5) Basic Milling operations

General. The milling machine is one of the most versatile metalworking machines in a
shop. It is capable of performing simple operations, such as milling a flat surface or drilling a
hole, or more complex operations, such as milling helical gear teeth. It would be impractical to
attempt to discuss all of the operations that a milling machine can do. The success of any
milling operation depends to a great extent upon judgment in setting up the job, selecting the
proper cutter, and holding the cutter by the best means. Even though we will discuss only the
more common operations, the machinist will find that by using a combination of operations,
he will be able to produce a variety of work projects. Some fundamental practices have been
proved by experience to be necessary for good results on all jobs. Some of these practices
are mentioned below.


Plain Milling.
(1) General. Plain milling, also called surface milling and slab milling, is milling flat
surfaces with the milling cutter axis parallel to the surface being milled. Generally, plain
milling is accomplished with the workpiece surface mounted to the milling machine table
and the milling cutter mounted on a standard milling machine arbor. The arbor is well
supported in a horizontal plane between the milling machine spindle and one or more
arbor supports.
Angular Milling.
(1) General. Angular milling, or angle milling, is milling flat surfaces which are neither
parallel nor perpendicular to the axis of the milling cutter. A single-angle milling cutter
(figure 14) is used for this operation. Milling dovetails is a typical example of angular
milling. When milling dovetails, the usual angle of the cutter is 45°, 50°, 55°, or 60°,
based on common dovetail designs.

h. Face Milling.
(1) General. Face milling, also called end milling and side milling, is machining
surfaces perpendicular to the axis of the cutter.
Straddle Milling.
(1) General. When two or more parallel vertical surfaces are machined at a single cut,
the operation is called straddle milling. Straddle milling is accomplished by mounting
two side milling cutters on the same arbor, set apart so that they straddle the workpiece.

Gang Milling.
Gang milling is the term applied to an operation in which two or more milling cutters are used
together on one arbor when cutting horizontal surfaces. The usual method is to mount two or
more milling cutters of different diameters, shapes and/or widths on an arbor as shown in
figure 18. The possible cutter combinations are unlimited and are determined in each case by
the nature of the job.

Form Milling.
(1) General. Form milling is the process of machining special contours composed of
curves and straight lines, or entirely of curves, at a single cut. This is done with formed
milling cutters, shaped to the contour to be cut, or with a fly cutter ground for the job.
(2) Operation. The more common form milling operations involve milling half-round
recesses and beads and quarter-round radii on the workpieces (figure 19). This
operation is accomplished by using convex, concave, and corner rounding milling
cutters ground to the desired circle diameter.
(3) Other jobs for formed milling cutters include milling intricate patterns on workpieces
and milling several complex surfaces in a single cut, such as produced by gang milling.

l. Woodruff Keyway Milling.
(1) General. Keyways are machined grooves of different shapes, cut along the axis of
the cylindrical surface of shafts, into which keys are fitted to provide a positive method
of locating and driving members mounted on the shafts. A keyway is also machined on
the mounted member to receive the key. The type of key and corresponding keyway to
be used depends on the class of work for which it is intended. The most commonly used
type of key is the woodruff.
(2) Operation.
(a) Woodruff keys are semi-cylindrical in shape and are manufactured in various
diameters and widths. The circular side of the key is seated into a keyway which is
milled into a shaft with a woodruff keyslot milling cutter having the same diameter.

m. Gear Cutting.
(1) General. Gear teeth are cut on the milling machine using formed milling cutters
called involute gear cutters. These cutters are manufactured in many pitch sizes and
shapes for different numbers of teeth per gear (table 4 on the following page).
(2) Operation. If involute gear cutters are not available and the teeth must be restored
on gears that cannot be replaced, a lathe cutter bit can be ground to the shape of the
gear tooth spaces and mounted in a flycutter for the operation. The gear is milled in the
following manner:
Drilling.
(1) General. The milling machine may be used effectively for drilling, since the
accurate location of the hole may be secured by means of the feed screw graduations.
Spacing holes in a circular path, such as the holes in an indexing plate, may be
accomplished by indexing the workpiece with the indexing head that is positioned
vertically.
(2) Operation. Twist drills may be supported in drill chucks that are fastened in the
milling machine spindle or mounted directly in the milling machine collets or adapters.
The workpiece to be drilled is fastened to the milling machine table by means of clamps,
vises, or angle plates. Remember, proper speeds and feeds are important functions to
consider when performing drilling operations on the milling machine.
o. Boring. Various types of boring toolholders may be used for boring on the milling
machine. The boring tool can either be a straight shank, held in chucks and holders, or
tapered shanks to fit collets and adapters. The two attachments most commonly used for
boring are the flycutter arbor and the offset boring head. The single-edge cutting tool that is
used for boring on the milling machine is the same as a lathe cutter bit. Cutting speeds,
feeds, and depth of cut should be the same as those prescribed for lathe operations.