Recommendations for an optical tachometer

lesrhorer

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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

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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
 

rwm

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lesrhorer

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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.
 

lesrhorer

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

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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.
 
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brino

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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!

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
 

rwm

Robert
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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
 

RJSakowski

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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.
 

Beckerkumm

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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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