Source for know RPM reading

[RJSakowski]

It may be my scientific background, but I've never been bothered by extra digits. Heck, I once made a frequency meter that had eight significant digits. I calibrated it to WWV, the US time standard at the time. At the time, I was setting the oscillator frequency of a digital stopwatch that I manufactured which had only five digits and would have done just fine using a six digit meter. My preferred measurement instrument for the lathe is my micrometer even though in many cases I only need accuracy to a few thousandths. You can always choose to ignore the additional resolution but if you don't have it, you can't use it when you do need it.

OTOH, As far as RPM of a lathe or mill spindle, it wasn't that long ago when you had six or so fixed speeds to choose from. My Grizzly had 150, 300, 560, 720, 1200 and 2400 OEM speeds to choose from. In most cases, a single digit would suffice. I certainly wouldn't obsess over having an RPM meter accurate to a single RPM. (and I tend to be OCD about such things)

 

[ChazzC]

It may be my scientific background, but I've never been bothered by extra digits. Heck, I once made a frequency meter that had eight significant digits. I calibrated it to WWV, the US time standard at the time. At the time, I was setting the oscillator frequency of a digital stopwatch that I manufactured which had only five digits and would have done just fine using a six digit meter.

Not to drift (I know you got the reference!) too far OT, but I used to use the color burst frequency (3.579545454 . . . MHz) on a small analog tv, tuned to a live studio network show (live sports supposedly didn't have as accurate a signal coming from the trailers) to calibrate the Heath 512 MHz Frequency Counter I built: easier to access than WWV. I haven't checked it, or used it, in a long time, but like several other pieces of gear can't bear to part with it.

 

[RJSakowski]

Not to drift (I know you got the reference!) too far OT, but I used to use the color burst frequency (3.579545454 . . . MHz) on a small analog tv, tuned to a live studio network show (live sports supposedly didn't have as accurate a signal coming from the trailers) to calibrate the Heath 512 MHz Frequency Counter I built: easier to access than WWV. I haven't checked it, or used it, in a long time, but like several other pieces of gear can't bear to part with it.

Exactly! That was the reason for the high degree of accuracy of the television signals mentioned. It didn't take much of a deviation tp screw up the color rendition The local crystal oscillator wasn't all that accurate but once synced into local, station or better yet, the network signal, it achieved their accuracy.