Recommendations for an optical tachometer

[pontiac428]

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.

 

[aliva]

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.

 

[mksj]

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.

 

[lesrhorer]

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.

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.

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.

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

-brino

Yeah, mm, not cm.

 

[lesrhorer]

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.

 

[aliva]

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.