View Full Version : Spindle tach for 11" lathe?

07-22-2006, 03:05 PM
My new Rockwell lathe has a dual belt and conical sheave varispeed drive. It originally had a somewhat cheesy (IMO) "tach" that was operated by a vertical "rack" attached to the intermediate jack shaft swing arm via. linkages and driving a spur gear with needle on the end. The needle tracked across the tach face which had dual bands, one for direct, the other for back gear.

Now, the bracket that provided the linear guides for the rack has broken out and there is no easy way to fix it. The broken part along with the rack and linkage is MIA, so I would have to disassemble the entire swing arm to remove the bracket, machine a new guide, find a way to attach it that will hold up, and recreate the missing rack itself (along with finding out it's TP, etc.). Doable without a doubt, but a lot of grungy work for a rough approximation. Seems to me it would be hard to “calibrate” as well since you would have to get the rack/pinion mesh on exactly the right tooth or be off by some 100 rpm or more.

I’ve seen in the past and found on google some relatively low cost optical input tachs that are relatively easily adapted to any spindle using light/dark alternating panels or interrupter/tone ring as a trigger. Some of the cheapest seem to have problems with low rpm resolution needing at least a few transitions a second for any accuracy. Others have arbitrary dividers so that you can have more than one light/dark transition per rotation and divide it out to get accurate readings. Lets say 4 light/dark pairs, so you divide the “count per minute” by 4 (or 8 depending on how “count” is calculated) to get rpm.

So that leads to my question.

What digital/optical (or magneto, etc?) systems have you guys seen or used that you like. What do you think about them and what did they cost? Any links to systems you have seen and like would be much appreciated as well.

Or would you go to the hassle of fixing the rough estimate gauge?

Seems the ease of implementation and vastly improved accuracy (really not needed though) make the spindle tach the obvious choice with the extra expense offset by the vastly reduced time to implement. Assuming that’s true, it’s just a matter of choosing the tach...

07-22-2006, 03:12 PM
Oh, and here is another post on this board with a similar question, though different orientation.

Thread (http://www.homeshopmachinist.net/bbs/showthread.php?t=15687&highlight=Spindle+tach)

Edit: BTW, I'm hoping for a simple and inexpensive permanent mount tach for reading between say 20 and 2500 rpms, preferably digital but analog is fine as long as the range is right for reasonable resolution. Most of what I've found through search is DIY, some hand helds, and some rather involved examples.

07-22-2006, 03:55 PM

07-22-2006, 04:07 PM
Ok? Appears to be a mechanical tach from somewhere...

07-22-2006, 04:11 PM
It's about WWI vintage. What makes it unusual is that it still works perfectly and has a range of zero to 10,000 rpm over three scales. It was made by Jones Motrola (note: not Motorola). It's pretty accurate, within a couple of percent.

Steve Stube
07-22-2006, 04:36 PM
25 or 30 years ago I used the small DC hobby motors as tech generators for spindle speed output to a 10 volt meter (1 or 2 ma movement). Back then, the hobby motors could be purchased at Radio Shack 2 for a buck and the voltmeters were 2 bucks each. The motors are very linear in output voltage versus rpm and all one needed was a strobe light or other means to measure rpm at a few points to calibrate the voltmeter to rpm. A ruber band around a small pulley on the motor and the outboard end of the lathe spindle provided the drive although I did also make some aluminum pulleys with O-ring tire setups to hinge and drop onto the spindle and other applications like bandsaw sfpm where the wheel rode directly on the blade. Depends on what a guy has available and how comfortable you are with experimenting a bit. For the calibration standard any old mechanical driven speed indicator - remember the ones that simply counted the revolutions and you timed the event with your watch and made the calculations to put it in terms of revolutions per minute. Think eBay - lots under $10.00 (unless they are the popular collector edition). Consider borrowing a digital tech to make your own based on the hobby motor. I know it's low tech but I doubt you can beat it for low cost. Plus it was fun for me.

Steve Stube
07-22-2006, 04:39 PM
Oh you guys are faster typing than me plus I reviewed my post, wanted to make a change and hit the back button. ALL GONE, start over!

07-22-2006, 08:29 PM
Baddog.. little machineshop.com, has a tachometer listed in their catalog,
that gives you rpm to 9999, and also claims it will give a conversion of sfm
up to 9999. listed at $99.95 u.s. Has anyone used this unit, and can tell us if this is a suitable unit? Ray in N. Ont.

07-22-2006, 08:37 PM
BadDog, Here's another thread from a few weeks back with some tach links.


07-22-2006, 08:55 PM
Thank you, I missed that one. The tiny tach for diesels uses a transducer for attaching to the crank and should work perfectly. I'll have to call to find out if the $65 includes a transducer and other details.



Some of the others would likely work, but have a "toy" look or seem incomplete or problematic with statements like "not accurate used indoors" and such. That tiny tach looks very neat and professional...

07-22-2006, 10:56 PM
The tiny tach for diesels uses a transducer for attaching to the crank and should work perfectly. I'll have to call to find out if the $65 includes a transducer and other details.

If it's the one in your link, it won't work. The transducer attaches to an injector line and senses the fuel pulse. There's no crank sensor.

07-22-2006, 11:10 PM
Argh, I knew things couldn't be that easy...

But if there is such a thing out there, than that's what I'm looking for. Compact, neat, self contained and professional appearing "kit" with proven components and not too costly (less than $100, pref way less) to risk being ruined on a piece of tooling in a non-AC shop that sometimes tops 130 ambient in the summer heat.

I've got enough projects waiting for this lathe to be up and running that I'm just not interested in taking it on as a DIY project.

Thanks for any and all help. If nothing proves promising, I may just have to opt for a hand held. At least then I could use it for other things too...

07-23-2006, 01:37 AM
Little Machine Shop has a tach that would probably work, but I imagine you would have to make a new interrupter wheel to fit your lathe. Here's a link:


You might also try looking at hobby shops that cater to RC airplane builders. They use tachs to adjust their engine speeds. A quick Google search turned up this:


Also recall reading about using a bicycle computer as a tach. It seems that there is a unit that is available in Great Britain that would read out directly in RPM. Found the posting, here's a link:



John Stevenson
07-23-2006, 06:11 AM
Or there is this article that was published in MEW using a digital multimeter.



Forrest Addy
07-23-2006, 06:33 AM
Use an automotive add-on tach running from a 4 lobed cam breaking points or from a pulse generator. If you use more lobes or a different cylinder setting and re-calibrate the dial face with a computer graphics stick-on applique you can make a nice looking installation.