Subtractive Machining Explained: Techniques, Applications, and Benefits

Dive too deeply into the world of machine tools and modern machining techniques, and you’re likely to come across a discussion of subtractive and additive machining methods. What are these techniques, how do they work, and how are they used?

We’ll explore both ideas in this blog, and then focus a bit more on subtractive machining.

Modern machining methods

Machining refers to the mechanical processes used to create and finish parts. Most machining is done with various machine tools – these include lathes, mills, routers, and similar machines.

Additive and subtractive machining are two different ways of achieving the same goal. Additive machining, as the name implies, combines material to create a part. One rough example of this process might be pottery, where a wheel (the machine tool) is used to combine and shape a material (the clay) into a finished product.

However, additive machining was always limited by the kinds of material. Clay works fine for additive processes, but wood is impossible. Various metals can be moulded, but only when hot, and casting and moulding processes are distinct from additive machining. 

For much of human history, that meant that parts for various machines needed to be created using subtractive machining.

Subtractive machining – the standard approach

Let’s use an example. Say you have a flat disk of metal, and you need to create a gear or cog. You could try to attach the teeth of the gear to the edge of the disc, but that might pose a challenge. You would need to heat and weld the teeth onto the disc, and even then, the finish wouldn’t be particularly smooth. To finish the gear and ensure it worked correctly, you’d have to go back through and remove any excess material. 

Of course, to save on time, you could simply create a gear by removing material from the disc in the first place. By cutting or grinding out excess material, you can create the raised pattern of teeth for the cog. And while you do it, you can use finishing processes to ensure a smooth, even finish.

By removing material, you’ve created a finished part – that’s subtractive machining. Historically, subtractive processes have been used with wood and metal. Think of a woodworker, turning a piece of oak on a lathe to create a beautiful chair leg, or Industrial Revolution-era metal mills grinding out iron supports for new factories.

Subtractive machining is usually used in one of two ways: to create a finished part out of a larger, unworked piece, or to finish an already-worked part to a more polished state.

Carving a leg out of a block of wood is a great example of the first method. Switching out the cutting tool on the lathe for a finer, smoother one would be a great example of the second. Both instances remove material, just in different amounts and for slightly different purposes. 

Subtractive machining in modern manufacturing

Today’s subtractive machine tools bear little resemblance to their early predecessors. Modern lathes and mills are extremely precise, high-powered, variable-speed, and frequently CNC equipped. With Computer Numerical Control (CNC) technology, operators can use computers to plan, program, and execute cutting operations with incredibly narrow tolerances. 

Many lathes and mills can move on three or even four axes, allowing parts to be cut, rotated, cut again, flipped, and cut once more, as many times as needed. What used to require multiple machines or multiple operations on the same machine can now be programmed and cut on a single machine tool.

Modern technology has added a number of new subtractive machine tools to a machinist’s arsenal. Electron Discharge Machines (EDMs) use electrical current to slowly section ferrous materials. For non-ferrous or coated parts, water jet cutters use concentrated streams to slice through. Laser cutters work on a variety of materials, sectioning parts as needed.

The future of subtractive and additive processes

While subtractive machine tools still occupy pride-of-place on most manufacturing floors, there is a growing push to integrate modern additive processes as well. Modern tools like 3D printers allow additive processes to be used with wood and metal alloys, adding a new dimension to modern machining. Subtractive machine tools can then be used in a finishing role, removing excess material or cutting geometries not currently possible with additive methods.

Moving forward, machinists will be able to use the two processes together. It may even be possible to create a computer program that will start a process on a 3D printer and then finish it in a lathe or mill, seamlessly combining the two approaches. In the meantime, machinists will be able to experiment with new additive processes while still relying on lathes, mills, and subtractive machining.