Mini Metal Lathe Digital Tachometer

[Paul Jones]

I was inspired by the “Lathe & Mill Tachometer” post by Catfish (see the 4-2-2014 post by Randy Richard) and decided to install the same type of product on my 7" swing mini lathe I bought new in 1993. Randy advised searching for “Hall Effect Tachometer” parts on eBay. I found many offered in the range of $13 to $20 and I purchased one with a blue LED display. The eBay purchase included the LED display with bezel and its attached circuit board that snaps into a rectangular cutout, the cabled Hall Effect sensor NPN with nuts and lock washers, and a small rare-earth magnet. The simple wiring diagrams are posted with the products photos on eBay. I used eBay “Buy It Now” and the parts arrived within two days.

I mounted the digital display into a rectangular hole I cut into the cowling covering for the change gears at the rear of the lathe. The cowling space had sufficient room for this thin electronic part and did not require mounting a separate enclosure. I cut the opening using a Dremel tool with a reinforced abrasive wheel. However, the enclosure near the top of the cowling has very little room inside for the cabled sensor. This restriction required installing the sensor from the outside at the back of the cowling covering. This mounting works well and allows less than a 2 mm gap from sensor end to the spinning magnet mounted near the rear of the spindle (sensor specs require a 1 to 10 mm gap). The north end of magnet must be facing the sensor for it to work correctly. You have a 50:50 chance doing this correctly so I delayed epoxying the magnet in place until this could be tested. I decided to use 5-minute type epoxy to make sure the magnet stayed in place when the spindle spins at over 3000 RPM.

The sensor and power supply wires were secured to the inside walls of the cowling with silicone caulk and this keeps the wires safely away from any spinning gears. Also, inside the cover there are several strengthening ribs that I drilled through for passageways to secure the wires very close to the side walls. The tachometer electronics requires 9V – 15V DC and I measured the current at less than 40 mA. I purchased a small 120 VAC to 9 VDC power supply listed on eBay. The power supply was mounted inside the lathe speed control box (see photo) and connected after the power switch into the 120 V AC screw terminal connector. Later I may consider adding a separate on/off switch for the display. Finally, I used an optical tachometer verified the digital tachometer output was correct.

[Catfish]

Paul,
Very nice clean install. I like having a tach on the lathe.
Randy

[Paul Jones]

Randy,

Thank you and thanks again for the tip on where to purchase the components. Today, I used eBay to purchase another HE sensor and tachometer display for my drill press. I plan to install it soon and will post it.

Also the tachometer versions I bought use one surface mount IC and a few surface mount resistors and capacitors. I choose this version vs. ones with more discrete components because I thought maybe the surface mount technology may draw less current. Originally wanted to tap in directly to the correct voltage somewhere on the lathe electronics to my motor speed regulator but would put a 0.1 quick-blow inline fuse in the connection to protect it. I decided against this only because I didn't want to chance destroying a $150 controller board for the less than $20 tachometer so I used a separate power supply. Just an FYI the tachometer with surface mount components draws current ranging from 25 mA at zero RPM and up to 36 mA at 3000+ RPM.

Paul

[Catfish]

We are off to Hawaii for 10 days soon, when I get back I will order one up for the Lance. I am surprised how well these units work.
Randy

[kbalch]

Thanks Paul! I've added your Mini Lathe Digital Tachometer to our Electrical and Measuring and Marking categories, as well as to your builder page: Paul Jones' Homemade Tools. Your receipt:

Mini Lathe Digital Tachometer by Paul Jones

tags: power supply, lathe, tach, electronics

[Hotz]

Paul congratulated for excellent finish assembly. Very interesting this accessory.
You gave me an idea for the future perhaps to the reader phase or ABS sensors (automotive) be able to pass a satisfactory signal.
Projects and ideas for the future, thanks for sharing.

Paul.

[DIYer]

Neat work! I love tidy jobs.

[Christophe Mineau]

Hi,
Maybe for a much slower application, I wonder if one could'nt use a bike computer, it's the same principle, isn't it ?

[Inchbeag]

Christophe, yes Bike computers do work but this one is illuminated so easier to read. Bike Computer LCDs are not good when looked at an angle.
Nice application Paul, just bought one on eBay

[Paul Jones]

Thank you DIYer. I made one more improvement by attaching with silicone caulk, a thin Plexiglas cover at the back of the display to protect the open circuit board from metal particles. Also, all the soldered connections are covered with heat shrink tubing to prevent electrical shorts. Installing the tachometer display and wiring took about an hour (before waiting for the silicone to harden) and was one of the easiest tool projects I have completed this year. Paul