Machining involves using cutting tools to remove material from a metal or plastic workpiece to give it the desired shape and dimensions. Unlike casting or forging, no material is added; excess material is removed in a controlled manner. For this reason, the method is also known as “subtractive manufacturing.”

Whether it’s taking a block of aluminum and machining a precision fastener from it or adjusting the diameter of a steel shaft to the micron level, all of these are tasks performed by machining. From automotive to defense, and from aerospace to medical applications, virtually every industry that requires precision relies on this method.

In this article, we explain how subtractive manufacturing works, its main methods, the advantages it offers, and when it should be preferred.

How does machining work?

The logic is simple: there is a rotating tool or a rotating workpiece; controlled contact is established between the two, and the cutting edge removes thin layers (chips) from the material. The tool’s rotational speed, feed rate, and depth of cut are predetermined.

In modern workshops, a computer controls these movements. The part’s 3D model is converted into toolpaths in CAM software, which are then translated into commands (G-code) that the machine can understand. The operator secures the raw material to the machine, starts the program, and the machine machines the part with micron-level precision. This automation is known as “CNC machining.”

Machining Methods

There is no single machining method; different processes are used depending on the part’s geometry. The most common ones are as follows.

Turning

In turning, the workpiece rotates at high speed, while the cutting tool moves along a fixed axis to cut the material. It is the ideal method for cylindrical and conical parts: shafts, bushings, pins, couplings, and flanges. In addition to machining outer diameters, drilling, grooving, and threading are also performed on a lathe. We’ve covered this topic in detail in a separate article: What Is a CNC Lathe?

Milling

In milling, the logic is reversed: this time, the tool rotates, while the workpiece is clamped to the table and moves. Flat surfaces, pockets, slots, gear profiles, and complex three-dimensional shapes are produced through milling. Milling is indispensable for prismatic and free-form parts.

Drilling, Reaming, and Tapping

Drilling a hole is the first step; however, the job isn’t finished there if a precise bearing bore or a clean screw thread is required. The hole size is refined with a reamer, and the thread is cut with a tap. These operations are often completed in a single setup on a lathe or milling machine.

Grinding

Grinding comes into play where the highest surface quality and tightest tolerances are required. An abrasive wheel removes very thin layers from the part, polishing the surface and fine-tuning the dimensions. Grinding is essential for hardened steels and parts requiring a surface roughness of less than Ra 0.2.

The Difference Between CNC and Conventional Machining

On conventional machines, the operator performs every movement manually; achieving the correct dimensions depends on the operator’s experience. These machines are still used for single-part or simple jobs.

On CNC machines, however, movements are controlled by a computer. The practical benefit of this is as follows:

  • Repeatability: The first part and the five-thousandth part come out with the same dimensions.
  • Precision: On a high-quality CNC machine, tolerances of ±0.01 mm—and even micron-level precision (±0.005 mm) with grinding—are easily achieved.
  • Complex geometry: Shapes that are impossible to achieve by hand can be produced using software.
  • Speed: In mass production, the machine operates without interruption.

In mass production and jobs requiring tight tolerances, CNC is almost always the preferred choice.

Advantages of Machining

There are several key advantages that make machining so widespread:

  • High precision. Methods such as casting produce rough shapes; the final dimensions and tolerances are often achieved through machining.
  • No molds required. With casting, you have to commission expensive molds for each part. In machining, parts are machined directly from raw material; this makes prototyping and low-to-medium-volume production cost-effective.
  • Wide range of materials. Many materials can be machined, from aluminum to titanium, stainless steel to engineering plastics.
  • Superior surface quality. The part emerging from the machining process is often ready for assembly without requiring additional finishing.
  • Flexibility. When a design change is needed, you update only the program—not the mold.

There’s another side to the coin: the resulting chips represent material loss, and at very high production volumes, casting may offer a lower unit cost. The right method depends on the part’s geometry and production volume.

Which materials can be machined?

Machining offers flexibility in terms of materials. The most commonly used materials are:

  • Aluminum alloys: Lightweight and easy to machine; widely used in the automotive and aerospace industries.
  • Steel and stainless steel: Used in parts requiring high strength.
  • Titanium: High strength-to-weight ratio; a favorite in the aerospace and defense industries.
  • Brass and copper: Known for their conductivity and machinability.
  • Engineering plastics: Such as POM, PEEK, and nylon.

Each material has its own specific cutting speed, tool selection, and cooling requirements. We cover material selection in a separate guide: Which Metals Can Be Machined with CNC?

In which industries is machining used?

Precision parts are needed almost everywhere, but some industries are particularly reliant on machining:

  • Automotive: Engine and transmission parts, fasteners, and specialized fixtures.
  • Defense industry: Parts with tight tolerances, critical safety requirements, and often requiring confidentiality.
  • Aviation: Components that require both lightness and strength, and certified production.
  • Machinery manufacturing: Shafts, gears, bearings, molds, and fixtures.
  • Medical and energy: High-precision, traceable parts.

When should you choose machining?

A practical rule of thumb: If a part requires tight tolerances, good surface quality, or complex geometry, and the production volume ranges from a single piece to a medium-sized series, machining is often the best method. It’s also ideal for avoiding tooling costs during the prototyping phase; plus, you can seamlessly transition to mass production using the same program you used for the prototype.

For parts with very high production volumes, simple geometries, and loose tolerances, however, methods such as casting or stamping may offer a cost advantage.

Frequently Asked Questions

What is the difference between machining and casting? Casting shapes the material by pouring molten metal into a mold and adding material. Machining, on the other hand, shapes the material by removing chips from a block of material. Casting is economical for high-volume production; machining excels in precision and flexibility. In most parts, both processes are used together: casting provides the rough shape, while machining achieves the final dimensions.

What tolerances can be achieved in subtractive manufacturing? On a good CNC machine, ±0.01 mm is a common target. With grinding and precision machining, tolerances of ±0.005 mm or less—that is, at the micron level—can be achieved. The required tolerance is determined by the part’s function.

Can I have a single part manufactured? Yes. One of the greatest advantages of machining is that it does not require a mold; therefore, both a single prototype and a production run of thousands of units can be manufactured cost-effectively.

What do I need to provide for production? Generally, a technical drawing of the part (including dimensions and tolerances) or a 3D model (in formats such as STEP or IGES) is sufficient. If you don’t have a drawing, production can also be carried out through reverse engineering using a sample part.

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