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Digital spindle tachometer for my lathe

John Conroy

John Conroy

member
Premium Member
After converting my lathe to a 3 phase motor and VFD I decided to install a cheap digital tachometer on the spindle to allow me to monitor the spindle speed at any VFD frequency setting. I picked up the kit on Amazon but you can get them on Ali Express or eBay also.

https://www.amazon.ca/DIGITEN-Digital-Tachometer-Proximity-Switch/dp/B00VKAT8A2/ref=sr_1_10?crid=3V0O58CEC1084&dchild=1&keywords=digital+tachometer+rpm+meter&qid=1604947575&sprefix=digital+tach,aps,217&sr=8-10

It came with the digital display panel and a hall-effect sensor speed pickup with a magnet. I machined a steel ring with and ID to slip over the threads on the left end of the spindle so it could be held in place with a M5 set screw. To keep the set screw from damaging the threads I machined a 1/2 round slot with a brass key that lines up with the screw. Then I milled a pocket to hold the magnet and epoxied it in place on the OD of the ring.















I put together a bracket to hold the hall-effect sensor to line up with the magnet.





I set the clearance between the sensor and the magnet at .060" and ran the wires up to the VFD cabinet. I cut a rectangular hole in the cabinet door to fit the display and connected the wiring as per the instructions. It works like a charm. I have been checking the magnet ring for any sign of the magnet coming loose or the set screw backing out and there have been no problems after about 20 hours of use.







The speed chart for this gear setting on the lathe says the rpm should be 910 but the display shows 963 which I have verified is accurate with a hand held tachometer. A fun little project for these boring COVID distancing days.
 
I really like this and not that expensive. Is there a reason you couldn’t thread it on and add a lock ring similar to the preload on the bearings?
 
I was just too lazy to single point the 55 mm X 1.5 mm thread but I will do that if I have any trouble with the set screw method.
 
Excellent post John, I was working on a similar project for my lathe. Back in January of this year I ordered 3 small items through AliExpress which took months to arrive. The one item was a digital LED tachometer complete with proximity switch, wiring, well so I thought. Unfortunately the package was missing the proximity switch and I got absolutely no where with AliExpress. My first and last time ordering stuff from China. That being said, I'm looking for another proximity switch however I don't understand the electrical specifics that I will require to marry up with mentioned tachometer. Perhaps I should simply bite the bullet and buy the set you listed.

Suggestions welcome!
 
John Conroy said:
Looks like that speed sensor will work Dusty.
Click to expand...

Hi John, I notice that you use a blue colored grease on your gearing. What kind or type of grease is it?

Also like the shield you made to prevent said grease from migrating to you V belts. Great idea!
 

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Dusty said:
To be on the safe side I also ordered the unit John posted because I have use for two sets.
Click to expand...

What are you mounting the Tach's on?

One of these would probably be useful to have on my mill, but I don't see any place on the spindle that I could attach the magnet to.
 
The manual for my lathe calls for a NGLI 1 grease, which is not as thick as most general purpose greases which are NGLI2. It is quite thin almost like a very thick oil but show little tendency to fling off. I have tried 2 brands and the blue max stuff seems to fling off less but there is still a little that gets around. I added the extra unpainted sheet metal to the existing belt guard to eliminate that.

20201111_171448.jpg
 
YYCHobbyMachinist said:
What are you mounting the Tach's on?

One of these would probably be useful to have on my mill, but I don't see any place on the spindle that I could attach the magnet to.
Click to expand...

Hi Craig, on my lathe something like John Conroy's although my spindle doesn't stick out like his so adding a steel ring is out of the picture. My Craftex hobby lathe came with a LED tachometer controlled by a half dial through the motor. Would like to put the second tachometer on my floor model drill press but like your mill almost no place to mount the magnets safely. We'll shall see what comes of that. Always planning with safety in mind. Should have my components from amazon by Friday, 20th of November.

Have you found the grub screws you were after? Beautiful sunny morning here, picks up one's spirits.
 
YYCHobbyMachinist said:
What are you mounting the Tach's on?

One of these would probably be useful to have on my mill, but I don't see any place on the spindle that I could attach the magnet to.
Click to expand...
I have one to install and I'm going to pull the spindle pulley and drill a hole in the underside to located the magnet. I'll drill a hole in the bottom of the shroud to mount the reader head.
 
That's funny I installed the same tach on my lathe and does show more that what the lathe diagrams says
The first one I got never worked so they sent me a replacement.
 
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