Taig Micro Mill DRO Tachometer & SFM
Last updated on Friday, February 02, 2018 12:34:02 PM Eastern US Time Zone


EO Sensor, Encoded Pulley, Infrared Reflectivity, Surface feet per Minute

This DRO modification has been updated for the CNC conversion but this information has been retained for reference.
See updated CNC Tachometer.

EO Sensor

A Fairchild QRB1114 E-O sensor is mounted using a milled Delrin plastic enclosure so the circuit will not short.
Mounted to the mill head using the two, pre-existing 10-32 tapped holes. Alignment is easily accomplished
 by moving the spindle up or down on the dovetail mount. The shielded power/signal cable exits to the left
through a heavily chamfered hole in the aluminum motor support.
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.

For the DPU-550, a 74LS14 Schmitt Trigger would be needed to make the tachometer work reliably.
Diagram. The IC 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.

Encoded Pulley

Did not need the black tape. The circuit easily picks up a pulse from the black set screw,
representing about a 3.5% duty cycle. The sensor is 0.286" from the pulley surface.

Sensor circuit enclosure with the cover removed. The two, flat-head screws are 3-48.
A nylon tie serves as cable strain relief & the hot glue keeps it from rotating.

Infrared Reflectivity

A near-infrared (NIR) camera picture, using the Sony DSC-F717 night shot mode, showing
the Fairchild NIR emitter glowing at 940 nanometers (nm) which is invisible to the naked eye.

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 set screw had high NIR reflectivity, 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.

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

EO Sensor, Encoded Pulley, Infrared Reflectivity, Surface feet per Minute