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PM 728V-T v2 2020-10
Copyright © 2020 Quality Machine Tools, LLC
Figure 3-15
Workpiece positioning example
TAPPING OPERATIONS
When threading a drilled hole it is essential to align the thread-
ing tap properly in the bore. The mill is often used for this
purpose, ideally with a dedicated (non-slip) tap holder or, for
production work, an auto-reverse tapping attachment, e.g.,
Tapmatic.
A drill chuck can be used instead for sizes up to (say) M6 or
1/4”, beyond which the chuck may not grip tightly enough to
avoid slippage. Tapping can be done under power, or by hand
turning the chuck.
For either method, it is essential to use a tapping fluid. Any cut
-
ting oil is better than none, but most users find Castrol’s Moly
Dee the most reliable for threading in steel.
If power tapping, bear in mind that the spindle does not stop
instantaneously, so be careful tapping blind holes.
Free the
quill locking lever
. Start with the lowest spindle speed. Coun-
teract the effect of the quill's return spring by applying gentle
downward pressure on the quill while feeding in the tap. (This
is to reduce lifting force on the workpiece, which tends to pull
the workpiece out of the chuck.) For easier tapping it may be
helpful to experiment with a larger hole than specified in the
In this illustration a hole is to be drilled exactly 0.25” on the
Y-axis relative to the front edge of a workpiece in a vise, or
otherwise clamped to the table, Figure 3-15.
1.
Install an edge-finder in collet or chuck (a tip diameter of
0.2” is assumed).
2. Lock the X-axis (optional).
3. If the reference edge is already to the back the spindle
centerline, do nothing; if not, rotate the Y-axis handwheel
clockwise to send the workpiece backwards (toward the
column).
4.
Engage the fine downfeed, Figure 3-13.
5. With the spindle running, lower the quill as necessary
using the fine downfeed control, then bring the table for
-
ward (counter-clockwise), stopping at the point where the
edge-finder just makes contact (the tip jumps out of line).
Stop the spindle.
6. While holding the Y-axis handwheel to prevent rotation,
zero the Y dial.
7. Raise the quill, then rotate the handwheel one exact full
turn counter-clockwise (0.1”) to bring the reference edge
to the spindle centerline.
8. Rotate the handwheel 2-1/2 turns counter-clockwise to
bring 50 on the dial opposite the datum; the spindle is now
precisely 0.25” to the back of the reference edge.
TILTING THE HEADSTOCK
In routine operations the user relies on squareness of the spin-
dle relative to both axes of the table. Front-to-back squareness
is set at the factory, and is not adjustable by everyday meth-
ods. In the other plane, the headstock can be set to any angle
up to 90 degrees either side of the normal vertical position.
Because re-establishing true vertical (tramming) on any mill is
a time consuming process, most machinists look first for other
ways of handling a project instead of tilting the head.
The headstock is secured by two nuts spaced on either side,
Figure 3-12. The headstock is top-heavy, and may swing sud-
denly to either side unless a helper is on hand to restrain it.
Testing for movability as you go, carefully loosen the nuts by
degrees. Be especially careful if the head has not been moved
before — the paint seal may let go without warning. (First-time
tilting may also call for unusual effort on the wrench.)
Set the headstock to the desired tilt using a digital angle indi-
cator such as Figure 3-16. Insert a length of bar stock, ideally
ground, in a collet of known accuracy (check for runout at the
tip). Angle indicators like this are typically good within ± 0.2
o
. If
the project calls for greater accuracy a more accurate means
of angle measurement will be needed.
Fine adjustment of tilt angle is made easier by a
worm shaft
& worm wheel
combination. The worm shaft, Figure 3-13 (6),
engages a worm wheel secured to the headstock casting, Fig-
ure 3-17 inset. Use a 21 mm wrench to turn the worm shaft.
Figure 3-16
Headstock tilt angle measurement
Use the indicator's relative mode to measure the angle
between the spindle and the surface of the workpiece.
usual tap drill charts. Typically, a 10% reduction in drill size
below nominal causes no real difference in holding power or
thread integrity under load.
Figure 3-17
Headstock worm wheel
