9x20 Lathe DRO Spindle Reflective E-O Tachometer & Surface Feet per Minute (SFM)
Last updated on Tuesday, September 19, 2023 04:33:03 PM Mountain US Time Zone


EO Sensor Placement,
Surface Feet per Minute

Surface Feet/Minute (SFM) Chart 1  
SFM Chart 2      SFM Chart 3  
Machinist Calculator
OpenDRO User's Guide Rev 5   
OpenDRO Supplemental Guide


EO Sensor Placement

Spindle housing shown with the front cover removed.
Cleaned the spindle (sandpaper, steel wool, then alcohol) &

placed a piece of electrical tape, length-wise. Primed &
then painted the spindle flat black. Removed the tape

leaving a shinny area. I used a variable 4.7K ohm resistor &
two wire leads from the circuit to facilitate adjustment.

Before installing, I reproduced the lathe install geometry
by placing an aluminum cylinder with a piece of black

electrical tape on it into a drill press chuck. Then mounted,
instrumented (voltmeter) & pre-tweaked the circuit

to get it working reliably. Mounted the reflective
tachometer pick-up off one of the bolts.

The cable loops over the spindle.
The IR sensor is the
Fairchild QRB1114.

For the DPU-550, a 74LS14 Schmitt Trigger was needed
to make this particular
tachometer setup work reliably.
The IC (unused pins removed) was spliced into the AUX IN
of the header wires, then shrink-wrapped.
Pin 7 is ground, pin 14 is +5VDC, pin 1 is the signal IN
from the sensor & pin 2 is the signal OUT to the DRO.

This circuit converts the sensor's somewhat noisy
waveform to a clean, well-defined, square-wave output.

The unused inputs do not have to be tied to ground
because the purported power loss is insignificant.

Objects that appear light or dark to the eye can exhibit
either high or low NIR reflectivity. This characteristic must

 be taken into account when selecting materials for use
in NIR reflective tachometer pick-up assemblies.

For example, if the axle's black paint had high NIR reflectivity
(like the metal), it would not have activated the

sensor properly. This principle applies to all materials
including: plastic, metal, paint, tape, etc. Selecting a material based
solely on its visual appearance can lead to a sensing failure.


Power switch housing cover was then
removed. Routed the tachometer cable

through a pre-existing hole. Used spiral
cable wrap, a cable tie-down & a grommet

to assure long-term vibration does not
abrade the shielded/grounded cable.

Surface Feet per Minute (SFM)
SFM Charts are only a starting point. When cutting metal,
one attends to (among other things):
speed, feed,
chip size, chip length, chip coloration, coolant, rigidity,
surface finish, sound, smell, & vibration.
There are numerous, interacting variables that are
unique to any given machine & setup that simply

can not be accounted for by SFM tables.
The rigidity,
& feed in a vertical machining center
is a bit better than a hand drill. So to say that one
SFM value should be the same for both is a stretch.

SFM = (RPM x pi x DIAMETER) / 12
where: pi = 3.14159 & the diameter is in inches

The DPU-550 DRO also calculates SFM. Now I can set
different speeds to obtain a specific SFM (e.g., 200) that
varies as diameter changes. Use Function 7 to toggle &
assign which line Tach or SFM are to be displayed on.

EO Sensor Placement,
Surface Feet per Minute