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Subtractive Manufacturing

“Subtractive Manufacturing” is a blanket term that has only recently been invented to differentiate it from Additive Manufacturing, also known as 3D Printing. It covers a wide range of tools and processes such as milling, turning, cutting, drilling – any process whereby the final shape is achieved through removing some amount of material.

This is still an extremely useful production method and indeed is the only way that many things can be made; 3D printed parts often struggle to stand up to the forces that a solid bar of steel can withstand, for instance. A good rule of thumb is that a 3D printed part is typically 80% as strong as a part that was machined from a solid block of similar material.

Shown here are two major tools in any machining operation: a Mill and a Lathe.

Image Credit: Market Prospects

Computer Aided Machining

Even though many of these processes have existed for hundreds or thousands of years, in recent times they have been automated with computers. Machines like the ones shown here can be automated with motors and computers and perform the same tasks that would be done by hand.

When using CAM, it’s a lot like a 3D printer:

  1. A part is designed on the computer using 3D modeling software.
  2. The part is run through a program that creates machine movement code (gCode).
  3. The code is sent to the machine which executes it.
Image Credit: Autodesk, Fusion 360

Toolpaths generated in Fusion 360 for machining on a Mill.

The biggest difference is that machining something requires a lot more knowledge about the process: it is possible to design things that are literally impossible to make, or parts that have to be repositioned several times during the cutting process.

Note that in the videos below there is a lot of fluid being dumped over the parts. This fluid is for cooling metal parts during machining to prevent them from getting damaged or warped.

Example Subtractive Manufacturing Machines

Milling

Milling is one of the most common and versatile processes available. Like 3D printing, a gantry moves a tool head and the bed in combination to create 3 directions of movement. Different drill bits are placed in the Spindle and the work piece is fixed to the Table so they don’t fly off during the process. The machine moves all three axis in concert to create sloping shapes.

This process is perfect for creating complicated geometries out of solid metal or even plastic, making it ideal for creating molds and structural components.

Image Credit: Basicmechanicalengineering.com

Turning

Turning is ideal for components that are symmetrical around their axis. These include as drive shafts in large ships, chair legs, threaded rods, etc. Turning is what a Lathe does: spins a work piece about its axis.

A piece of material is mounted in the Spindle (aka “Chuck). The Tail Stock is slid up against it to prevent it from wobbling and the machine is turned on, spinning the part at a high rate of speed. Then the Tool Post, containing the cutting tool is moved against the part to remove material.

In advanced CNC machines they have automatic tool changers (Tool Turrets) that have multiple cutting tools on them which rotate into place as necessary.

Image Credit: Hwacheon

When is Subtractive Manufacturing a good idea?

  1. The part needs to be extremely strong: machined parts will almost always be stronger than 3D printed parts.
  2. It needs to be made out of a solid block of material and any voids would cause a problem.
  3. A perfect surface finish is required. While 3D prints can be sanded down, machined component can be made super smooth.
Image Credit: Source Horizon
Image Credit: Reality Daydream

When should you consider something else?

  1. The part needs to be mass manufactured. While CNC can often mass manufacture parts, molding or other processes is often cheaper.
  2. When extremely complex geometry is required.
  3. Working with CAM machines can be dangerous and require training to use safely.
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