What is Milling?- Definition, Process, and Operations

Milling is one of the most widely used machining processes. Milling machines can be found in virtually every machine shop. The benefits and capabilities of these machines are vast.

Newcomers to the industry typically ask what milling is, how it works, and what types there are. This paper will provide a high level of detail around the milling process and technology.

This paper will include a lot of helpful information for both novices and professionals. This paper also includes a lot of helpful suggestions on improving milling practices and quality.

What is Milling?

Milling is a machining process that uses rotating cutters to remove material from a workpiece by advancing a cutter into the workpiece.

Milling encompasses many (different) operations on a variety of machines, from small individual parts to large heavy-duty gang milling operations. It is one of the most widely used processes for machining custom parts to exacting tolerances.

Milling can be done in different ways (including in different directions) on one, or more axes, and with different cutting head speeds and pressures. There are many different machine tools that can perform milling.

The main pieces of equipment needed for the milling process are a milling machine, a workpiece, a jig, and a milling cutter. The workpiece is simply a piece of pre-shaped material that is attached to the jig, which is, in turn, mounted on a platform in the milling machine.

A milling cutter is a cutting tool that has sharp teeth and is also mounted in the milling machine and rotates at a high-speed. The workpiece is fed into the milling cutter, and in the process the material is cut from this workpiece in the shape of small chips, to produce the desired shape.

Milling is used mainly to make parts that are not axially symmetrical, and that have many features, such as holes, slots, pockets (or cavities), or three-dimensional surface contour.

Parts made completely by milling are typically characterized by the parts being produced for use, in small amounts, possibly for prototyping, such as custom fasteners or brackets.

Another strong application of milling is the fabrication of tools for other processes. For example, three-dimension shapes are usually is milled. Milling is also a regular secondary for adding or refining features on parts made with another process.

The capability for milling to make parts with high tolerances, and high surface finishes works well for adding precision features to a part that has already been created, with another process.

Milling Process

Milling is a cutting operation which uses a milling cutter to make a cut on the surface of a workpiece. A milling cutter is a rotating cutting tool, typically having several cutting edges. Milling is different from drilling in that the tool is usually advanced at a right angle to the axis of rotation, meaning that the cutting action is occurring on the circumference of the milling cutter rather than the end.

When a milling cutter advances into a workpiece, the cutting edges of the tool are cut into and out of the workpiece again and again, so chips are scraped off of the workpiece with each revolution of the cutter.

The cutting action is a shear strain; the material is propelled from the workpiece in little lumps which are more or less joined together as chips. This makes cutting metal a little different process than cutting soft materials with a blade.

The milling process senses the removal of material through many separate small cuts, this is accomplished through the use of multi tooth cutters, high speed rotation of the cutters, and/or the slow advancement of the material; typically a combination of these three actions.

Different speeds and feeds are used to incorporate a combination of different variables. The speed that the workpiece is permitted to be advanced through the cutting device is called the feed rate, or just feed; typically measured as distance per time although sometimes distance per revolution or per tooth of the cutter is utilized as a measure.

There are two major classes of milling process:

In face milling, the cutting action typically occurs on the end corners (ends) of the cutter. The main use of face milling is to produce flat surfaces on the workpiece or to cut flat-bottomed cavities.

In peripheral milling, the cutting action mainly occurs in a peripheral manner (in the flat sides) of the milling cutter so that the cross-section of the milled surface takes the shape of the milling cutter.

The blades of the milling cutter can be thought of as having “excised” material from the workpiece. Peripheral milling is suitable for cutting deep grooves, threads, and gear teeth.

What Is Milling Cutter?

Milling cutters are cutting tools often used in milling machines or machining centres to mill or cut material from a work piece. They remove material by moving it in the machine or removing it directly from the cutting die.

All milling machines have a milling cutter. In a typical milling operation the milling cutter is moved at a right angle (90°) to its axis, allowing it to remove material from the circumference of the milling cutter (cutting action).

The milling cutter’s function is to remove material from a work piece. Cutters are not made from just one blade. In lathe operations, a turning operation typically uses a cutting tool with one edge.

A milling cutter has multiple edges. This allows for a more continuous cutting edge to remove material in the act of scraping it away, rather than cutting it.

The milling cutter is usually made from hard strong materials to keep from breaking or being otherwise damaged from significant stress and wear.

Milling Operations

A variety of processes can be performed upon the workpiece during the process cycle to achieve the desired shape of the part. The following processes are each defined by the type of milling cutter employed and the path taken by this cutter to remove material from the workpiece.

• End milling: An end mill utilizes either peripheral or groove cuts, based on the distance of the infeed, over the workpiece to machine a specific feature such as a profile, groove, pocket, or even an intricate surface contour. When machining a depth, this feature can be machined to depth with a single cut or achieved by machining with reduced axial depth of cut and multiple cuts.
• Chamfer milling: A chamfer cutter will machine a peripheral cut adjacent to the edge of a workpiece or feature to form a sloped surface called a chamfer. Chamfers are typically at a 45-degree angle but can be different angles and may be machined on the outside, or inside of a part and follow a straight, or curved surface.
• Face milling: An end mill will machine a flat surface of the workpiece forming a smooth surface. The normally very small depth of the end face can be machined by depth with a single cut or by machining with reduced axial depth of cut and the number of cuts as well.
• Drilling: A drill will penetrate the workpiece axially and cut a hole with a diameter equal to that of the tool. A drilling operation can machine a blind hole, which is a hole stopping at depth within a workpiece, or a through hole, which extends completely through the workpiece.
• Boring: A boring tool penetrates the workpiece axially and cuts along an interior surface to create various features. A boring tool is a single-point cutting tool and can be set to the desired diameter by an adjustable drill head. Boring is usually performed following the drilling of a hole to increase the diameter or provide more precise dimensions.
• Counterboring: A counterbore tool penetrates axially into the workpiece and enlarges the upper part of an existing bore to the diameter of the counterbore tool. Counterboring is often performed following drilling to allow for the head of a fastener, such as a bolt or insert screw, to sit below the surface of a part. The counterboring tool has a guide at the end to guide the tool directly into the existing hole.
• Countersinking: A countersink tool penetrates axially into the workpiece, and enlarges the existing bore to a tapered opening at the upper area of the existing bore. Countersinking is often performed after drilling to create enough room for the head of a fastener, such as a screw, to be flush with the workpiece surface. Common included angles for a countersink are 60, 82, 90, 100, 118 and 120 degrees.
• Reaming: A reamer penetrates axially into the workpiece and enlarges an existing hole to the diameter of the reamer. Reaming makes minimal material removal only and is typically performed after drilling, to achieve accurate diameter and smooth internal finish.
• Tapping: A tap penetrates axially into the workpiece, and cuts internal threads into the existing hole. The existing hole is generally drilled with the required tap size to allow for the chosen tap. Threads can be cut to a certain depth inside the hole (bottom tap), or to entire depth of a through hole.

FAQs.