East Texas Woodturners' Association

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

http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200322901_200322901&issearch=160585

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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