Milling is a machining process in which a rotating multi-edge cutting tool removes material from a workpiece. Relative movement between the cutter and workpiece can be used to produce flat surfaces, shoulders, slots, pockets, profiles and many other engineering features.
Milling machines range from traditional horizontal and vertical machines to modern CNC machining centres capable of controlling several axes and performing multiple operations within a single setup.
Milling machines have traditionally been classified into several broad groups:
Although modern CNC machines differ considerably from traditional manual milling machines, the underlying principles of cutter rotation, feed, chip formation and workholding remain applicable.
A wide range of milling cutters is available. Cutter selection depends on the feature being produced, workpiece material, machine capability and required surface finish.
Traditional and modern cutter forms include:
T-Slot Milling Cutter
Shell End Mill
High Helix Cylindrical Cutter
Slotting Cutter
On a vertical milling machine the cutter spindle is arranged vertically. The machine provides controlled relative movement between the workpiece and cutter, allowing operations such as face milling, end milling, slotting and drilling to be performed.
Traditional vertical machines normally provide movement in three principal linear directions. Modern CNC vertical machining centres apply the same basic coordinate principle using programmable machine axes.
On a horizontal milling machine the cutter spindle is horizontal. Traditional horizontal machines can carry cutters on an arbor and are particularly suited to operations using peripheral, side-and-face and form cutters.
Horizontal machines can provide good cutter support and efficient material removal. The suitability of a horizontal or vertical configuration depends on component geometry, cutter access, workholding and the operations to be performed.
Fixed-bed milling machines use a more rigid machine structure than the traditional knee-type arrangement. Machine movements are provided by the table, spindle head or a combination of controlled axes depending on the machine configuration.
The increased structural rigidity can be advantageous for larger components, heavier cuts and production machining.
The relationship between cutter rotation and feed direction affects chip formation, cutting forces and the behaviour of the machine and workpiece. The two basic arrangements are commonly called up milling (conventional milling) and down milling (climb milling).
In up milling, the cutter motion at the point of engagement is opposed to the direction in which the workpiece is fed. Chip thickness begins close to zero and increases as the tooth passes through the cut.
The cutting action can produce a component of force which tends to lift the workpiece from the table, making secure workholding important. Traditional machines with significant feed-screw backlash have often been operated using this method.
In down milling, cutter motion at the point of engagement is in the same general direction as the workpiece feed. The tooth enters with a relatively large chip thickness which reduces towards the end of the cut.
The cutting force tends to pull the workpiece in the feed direction and can also act towards the supporting surface. On machines with uncontrolled backlash this can cause the table to be drawn into the cutter unexpectedly. Modern machines with suitable feed systems and controlled backlash can use climb milling effectively.
Cutting speed is the peripheral speed of the effective cutting diameter relative to the workpiece and is normally expressed in metres per minute.
For a cutter diameter D in millimetres rotating at N revolutions per minute:
Vc = πDN / 1000
where:
The required spindle speed can therefore be calculated from:
N = 1000Vc / (πD)
For some cutter geometries the effective cutting diameter at the actual depth of cut should be used rather than simply assuming that the nominal outside diameter represents the cutting diameter.
Milling feed is commonly related to the feed per effective cutting tooth. Once the required feed per tooth and spindle speed are known, the table feed can be calculated from:
Vf = fz × N × zc
where:
The feed per tooth can therefore be calculated from:
fz = Vf / (N × zc)
These relationships are fundamental milling calculations. The appropriate values of cutting speed and feed per tooth, however, depend on the actual cutter, cutting-tool material or insert grade, workpiece material, engagement and machining conditions.
For production machining, obtain starting cutting data from the current manufacturer's information for the specific cutter or insert being used, then apply the cutting-speed, spindle-speed and feed relationships above.
Milling conditions are also affected by the axial depth of cut and radial engagement or width of cut. These determine how much of the cutter is engaged with the workpiece and influence cutting forces, chip thickness, heat generation, tool loading and material removal rate.
Consequently, cutting speed and feed should not be considered independently of cutter engagement. Conditions suitable for a lightly engaged finishing operation may be inappropriate for full-width slotting or heavy roughing.
Feed per tooth describes the programmed movement associated with each effective cutting tooth, but it does not always equal the actual maximum chip thickness.
Cutter entering angle and radial engagement can alter chip thickness. This is particularly important with modern face mills, high-feed cutters and operations using relatively small radial engagement.
Milling cutters may use high-speed steel, cemented carbide and other cutting materials depending on the cutter design and application. Indexable cutters allow replaceable inserts with grades and geometries selected for particular workpiece materials and machining conditions.
For further information see:
Milling produces changing cutting forces as individual cutter teeth enter and leave the workpiece. The component must therefore be supported and clamped securely while maintaining access for the cutter.
Machine rigidity, workholding rigidity, spindle condition, toolholder stiffness and cutter overhang all influence dimensional accuracy, tool life and the tendency for chatter.
Traditional milling machines require the operator to control the machine movements directly. CNC machines control these movements from a programmed tool path and can coordinate several axes simultaneously.
Modern machining centres may combine milling, drilling, boring, tapping and related operations in a single setup and can include automatic tool changing, probing, pallet systems and automated workpiece handling.
Despite this development, the fundamental relationships between cutting speed, spindle speed, feed, cutter geometry, workholding and machine rigidity remain applicable.
Important considerations when planning a milling operation include:
Component design can have a major effect on milling time and cost. Features should provide reasonable cutter access and should avoid unnecessarily deep, narrow pockets or internal corners which require very small or excessively long cutting tools.
The designer should also consider how the component will be located and clamped and whether several features can be machined from a common setup. Reducing the number of setups can improve both production efficiency and dimensional consistency.
The following external resources were included on the original RoyMech page and provide additional information relating to milling and machining.