9x20 Lathe
Spindle Reflective E-O Tachometer & Surface
Feet per Minute (SFM)
Last updated on
Thursday, October 21, 2010 06:30:44 PM
Eastern US Time Zone
EO Sensor Placement, Cable, Surface Feet per Minute
Spindle housing shown with the front cover removed. Cleaned the spindle
(sandpaper, steel wool & then alcohol) and
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.
Circuit
is from the DRO-350 site. 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 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.
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.
Cable
WARNING: SHOCK HAZARD.
FIRST UNPLUG THE LATHE MOTOR FROM THE 110 VAC OUTLET
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.
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.
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: π = 3.14159 & the diameter is in inches
Operation is similar to the Mill Tachometer & SFM
For the ShumaTech DRO-350L, AUX ON in setup (Function 0)
Select
"no tool offset" (Function 6, 0)
Move tool bit cutting tip to the
part's radius or use the preset
function
Zero DRO scales
Define tool offsets (Function 5,
#1~9, X part radius, Z1 offset)
Select tool offset (Function 6,
#1~9)
Diameter of part should appear
on X-Axis scale
Function 7 displays RPM
& Function 8 displays SFM.
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
EO Sensor Placement, Cable, Surface Feet per Minute
