3D Printing vs. CNC Machining: Which is Better for Prototyping and End-Use Parts

Are you wondering whether 3D printing or CNC machining is the better choice for your custom part applications? In this article, we delve into the practical differences between these two technologies and help you determine the right manufacturing method for your prototypes, end-use parts, and everything in between. Explore the distinct advantages and considerations of 3D printing and CNC machining, empowering you to make an informed decision based on your specific requirements.

CNC machining is a widespread subtractive manufacturing technology. It employs several rotating tools and cutters to convert a solid block of raw material, also known as a blank, into nearly finished parts.

It is a favourite manufacturing method for small one-off jobs and medium to high-volume production, offering outstanding repeatability, high accuracy, and a broad range of materials, post-processing options, and surface finishes.

3D printing (3DP), which is a form of additive manufacturing (AM), builds parts in a layer-by-layer manner. The benefit of 3D printing and other AM processes is that there is no need for special tooling or fixtures, which results in low initial setup costs compared to CNC machining.

While the two technologies have distinct operational mechanisms, there are significant areas of synergy in their application, particularly concerning prototypes and operational end-use parts (constructed from plastics and metals).

This article details the fundamental technological factors you should consider to select the appropriate technology for your bespoke component requirements.

3D Printing vs CNC: How do you choose the right manufacturing technology? #

When selecting the best manufacturing technology for a specific application, there are numerous factors to consider. Thankfully, we have created some effortless rules to help you choose between 3D printing, CNC machining or both in some cases.

As a general guideline, any product that can be constructed without difficulty through subtractive manufacturing processes should commonly be produced via CNC machining.

CNC machining provides greater dimensional accuracy than 3D printing (which may change with further innovations in AM) and produces parts with superior mechanical properties in all three dimensions. However, CNC machining typically carries a higher price, especially for smaller volumes of parts.

3D printing is ideal in numerous situations, including but：

• not limited to: when traditional methods prove inefficient, cost-ineffective or not feasible to manufacture parts with highly complex, topology-optimized geometries.
• Additionally, when a speedy turnaround is crucial, 3D-printed parts can be delivered within 24 hours.
• Furthermore, 3D printing is generally less expensive than CNC machining for small volume production, making it ideal for budget constraints.
• When you require a limited number of indistinguishable parts (ten or less)
• or your parts necessitate materials that are difficult to machine, such as metal superalloys or flexible

TPU, 3D printing or CNC machining may be appropriate. However, if hundreds or even thousands of separate components need to be produced, traditional forming approaches like injection molding are likely the most cost-effective option at scale.

How many parts do you intend to manufacture? The quantity of parts #

is a critical factor in deciding between 3D printing and CNC machining. Refer to the table below for guidance related to the amount of parts, materials, and part features. We also provide substitute options in addition to our primary recommendations.

Number of Parts	1-10	10-100	100-1000	1000+
Plastic	3D printing	3D printing (consider CNC)	CNC machining (consider injection molding)	Injection molding
Metal	3D printing & CNC machining*	CNC machining (consider 3D printing)	CNC machining (consider investment casting)	Investment or die casting
	(dependent on part geometry)

Which is more accurate: 3D printing or CNC machining? #

CNC machining offers tight tolerances and excellent repeatability. CNC machines can accurately produce very large to very small parts. Although internal corners will always have a radius due to the shape of most cutting tools, external surfaces can have sharp edges and can be machined very thinly.

The dimensional accuracy varies across 3D printing systems. Industrial 3D printers can produce parts with very good tolerances. If tight clearances are necessary, the key dimensions can be produced oversized using 3D printing, then machined during post-processing.

The smallest wall thickness of 3D printed components is limited by the end effector size (to the nozzle diameter in FDM or the laser spot size in SLS). As components are constructed layer by layer, layer lines may be visible, primarily on curved surfaces. The maximum size of a part is typically limited as 3D printing demands precise environmental control.

