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
groups.io wrote:
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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 groups.io 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
