Types of Milling Explained: Know All Milling Operations

How Does CNC Milling Work?

Milling is a machining process that produces complex geometric forms from solid blocks of material using a CNC milling machine and milling cutters. The milling process comprises a stationary workpiece and a rotating cutting tool. Additionally, a CNC milling machine has linear and rotary axis motions to generate complex tool paths between the workpiece and milling cutter.

CNC milling technology is automatic and thus highly precise. The motion of the tool on the workpiece (the tool path) is programmed via CAD/CAM software. In a typical CAM software environment, engineers have a multitude of types of milling operations to choose from, each having its functional use.

Basic Types of Milling Operations

Let us begin with some of the basic and most common types of milling operations prevalent in a machine shop.

Typically, these milling operations are also characterizable by part geometry, as each process achieves specific geometric features on the workpiece.

Plain Milling

Plain milling, or slab milling, is a core milling process where a horizontal milling cutter machines flat surfaces on the workpiece. The cutter’s direction of rotation aligns with the feed direction in plain milling. In other words, the tool axis is parallel to the cut surface.

Generally, plain milling is quite useful for roughing cycles at the beginning of a manufacturing plan. It is effective in converting rough material stocks to fine prismatic, square geometries with accurate dimensions and smooth finishes, ready for further milling operations.

In addition, slab milling offers a good material removal rate and high process stability due to the horizontal orientation of the cutting tool.

Face Milling

Face milling, like plain milling, produces flat and smooth surfaces on the workpiece. The difference is in the tool orientation. The cutting tool in face milling is perpendicular to the cutting surface, which means that the bulk of the cutting action happens at the face of the cutting tool rather than its side edges.

This makes face milling an ideal operation for achieving fine surface qualities and high dimensional accuracy. Due to the tool axis being perpendicular to the cutting plane, it is less susceptible to geometric deviations and tool vibrations, leading to high workpiece parallelism.

Side Milling

Side milling is among the common types of milling processes used to cut the sides of a workpiece. It primarily utilizes the periphery, or side edges, of the cutter to remove material. Generally, side milling is useful to cut profiles, slots, and vertical walls in the workpiece.

Due to the cutting forces acting perpendicular to the tool axis, machinists need to exercise care in managing the cutting load on the tool. Excessive cutting forces or very deep cuts can cause tool deflections and even breakage.

Shoulder Milling

Shoulder milling is a milling process that produces vertical walls and flat floors/bottoms with accurate, perpendicular shoulders between them. It utilizes both the face and peripheral edges of the milling cutter, typically an endmill. This kind of cutting allows simultaneous cutting of the floor and the wall, allowing for cutting of sharp corners in the shoulder geometry.

The useful of shoulder milling in producing high-precision parts with prismatic features like molds and dies, where sharp corners are essential to maintain rigid assemblies.

Angular Milling

Angular milling machines make angular cuts and features into the workpiece. Most commonly, such angular features include chamfers or grooves along the edge of the workpiece.

Machinists might utilize multiple milling techniques to perform angular milling operations. For example, they may use an angle fixture to hold the workpiece at the desired angle, or they may use purpose-built milling cutters with angled cutting edges, or even multi-axis machines.

Generally, angular milling is a finishing operation to remove sharp edges or produce aesthetic features. For high-precision parts like molds and dies, it is quite useful in accurately producing the correct draft angles in the workpieces.

Form Milling

Form milling is a milling process that involves specifically shaped cutters to produce features with special geometries. Form milling tools can produce complex contours like curves, arcs, and other nonconventional shapes. The figure below, for example, shows a form milling cutter with a round outline, capable of producing an arced cavity inside the workpiece.

A common application of form milling is in mold making and tooling, where engineers design their own form milling cutters to cut custom geometric profiles in mold and die stocks.

Profile Milling

Profile milling is an advanced milling process that produces complex contours on the workpiece. These contours generally comprise a combination of curves, arcs, and lines. Profile milling is generally a CNC milling machine operation, as it is impossible to achieve intricate profiles manually.

There are several applications of profile milling in industrial manufacturing of geometrically complex parts. For instance, aerospace turbine blade profiles and automotive exteriors require extensive profile milling operations.

Gang Milling

Gang milling is a milling process that maximizes productivity by utilizing several cutting tools at the same time. These tools are mounted on a single arbor in a specific manner. This single arbor then rotates all the cutters, each of which machines a different portion of the workpiece surface. The figure below illustrates this.

The gang milling is very common for parts with multiple cuts in the same orientation. It reduces the total machining time while improving throughput.

Gear Milling

Gear milling, as the name suggests, is an exclusive milling process for producing gears. Gears have highly intricate geometric profiles that are difficult to achieve with standard milling cutters.

Thus, gear manufacturers use special gear milling cutters with the exact profile they require. This approach helps maximize precision and productivity.

Thread Milling

Thread milling, like gear milling, is among the exclusive types of milling operations. It is used to cut threads inside a hole in the workpiece. Thread milling operations utilize special milling cutters in the specific shape of the thread profile.

Moreover, thread milling is a high-accuracy milling process as it requires a helical toolpath in order to cut the thread at the correct pitch. Typically, machinists perform thread milling on a CNC milling machine using specialized toolpath generation cycles.

Producing threads using milling techniques instead of traditional methods like taps offers greater flexibility as the process is conveniently adaptable to any hole size, pitch, and profile.

