Adding LED RPM Meter to a Oneway Lathe

I've measured my lathe's actual RPM using a magnet, a coil, and a digital oscilloscope (in my laptop). The computer part is easy (for me at least). The tricky part is the sensor. I don't expect people would want to buy a digital oscilloscope just to measure RPM ;-)

DJ Delorie wrote: I've measured my lathe's actual RPM using a magnet, a coil, and a digital oscilloscope (clip) ^^^^^^^^^^ I recall reading somewhere about using a bicycle speed meter (cyclometer.) You mount a magnet somewhere, like on the handwheel, and a little reed switch next to it. Cyclometers have a setting for wheel size, which should allow you to make the thing direct reading. It won't feed your competer, though.

There isn't much discussion about adding an electronic RPM gauge to the lathe in this article, but it does have a person you may be able to contact for information. And it says he bought the tachometer from MSC.

Recently I found an old Sears hand held tachometer that is pressed against the end of the spinning shaft. (See photograph #2.) This gives a direct readout while it is pressed into your morris taper. Again with a little practice you will learn what your RPM is by the sound of your lathe using the hand held tachometer as a reference. In this way one small inexpensive hand held tachometer could be used by your entire woodturning club. Check with your machinist supply company to find this tachometer or a pawn shop to find a used one. (Of course not everyone in the club will need the tachometer. Some people do not take their lathes apart and put them back together again differently.) (See photograph #3 where I made a jack shaft to reduce the RPM of my Powermatic lathe.)

This is the contact information from the Woodturning Plus website for the Ron Hampton author who wrote the RPM article.

If you are an author or would like to be an author and would like to have your material posted here please contact me at snipped-for-privacy@gte.net Ron

The Balancer may be purchased from Ron Hampton at 903-794-5386

I would go with an analog readout. It would be too distracting to watch the numbers flicker up and down. I don't like LED speedometer gauges in cars for that reason, even though they're compensated for "lag." That, and they're not good for those of us of the bifocal persuasion.

I forgot all about reed relays. I wouldn't expect them to be able to reliably switch fast enough for the higher speeds (100 times/sec), though. Maybe a hall effect sensor. The usual sensors are either hall effect (magnetic) or infrared (paint a dark/light pattern on your spindle). Or a coil, but that requires more pre-conditioning of the signal, and they're bigger.

Sure, and glad to know others get spooked by things apparently moving out of the "corner of the eye."

Probably not accurate enough for Steven, but I bought a cheap tachometer (handheld) that model aircraft hobbiests use to determine propellor RPM.

Amazing. The effects of the N.E. blackout reach all the way to the N.W. and the S.E. 'On topic' posts from both Leif & Arch in the same thread, ..and not a single ;) nor 'G'. Hallelujah! Arch

Fortiter,

Steven, For what you've described, I think you have 2 optrions:

1. An automotive electric speedometer with digital readout should be able to get you in the ballpark with a little calibration. My wife gave me a device a few years back that used a magnet attached to the drive shaft with a magnetic pickup mounted on the bottom of the chassis. I don't remember the calibration procedure, but it could be set to read out correctly for different gear/tire size combinations. My race car had a direct drive high gear and an rpm range up to 7000 RPM, so it should be within the range of your lathe. You can call some of the speed shops in Houston proper, or down along I-45 near Houston International Raceway (near NASA), and get up to date info on what is available now. Seems like there used to be a custom engine builder up around I-45 near Spring, also. Seems like the cost on the unit I had was around 0 about 6-7 years ago. Should be able to get the same cheaper now.

1. Electronic test equipment dealers, who handle used equipment should be able to furnish you with an electronic counter with data output. The big down side here is possibly the cost may be higher than you want to expend. They would require some sort of sensor also, with the magnetic being the easiest to set up. There is a new/used dealer in Garland called Tucker Electronics. They have a good used supply of most commercial equipment, as well as several lines of new equipment. I don't have their 800 number any more, but they have a website. I worked for them back in the 70's, and the owner is a good guy, and they stand behind their stuff. A lot of it is used gov't equipment (NASA, FAA, etc.) and is in good shape when they receive it, then they go over it and recalibrate it to original specs and give you a certificate traceable to NIST (formerly National Bureau of Standards). Hope this helps.

