CNC Machining vs AM for Metal Prototypes
Dec 10, 2019
It’s easy to make affordable metal prototypes with rapid prototyping technologies, but is it better to use CNC machining or metal 3D printing? This article considers the respective benefits of each manufacturing process.
It can be tempting to think of CNC machining as a production process and 3D printing as a prototyping process. After all, CNC is capable of creating high-precision parts from industrial-quality metals, while additive manufacturing has low startup costs and is optimized for on-the-spot, one-off parts.
But the reality is more complicated than that. Yes, metal 3D printing is an excellent prototyping tool, but with the right approach, CNC machining can be too. It can, therefore, be difficult to choose between the two.
When comparing CNC machining with AM for metal prototypes, there are numerous factors to consider: the cost of the process, material availability, scalability, bridge production and much more.
This article looks at the respective benefits of CNC machining and additive manufacturing and considers situations in which one is better than the other.
CNC vs AM: Premium vs economy?
At first glance, CNC machining may seem like a greater investment than 3D printing. 3D printers are commonplace, even in small offices, while CNC machines are expensive pieces of professional equipment used only for serious projects. Low-end desktop 3D printers can cost just a few hundred dollars; all CNC machines cost far more.
With metal prototypes, however, the distinction isn’t so clear. Metal additive manufacturing systems are not available in the consumer price range, so metal 3D printers and CNC machines are comparable in price.
Moreover, when ordering prototypes from a manufacturing network like RapidDirect, the cost of the parts is the only thing that matters. You, the client, do not need to worry about the machine shop’s investments.
So is there a big price difference between machined and 3D printed metal prototypes? In short: no. Advances in CNC machining have made the process more efficient than ever, which means engineers can create machined parts quickly and in large volumes. Additionally, metal AM remains at the more expensive end of the 3D printing spectrum.
Therefore, when deciding between CNC machined and 3D printed prototypes, the budget shouldn’t be your only consideration.
Advantages of CNC machining for metal prototypes
CNC machining is a tried-and-tested manufacturing process for all kinds of metal parts across a range of industries. It’s also widely used for both production and prototyping, with machine shops around the world frequently taking orders for visual, mechanical and final-stage metal prototypes.
Advantages of CNC machining include strong parts, scalability and tight tolerances, making the process preferable to additive in many situations.
CNC machining and AM manipulate metal in fundamentally different ways. Where are processes like Selective Laser Melting (SLM) build parts layer by layer from melted powder, CNC cuts material away from a chunk of extruded metal known as a “blank.”
While AM can be used to make fairly strong prototypes, the mechanical advantages of CNC are significant. The extruded metal blank is physically consistent and requires no adhesion between layers. It is a solid block, and it can, therefore, be used to make strong, rugged prototypes with minimal deformities.
In very, very low quantities, it can be faster and more affordable to choose AM over CNC machining for metal prototypes. That’s because 3D printing has short setup times, despite the relative slowness of the printing process.
However, for anything more than a handful of units, CNC becomes cheaper and more efficient than the alternative.
This has several implications. Even if a one-off prototype is cheaper to make using 3D printing, CNC machining may be much more economical in the long term. Because if the project needs to be scaled up and more units are required, it is easier and faster to increase the order size using CNC machining.
A machined prototype is a shortcut to mass production; a 3D printed prototype is not.
In general, CNC machining is far more accurate than 3D printing, which means metal prototypes can be machined with much tighter tolerances using a CNC machine.
CNC tolerances can be as low as ±0.025 mm, while high-end metal 3D printers can generally only go as tight as ± 0.200 mm.
Metal parts are also highly repeatable when made with a CNC machine. This is more important during production, but can also be advantageous when creating multiple prototypes for mechanical testing.
Advantages of AM for metal prototypes
For most prototyping situations, CNC machining produces better metal parts that are amenable to upscaling and bridge production.
Additive manufacturing does, however, offer several important advantages.
Speed and price for one-offs
For prototypes that must be made in an incredibly short timeframe, there may simply not be enough time for CNC machining, which makes AM the obvious solution. Setup times are short, so parts can be 3D printed and delivered within days.
3D printing technologies can create very complex shapes with unusual geometries — even on the inside.
( Image from Autodesk )
Such internal geometries are impossible to recreate with a CNC machine since the cutting tool of a machine cannot reach the interior of a part without penetrating the outside. 3D printing, meanwhile, builds up a part in cross-sectional layers, so there are virtually no geometrical limitations.
When to choose CNC & when to choose AM
When creating metal prototypes, it is generally preferable to use CNC machining, since the process creates repeatable, precise parts with excellent mechanical properties. However, choosing the ideal process depends on your situation and specific requirements.
Situations in which CNC may be preferable to AM include:
• When the metal prototype needs to be particularly strong
• When tight tolerances are required
• When many copies of the prototype are required
• When there is a possibility of scaling up the project in the future
• When the prototype requires rounded edges or radii
Situations in which AM may be preferable to CNC include:
• When one or just a handful of prototypes are needed and the criteria from the above list do not apply
• When the prototype requires a complex internal geometry
• When there is a very tight deadline
If it is still unclear which manufacturing process represents the best course of action for your metal prototypes, the simplest solution is to get in touch with a RapidDirect expert who can guide you through the process.
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