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Milling Machines
Milling Machines
A milling
machine is a power driven machine that cuts by
means of a multitooth
rotating cutter.
The mill is constructed in such a manner that the
fixed work piece is
fed into the
rotating cutter. Varieties of cutters and holding
devices allow a wide rage
of cutting
possibilities.
The
mills
in
the
Student
Shop
are
vertical
milling
machines,
commonly
called
style
mills.
These
versatile
mills
are
capable
of
performing
many
operations,
include
some
that
are
similar
to
those
performed
on
the
drill
press
like
drilling, reaming, countersinking, and
counter boring. Other operations performed on
the mill include but are not limited
to: side and face milling, fly cutting, and
precision
boring.
Mills
are
classified
on
the
basis
of
the
position
of
their
spindle.
The
spindle
operates in either
vertical or horizontal position. The amount of
horsepower the mill is
able to supply
to the cutter is also often important.
Mill Construction;
The vertical milling machine is made up
of five major groups: base and column,
knee,
saddle,
table,
and
head,
(see
figure).
The
base
and
column
are
one
piece
that
forms the major
structural component of the milling machine. They
are cast integrally,
ad
provide
the
mill
with
its
stability
and
rigidity.
The
front
of
the
column
has
a
machined face which provides the ways
for the vertical movement of the knee. The
knee supports the saddle and table. It
contains the controls for raising and lowering
the
saddle.
Sitting
atop
the
knee
is
the
saddle
which
supports
the
table.
The
saddle
slides in dovetailed grooves into and
away from the machine, providing the mill with
its Y-axis movement.
On top
of the saddle sits the table. Being
moved side-to-side,
left-right,
over
the
saddle
furnishes
the
mill
with
its
X-axis
movement.
The
work
piece is
secured to the table through the use of various
types of holding devices.
The head is the most complex assembly
in the major parts groups. This contains
The following components:
1. The drive motor and
on/off switch.
2. Drive
belt, gear train, and range lever selector.
3. Quill, spindle, and draw
bar.
4. Quill feed, lock,
and digital depth read out (Z-axis).
The
on/off
switch
is
an
electrical
switch,
with
positions
marked
high
and
low.
This is nothing more than an electrical
reversing drum switch. Its position selection is
done to match that of the range lever
selector. Changing the range from high to low or
vice-versa is done through the use of
the range lever selector and not the on/off drum
switch. This is a static adjustment,
and done with the motor turned OFF! Check to be
sure
the
gears
are
fully
engaged
before
turning
on
the
motor.
Do
this
by
manually
rotating
The
spindle: Spindle speeds are adjusted with the hand
wheel on the right front
part of the
head. This is a dynamic adjustment, and done with
the motor ON!
The
spindle
is
located
within
and
moved
up
or
down
by
the
quill.
This
is
the
Z-axis
movement for plunge operations on the vertical
mill. The quill is moved by the
quill
feed ever, and can be locked in place with the
quill lock. Depth of plunge moves
are
measured with the electronic digital read out
located on the front of the mill head.
Procedures;
Proficiency in milling operations
involves more than simply cutting metal. There
are
two
main
categories
of
procedures
when
machining
on
a
milling
machine:
the
preliminary operations,
and the machining operations.
Preliminary
Operations-
Cleaning-
The
first,
(and
last),
procedure
in
any
machining
operation. Without clean equipment and tools, the
accuracy of the finished
product
diminishes quickly. The accuracy, durability, and
longevity of the equipment
and
tools
depend
on
being
kept
clean.
In
today's
high
tolerances
in
engineering,
cleanliness is
critical.
Set-up- For most
jobs performed on a milling machine, setting up
the work piece
is the most difficult,
critical, and time consuming part of the job. The
work piece must
not
only
be
securely
clamped,
but
held
in
such
a
way
so
that
very
surface
to
be
machined
will
accurately
align
with
other
surfaces
when
finished.
Several
types
of
holding
devices are used in mounting the work piece o the
milling machine. The most
common used
in the Student Shop are the vice, table clamps,
index chuck, and rotary
table. The vice
is probably the most widely used fixture. There
are two configurations
for the vise.