*According to the specified level of tolerance

Technology	Tolerance	Minimum Wall Thickness	Maximum Part Size
CNC Machining	± 0.025 - 0.125 mm*	0.75 mm	Milling: 2000 x 800 x 1000 mm Lathe: Ø 500 mm
SLS	± 0.300 mm	0.7 - 1.0 mm	300 x 300 x 300 mm
FDM	Desktop: ± 0.500 mm	0.8 - 1.0 mm	Desktop: 200 x 200 x 200 mm
	Industrial: ± 0.200 mm		Industrial: 900 x 600 x 900 mm
SLM/DMLS	± 0.100 mm	0.40 mm	230 x 150 x 150 mm
Binder Jetting	± 0.200 mm	2.0 mm	380 x 355 x 735 mm

Comparing materials: machining metals vs. printing plastics. #

3D printing and CNC machining technologies work with metals and plastics, but the technologies differ in their manipulation of these materials.

CNC machining is primarily used to produce metal parts, but this process is also flexible enough to manufacture parts from thermoplastics, acrylics, woods, modeling foams, and machining wax.

Material considerations are important when utilizing CNC machining.

• Great mechanical and thermal properties with complete isotropic behaviour.
• Limitations in dimensions from blank size (employing a non-standard blank size will amplify the manufacturing cost).

What are the most common CNC materials?

Plastics	Metals
ABS	Aluminum
Nylon	Stainless Steel
Polycarbonate	Titanium
PEEK	Brass

3D printing is mainly used for manufacturing parts using thermoplastics and thermosets; however, certain technologies allow for the printing of metal parts. Moreover, various 3D printers are capable of producing parts made of ceramics, wax, sand, composites, and an increasing variety of bio-materials.

Material considerations for 3D printing

• A wide variety of materials with different physical properties are available for 3D printing.
• This includes materials that are difficult to machine, such as TPU and metal superalloys.
• However, it's important to note that 3D printed parts may have potentially lesser mechanical properties when compared to CNC parts, as they are typically not fully isotropic.

What are the common 3D printing materials?

Plastics	Metals
Nylon	Aluminum
PLA	Stainless Steel
ABS	Titanium
ULTEM	Inconel
ASA
TPU

What is the most efficient method for producing intricate components? #

The intricacy of a part is a significant aspect to weigh when selecting between CNC machining and 3D printing. Both procedures exhibit design constraints, but CNC machines are limited to far fewer geometries.

CNC manufacture involves certain noteworthy restraints, such as tool access and clearances, hold or mount points, as well as the incapacity to shape rectangular corners owing to the tool geometry. Some geometries are impossible to manufacture with CNC as the tools cannot access all surfaces of the part. This applies to 5-axis systems too.

Typically, the operator must rotate the part for the tools to access different sides and angles. Repositioning leads to longer processing and labour times, and in some situations, jigs and fixtures are necessary. All these factors increase the final cost of the part.

In comparison, 3D printing can make parts with minimal geometric limitations. Support structures may be necessary for processes such as FDM, however, the significant design flexibility and capacity for intricate designs obtained through 3D printing are not diminished by the additional post-processing.

Additionally, polymer-based powder bed fusion processes like SLS and MJF can generate freeform, organic geometries without the requirement for support structures. The ease with which highly complex geometries can be produced is one of the primary advantages of 3D printing.

Comparing manufacturing workflows for 3D printing and CNC machining #

it's essential to comprehend the workflows for both types. Let's break down the workflows for 3D printing and CNC machining.

A labour-intensive process tends to characterize CNC manufacturing. The machine operator must first decide upon tool selection, spindle speed, cutting path and potential part repositioning. The operator must manually configure the block in the machine, considering all the relevant factors. It is also important to determine if the component requires post-processing after machining or if it will be ready to use. These concerns affect both the part's quality and production time.

In 3D printing, the operator prepares the digital file first by selecting the appropriate orientation and adding supports as needed. Then the file will be sent to the machine, where the printer carries out the building process with minimal human intervention. Once printing is finished, the part will require cleaning and post-processing, which are the most labour-intensive stages in the 3D printing manufacturing workflow.

Comparing post-processing for 3D printing and CNC machining #

There are many post-processing methods that can be applied to both 3D printing and CNC machining to improve the functional and cosmetic qualities of parts. Let's cover the most common post-processing techniques.

Post-processing methods for CNC machining:

• Bead blasting: A process that propels fine beads at the surface of the part to create a smooth and uniform finish.
• Anodizing (Type II & Type III): An electrochemical process that forms a protective oxide layer on the surface of the part, offering improved corrosion resistance and aesthetic options.
• Powder coating: A technique where a dry powder is electrostatically applied to the part's surface and then cured to create a durable and attractive finish.