Slot Milling

Slot milling is a common milling process useful in producing slots in workpieces. It can work with a variety of cutter types, including T-slot cutters, endmills, or disc cutters, depending on the geometry of the slot.

Some common applications of slot milling include the machining of keyways, pockets, and grooves in structural components.

Special Types of Milling Operations

Milling operations are not limited to the standard processes discussed above. With the latest advances in CNC technology and tooling, milling has evolved significantly. A modern-day CNC milling machine can handle highly complex part geometries with intricate features.

This section will discuss some of the advanced types of milling techniques.

Multi-Axis CNC Milling

Multi-axis CNC milling is an advanced milling technique that utilizes a multi-axis CNC milling machine. 5-Axis CNC milling centers are the most popular choice for advanced manufacturing machine shops, which generally comprise three linear axes (X, Y, Z) and two rotary axes (any two of A, B, C), which the machinist can manipulate simultaneously. The figure below illustrates this configuration.

CNC multi-axis milling is capable of machining high-precision parts with geometric features like spline curves, profiles, surfaces, and undercuts. It is a crucial role of demanding industries like aerospace, automotive, and medical devices, where non-prismatic parts and tight tolerances are common.

Turn Milling

Turn milling is a hybrid machining technique that combines features from both turning and milling in the same machine, known as a turn-mill. Turn milling centers typically have a chuck and a tail stock to mount and rotate the workpiece, as in a turning machine. In addition to this, they also have specialized tool-holding devices with rotating cutting tools, as in a milling process.

The milling cutter in turn-milling centers is maneuverable in multiple directions, including both parallel and perpendicular to the workpiece axis (see figure below).

This type of dual functionality allows turn-milling centers to produce parts with both cylindrical and prismatic features in the same setup. This boosts productivity, accuracy, and saves costs.

Thin-Wall Milling

Thin-wall milling is a special branch of advanced milling techniques. Certain parts, like turbine blades and product enclosures, have thin-walled features. Thin walls are characterized by their low height-to-width ratio.

The delicate geometry of these thin-walled features makes them susceptible to issues like vibrations, permanent deformations, and machining chatter. To avoid these problems, machinists utilize special milling techniques like dynamic milling, adaptive feed rates, and variable engagement toolpaths for maximum precision and surface quality.

Two Typical Milling Operations: Conventional vs Climb Milling

Conventional and climb milling are two types of milling operations that define how the cutter engages with the workpiece. The main difference between them is how the workpiece feed direction is with respect to the cutting tool’s rotation.

In conventional milling, or up milling, the feed direction opposes the cutter’s rotation. This causes the cutting edge to initiate the cutting as rubbing and then slowly transition to pure shear. As a result of the cutting direction, the chip width starts from zero and is maximum when the cutter exits the workpiece. Generally, conventional milling is more stable in terms of vibration and dealing with machine backlash.

In climb milling or down milling, on the contrary, the feed direction aligns with the cutter’s rotation. The chip width starts at its maximum and gradually goes to zero, leading to a smooth cutting load trend. Climb milling is the preferred milling technique as it yields a better tool life, surface finish, and low cutting loads.

Key Factors in Selecting the Right Milling Type

With so many types of milling operations to choose from, making the optimal choice can be a challenging task. Generally, there are several factors that engineers must consider when selecting milling operations for their manufacturing plans.

Part Geometry

The geometric features of the part are the first thing to look at when selecting milling processes for a part. Specific features like angled surfaces, curvature, and threads can help shortlist the milling process to a manageable number.

For instance, an automotive panel, owing to its complex curvature, would most certainly require advanced milling techniques like profile milling with a CNC milling machine.

Machine Tool Capability

Different milling machines have different machining capabilities. A manual milling 3-axis machine is perfect for simple jobs like squaring stocks, cutting angled surfaces, or producing planar surfaces. A more complex operation, like profile milling, however, may not be possible without a multi-axis CNC milling machine.

Similarly, a turn-mill center is a specialized machine tool that can efficiently produce primarily cylindrical parts with prismatic features.

Quality Requirements

Quality requirements like surface finish and tolerances also dictate the choice of the milling process. Climb milling, for example, would be more suitable if high-quality production is desired. A CNC milling machine is the preferred choice if tight tolerances are required.

Cutting Parameters in Different Milling Operations

The milling process is defined by milling cutting parameters, which engineers use to control factors like cutting forces, surface quality, and manufacturing tolerances. The three main cutting parameters in milling are as follows:

Feed

The feed rate determines the rate at which the cutting tool and workpiece move relative to each other. Engineers use various ways to represent it. The common units are in terms of distance/time (mm/min), distance/revolution (mm/revolution), and distance/teeth (mm/teeth). Higher feed rates in milling operations achieve high productivity but decrease tool life and surface quality.

Speed

Cutting speed defines how quickly the tool rotates over the surface of the workpiece. Its typical measurement units are mm/min or RPM. Higher cutting speed in milling techniques promotes productivity and surface quality but can lead to excessive tool wear and thermal issues.

Depth of Cut

The depth of cut determines how deep the cutting tool goes into the workpiece surface. In a milling process, since the cutter is often cutting in both the radial direction and axial direction, there are two depth of cut parameters, namely radial depth of cut (RDOC) and axial depth of cut (ADOC). High depth of cut increases cutting forces, vibrations, and heat generation, but it also leads to greater productivity.