Difference between 3D printing and CNC machining

3D printing vs CNC machining: what is the best way to make your part? which is better?

CNC machining and 3D printing are two very different production methods used to build net shape parts. Their common point is the ability to provide products quickly, making them the technologies of choice for the rapid production of prototypes.

On a high level, CNC machining and 3D printing use the same workflow: the part is designed with a CAD software and there is the generation of a G-code that defines the instructions for the machine operations. Then, both the machines need to be prepared for the process and, after the part is manufactured, post-processing might be performed. Even if these processes might have common aspects, the conditions are different in terms of production time, production cost, manpower and effort.

CNC and 3D printing are used to produce different polymers and composites parts, showing multiple advantages and disadvantages.

Difference between CNC and 3D printing

The main difference between CNC machining and 3D printing is in the nature of the technology: while CNC machining subtracts material from a billet to form a net shape part, 3D printing adds material, layer by layer. The difference in the nature of the process results in different outputs in the finished parts, as well as different possibilities in terms of geometry and precision requirements.

Generalizations have been made to show the differences between machining and 3D printing. Here’s a table showing the main differences between these two technologies.
The most commonly used CNC machines in the industry are milling, since they easily bring parts to precise fine tolerances. Usually, when talking about CNC, one refers to 3-axis Milling CNC machines.

CNC machines: what they are and how they work

Computer numerical control (CNC) machines transform blocks or rods of material to the net shape by following a coded programmed instruction.
CNC were discovered about 80 years ago, when it was possible to combine the machining processes together with the precision of computer numerical control. These CNC machines are now able to achieve very fine tolerances and smooth surfaces as well.
Depending on the type of machine, usually CNC machines don’t require a manual operator directly controlling the machining operation all the time. Machining is the most common production method for metals, wood, and ceramics, and it is also common for low to mid-volumes of plastics and composites. It provides the desired shape with very good accuracy and precision but tends to waste material in the generation of removed chips.

What are CNC machines used for?

CNC machines are used in various industries to produce finished parts. The reason why they boomed in the recent years, is because they can guarantee very high repeatability and precision in the order of thousandths of mm.

They have been the preferred production technology for high-repeatable industries, like:

• Medical
• Aerospace
• Transportation
• Oil & Gas
• Electronics

Advantages and disadvantages of CNC machining

Benefits and limits of CNC machining might vary according to the different equipment, but some common advantage and disadvantages of 3-axis CNC milling machines are as follows. 

Here's the advantages of CNC machining of polymers and composites:

• Generally guarantees fine dimensional tolerances and very low surface roughness, when integrated with finishing steps,
• Good for low to mid-volume production,
• Custom basic shapes are obtainable using custom machining steps,
• Short to mid-range lead times,
• Starting with a block, the properties will be the same in every direction, achieving isotropic parts,
• Very wide material choice (metals, polymers, composites, wood, ceramics).

Here’s the disadvantages of CNC machining of polymers and composites:

• Much waste material: in some cases, it is difficult to recycle because of processing oils. In some sectors, like Aviation, waste material can account for 90% of the whole part. When talking about cost intensive materials, the lower the waste, the higher the saving. Since this technology creates high amounts of scrap material, it is not always an efficient production method,
• Complex geometries are expensive because they require multiple repositioning steps,
• Geometry constraints, no hollow geometries, and undercuts are difficult and costly to achieve,
• Due to the limit in the complexity of the designs, no integrated assemblies are manufacturable,
• Speed depends on part complexity, hence complex parts will take longer to be manufactured,
• Labor intensive and time consuming for multiple repositioning: the operator needs to reorient and re-fixture the workpiece for each repositioning (if there is no 5 axis CNC), this might lead to a loss in precision, usually overcome with the use of centering devices,
• Need for trained workers to operate the machines,
• Difficulty, machine parameters and tools are determined by the material to be machined. The parameters to set up are how to fix the part, number of machining steps, cutting tools, speed and feed rate: therefore, the need for trained technician for designing the process, taking up to 7 hours. Preparation times for the device and programming of the cutting operations increase as the complexity of the object increases.
• High equipment costs in terms of initial machine investment as well as the costs of fixtures and tools
• In the absence of a fully automated system, the presence of a specialized operator is required throughout the entire process, so that they can change tools and reposition the object on the plane whenever the face to be machined is not accessible to the cutting tools.
• Costs for preparation and operation might be high, so the overall production cost results advantageous for larger number of parts to manufacture.

Examples of CNC machines

Depending on the type of machining operation to be performed, there are multiple types of CNC machines that perform on polymers and composites:

• Milling Machines: Milling is a machining process that utilizes rotary cutters to remove material by advancing a cutter into a workpiece. This can be done changing pressure, cutter head speed and direction on one or different axes;
• Lathes and Turning Machines: commonly performed with a lathe, turning is a machining process used to reduce the diameter of parts to a specified dimension. It is used for the production of cylindrical parts in which the cutting tool moves in a linear fashion while the workpiece rotates;
• Laser Machines: laser machining is a machining process that uses heat directed from a laser beam. This process removes material from surface through the use of thermal energy. Heating, melting and vaporization of the material surface happens due to impact of photons.

When talking about CNC machines, one usually refers to milling CNC machines, since they are the most common. There are more types of machines on the market, depending on the number of axis in motion: there are 3-axis, 4-axis and 5-axis CNC machines. The main difference between these types of CNC machines is in the complexity of the movement between the workpiece and the cutting tool. The higher the number of axis, the higher the design freedom obtainable. The most flexible ones are the 5-axis CNC machines. They offer high flexibility in the production of complex part, requiring fewer machining steps.

Is 3D printing cheaper than CNC machining?

3D printing can be cheaper than CNC machining, especially when referred to 5-axis CNC machines. The most frequent ones are, however, 3-axis CNC machines, because the cost of the investment is lower. Recently, with the advent of 5-axis CNC machining, the efficiency of this subtractive technology has considerably improved. Despite this, the investment cost for a 5-axis CNC machine is very high, therefore most of the companies are equipped with 3-axis CNC machines or manual drills and mills.

What are some advantages of 3D printing over CNC?

FFF (Fused Filament Fabrication) is the most common 3D printing technology. It extrudes continuous filament of thermoplastic materials or composites in order to form an object, layer by layer. Roboze is leader in the FFF technology for high performance polymers and composites (check FFF of PAEK polymers)

Like explained in the article about additive manufacturing vs traditional manufacturing, FFF technology has its advantages and disadvantages.

What to choose between 3D printing and CNC machining? How to integrate 3D printing with CNC machining?

CNC machining and 3D printing can go hand in hand for the quick production of complex parts to be slightly finished on some surfaces. Depending on the material, 3D printed parts can be combined with machining to achieve very low roughness or to meet very low tolerances. PEEK and Carbon PEEK, for instance, have a good machining behavior that allows them to benefit from both technologies. The most common CNC machining operations performed on 3D printed parts are for finishing O-rings grooves, threaded holes and inside and outside diameters.
Thanks to the combination between these two technologies, one can get the advantages of both with the accurate production of complex parts.

Our team of engineers here at Roboze will support you in the evaluation of the best options for your application.