Introduction

Selecting the right manufacturing technology for a particular application can be hard, even to the most experienced designers. With rapid developments in digital manufacturing technologies, like 3D printing, the potential benefits for designers can easily be overlooked without sufficient knowledge of the subject.

The purpose of this article is to identify how 3D printing is positioned in the current manufacturing landscape and compare it against CNC machining. After reading, you should be able to quickly assess whether to use 3D printing or CNC machining for your custom parts.

Classification of manufacturing techniques

Most manufacturing technologies can be categorized into one of 3 groups. At the simplest level, these groups can be defined as:

— Formative manufacturing: best suited for high volume production of the same part, requiring a large initial investment in tooling (molds), but then able to produce parts at a very low unit price.

— Subtractive manufacturing (e.g. CNC): best suited for parts with relatively simple geometries, produced at low to mid volumes.

— Additive manufacturing (or 3D printing): best suited for low volume, complex designs that formative or subtractive methods are unable to produce. Common usage is for unique, one-off rapid prototypes or end-use parts.

A schematic comparison of how formative (top), subtractive (center) and additive (bottom) manufacturing techniques produce parts

Cost and part geometry are often the governing factors behind the selection a particular method of manufacturing. The figure below gives some general insight into how the cost per part varies based on the size of production (assuming the geometry can be produced with each technology).

The total number of required parts is a key design consideration when selecting a manufacturing technology

In this article, we will focus on short production runs (relative low number of parts). This is the area where Additive and Subtractive manufacturing are particularly price competitive.

Note: Recent developments in 3D printing have lead to some form of economies of scale (such as Carbon and HP's MJF). However, these technologies still need time to mature, so for simplicity are not discussed in this article.

3D Printing vs. CNC Machining 

When choosing between CNC and 3D printing, there are a few simple guidelines that can be applied to the decision making process.

As a general rule of thumb:

"Parts with relatively simple geometries, that can be manufactured with limited effort through a subtractive process, should generally be CNC machined."

Switching to 3D printing makes sense in the following cases:

— Prototyping: For very small volumes or single part production. In these cases, 3D printing is generally more price competitive than CNC, especially for plastics.

— Complex geometries: When subtractive methods are not able to produce the part, due to its complexity (for example for topology-optimized geometries).

— Speed: When a fast turn-around time is critical; 3D printed parts can be ready for delivery within 24h.

— Specific Materials: When materials are required that cannot be easily machined, like metal superalloys or flexible TPU.

To summarize:

"CNC offers greater dimensional accuracy and produces parts with better mechanical properties than 3D printing, but this usually comes at a higher cost for low volumes, more design restrictions and at slower turn-around times."

If high volumes are needed (100's or more), neither CNC nor 3D printing are likely to be a suitable options. In these cases, traditional forming technologies, such as investment casting or injection molding, are more economically viable due to the mechanisms of economies of scale.

Recommended process reference table

For quick reference, use the table below. In this simplification it is assumed that all technologies are able to produce the parts in question. When this is not the case, 3D printing is generally the preferred method of manufacturing.

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Process Characteristics

Dimensional accuracy

CNC machining offers tight tolerance, excellent repeatability and relatively few size restrictions: very large to very small parts can be accurately CNC machined.

Different 3D printing systems offer different dimensional accuracy and industrial machines can produce parts with very good tolerances and repeatability, comparable to CNC. Since parts are fabricated one layer at a time, layer lines might be visible, especially at curved surfaces. The maximum part size is relatively small, as 3D printing processing often require close environmental control.

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Materials

Common CNC materials

Plastics: ABS, Nylon, Polycarbonate, PEEK

Metals: Aluminum, Stainless Steel, Titanium, Brass

CNC materials are suitable for a wide range of industrial applications. Metals and plastics (thermoplastics or acrylics) are most commonly used, but other materials are also available, such as softwoods and hardwoods, modeling foams and machining wax.

+ Large selection of well-established, common engineering materials available.

+ Well-understood mechanical properties (fully isotropic).

- Very hard or very soft materials (such metal superalloys and flexible TPU) are difficult to machine and increase the cost significantly.

Common 3D printing materials

Plastics: Nylon, PLA, ABS, ULTEM, ASA, TPU, Resins

Metals: Aluminum, Stainless Steel, Titanium

In the past, 3D printing was predominately used for plastics, but now metal 3D printing is becoming a standard manufacturing process for many industries (for example, aerospace). Other available 3D printing materials include composites, ceramics, wax and sand.

A comprehensive list of 3D printing materials and their associated properties can be found in this publicly available Material Index.

+ Wide variety of materials with very large range of physical properties.

+ Materials that are difficult to machine (such flexible TPU and metal superalloys) can be 3D printed.

- May have lesser mechanical properties compared to CNC parts (they are typically not fully isotropic).

Case study: prototyping a plastic enclosure

Low-cost enclosure prototypes 3D printed in FDM

Electronic enclosures are commonly prototyped during the development of electrical appliances. To accelerate this development and allow for a large number of design iterations, fast lead times and low cost are the main selection criteria.

Enclosures often have snap fits, living hinges or other interlocking joints and fasteners. All these features are relatively difficult to CNC machine, but can be easily 3D printed with FDM or SLS and tested for form, fit and function. CNC is excellent though for creating enclosures with press fits (or other interference fits), due to its higher dimensional accuracy.

CNC and SLS are recommended for prototypes of high accuracy, but desktop FDM offers much shorter lead time at significantly lower cost. Since mechanical performance is not the main criterion here, the benefits of CNC and SLS are usually not worth the extra cost and time. If a metal enclosure is required, CNC machining is the recommended solution.

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Case study: manufacturing metal brackets and components

Metal components with simple geometry manufactured with CNC machining and powder coated

Metal brackets and mechanical components are commonly used to bear high loads and operate at elevated temperatures. In this case, dimensional accuracy and excellent material properties are the main objectives.

If the model has a simple geometry (like the components of the image above), then CNC is the most cost competitive option, offering great accuracy and excellent material properties..

When the geometric complexity increases or when more "exotic" materials are required, metal 3D printing must be considered. Components optimized for weight and strength have organic structures that are very difficult and costly to machine.

SLM/DMLS and Binder Jetting are currently the two main metal 3D printing processes. New, low-cost extrusion-based metal 3D printing systems are planned for release in 2018.

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Topology optimised brackets manufactured in Titanium with SLM. Courtesy: Formula Student Team TU Delft

Conclusions

The guidelines and tables of this article should give the reader a basic understanding and reference for choosing between CNC machining and 3D printing.

If you want to learn more specifically about producing custom metal parts, the Metal Kit is available for free download and covers the key aspects of metal manufacturing for short run, custom parts.

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