RF-25 Mill DRO Spindle Reflective E-O Tachometer & Surface Feet per Minute (SFM)
Last updated on Tuesday, September 19, 2023 03:57:39 PM Mountain US Time Zone

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Encoded Pulley, Infrared Reflectivity,
EO Sensor, Cable Routing, Surface Feet per Minute

Encoded Pulley

Mark the pulley edge inside the housing
so you know where to mount the E-O sensor.
To keep the spindle from turning, I protected a fully seated
(tightened) boring head with tape then lightly clamped
 it in the vise. The spindle nut is about 1.9" & has a
left-hand thread. Next, I used a large gear puller to slowly
separate the pulley from the tapered spindle. The spindle
had a little paint overspray from the factory, so I cleaned
it & the threads. I used blue Loctite when the nut
was replaced. Do not over tighten the pulley nut.

Masked off half the bottom pulley edge.
Knocked the surface shine off with a scrub pad &
then cleaned with alcohol so the flat-black
enamel paint would stick really well.




A 50% duty cycle encoded pulley, results.
A much lower value, like 5%, should work, too.

Infrared Reflectivity
Objects that appear light or dark to the eye can
exhibit either high or low NIR reflectivity/transmissivity.
This characteristic must be taken into account when selecting
materials for use in NIR reflective tachometer pick-up assemblies.
For example, if the pulley'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.
NIR picture of tachometer sensor.

EO Sensor

Used a Unibit to drill an access
hole for the optical sensor mounting.
Squared the hole with a file.

The Fairchild QRB1114 IR electro-optical (E-O) sensor &
perf board circuit are mounted in a small plastic case.
Circuit design is from the DRO-350 site. The signal cable has
an internal strain relief. Access is via cover screw removal.
The sensor's full length projects through the squared hole
in the mill's plastic, double-walled belt guard housing.

For the DPU-550, a 74LS14 Schmitt Trigger was needed to
make this particular tachometer setup work reliably. Diagram.
The IC (unused pins removed for compactness) was
spliced into the AUX IN to 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.


Cleaned the painted surface & box
with alcohol to remove any oils.
Sensor was mounted using thin,
double-sided foam tape.

Mounted E-O sensor with cover removed.

Cable Routing

Sensor shown aimed up at the bottom of the encoded
pulley rim. The other hole is for the gray signal cable.
The cable was then dressed along the same path as the
lathe Z-axis cable & plugged into the AUX input jack.

IR photo showing the E-O emitter (left)
glowing at 940nm. The pickup, after
the reflection, is just to the right.

The gray cable is for the tachometer &
the black cable is for the Z-axis scale. I know
what the fixed speeds are, though it's
easier to look at the DRO RPM than the chart,
but it's the SFM calculator that I find useful.
I have since removed the three quill levers.

Surface Feet per Minute
SFM is 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,
coolant, & 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

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

Encoded Pulley, Infrared Reflectivity,
EO Sensor, Cable Routing, Surface Feet per Minute

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