DRO Spindle Reflective E-O Tachometer & Surface
Feet per Minute (SFM)
Last updated on Sunday, September 07, 2014 06:03:57 AM Eastern US Time Zone
EO Sensor Placement, Cable, Surface Feet per Minute
WARNING: SHOCK HAZARD. FIRST UNPLUG THE LATHE MOTOR FROM THE 110VAC OUTLET
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. Diagram.
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 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 · π · DIAMETER) / 12 where: π = 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.
Surface Feet/Minute (SFM) Chart 1
OpenDRO User's Guide Rev 5 OpenDRO Supplemental Guide
EO Sensor Placement, Cable, Surface Feet per Minute Machinist Calculator