# Delta 46-715 Lathe Repower #6: Delta 46-715 Lathe Repower #6: Tachometer Hack from \$3 Bicycle Computer

 Blog entry by Pete_LJ posted 03-12-2017 06:32 PM 1685 reads 0 times favorited 0 comments
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03122017

I got the tachometer installed on the lathe and it works pretty nicely. My concept was to use a \$3 Chinese bicycle computer for the tachometer. However, such computers typically measure speed primarily (sometimes cadence, bike-speak for rpm at the pedals, which is typically limited to 199). The computer that I used was purchased off ebay for less than \$3. Here is a link for the bike computer(“BC”) that I used: http://www.ebay.com/itm/14-GKunctions-Cycling-Bike-Cycle-LCD-Computer-Odometer-Speedometer-Waterproof-LJ-/272549666972?hash=item3f7539e09c:g:UuUAAOSwpdpVYzIi

My concept was to program the BC so that the mph function for speed would instead read the rpm of the lathe’s drive pulley (which is attached to the lathe spindle). Basically, my driven pulley is 4” in diameter and was rescued from an old treadmill (and, as a result, has a magnet built into the pulley from when it was part of a treadmill). I mounted the pickup (AKA sensor) to a 1/2” dowel which I then inserted in a 1/2” hole in 3/4” plywood. This pickup assembly was then mounted to the plywood box holding the motor and about 12/1000” from magnet embedded in the pulley.

My calculation for the programming:

diameter drive pulley a 4 inches
pi b 3.1415
circumference driven pulley c 12.566 inches “c”=”a x b”
conversion factor mm/in d 25.4
circumfernce drive pulley e 319.1764 mm “e”= “c x d”

hypothetical1 hypo speed of driven pulley f 1000 rpm
circumfrence drive pulley e 319.1764 mm
speed (minutes) h 319176.4 mm/minute “h”=”f x e”
conversion factor i 60 minutes/hour
speed (hour) in mm j 19150584 mm/hour “j”=”h x i”
conversion factor k 0.000001 km/mm
speed (hour) in km l 19.150584 km/hour “l”=”j x k”
conversion factor m 0.621388181 miles/km
speed (hour) in miles n 11.89994656 miles/hour “n”=”l x m”
(AKA MPH)

Hypothetical2
Correction factor o 0.840339908 “o”=”10mph/n”
hypo speed of driven pulley p 1000 rpm
circumfrence driven pulley e 319.1764 mm
“corr” circ. driven pulley r 268.2166667 mm “r”=”e x o”
speed (minutes) s 268216.6667 mm/minute “s”=”p x r”
conversion factor i 60 minutes/hour
speed (hour) in mm u 16093000 mm/hour “u”=”s x i”
conversion factor k 0.000001 km/mm
speed (hour) in km w 16.093 km/hour “w”=”u x k”
conversion factor m 0.621388181 miles/km
speed (hour) in miles n 10.000 miles/hour “n”=”w x m”

Please note, “g” and “q” have neen intentionally omitted.

Thus, with this programming of the speedo set forth in hypo2, the speedo will display 10.0mph when turning 1000rpm. And, as expected the speedo will display 5.0mph when turning 500 rpm. This was all accomplished by using a corrected circumference for the driven pulley of 268mm (AKA “r”) for a 4” pulley. As a result, if I had a 6” pulley, I would instead program the circumference with 402mm (268mm x 6”/4”). Negatives:* Need to adjust gap between magnet and pickup as it is rubbing a little and does not read accurately at very low speeds (namely below 200rpm).

Clearly, I need to get wiring secured for tach (and motor) secured into final position.