Re: Hall Effect Tach on SB 9" - Solved

Mike Poore

Here is the working version pictured below. I made an aluminum collar to go over the nut on the end of the spindle to hold the magnet and to create some space between other steel that could potentially have some magnetism. The bracket holding the sensor was installed in a bolt hole already present on the headstock, but unused on my lathe. I am not sure why that bolt hole is there. The metal case is a little unattractive, but does the job I wanted by keeping all of  the vfd and tach controls within easy reach. The original forward/reverse switch is wired into the VFD to control the motor. The ferrite choke is inside that control box. The aluminum tape on the sensor and wire was part of an experiment to eliminate the emi. It did nothing, but now I don't want to bother removing it. The only lathe modification was to file down the end of the bolt for the reverse gear flush with the nut for clearance.

On 6/23/2022 9:36 PM, Mike Poore via wrote:
Thanks everyone for thoughts. I already had a choke similar to your picture, so I installed it 1" from the display. Problem solved! I did have to solder in some extra wire to wind it, but that was the only modification other than the choke. As my original post indicated, this was my suspicion, but your info helped a lot in doing it properly. Without that info, it is likely I would have done it wrong.

I will post some pictures now that I know it works in case anyone else would like to add a tach to their 9". It was a little tricky finding a spot to install it without physically altering the machine. I always try to avoid doing anything to old machines  that can't be undone such as drilling holes.

On 6/23/2022 3:41 PM, Gary Johnson via wrote:
My experience is with the very nice MachTach unit, sadly no longer available. I'd agree that installing the strong, rare-earth magnets is helpful, and make sure they are close to the sensor when they pass. I expoxied 4 magnets on the face of the big gear in my heavy 10 to get 4 pulses/rev. Flawless operation.

EMI from your VFD can definitely be the root cause of your problem. Ferrite common-mode chokes should be installed on both the sensor cable and the power cable, and any other cable that connects to the display unit. Choice of ferrite material and winding are important. At the expected low frequencies a Fair-Rite type 75 toroid, p/n 2675821502 is a good choice. This part is 1.22" OD and .75" ID, suitable for small cables. Wind as many turns as you can, in neat order, tightly through the core and fix them in place with a couple of cable ties. They should be installed close to the display unit. If a larger toroid is needed for fatter cable, a type 31, 2.4" OD is also a good choice. Fair-Rite p/n 2631803802. Good sources for these ferrites are Mouser and Digi-Key.

Routing the sensor cable further away from the motor leads could be helpful. Also, do not coil excess cable; that acts as a magnetic loop antenna. Cut off any excess and re-terminate.

-Gary NA6O

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