell sub-$20 elapsed-time meters for gasoline engines that are
only activated when vibrated by the running motor.

Lloyd

"Lloyd E. Sponenburgh" lloydspinsidemindspring.com fired this volley in
. 3.70:

There are lots of them out there that consistently tach
29Krpm.



um... and some that do 40K.

Lloyd