The second style is with the swivel-base mounted
under the vise allowing
it to be
rotated and set at a variety of angles. Large work
pieces can be held directly
on
the
table
surface
through
a
combination
of
T-nuts,
bolts,
and
clamps.
An
index
chuck permits the
rapid positioning of the work, usually indexing in
15°
increments.
The rotary
table gives the mill its 4th axis with its
circular movement. Circles, partial
curves,
angularly
spaced
holes,
curved
slots,
and
O-
ring
grooves.
This
fixture
is
graduated in degrees and minutes of a
degree.
Tooling-
End
mills
are
the
most
common
cutter
used
on
the
vertical
milling
machine.
They
are
extremely
versatile
in
that
they
can
be
used
for
surface
cuts,
slotting, and side (or profiling) cuts.
End mills come in many types, each being suited
for a particular application. End mills
are fluted, much like drills, and the number of
flutes
determines
what
the
end
mill
can
do.
Two
fluted
end
mills
are
used
for
machining aluminum, and
are favored for plunge cuts. Four fluted end mills
are used
in machining the harder metals
like steel. Generally, it is not a good idea to
use a four
fluted end mill when
machining aluminum or brass however, since the
flutes can fill
with material, and no
longer cut. The other main characteristic of the
end mill is the
cutting end of the tool
its self. Some end mills are bottom cutting,
meaning they can
be plunged into
material much like a drill, while some are not,
and are only useful for
cutting on the
side. Be careful not to plunge an end mill that is
not bottom cutting.
Other types of end
mills includes the ball end mill, which has a
radioed end used to
produce
a
fillet,
and
corner
radioing
end
mills,
used
to
round
the
edges
of
a
work
piece.
Other
types
of
cutters
include
slitting
saws
for
cutting
grooves,
shell
milling
cutters
for
faster
milling
of
surfaces
than
is
possible
with
an
end
mill,
fly
cutters
(which
are
single
point
cutters
for
facing
large
work
pieces),
formed
cutters
for
cutting
special
shapes like gears; and groove cutters like T-slot
and dovetail cutters.
Milling
cutters
are
expensive
and
easily
ruined
if
not
taken
care
of
when
using
or
storing. Failure to
obtain satisfactory results on a job can many
times be attributed to
inappropriate
selection of the proper milling cutter.
Machine
Controls-The
speed
or
RPM
of
the
spindle
is
set
through
a
variable-
speed changing
mechanism.
This
is
a dynamic adjustment. Speed changes are
done
only with the motor running. This
is different than the range changes, which are
done
only with the motor turned off,
static. The feed is the rate that the work piece
is fed
into the cutter. The shop mills
are equipped with a variable power feed. This
means
that while it is easy to feed at
a very slow feed, it is also easy to accidentally
feed too
fast when using a small
cutter, possibly breaking the cutter while in
operation. May
factors and conditions
influence the speed and feed at which the material
is worked.
The operator must take into
account rigidity of the work piece and cutter, the
depth of
cut, and the desired finish.
It is generally a good idea to start with a slow
feed rate,
and increase it until an
efficient removal rate is achieved. Feed rates are
decreased for
finer finishes or greater
depths of cut, and maximized for roughing cuts.
Machining Operations;
Once the preliminary
operations and selections have been accomplished,
a quick
check
should
be
made
to
be
sure
that
work
and
fixtures
will
clear
any
parts
of
the
machine, and that the cutter will not
strike the table or fixtures. All table movements
that will not be used on a cut should
be locked, and those that will be used should be
unlocked.
The
head
controls
should
be
checked
for
proper
range
and
speed.
When
starting the motor, make certain the
cutter is rotating in the proper direction. Do not
stop
The cutter
in mid cut and make no adjustments with the cutter
in contact with the
work piece. There
are two types of milling to be discussed.
Conventional milling is
where the work
piece is fed opposite the direction of
the rotation of the cutter, and
climb
milling is when the work piece is fed in the
direction of rotation of the cutter.
Each has its own advantages and
disadvantages. Climb milling draws the part into
the
cutter, and can violently take up
any backlash in the table. However, it does
produce a
smoother finish. Conventional
milling is the more preferred method, and will be
used