Additive or Subtractive Manufacturing?

Before prototyping or manufacturing a product, it is important to consider all the different manufacturing processes which are available, as well as the strengths and weaknesses of each. As a designer it is your goal to understand how you can utilize these different tools when creating new projects. When choosing between additive or subtractive manufacturing processes, it is important to understand the differences in production from: possible geometries, material strengths, and cycle time. Each method has its own pros and cons. In this article we’ll review some of the differences in choosing between additive or subtractive processes.

Cutting down to size

Traditional manufacturing processes, also known as “subtractive” processes, remove material from a workpiece to create the desired geometry; examples of these processes include milling, turning, or sawing. Subtractive manufacturing begins with a solid block of material that is modified by a series of processes such as cutting, drilling, and milling to remove material. One such example is using an advanced CNC machine, where the machinist is able to rotate the block around multiple axes (x, y and z) to make the necessary cuts, channels, and holes, and any other features that can be produced by material removal. Afterwards products often need to go through several steps of machining and assembly before they are finalized.

With subtractive manufacturing, there are a variety of options in terms of materials and processing methods. Softer materials may be easy to machine to their final shape, but will wear more quickly. Harder materials may be more difficult to work with and achieve complex geometries, Extensive attention to processing parameters such as tool wear might also be required.

Building up from nothing

“Additive manufacturing” is a term used to describe 3D printing or any similar process where a shape is created by adding successive layers of material. A 3D printer works by “drawing” a shape, one thin layer at a time, onto a build plate. This process, while lengthy, allows parts to be built up slowly while also simultaneously printing a removable “scaffold”to allow bends and curves to be supported as the material cures.

Manufacturers look at additive manufacturing as a way of reducing the number of steps in a process and reducing waste and scrapped material. It is also possible to print geometries that cannot be produced using subtractive processes. As 3D printing technology develops, we continue to see an ever changing variety of materials that can be printed, including some metals!

The primary benefit of additive manufacturing is the ability to produce very complex parts quickly without specially made fixtures and with minimal secondary processing. This makes additive manufacturing an excellent choice to produce prototype parts for demonstration or test purposes.  The cost of 3D printers has dramatically decreased over the last decade, with smaller desktop versions selling for as little as several hundred dollars. MakerBot, a popular model for hobbyists, starts around $1,500. The low capital costs have persuaded many companies to bring prototyping in-house.

Additive or Subtractive manufacturing summary

Subtractive processes	Additive processes
Wide variety of materials Stronger materials Tooling and fixturing wears Requires multiple steps or assembly	Manufactures complex shapes Low entry cost Can make multiple parts at once Manufacturing speed is slower per part

Both of these manufacturing types have their own strengths and weaknesses. It is important to understand how to utilize each process when it is most effective and efficient. Whilst additive processes are currently limited by material selection and speed, the ability to produce complex parts cheaply with no special fixtures makes it great for small batch and prototyping applications, where quick iterations are needed.

On the other hand, subtractive manufacturing can’t be beaten for producing large volumes of simple parts (pipe plugs, bolts, screws) or for working with certain materials, especially metals. As a designer, you should consider the differences between additive or subtractive manufacturing when selecting a process to produce your design.

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