Ken Moon Webberville, TX

PS. Are you going to be at SWAT Labor Day weekend?

Leo, I believe those speedometers use a magnetic sensor, not a reed switch. The one I have is adjustable for wheel size, but does not go anywhere small enough. They are cheap, though.

Graingers has digital rpm meters.

I have an rpm attachment for my DVM.

Dan

steve - the best way to do this would be to machine a "castle" shaped piece of soft iron (think of a castellated lock nut) that you can attach to the lathe - the number of castle "points" depends on the minimum RPM, but your car uses this kind of technkque and works down to 1/2 cranking speed - e.g. about 100 rpm - in fact you might be able to dismantle an automotive distributor to get the parts - that would give you a pickup coil too. There are some meters that will count pulse rate directly from the coil, but if you wanted to build your own, do the following:

1. signal condition with an op amp with unity gain to reject common mode voltages, then run into a second op amp with some positive feedback to generate clean pulses. add a little lag to the first op amp to help remove noise.
2. feed the pulses into a trigger circuit to make nice digital pulses. Feed the pulses into a BCD counter that drives your LED display (or buy an LED counter assembly)
3. reset the display based on a reasonably stable time base - you can use a cheap clock circuit to generate pulses every 10 ms (100 times per second).

quick calculation - if you use a 10 ms counting interval, and you want the thing to read in RPM, then I'd suggest having 10 castle peaks - you will thus read "10" for your 100 rpm. If you use an automotive 8 cylinder distributor, you'll get 8 pulses per revolution, so you would use 100/8=12.5 ms as your time base for resetting the counter.

oh, did you mean you wanted to BUY something ready made? There are lots of tach assemblies available - Minarek is a good source for starting out.

sorry

The some bicycle computer have 2 sensor >> Steven, Using speedometers are going to be a hassle because of the

"Steven D. Russell" wrote in news:2s6%a.436$OG1.301 @newssvr32.news.prodigy.com:

There are several possible methodologies you could use for RPM measurement. Bicycle cyclometers and many other tachs are based on hall effect switches which are simply small magnets which generate a pulse as they are passed by a pickup sensor. The cyclometer idea may or may not work because they convert RPM to MPH based on the circumference of the bicycle wheel (usually measured in centimeters).

You could build one using a simple microcontroler, e.g. a Pic or Basic Stamp. There are many articles on the web aboub Pic conctrollers and "Basic Stamp" controllers, but, a certain knowledge and investment in time may be required to take this approach.

Commercially you could buy a handheld tach using LED's to sense the motion. LED based measurements count pulses based on the change in light reflected off the object being observed. An example of this would be a Testo 470 or

Or you could trade in your oneway for a DVR 3000 which indicates speed...probably not what you had in mind ;)

If you need any microcontroller info you can contact me via email.

steve

Hi Steve,

Had the pleasure of meeting you (and learning from you) at the Detroit Woodworking Show last December, and also purchased your helpful CD.

Skip Campbell at MKC Tools (a fellow Texan, though I don't know how close to you, with the size of Texas - but he seems to be very nice and knowledgeable, enjoys machinery challenges) sells a digital lathe tachometer for under $100. If it wouldn't be readily mountable to your Oneway, I would bet that he would be willing to work with you on it.

Here is a link to the specific page (he does these for Shopsmiths, as well as others):

Hello Fred,

Thanks for the kind words... :-) I'm glad you enjoyed my CD. I will contact Skip and see if we can put our heads together on this... Thanks again and all the best to you and yours!

-- Better Woodturning and Finishing Through Chemistry...

"Woodturning with Steven D. Russell" CD now available! More than 93,000 words, and 500+ photos in 21 articles, email for details.

Steven D. Russell Eurowood Werks Woodturning Studio The Woodlands, Texas

I have such a device that plugs into my DVM. It works very well, but is a tad on the expensive side and a bit combersome for a permanent solution.