Milling includes a number
of versatile machining operations, including: slotting,
facing, and pocket removal. These operations are carried out
by a cutting tool that revoloves around its central axis. A workpiece
or stock is clamped on a bed, and the cutting tool moves
into it, removing material as it goes. Either the tool itself is
moved ( usually it moves along the axis of the tool) or the bed
the stock is attaced to moves, bringing the workpiece into contact
with the tool.
Another way that milling
is versatile is that it is used to make parts from raw stock as
well as adding high tolerance features to parts made with another
process- such as sand casting.
Parts such as the ones
shown in sample parts can be machined efficiently
and repeatedly using various milling cutters.
Depending on the geometry
of the cutter and the motion between the cutter and the workpiece,
we distinguish the following milling categories: slab
miling, face milling, end
milling, straddle milling, and
In slab milling, also
called peripheral milling, the axis of cutter rotation is parallel
to the workpiece surface to be machined (Fig. 1a). The cutter, generally
made of high-speed steel, has a number of teeth along its circumference,
each tooth acting like a single-point cutting tool called a plain
Cutters used in slab
milling may have straight or helical teeth producing orthogonal
or oblique cutting action.
1. (a) Schematic illustration of conventional milling and
climb milling. (b) Slab milling operation, showing depth of
cut d, feed per tooth f, chip depth of cut, t, and workpiece
speed v. (c) Schematic illustration of cutter travel distance
t, to reach full depth of cut.
The cutting action occurs
either by conventional milling or climb milling.
In conventional milling,
also called up milling, the maximum chip thickness is at
the end of the cut. This is the dominant method of milling.
- Tooth engagement is
not a function of workpiece surface characteristics.
- Contamination or scale
on the surface does not affect tool life.
- The cutting process
is smooth, provided that the cutter teeth are sharp.
- The tool has the tendency
- The workpiece has
the tendency to be pulled up, thus proper clamping is important.
In climb milling, also
called down milling, cutting starts with the chip at its
thickest location. A typical application is in finishing cuts on
- The downward component of cutting forces holds the workplace
in place, particularly for slender parts.
- Because of the resulting
high impact forces when the teeth engage the workpiece, this operation
must have a rigid setup, and backlash must be eliminated in the
table feed mechanism.
- Climb milling is
not suitable for machining workpieces having surface scale, such
as hot-worked metals, forgings, and castings.
- The scale is hard
and abrasive and causes excessive wear and damage to the cutter
teeth, thus reducing tool life.
In face milling the cutter
is mounted on a spindle having an axis of rotation perpendicular
to the workpiece surface and removes material in the manner shown
in the figures below.
|Face milling operation
showing (a) action of an insert in face milling, and (b) climb
milling. Note that the width of cut w is not necessarily the
same as the cutter radius.
The cutter rotates at
a rotational speed N and the workpiece moves along a straight path
at a linear speed v.
When the cutter rotates
such that its linear velocity is in the same direction as that of
the workpiece, the operation is climb
milling; when it rotates in the opposite direction, the operation
is conventional milling.
The cutting tools are
usually carbide or high-speed-steel inserts and are mounted on the
Because of the
relative motion between the cutting teeth and the workpiece, a face
milling cutter leaves feed marks on the machined surface,
much as in turning operations. Surface roughness depends on insert
corner geometry and feed per tooth.
The cutter in end milling
has either straight or tapered shanks. The cutter usually rotates
on an axis perpendicular to the workpiece, although it can be tilted
to machine tapered surfaces.
Flat surfaces as well
as various profiles can be produced by end milling. The end faces
of some end mills have cutting teeth, and these can be used as a
drill to start a cavity.
Ball nosed end mills
have hemispherical ends for producing curved surfaces, such as cutting
die cavities. Hollow end mills have internal cutting teeth and are
used for machining the cylindrical surface of solid round workpieces,
as in preparing stock with accurate diameters for automatic machines.
End mills are made of high-speed steels or have carbide inserts.
A typical application
is milling aluminum-alloy aerospace components and honeycomb structures,
with spindle speeds on the order of 20,000 rpm. Chip collection
and disposal can be a significant problem in these operations.
Two or more cutters are
mounted on an arbor and are used to machine two parallel surfaces
on the workpiece.
Cutters with very specially
shaped teeth are used to produce curved profiles. Such cutters are
also used for cutting gear teeth.
Manufacturing Proceses for Engineering Materials.