RF-25 Mill
Spindle Reflective E-O Tachometer &
Surface Feet per Minute (SFM)
Last updated on
Wednesday, September 15, 2010 07:44:41 PM
Eastern US Time Zone
Encoded Pulley,
Infrared Reflectivity,
EO Sensor,
Cable Routing,
Surface Feet per Minute
RF-25 Mill
DRO
X Y Z
Axes Scales
9x20 Lathe DRO X
& Z1 Axis Scale.
9x20 Lathe
Tachometer & SFM
Taig Mill
Taig Mill DRO &
X Y
Z Axes Scales.
Taig CNC Mill DRO
Taig
Mill Tachometer & SFM Taig Micro
Lathe DRO
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. 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 an
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.
The 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.
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 &
where the second
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.
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, and 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·
p·
DIAMETER)/12 where:
p = 3.14159 &
the diameter is in inches
Operation is similar to the
Lathe
Tachometer & SFM
For the ShumaTech
DRO-350M, AUX ON in
DRO setup (Function 0)
Select
"no tool offset" (Function 6, 0)
Zero DRO scales
Define tool offsets (Function 5, #1~9,
diameter of end mill (or
other) , Z offset)
Select mounted tool offset (Function 6,
#1~9)
Function 7
toggles display between RPM & Z-Axis DRO
Function 8 toggles display between
SFM & Z-Axis DRO
Notes: Scales
directions & polarities must be
correctly defined in the DRO setup.
SFM changes as the
cutter diameter
changes.
Larger diameters have higher SFM than
smaller diameters for a given RPM.
Double check
initial DRO calculated
SFM values by using
hand calculations.
SFM is applied equally for either mill or lathe operations.
Surface Feet/Minute (SFM) Chart 1
SFM 2
SFM 3
RF-25 Mill DRO &
X
Y Axes
Scales.
Encoded Pulley,
Infrared Reflectivity,
EO Sensor,
Cable Routing,
Surface Feet per Minute