Measure Rpm

[jhwatts]

I have a nice home brew table top mill that I am currently retrofitting. I would like to be able to know how fast my spindle is turning so I can use speed and feed calcs. Do they make something I can use that will attach to my spindle and tell me it’s RPM? I have a NI DAQ card that I can use to display this information on the PC I am using if had some kind of output voltage or something and know how that voltage relates to RPM.

[unterhaus]

photointerrupter comes to mind. These seem to be everywhere inside the junk I take apart.
Alternatively, you can hook up a dc motor across a resistor and capacitor. It will be noisy, and you have to calibrate it. This is the equivalent of a tachometer.

[Al_The_Man]

If you want to use the DAC card and the top spindle revs are not above around 3K to 4K. you could pick up the smallest DC servo motor you could find and use it as a tach, the voltage out is proportional to speed. The other way is to rig up a digital tach with a prox switch and detect a mechanical tab or gear teeth.
Al.

[roonster71]

here is a simple guage made from paper if you use florecent lighting its good for up to 2400 rpm

http://www.sherline.com/rpmgage.pdf

also try your local model airplane store they usually have digital tachs for high rpm propellers for under $50.00

hope this helps

[jhwatts]

It is a gear driven mill. I could open the gear box and look at the number of gears, the number of teeth, find the gear ratios and determine the RPM based on the motor speed. I just need the relationship between voltage and motor speed. It is a dc motor and the sticker says 2500 rpm max. Can any body give me any insight on how input voltage to the motor relates to motor speed?

[gar]

050913=0523 EST USA

jhwatts:

The theoretical unloaded speed of a dc motor is RPM = K * Varm. Where K is a constant, and Varm is the voltage across the armature. This is for constant field excitation. A permanent magnet motor is the equivalent of constant field excitation.

This is modified by frictional losses and some other losses. For your purposes you can proibably ignore these factors.

It is the internal counter EMF that controls speed and you can not directly measure this. But you can estimate this by Varm-in minus ( Iin * Rint ) = Varm. Where Varm is the internal counter EMF, Varm-in is the terminal voltage applied to the motor, Iin is the input current, and Rint is the dc internal resistance of the armature.

With the strobe disk on your spindle and a flourscent light you can determine unloaded spindle speed at some maximum motor speed and the motor voltage. Then use the above equations to calculate spindle speed at most speeds an loads.

.

[jderou]

you could measure the rpm using the technique I used in this post:
http://www.cnczone.com/forums/showth...ibrate+spindle
requires an audio editing program such as cool edit pro.

[jfong]

Since you have an National Instruments DAQ card, depending on the model some of them have built in digital counters. The ones I have here do. Using the counter would probably give you more accurate RPM than trying to read the voltage (then converting it to RPM) from a DC motor. I used a hall effect sensor and a small magnet epoxied to back gear of my lathe spindle to read RPM.

At every rotation, the magnet triggers the hall effect sensor. This is a TTL pulse the DAQ card can then count. Instead of using a PC and one of my DAQ cards, I wrote a small program for a PIC microcontroller and it displays the RPM on the LED panel.

Jim

[jhwatts]

I thought about doing something similar to that, I thought about making some type of switch that would trip every rotation and make some type of timer that stop counting after a minute. Could you give the name of a supplier that can get me a hall effects sensor?

[jfong]

I used a SGS Sprague UGS3077 since that was already in my "junk" box.

A quick look at Digikey.com comes up with an Opteck hall effect that should work, $1.50. I used a tiny rare earth magnet purchased at Radio Shack.
Digikey part#365-1037-ND

I used a latching type hall effect since it was a bit easier to write the PIC program. The Opteck is a pulse type.

Jim

Originally Posted by jhwatts I thought about doing something similar to that, I thought about making some type of switch that would trip every rotation and make some type of timer that stop counting after a minute. Could you give the name of a supplier that can get me a hall effects sensor?

[rustyolddo]

There are some handheld optical tachs on ebay that go for $40. They use a piece of reflective tape that you can stick anywhere on the spindle. Automation Direct has some 5mm Proximity sensors that are good to 5kHz but you'll need a freq counter or software to measure the output.

[jderou]

Does anyone know if mach2/3 can convert the output pulses? This sounds like it could be a nice addition to my mill, possibly even allow me to rigid tap?