Post-processing methods for 3D printing:

• Media blasting: Similar to bead blasting, this process uses media (such as sand or glass beads) to remove layer lines and create a smoother surface finish.
• Sanding & polishing: Manual sanding or polishing techniques are employed to smooth out the surface and remove imperfections.
• Micro-polishing: A finer polishing technique that utilizes abrasive compounds and tools to achieve a high-gloss, mirror-like finish.
• Metal plating: Involves depositing a thin layer of metal, such as nickel or chrome, onto the surface of the printed part for enhanced aesthetics or functional properties.

These post-processing methods play a crucial role in enhancing the quality, appearance, and functionality of parts produced through both CNC machining and 3D printing.

3D Printing vs CNC case study: what if you’re prototyping a plastic enclosure? #

If you are developing a new consumer electronics product, creating prototypes for the enclosure is a crucial stage before commencing mass production. The primary goals are speedy lead times and minimal costs for expediting development.

Enclosures for electronics usually have snap fits, living hinges or other interlocking joints and fastenings. These features can all be produced using FDM and SLS 3D printing or CNC machining.

CNC and SLS are capable of producing accurate and aesthetically pleasing prototypes. However, for desktop purposes, FDM is more cost-effective and efficient due to its shorter lead times. Considering that mechanical performance may not be the primary concern for this specific project, the additional cost and time required for CNC and SLS are generally not justified.

Factor	CNC machining	Desktop FDM	SLS
Cost	$$	$	$$
Common materials	ABS, Nylon	PLA, ABS, Nylon	Nylon
Lead time	1-2 weeks	1-3 days	Less than a week
Accuracy	± 0.125 mm	± 0.500 mm	± 0.300 mm

3D Printing vs CNC cast study: what if you’re manufacturing metal brackets and mechanical components? #

Metal brackets and other mechanical components are capable of bearing heavy loads and operating at extreme temperatures. If you are producing such parts, your primary objectives will likely be good material properties and dimensional accuracy.

CNC machining is the most suitable option in terms of accuracy, cost, and mechanical accuracy if the geometry of your model is simple (like the components shown above).

When geometric complexity increases or less common materials are needed, metal 3D printing may be a suitable option. Components designed for weight and strength, such as the brackets shown below, have organic structures that are challenging and expensive to machine using CNC systems.

It is possible to combine CNC machining and metal 3D printing to produce parts with complex shapes and precise tolerances at crucial locations on the components.

Factor	CNC machining	CSLM/DMLS	Binder Jetting
Cost	$$	$$$$	$$$
Common materials	Aluminum, stainless steel, brass	Stainless steel, aluminum, titanium, Inconel, cobalt-chrome	Stainless steel, Inconel, cobalt-chrome, Tungsten carbide
Accuracy	± 0.025 mm	± 0.100 mm	± 0.200 mm
Mechanical properties	Very good	Very good	Good

V1’ top tips & tricks for choosing between 3D printing & CNC machining

Selecting the appropriate manufacturing technology for your bespoke components might appear to be an overwhelming task, but it need not be. We have compiled a few fundamental guidelines to adhere to when confronted with this choice.

1. Consider quantity and complexity: If you need to produce medium to high quantities (around 250-500 parts) with relatively simple geometries, CNC machining is a suitable choice. On the other hand, for lower quantities or single prototypes with complex geometries, 3D printing is more suitable.
2. Evaluate cost and geometry limitations: When it comes to metals, CNC machining can still be price competitive even for low quantities. However, keep in mind that there may be limitations on the complexity of the geometry that can be achieved with CNC machining.
3. Explore alternative technologies for large quantities: If you require more than 500 parts, it may be worth considering other manufacturing technologies like injection molding. These processes can be more efficient and cost-effective for high-volume production. Alternatively, you can also consider a combination of 3D printing or CNC machining with a forming process to achieve the desired results.

By considering these tips and evaluating your specific requirements in terms of quantity, complexity, cost, and available technologies, you can make an informed decision between 3D printing and CNC machining for your custom parts.

Frequently asked questions #