Tachometer project for the PCNC 1100

[MFchief]

A while back there was a thread about calibrating the spindle RPM that had a side discussion about hand held tachs. I had one, but it was old and made for RC airplane propellers with a minimum of two blades, always a pain to use.

So looking around at those on Amazon, I came across a panel mounted setup that was really cheap, 19 bucks, so I said, what the heck.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1


I checked it out against the mill specified speed and my hand held, and it seemed pretty close, so I installed it in my control panel. It has a magnetic sensor, so I mounted the magnet on the spindle pulley, with an equal counterweight at 180 degrees. Both the tach and the sensor are +12 VDC, with little current draw, so I didn't use a power supply, instead picked up the 12 volts from the DIN aux jack on the control panel.

Every thing seems to work well, the RPM is at least as accurate as the hand held, and when the spindle is calibrated at 500 RPM, it's just about dead on.

Terry

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[Uman]

That is a wonderful method to monitor the spindle RPM. Do you think it could also be used as a load meter, since it would indicate a drop in RPM at excessive load?

[GLCarlson]

That is a wonderful method to monitor the spindle RPM. Do you think it could also be used as a load meter, since it would indicate a drop in RPM at excessive load?

Nitewatchman documented that use several years ago.

[GLCarlson]

Terry,

How stable is the reading? I see about 5 rpm wander (up & down) around a midpoint after my spindle is stabilized and warmed up. More when it's cold of course.

[MFchief]

I think it would be a pretty course monitor of load, but I certainly expect that the rpm would drop some as load increases. I suspect your ears would tell you about as much, but if I saw rpm steadily decreases toward zero, I would probably hit the pause button. LoL

I haven't run the machine enough to warm the spindle since I installed the monitor, but just while I was checking the 500 rpm calibration, it would go up and down 2 or three rpm. Next opportunity to get the machine warm I'll report back.

I think the biggest problem with all of this is that 99% of us don't have a reliable RPM standard to compare against, so the best we can do is relative numbers between the machine cmd rpm and the various tachometers we buy.

Terry

[SCzEngrgGroup]

It's kinda pointless to obsess over RPM accuracy - it really doesn't matter one bit. All feed and speed calculations are simply educated guesses, and even a 10-15% variance in RPM is near meaningless. Repeatability is much more important, if you're taking the time to really optimize your feeds and speeds for maximum performance on your machine.

Regards,
Ray L.

[MFchief]

I totally agree Ray, I got along without a tach for years, it's just not necessary for what I do. But, this was 20 bucks and an afternoon, and it's cool to have the nice red display just for the fun of it.

Terry

[GLCarlson]

I think it would be a pretty course monitor of load, but I certainly expect that the rpm would drop some as load increases. I suspect your ears would tell you about as much, but if I saw rpm steadily decreases toward zero, I would probably hit the pause button. LoL

I haven't run the machine enough to warm the spindle since I installed the monitor, but just while I was checking the 500 rpm calibration, it would go up and down 2 or three rpm. Next opportunity to get the machine warm I'll report back.

I think the biggest problem with all of this is that 99% of us don't have a reliable RPM standard to compare against, so the best we can do is relative numbers between the machine cmd rpm and the various tachometers we buy.

Terry

Having watched my tach as I play with cuts, I have concluded it beats my ears. I haven't done any formal studies, but there is a consistent and repeatable change in rpm when a cut changes, heavier or lighter. I can even see the difference in straight cuts and going around a corner, if the corner is big enough or the speed slow enough to let the tach update a few times. The tach allows me to know actual RPM for a given DOC/WOC, not just "what I asked for" but "what's actually happening".

As far as absolute or relative rpm goes, we all have a fairly precise frequency standard at every wall socket. That 60 Hz stuff is tightly controlled. But, beyond that, there is a time base in all of those counters. I don't know for sure, but expect that they use the same crystal controlled (or resonator, kinda the same thing) that one finds in the average dirt cheap watch or phone; commonly those run at kHz to low MHz frequencies. My cheap tach claims 0.1% to 0.01% accuracy- plenty good enough to allow comparison across machines. I have several of these cheap tachs, and they all give the same reading (within claimed accuracy and digit bounce) when I hook them up to the same sensor sequentially.

I also agree that obsessing over RPM accuracy is pretty much pointless, except for tapping where a TC tapping head needs to be fed a speed and RPM that don't take it outside its ability to compensate (my machine's actual RPM vs commanded RPM is very close to or over that line). Repeatability is much more important, true. Having a tach that actually reports on spindle speed is a useful way to know that.

And it DOES add to the cool factor...

[MFchief]

I agree there are some things around the house with precise frequency control, and certainly 60 hz ac is held pretty tightly. But I haven't found any easy ways to use those sources. Pointing my hand help tach at an incandescent bulb doesn't get me 60 hz. I have read on-line that pointing at an "old style" (by which I believe they mean magnetic ballast) fluorescent bulb should read 7200 rpm. I have had very inconsistent results trying that one, sometimes it does read 7200, but the next reading on the same bulb will be completely different.

If you have some checks that I could use, I'd love to hear about them!

When I tested the unit I installed and used the handheld at the same time, on a spindle setting of 500 rpm, they were within about 6-7 rpm of each other, so I believe most all of them are close enough for what we are doing with our mills.

Terry

[popspipes]

I have one of those cheapie ebay handheld tachs, I tested it tongue in cheek, for accuracy, the thing was for all practical purposes right on!
I had a time base with a 4 mc crystal, count down timer with divide by 10 counters to 1 cps, I soldered led's at various points in the divider chain, it would pick up on those relatively easy.

