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Processes
- Milling
The
process of milling is, at its most basic the removal of material
by a spinning multi toothed tool to give a flat surface, channel,
groove or hole in the work piece.
There are two basic types of milling machine: vertical and horizontal.
Both are similar in their holding and movement of the work piece,
the difference lies in the positioning of the cutter.
On a horizontal miller the tool is mounted horizontally, so
its rotation is perpendicular to the ground. On a vertical miller
the cutter is mounted vertically and hence its rotation is in
the same plane as the ground. Of these two sorts of miller,
the vertical miller is considered to be the more versatile,
(and is probably more common) as it is capable of milling profiles.
As with most machine tools, millers are available in a range
of sizes and capabilities; from the small manually operated
hobby type machines to the very expensive and specialised CNC
controlled industrial units.
The most common type is the vertical or universal mill. |
The milling gallery shows a medium
sized Bridgeport milling machine; a good example of a manually
operated vertical mill.
On a basic level, the vertical miller can be thought of as being
similar in layout to a the pillar drill detailed in a previous
section – the motor is usually positioned at the back of the
unit behind the spindle, the work is held on a table or bed
beneath the cutter.
The milling machine differs from the pillar drill in that it
is far more robust and complex; basic differences are the moving
bed on the mill to allow the movement of the work, the different
types of cutters used and the ability on most mills to tilt
the spindle head to allow machining at an angles.
The
more basic and common features of the milling machine are detailed
above, the digital readout (DRO) measures and displays movement
in both X and Y axes, and is hence valuable for precise machining.
The readout on this particular machine measures to 0.001mm.
The digital readout is not usually present on older or cheaper
millers.
The Basic
Cutting Tool
Unlike drill bits which are held in a universal Jacobs’ chuck,
milling cutters are held in collets. A collet has a tapered
body at the end of a parallel section, which is female threaded
at the opposite end.The tapered section is divided along its
length into three equal segments.
The threaded part of the collet screws onto the draw bar (which
runs down the central length of the spindle) as the collet is
pulled up into the spindle the tapered section engages into
a matching taper in the spindle of the machine, and due to the
longitudinal cuts closes up on the cutter which is located inside
the hole – therefore holding it securely.
The diagram in the
gallery shows the basic shape and features of a collet.
Although likened
to a pillar drill earlier, the cutters used in vertical milling
are considerably different to standard drill bits.
There are two main sorts of cutters commonly used for vertical
milling, these are slot drills and end mills: Slot drills have
two cutting edges similar to normal drill bits, but the cutting
edges are unequal so that holes are cut across the centre. are
shorter and the tips are square as opposed to being angled in
relation to the length of the bit. Slot drills are used for
cutting slots and holes, they have cutting edges that are unequal
in length, this enables them to produce flat bottomed holes.
End mills usually
have more cutting edges than slot drills (typically 4 or 6)
and the blades end square at the tip. End mills are used for
machining flats and grooves, as the greater amount of cutting
surfaces and flat end gives a better finish on the work.
Other Cutting
Tools
As well as the commonly used slot drills and end mills, there
is a range of other cutting tools that are used for tasks for
which these are not suitable or less efficient.
Face mills can be thought of as large diameter, shallow end
mills; they are used to cut large flat surfaces on materials
as due to their large diameter disk shape, give a more accurate
and clean finish.
If an oddly sized hole is required in a piece of work, a boring
head can be used – this is basically a chuck that is capable
of holding a range of different sized single point cutters,
offset to the central axis of the miller spindle. This causes
them to spin off centre - the distance of the cutter from the
central axis can be finely adjusted, allowing it to create a
hole of virtually any size.
When using the boring
head a hole must first be made in the required place with a
conventional slot drill, the boring bar is then used to gradually
increase the diameter of the hole until it reaches the desired
size.
Milling Speeds
As with turning and to some extent drilling
described earlier, both the speed at which the cutter spins
and how quickly the work is fed into the cutter is critical
to the preservation of the tools, quality of the work and general
safety. As described earlier, slower speeds and feeds are used
when cutting harder materials, and faster speeds are used on
softer materials. Also in common with drilling, the larger the
cutter being used, the slower it must be used.
The use of correct
lubrication while milling is especially important, as the removal
of large amounts of material generates considerable heat and
friction.
Workholding
The basis of workholding on the milling machine
is the table - if a large piece of work is to be machined, it
can be bolted directly to the table using G-clamps or similar.
If large cylindrical objects are to be machined along their length
they can be held in V
– blocks as well as being bolted to the table, to ensure that
they do not move.
If smaller work is
to be machined it can be held in a machine vice which bolts
into several "T" section slots running along the length
of the table. To ensure that the work sits perfectly horizontal
in the vice, a set of parallels is used – these are a pair of
matching precision ground rectangular plates which are placed
at each end of the vice. The work is placed onto the parallels
and tapped down with a soft hammer to make sure it is fully
seated. The vice is then fully tightened up and the parallels
removed if necessary.
A rotary table can
be used if work needs to be milled accurately in a circular
configuration –this allows the piece to be held securely while
it is rotated around a point by winding a handle either clockwise
or anticlockwise to rotate it accordingly. An example of work
machined on a rotary table can be seen in the photo gallery.
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