Subtractive Manufacturing vs Additive Manufacturing

What is subtractive manufacturing

In subtractive manufacturing, objects are created by progressively removing material from a solid block or sheet. The material is removed from the starting material by cutting, drilling, boring, or grinding. While these processes may be carried out manually, they are more commonly achieved using computer numeric control. Today, the most popular subtractive manufacturing process is CNC machining.

In CNC machining, a computer-numerically controlled cutting tool mechanically removes material to achieve a geometry. It involves the use of CAD to design a model to be machined, as well as the use of CAM to instruct the CNC machine on how exactly to go about material removal. There are three major machining processes that concern removing material according to 3D models, namely: turning, drilling and milling. Other subtractive manufacturing processes are laser cutting, waterjet cutting, electrical discharge machining, and plasma cutting. These methods are mainly used for 2D machining.

Advantages of subtractive manufacturing

It applies to a wide variety of materials including metals, plastics and plastic composites and even wood.
It can be used to obtain almost any geometry such as flat surface, holes, cylinders, screw thread, slots etc.
It can produce high accuracy with close tolerance of about 0.025 mm
Smooth surface finish is obtainable

Limitations of subtractive manufacturing

There is material wastage as the chips formed are wasted. Even if the chips can be recycled, they are still waste material.
It takes more time per part than additive and formative manufacturing methods.

What is additive manufacturing

Additive manufacturing is the process of joining materials to make parts from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing and formative manufacturing methodologies. Although a majority of the current global activity in additive manufacturing involves polymer-based systems, there has been activities and interest in metallic part fabrication.

Additive manufacturing is widely known as 3D printing. It is carried out using machines known as 3D printers. The term 3D printing covers a range of processes that differ by feedstock material and energy source. However, there are fundamental steps common in all operations.

A CAD model of the part to be printed is designed.
Specialized slicing software then slices this model into several cross-sectional layers.
Depending on the technology used, the 3D printer proceeds to melt, fuse, or cure feedstock material and deposit it layer by layer until the desired geometry forms. The feedstock material may be powder, wire, sheet, or liquid.

3D printing processes include Stereolithography (SLA), Material jetting, Selective laser melting (SLM) or direct metal laser sintering (DMLS), Stereolithography (SLA), Electron beam melting (EBM), and Binder Jetting.

Advantages of additive manufacturing

It is highly efficient due to the elimination of waste.
It is comparatively faster to go from the design stage to production
Produces intricate and complex designs with ease

Limitations of additive manufacturing

It has a limited range of materials when compared to other manufacturing processes.
It is expensive when dealing with metals.
It is not suitable for high volume production

Comparison of additive manufacturing and subtractive manufacturing

	Additive manufacturing	Subtractive manufacturing
Achievable complexity	Can produce parts with highly complex and intricate geometries, even better than 5-axis CNC machining.	Better suited to relatively simple geometry.
Producible features	Cannot produce features such as holes and threaded features effectively	Effectively creates holes and threaded sections.
Properties of parts produced	Parts produced may have insufficient mechanical properties. Because the parts are created layer by layer, structural weaknesses arise between these layers, thereby compromising specific properties.	Parts produced may have excellent mechanical and thermal properties.
Accuracy	Can achieve less dimensional accuracy. The most accurate AM process, SLM/DMLS, can produce tolerances as tight as 0.100 mm.	Can achieve greater dimensional accuracy. Tolerances as tight as 0.025 mm are possible.
Production materials	Works predominantly with plastics and to a small extent, metals.	Works with a wide range of materials, including plastics, metals, wood, foam, glass, and stone.
Finishing	Parts produced always require finishing processes.	Parts produced may not require finishing.
Setup	Requires minimal setup which results in shorter time per part, from design to production. After designing a CAD model of the part and converting it, all you need to do is setup the feedstock material, and the 3D printer does the rest.	Requires more effort and time in setting up. In CNC machining, for example, after designing a CAD model and converting it to G-Code, you need to set several aspects and parameters of the CNC machine. These include placing the workpiece on the work table, selecting and preparing the appropriate cutting fluid, selecting and affixing the cutting tool, and setting the right speed, feed, and depth of cut.
Scalabitlty	Cost of production is directly proportional to production quantity. As production quantity increases, however, production costs rise significantly.	Cost of production is inversely proportional to production quantity. As production quantity increases, production costs reduce.
Speed and cost	Faster and less expensive for geometrically small parts, plastics, and small production runs.	Faster and less expensive for relative large parts, metals, and large production runs.

Choosing between additive manufacturing and subtractive manufacturing

Both additive and subtractive manufacturing are suitable for prototyping. However, while 3D printing is better suited for prototyping using plastics, additive manufacturing is preferable for metal prototyping. For finished products, subtractive manufacturing is more suitable. This is because parts produced by subtractive manufacturing do not always require finishing and have better mechanical properties.

For small production quantities such as 1-10 identical parts, 3D printing is more appropriate as it is less expensive for such quantities, given that the material of part is a polymer. Production runs of 10-10000 identical parts are more cost-effective using subtractive manufacturing. For quantities larger than 10000, you should consider formative manufacturing processes.

Smaller sized parts are better produced with additive manufacturing. Subtractive manufacturing, on the other hand, is better used for larger parts.

Materials such as metals, wood, glass, stone, and foam are either very expensive or impossible to manufacture using additive manufacturing. Such materials require subtractive manufacturing. On the other hand, materials that are difficult to machine, such as flexible TPU (Thermoplastic Polyurethane) and metal superalloys can be 3D printed.

Additive manufacturing is more cost-effective than subtractive manufacturing for small quantities of plastic. For metals, however, SLM/DMLS and binder jetting cost 100% and 50% more than CNC machining respectively.

In summary, additive manufacturing is best suited to prototyping and small-scale production of small, highly complex parts made from plastic. Subtractive manufacturing is better for large-scale production of relatively simple, large parts made from a wide range of materials.