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WORK EDGE AND HOLE CENTERLINE LOCATING

After a workpiece has been set up on the mill, it may be necessary
to position the machine spindle relative to the edge of
the part or to center the spindle over an existing hole. Edge finding
and centerline finding are common operations that you will
encounter almost daily in general milling machine operations.
Using an Edgefinder
A useful tool for edge finding is the offset edgefinder (Figure
J-38).

HOLE CENTERLINE LOCATING J-38
The edgefinder consists of a shank with a floating
tip that is retained by an internal spring. The edgefinder tip
is accurately machined to a known diameter, usually .200 or
.500 in. Follow this procedure for using the edgefinder.
Step 1 Secure the edgefinder in a collet or chuck in the
machine spindle.
Step 2 Set the spindle speed to about 600 to 800 rpm, and
slide the edgefinder tip over so that it is off center.
Step 3 Start the spindle and lower the quill or raise the
knee so that the edgefinder tip can contact the edge of the
part to be located.
Step 4 Turn the table or saddle cranks and move the workpiece
until it contacts the rotating edgefinder tip.

HOLE CENTERLINE LOCATING J-39
Continue to slowly advance the workpiece against the edgefinder tip
until the tip suddenly moves sideways. Stop movement at
this moment (Figure J-39). The machine spindle is now
positioned a distance equal to half of the edgefinder tip
diameter from the edge of the workpiece. If you are using a
.200-in.-diameter tip, the centerline of the spindle is .100 in.
from the workpiece. Repeat the edgefinder-to-workpiece
approach at least two times to confirm the position of the
workpiece and the readings of the machine dials or digital
readout display.
Step 5 When you are sure that the positioning is correct,
lower the workpiece or raise the spindle and set the DRO
display or the table and saddle micrometer collars to zero.
Then, move the table or saddle the additional .100 in. in the
same direction that it was moving as it approached the
workpiece. This will prevent introduction of a backlash error
owing to slack in the table or saddle nuts.
When work requires accurate table movements, backlash
must be considered.
When a digital readout is not available on your milling machine, use a dial indicator with a measuring capacity of 1 in. or more. When this
dial indicator is fitted with a magnetic base and clamped
on the machine, it will measure exact table movement
regardless of the play (backlash) existing between the
nut and leadscrew of the machine. Watch the dial while
tightening the machine locking levers to tell if unwanted
table movements are happening, and make corrections
before making a wrong cut.
Hole Center Locating
Many vertical milling machine operations require that the
machine spindle be positioned over the center of an existing
hole in the workpiece. To locate a hole centerline, use the following
procedure.

HOLE CENTERLINE LOCATING J-40
Step 1 Mount a dial test indicator in the machine spindle.
Step 2 Move the indicator contact point so it touches the
side of the hole (Figure J-40). Set the indicator bezel to zero.
Step 3 Rotate the spindle 180 degrees. Compare the two
indicator readings and split the difference between them by
moving the machine table. Locate the spindle center first in
one axis, then in the second axis.
Step 4 If the hole is not round or cylindrical, the spindle
still has to be centered in both the table and saddle axis. In
this case, the indicator readings will be identical, 180 degrees
apart, but will vary from axis to axis.
Step 5 Always double-check the readings on the indicator
and the machine dial settings before performing any additional
machining.
Figure J-40 Dial indicator used in locating the center of a hole.
SHOP TIP

HOLE CENTERLINE LOCATING J-41
A useful tool for aligning a machine spindle with an
existing hole or raised hub is the coaxial dial indicator
(Figure J-41). With this indicator mounted in the
machine spindle, the spindle turns at 500 to 600 rpm.
The operator holds the indicator arm with one hand.
The dial indicator face is stationary. Because the dial
indicator contact point is touching the surface being
centered while the spindle is rotating, any table movement
will either increase or decrease the indicator hand movement.
The operator moves the machine table slowly, one axis at a time, until the indicator hand stops moving and stands still. At this time the spindle is
centered over the hole or hub. Figure J-42 shows a job
contact probe used to inspect hole sizes and hole locations
on a workpiece.
The same tool is used to accurately
set up a workpiece prior to machining.
This probe uses electrical conductivity to sense contact with a
metallic object. An LED is illuminated when the stylus
touches the surface. The stylus indicates workpiece
contact in the table X, saddle Y, or spindle Z axes.

Figure J-42 A touch-sensing probe used to inspect the
workpiece on a milling machine. (This photograph is reproduced
with the kind permission of Renishaw plc, all rights in the
photograph are reserved to and by Renishaw plc).
Machining Holes in Vise
Body (Figure J-43)

HOLE CENTERLINE LOCATING J-43
Step 1 Align the workhead square to the machine table.
Step 2 Align and fasten a machine vise on the table so that
its jaw is parallel to the long axis of the table.
Step 3 Mount the vise body in the machine vise with the
bottom surface against the solid jaw of the machine vise.
Step 4 Mount an edgefinder in a spindle collet and align
the spindle axis with the base surface of the vise body.
Step 5 Move the table the required .452-in. distance and
lock the table cross slide.
Step 6 Pick up the outside of the solid jaw of the vise body.
Step 7 Move to the first hole location 1.015 in. from the
outside edge.
Step 8 Center drill this hole.
Step 9 Use a -diameter twist drill and drill this
hole in. deep.
Step 10 Repeat steps 8 and 9 for the remaining eight
holes. Accurate positioning is done with the micrometer
dials or DRO system.
Step 11 Remove the workpiece from the machine vise.
Turn it over so that the just-drilled holes are down, and
the bottom surface of the vise body is again against the
solid jaw.
Step 12 Use the edgefinder to pick up the two sides, as for
the first drilling operation.
Step 13 Position the spindle over the first hole location,
again with the first hole on the solid jaw side.
Step 14 Center drill this hole.
Step 15 Drill this hole with a -diameter drill deep
enough to meet the hole from below.

HOLE CENTERLINE LOCATING J-44
Step 16 Switch to an -diameter drill and drill com-
pletely through the vise body (Figure J-44). The hole
acts as a pilot hole to let the drill come out
in the correct place on the bottom side.
Step 17 Change from the drill to a -diameter
machine reamer and ream this hole completely through also
(Figure J-45).

HOLE CENTERLINE LOCATING J-45

Step 18 Repeat steps 14 to 17 for the remaining eight
holes.
Step 19 Reposition the workpiece so that it is upright in
the machine vise with the solid jaw of the vise body up.
Step 20 With an edgefinder, pick up the edges of the
workpiece.
Step 21 Position for the two hole locations and drill the
-diameter holes with their -diameter counter- bores (Figure J-46).

HOLE CENTERLINE LOCATING J-46
Step 22 Remove all burrs. One of the most desirable ver-
tical milling machine attachments is a digital readout (DRO)
(Figure J-47). A DRO increases the speed with which accurate
dimensional movements can be made.

HOLE CENTERLINE LOCATING J-47
Movements can
be measured in either inches or millimeters in both the
positive and negative direction. Because actual machine
movements are measured, backlash between feed screws and
nuts can be ignored.

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