# Recommendations for an optical tachometer



## lesrhorer (Oct 21, 2021)

I would like to outfit my mill with a tachometer.  There are tons of magnetic tachs out there, but fitting a magnet to the spindle would be a challenge, and I am a bit worried it would come loose during operations.  Well, more than a bit.  The only practical place to mount the sensor would seem to be in the pulley housing, and the pulley at the most practical location is over 120mm in diameter.  Given rotational speeds of nearly 3000 rpm the centripetal force is significant, to say the least.  Add to that the occasionally vigorous vibration of the spindle, and any attachment to the pulley is precarious, so say the least.

Hand held optical tachometers are extremely common, but I want a continuous panel display, and I really want to stay away from batteries.  I haven't been able to find a suitable optical panel display tachometer.  Can anyone recommend one?


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## Lo-Fi (Oct 21, 2021)

Honestly, for the amount I use it, the battery hand held is overkill. Speed and feed charts are all very well, but I quickly learned that experience and feel beats them hands down every time. A little strip of white paint works well on the end of the spindle on my Bridgeport


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## rwm (Oct 21, 2021)

I would go this route:
https://www.ebay.com/itm/-/384447305681?mkevt=1&mkcid=16&mkrid=711-127632-2357-0&media=GMAIL
Many of us run these on our spindles.
I highly doubt you will shake a small magnet off your pulley if mounted well. Get one of the small 6mm magnets and drill a shallow hole in the pulley. Then embed this in epoxy. If you insist on optical, you could probably modify the sensor to use a phototransistor.
Robert


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## lesrhorer (Oct 21, 2021)

Lo-Fi said:


> Honestly, for the amount I use it, the battery hand held is overkill. Speed and feed charts are all very well, but I quickly learned that experience and feel beats them hands down every time. A little strip of white paint works well on the end of the spindle on my Bridgeport


I have the charts in my head, so your point is rather moot.  What I don't have in my head is the ability to reliably tell the difference between 355 and 545 rpm when I am pushing the limit of a tool.


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## lesrhorer (Oct 21, 2021)

rwm said:


> I would go this route:
> https://www.ebay.com/itm/-/384447305681?mkevt=1&mkcid=16&mkrid=711-127632-2357-0&media=GMAIL


Like I said, the magnetic  route has tons of options.


rwm said:


> Many of us run these on our spindles.


On my mill, the spindle isn't easy to access, and attaching the sensor nearby is even harder.  The spindle receedes all the way up into the quill housing.  There isn't really any space below the pulley, either, for either the magnet or the sensor.


rwm said:


> I highly doubt you will shake a small magnet off your pulley if mounted well. Get one of the small 6mm magnets and drill a shallow hole in the pulley. Then embed this in epoxy.



I don't.  If I had access to the pulley hub, then I would agree, but the only access to the pulley  is on the very rim of the largest pulley.  The centripetal acceleration there is over 3500 m/sec^2, or 360G, and that is before taking vibration into account, which could easily add another 200G or more.  That, not to mention anything penetrating the rim of the pulley is going to interfere with / be hit by the belt.


rwm said:


> If you insist on optical, you could probably modify the sensor to use a phototransistor.
> Robert


Now, that is a thought.


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## mksj (Oct 21, 2021)

You might look at the Tachulator optical Tachometer, I use this one on my lathe. I had also used the MachTach, but they are no longer available. In the past I have also used a small magnet embedded in an aluminum ring  on the nose spindle, they worked well spinning to 5000+ RPM. 





						tachoptions
					






					www.mkctools.com


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## brino (Oct 21, 2021)

lesrhorer said:


> and the pulley at the most practical location is over 120cm in diameter



Wait, what?!?!

30cm ~= 12 inches
so 120cm is around 48" or four feet in diameter!



lesrhorer said:


> Given rotational speeds of nearly 3000 rpm the centripetal force is significant, to say the least.



Centrifugal?
Yeah you bet, what pulley material would stay together?

I suspect a typo or units error here somewhere..........

-brino


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## rwm (Oct 21, 2021)

Good find mksj. I was looking for that. I think that would work.
Can I just add: the magnet I have is 3mm think and weighs 500mg. I'll spot you 1000Gs of acceleration. The force on that at 1000Gs is minimal. (1.1 lbs) Could this magnet fit on the edge of the pulley without breaking into the sheave? It would not even have to be completely recessed.
Robert


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## RJSakowski (Oct 21, 2021)

I embedded small magnets in the side of a lathe pulley to make a rotary encoder.  Different application but similar principle.  The magnets that I used were 4mm in diameter and 2.5 mm thick I cut pockets to fit so the magnets are flush.  A small drop of super glue keeps them in place.


