DRO Spindle Reflective E-O Tachometer &
Surface Feet per Minute (SFM)
Last updated on Wednesday, January 03, 2018 06:48:30 AM Eastern US Time Zone
Surface Feet per Minute
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.
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 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.
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
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.
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
OpenDRO User's Guide Rev 5 OpenDRO Supplemental Guide
Encoded Pulley, Infrared Reflectivity, EO Sensor, Cable Routing, Surface Feet per Minute