Source for know RPM reading

Could you not verify the rpm gauge by checking it against your car engine. Your car has a tachometer. Might need two people but then you can get a wide range of rpm’s to check it against.
 
What is the spec for the tachometer? Are you wanting to know how close enough it is? Do you need to know exactly what the speed is? Or is 4% good enough? What's the application?
 
60 Hz line is pretty accurate. Just hook up a resistor in series with an LED to the 110 volt mains and it will blink at 60 times a second => 60*60sec/min = 3600 RPM. Most small LEDs will easily carry 5-10 milliamp of current. So 110Volt line / 5ma = 22,000 Ohm resistor.

In fact, if you happen to have a string of LED Xmas lights that plug directly in to the 110 line (no power supply) they probably blink at this rate. The eye cannot see things changing this fast so you do not notice the blinking. I just checked, you can get a string of lights from Lowes for ~$10.
 
Another way to get an accurate measurement of rpm on a lathe would be to use the lead screw. Set the feed for a high number of threads per inch and using the DRO, note the starting point of the carriage. Simultaneously engage the half nuts while starting a stop watch (a stop watch feature is available on smart phone clock apps). Disengage the half nuts while simultaneously stopping the stop watch. The spindle rpm is equal to the distance travelled by the carriage x 60 sec/min / (the feed rate in inches/rev x the time in sec.).

Sans DRO, an accurate measurement can still be made by setting a carriage stop at the start position and measuring the distance between the carriage stop and the carriage at the stop position. The greatest source for error will be starting and stopping the stopwatch in sync with the half nuts but one should be able hold this to a few tenths of a second. Running at 200 rpm and a .01"/rev., it would take 300 seconds to travel 10" and you should be able to hit a value +/-.0.1%.
 
B2 said:
60 Hz line is pretty accurate. Just hook up a resistor in series with an LED to the 110 volt mains and it will blink at 60 times a second => 60*60sec/min = 3600 RPM. Most small LEDs will easily carry 5-10 milliamp of current. So 110Volt line / 5ma = 22,000 Ohm resistor.

In fact, if you happen to have a string of LED Xmas lights that plug directly in to the 110 line (no power supply) they probably blink at this rate. The eye cannot see things changing this fast so you do not notice the blinking. I just checked, you can get a string of lights from Lowes for ~$10.
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Many cheapo LED's have low Peak Inverse Voltage (PIV) rating - far under 110V. What I am saying is many LEDs (5050, 3030 white) will fail within 1/60 of a second, since their PIV was exceeded ( 3030 PIV is 5V), unless it was designed to be connected to line voltage. LED Christmas lights are a good choice since they are made for line voltage.
 
Iceberg86300 said:
Jake is spot on re the slip & to stay with simple: no load = no slip = theoretical speed based on motor & power properties, and it's pretty easy to calculate.

And 1800rpm is exactly what's expected out of an idealized 4 pole AC induction motor on 60Hz mains.
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Idealized is the key word there. An idling electric motor is not idealized. You never see 1800 RPMs. And you never see the data plate RPMs until it's loaded "some". At a larger load, it's well under.
 
B2 said:
60 Hz line is pretty accurate. Just hook up a resistor in series with an LED to the 110 volt mains and it will blink at 60 times a second => 60*60sec/min = 3600 RPM.
Click to expand...
For this purpose, that's approaching the reinvention of a fluorescent light bulb. (Unless you didn't have said bulb, of course)

Here's one of meters pointed at a fluorescent bulb:
20241027_215358.jpg

Jake M said:
Idealized is the key word there. An idling electric motor is not idealized. You never see 1800 RPMs. And you never see the data plate RPMs until it's loaded "some". At a larger load, it's well under.
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Exactly. I went back & bolded "idealized" & added the conclusion I accidentally left out:

B/c that doesn't exist, you're left with an expected RPM somewhere between 1725 - 1800.
 
That is really cool!

I tend to jump all in with well executed bleeding edge instruments but my love for well executed old analog & mechanical indicators will never die.

Sent from my SM-N975U using Tapatalk
 
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