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## Beckerkumm (Oct 21, 2021)

The Tachulator doesn't need a magnet.  I bored aluminum round and painted the whole thing white.  Then I masked off the ring with strips of tape cut in width to accumulate to the diameter of the ring by an even number.  Then took half the strips off and sprayed black paint.  Dave


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## pontiac428 (Oct 21, 2021)

Calculating the force from a point mass on the pulley isn't the right number to determine whether it off-balances the rotating assembly.  Instead, take 1/2 the mass of the pulley plus (the mass of the magnet minus the mass of displaced aluminum pocket) divided by half the mass of the pulley.  Suddenly that mass' contribution to imbalance looks very small.  Think about it, if your calculation of  the point mass is correct, your wheel would go flying off your car if you picked up a pebble in your tread.


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## aliva (Oct 21, 2021)

Since your mill is variable speed the dials for speed adjustment should be good enough. My mill and I'm sure most mills have the speeds shown in 50rpm incriments. +- 50 rpm is not going to make a lot of diffrence in milling .You could get a cheap manual tach to verify that the dials are correct or at least close. There are many tachometer phones apps out there. As mentined above feel and expreince will keep you in the optimal range.
Remember speed and feed cahrts are just suggestions.


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## mksj (Oct 21, 2021)

Tachulator is designed to be used with an optical sensor, it can be used with a Hall sensor, but you need the correct type and know how to wire it in. They often are not directly interchangeable, the optical needs a dropping resistor. It can be set for different pulse numbers/rev. (see manual). I tend not to use optical in dirty environments, so typically use a Hall sensor with a small neodymium magnet. 

The Tachulator only goes down to 50 RPM with a single magnet, I use 4 on my lathe, but you could get by with 2 which should be good for 25 RPM. Optical you would need accurate spacing, there are also some that can use a toothed disk/plate otherwise reflective tape. If using a magnet system, then the Tachulator is more expensive unit, it does calculate SFM which is more useful on the lathe. I purchased the board and built my own enclosure. If you want to use it with a Hall sensor I would need to look at my notes as to what I did.


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## lesrhorer (Oct 22, 2021)

brino said:


> Wait, what?!?!
> 
> 30cm ~= 12 inches
> so 120cm is around 48" or four feet in diameter!


I meant mm, not cm.  I corrected the original post.


brino said:


> Centrifugal?


Centripetal.  "Centrifugal" is a fictitious force.  In the accelerated frame of reference of a spinning system, one "feels" a force trying to fling one outward.  In reality, there is no such force.  There is only inertia, and its tendency to require one to move in a straight line, rather than in a circle.  If one is restrained from moving straight, then there must be a force INWARD, preventing the object from moving in a straight line, forcing it to accelerate toward the center of the circle (not outward).  This is "Centripetal" force, and it is real.  As one can see it is in the opposite direction of the fictitious "Centrifugal" force.  There is no such thing as "Centrifugal" acceleration, at all.  The object is not accelerated outward in any sense.


brino said:


> Yeah you bet, what pulley material would stay together?


'Not a simple one made of Aluminum, that's for certain.  I believe there dynamically balanced steel flywheels that size which can exceed 3000RPM.


brino said:


> I suspect a typo or units error here somewhere..........
> 
> -brino


Yeah, mm, not cm.


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## lesrhorer (Oct 22, 2021)

aliva said:


> Since your mill is variable speed the dials for speed adjustment should be good enough. My mill and I'm sure most mills have the speeds shown in 50rpm incriments. +- 50 rpm is not going to make a lot of diffrence in milling .You could get a cheap manual tach to verify that the dials are correct or at least close. There are many tachometer phones apps out there. As mentined above feel and expreince will keep you in the optimal range.
> Remember speed and feed cahrts are just suggestions.


My mill is not variable speed.  It is a 9 speed mill with the rotation set by pulleys.


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## aliva (Oct 22, 2021)

lesrhorer said:


> My mill is not variable speed.  It is a 9 speed mill with the rotation set by pulleys.


You only have 9 possable speeds why not check each one  with a photo tach and make a chart that corrosponds to each pulley position.


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