3D Printing Vs CNC Machining – When to use what?

So, you have designed an awesome product and now wondering which manufacturing technology to use for manufacturing the product? Both 3D Printing and CNC (Computer Numerical Control) Machining are competing technologies and selecting one over the other can be tricky.

Well, look no further. This article will help you decide which technology will be the best choice for manufacturing your product. First, let us understand the fundamental difference between the two by doing some simple math.

Addition Vs Subtraction

3D Printing is additive manufacturing where a part is produced one layer at a time by adding materials such as plastic filaments, resins, metal or plastic powders etc. A source of energy such as a laser is used to solidify the raw material to make the product. This process does not require specialized tools hence, the initial set up cost is less compared to CNC machining. All 3D printers start from scratch and build the object layer-by-layer.

Process flow diagram depicting the basic difference between additive and subtractive methods. 

The image above clearly shows for a 3d model with geometry very complex, the wastage will be more & the machining time will increase. So to manufacture the above model & a similar model it is advisable to use 3D Printing. Smaller model with more complex geometry, always choose 3D Printing.

The image above clearly shows for a 3d model with geometry less complex, the wastage will be less & the machining time will decrease. So to manufacture the above model & a similar model it is advisable to use CNC Machining. Smaller and larger models with less complex geometry, always choose CNC Machining. 

Read more about designing for 3D Printing here!

CNC machining, on the other hand, is subtractive manufacturing. In this case, cutters and special tools are used to shape a block of material, known as “blank”. The machine tools cut the extra material away to create the finished part or product. CNC machining offers great dimensional accuracy and can be used on a variety of material such as wood, plastic, metal etc. CNC machining tools and cutters are computer controlled. There are specific codes to guide and control the sharp and precise tools.

Apart from the basic difference, as outlined above, there are many factors to consider while deciding when to use which manufacturing technology. Important factors are listed below-

Material Choice

CNC Machining is mainly used for metals, but it can be used on a variety of materials such as-

The advantage with CNC Machining is that you can make the prototypes in the same material as the final product hence speeding up the testing process. Different tools are needed for different materials, but the CNC machines are designed in such a way that the tools can be exchanged easily.

3D Printing is generally used for plastics. Choice of materials for 3D Printing is limited compared to CNC. Few materials which can be used for 3D Printing are-

Thermoplastics can be mixed with other materials such as wood, metal and ceramics to be used in 3D Printing. Objects made from such composites are not as strong as the original metal products made using CNC.

Material which cannot be machined, like TPU or superalloys, can be 3D printed. Some metals such as Stainless steel, Aluminum, Titanium can be 3D printed using niche and very expensive industrial machines.

Another challenge with 3D Printing is the compatibility of the printer. For example, a thermoplastic 3D printer is entirely different than a Resin 3D printer. So, you need a specific printer based on the material being used.

Geometric Complexity

There are many limiting factors in CNC machining. Blank size is the biggest and most important consideration. Other limitations to consider are tool access and clearances, mount points, inability to machine square corners due to tool geometry etc.

In the case of CNC, most geometries require the rotation of the part to access the different sides. Repositioning adds to the processing and labour time and custom fixtures may be required, affecting the final price. Therefore, some geometries cannot be created using CNC machining as the tools cannot access all sides of the part.

3D Printing, on the other hand, can easily create parts with highly complex geometry.  There are many types of 3D Printing technologies such as FDM, SLS, SLM, DMLS etc. Most of them require support structures which can be removed during post-processing. 3D Printing is well known to create parts with geometries which no other traditional manufacturing can produce.

Accuracy and Size

CNC machining provides excellent precision and is better than all 3D Printing technologies in terms of accuracy & tight tolerances. Internal corners of the part will always have a radius due to the shape of the cutting tool, but external surfaces can have sharp edges and can be machined very thin.

In the case of 3D Printing, tolerance and dimensional accuracy depend on the technology used. Industrial printers can provide good tolerances. Alternatively, a part can be 3D printed and later can be machined during post-processing, to get tight tolerance.

The end effector (for example the nozzle diameter in FDM or the laser spot size in SLS) restricts the size of the minimum wall thickness of 3D printed parts. Sometimes layer lines might be visible, especially at curved surfaces, because the parts are fabricated one layer at a time. As 3D Printing needs to be done under tightly controlled environment; the maximum part size is relatively small.

Following table lists tolerance, wall thickness and maximum part size for various technologies-

Manufacturing Process Flow

3D Printing is easier to use compared to CNC machining. In the case of 3D Printing, the operator needs to prepare the digital file, select the part orientation, infill and support if needed and send it to the printer. Once the printing starts, the product is printed with little or no human intervention. After the product is ready, it needs to be cleaned and post-processed.

CNC machining process is more complex and needs to be done by a skilled operator. He/she needs to select specific tools, the rotational speed of these tools, cutting path and repositioning of the part. The process is labour intensive and experience based, and all these factors affect the final quality and build speed of the final product. Once the machining is done, the product is ready for post-processing.

Post Processing

To finish the final product and improve its aesthetics various post-processing techniques can be applied in CNC as well as 3D Printing. Some are listed below-

Cost

Cost in 3D Printing is dependent on numerous factors. Some of them being volume, material and the time needed. 3D Printing is usually less expensive to get started. You can get a decent desktop 3D printer for around $500 while CNC mills are more expensive, but if one were to opt for Industrial 3D Printers, then those are more expensive than CNCs.

3D Printing is a preferred method for low volume production as it is cheaper and faster. However, for large volumes, CNC machining is the preferred option.

Other factors affecting cost in CNC machining are repairing and replacing the machine parts such as drills, lathes, and heads. These mechanical parts can get worn out due to regular use and need to be replaced regularly.

Noise and vibrations

Noise: Depending on the material used, type of cutting tools and machine rigidity, CNC Machines can be noisy. Cutting metal or wood using a large-diameter tool can be ear-deafening. Whereas, the noise from a desktop 3D printer without casing is gentle in comparison. However, when cutting wax models, the noise from a CNC mill is hardly audible.

Vibrations: A CNC machine vibrates heavily when working on a metal or wood block – you wouldn’t want to have it on the desktop near you. Vibration normally is no issue with 3D Printing.

Eco-Friendliness

CNC Machining starts with a block of material where excess material is removed using special tools. As a result, there is a lot of mess in the form of fine dust and small pieces of material which cannot be recycled. On the other hand, in 3D Printing, the raw material is fed into the machine and product is built in layers. This technology uses raw material only on an as-needed basis. There is no mess or wastage in 3D Printing hence, it is more eco-friendly manufacturing.

Conclusion

Now that you have a better understanding of various factors to consider while selecting between CNC machining and 3D Printing, you can easily decide which technology would be better for your product.

Generally speaking, each has its own merits and specific uses. 3D Printing is cheaper, easier to use, quicker, and can create parts with complex geometries. While CNC machining has the advantage of a wide variety of materials it can work with, better accuracy and suitability for mass production.

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