Tachometer for Powermatic 3520A
by Paul Coppinger
Updated Jun29, 2007
For hand chasing threads in wood, accurate, repeatable lathe speed is
crucial. The Powermatic 3520A and many other
lathes do not have a tachometer to allow setting and repeating lathe
speeds. My solution utilizes a SenDec
Magnetic Surface-Mount tachometer (Model #SFT806-0222) from NorthernTool.com
which costs about $80 with shipping
and has a resolution of 10 rpm. A rare earth magnetic which is supplied is
epoxied to the spindle
The sensor is mounted in a handmade
“Z” bracket
which is attached to the headstock
with less than ¼” clearance to the magnetic
by drilling and tapping two 8-32
holes. The readout has a store-bought
“L” bracket attached to one mounting hole with another rare earth magnetic, not
supplied, epoxied to the bottom of the bracket
The readout is magnetically attached
to the top of the headstock and shows the spindle speed of 340 rpm
The sensor magnetic could also be attached
to the hand wheel side of the spindle but could interfere with future spindle
removal.
A 10 rpm resolution is more than
adequate for thread chasing and the hour-meter could be useful to track belt
life, time-at-the-lathe, etc. In speed
comparisons to the Powermatic Freq-Speed chart supplied in the Owner’s Manual,
the SenDec tach speed is identical and doesn’t require 2 people or a mirror to
set the frequency of the phase inverter.
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