I also checked it against an led light bulb, it did lock a few times but was kind of iffy.

I built a Heathkit tach back in the 60's for checking engine rpm on an rc airplane and it had to be calibrated against a fluorescent bulb I believe, I may be wrong as its been awhile ha!

I used it to see what the max rpm was and to see how far it was off on my tapping cycles, it stays in the box now, but I do have one if I should need it ha!!

MF Chief, Makes for a nice display thats for sure!

[C*H*U*D]

It's kinda pointless to obsess over RPM accuracy - it really doesn't matter one bit.

I also agree that obsessing over RPM accuracy is pretty much pointless, except for tapping where a TC tapping head needs to be fed a speed and RPM that don't take it outside its ability to compensate (my machine's actual RPM vs commanded RPM is very close to or over that line). ...

Tapping was the first thing to pop into my head too. I can get away with M3 pretty easily, but I'd like a better understanding of my RPM for M2 and M2.5.

[SCzEngrgGroup]

Tapping was the first thing to pop into my head too. I can get away with M3 pretty easily, but I'd like a better understanding of my RPM for M2 and M2.5.

The ONLY time I can see where RPM is critical for tapping is if you're using a Procunier tapping head, as they have no ability to absorb over-feed once the clutch engages. ALL other tapping heads will easily absorb much more than a 10% feed error in either direction - many will absorb as much as a 30% over-feed error, and ALL will absorb almost any amount of under-feed. On the Procunier, simply reducing the feed by enough to deal with the over-feed will work fine, and you lose nothing other than a fraction of a second of tapping time.

Regards,
Ray L.

[MFchief]

Mike, your Heathkit reference really takes me back! I was a Heathkit addict, I built good bit of test equipment, even a 25" color television set. I've got a bunch of gear on my basement shelve that is totally useless these days, postmarker/sweep generator, color bar/dot generator, cats-eye capacitance meter, even a whopping 10 mhz freq counter. Lol

Terry

[popspipes]

Mike, your Heathkit reference really takes me back! I was a Heathkit addict, I built good bit of test equipment, even a 25" color television set. I've got a bunch of gear on my basement shelve that is totally useless these days, postmarker/sweep generator, color bar/dot generator, cats-eye capacitance meter, even a whopping 10 mhz freq counter. Lol

Terry

Terry,
I still have 2 Heathkit radios, probably still work other than batteries ha! I also have some of dads kits that he built as well.

I was into airplanes a few years, then went to boats 1968 or so, I used the Futaba 3PK wheel radios, no radio problems, no frequency conflicts FASST spread spectrum. Things have really changed in the last few years radio wise. The Tormach came along in 2012 and that was the end of the RC stuff.
I loaned the tach to a fellow and never got it back, wasnt a great loss ha!

Freq counters, I have had a couple of those, the last one goes to 500 mc, not Heathkit though...... I am a ham as well since 1977, callsign WE0L.

[MFchief]

Mike,
It's kind of odd but even though I was really into electronics, I never got the ham bug. Guess I was anti-social, or something. LoL

Did the airplane thing however, started with control line planes, got into RC when I was able to afford it. Lots of fun, but haven't done it for many years.

Terry

[GLCarlson]

...

If you have some checks that I could use, I'd love to hear about them!

...

Terry

A transformer, bridge rectifier/diode, and a differentiator (capacitor and resistor) will get you some 60 or 120 hz spikes at a voltage that won't fry the input. Problem is that while the AC line is held very tightly, it's still low frequency and minimally useful as a time base- all of which I should have said. Today, a cheap Chinese timebase generator is 5 bucks...assembled.

It's fairly clear that these little tachs are good to within 5 rpm or so on our machines. Good enough, don't obsess.

And, yeah, I miss the Heathkits too. But that's nostalgia, not engineering. I'll take today's cheap, reliable, functional tech -even if it's all throw-away black boxes- over two weeks putting a tube type marker generator together with a pound of solder. Sure was fun, though.

[MFchief]

It really is amazing what you can buy for next to nothing these days. Robot assembly lines and mass production go a long ways.
Terry

[MFchief]

GL,

Just ran the machine for a few minutes, tried 500, 1500, and 2000 RPM. I had a bounce of 2-4 rpm most of the time, changed about every 10 seconds or so. Letting the spindle run for about 20 minutes had no affect, same bounce cycle, no rpm change.

Also tried a cut in 6061 with a 0.5" 2 flute, not new, end mill. Cut .25 DOC and .25 WOC at 1990 RPM and 16 IPM. I know not an ideal cut, but I didn't feel like changing the belt to high. The cut was about 3.5" long and the spindle dropped about 20 rpm during the cut and recovered immediately at the end. Actually thought the rpm would drop more than it did.

Terry

[popspipes]

A transformer, bridge rectifier/diode, and a differentiator (capacitor and resistor) will get you some 60 or 120 hz spikes at a voltage that won't fry the input. Problem is that while the AC line is held very tightly, it's still low frequency and minimally useful as a time base- all of which I should have said. Today, a cheap Chinese timebase generator is 5 bucks...assembled.

It's fairly clear that these little tachs are good to within 5 rpm or so on our machines. Good enough, don't obsess.

And, yeah, I miss the Heathkits too. But that's nostalgia, not engineering. I'll take today's cheap, reliable, functional tech -even if it's all throw-away black boxes- over two weeks putting a tube type marker generator together with a pound of solder. Sure was fun, though.

I always try to stay abreast of technology in electronics especially, one of the only things that gets cheaper as it progresses, at least in some areas.