Despite the advances of 3D printing, CNC machining is the most cost-effective method of on-demand manufacturing, especially for metal parts. Let’s break down their costs.
CNC machining is an established digital manufacturing process that produces high-accuracy parts with excellent physical properties directly from a CAD file. Although it’s been around since the early 50’s, recent technological advancements in digital supply chains have reduced the cost of CNC machining drastically and made it easily available to more professionals.
3D printing or additive manufacturing is another digital manufacturing technology that can produce parts on-demand. Since 3D printing requires no tooling, start-up costs are low, making it particularly competitive for low volumes or one-off custom parts.
The extensive mainstream media coverage on 3D printing may have lead to inflated expectations from this technology in applications where it is not the most suitable — especially for metal part production.
In this article, we’ll take a closer look into the costs of CNC machining (machine, labor and material costs) and compare it to 3D printing through some back-of-the-envelope calculations and practical examples to gain a better understanding of the current state of these two technologies.
Overhead machine cost corresponds to approximately two thirds of the overall cost of both CNC machining and 3D printing. Generally, the machine rate (cost per hour) is calculated by dividing the cost of purchase by the total hours the machine is expected to operate (typically eight years for 5,000 hours per year).
CNC machines come in different variations. Depending on their machine architecture and capabilities the typical machine per hour differs.
The typical rate of a 3-axis CNC milling machine (excluding the operator salary) is $40 per hour. To this, the machine operator salary should be added, which usually range at around $35 per hour. CNC turning is usually priced lower at $35 per hour, while the machine cost per hour of multi-axis CNC machining typically ranges between $75 and $120 or higher.
On the other hand, the hourly rate of industrial 3D printers varies between $10-20 for industrial SLS or FDM machines to more than $100 per hour for metal SLM and DMLS 3D printing systems. To this, costs related to the risk of print failure should also be included; according to a study, risk-related costs can double the operating cost of 3D printing.
This means that the total cost of owning and running a CNC machine is comparable to (or in the case of metal much lower than) that of an industrial 3D printer.
The table below summarizes the price of stock metal alloys and plastic materials commonly used in CNC for a sheet with dimensions of 6'' x 6'' x 1'' (or approximately 150 x 150 x 25 mm).
The same volume of material corresponds to approximately $40 worth of Nylon 12 powder for SLS 3D printing — or 650 grams at $60 per kg — or $1300 worth of stainless steel powder for DMLS or SLM metal 3D printing — or 4.3 kg at $300 per kg.
Of course, CNC machining, being a subtractive manufacturing technology, has considerable waste as material is removed from the original block. Also, parts designed for (especially metal) 3D printing should be topology optimized to reduce their weight, so they use as little material as possible.
What is often overlooked is that 3D printing can also be wasteful: depending on the process, only 50-80 percent of the unused powder (in the case of SLS and DMLS/SLM) can be reused. The price one pays for this piles up fast, especially considering the cost of 3D printing materials.
So, generally speaking, the cost of materials in 3D printing is considerably higher than in CNC.
The labor cost for the 3D printing machine operator are relatively low, as the process is mostly automated. Engineering and design costs for 3D printing are high, though, as the parts may require re-design and optimization. Also, considerable manual work is required for post processing and finishing a 3D printed part (cleaning, support removal, surface polishing).
On the other hand, labor in CNC machining is primarily connected to start-up costs (CAM programming, process planning), but these are one-off costs and are eliminated for higher production volumes. Quoting traditionally demanded considerable resources from the machine shops, but this is not the case anymore. Let’s see why this is the case.
CNC machining technology has not changed significantly during the past few years. Incremental improvements in machine design, tooling, consumables and CAM software lead to an optimization of the production cycles, improving quality and but not significantly affecting the cost of CNC machined parts.
What has changed are the manufacturing supply chain networks surrounding the CNC machining process.
For example, online manufacturing networks are becoming smarter and faster. A big step forwards was the automation of the quoting process for both clients and manufacturers. Using machine learning and artificial neural networks, the cost of manufacturing a part with CNC machining can be predicted (based on its geometry, material and other specifications) and an instant quote to the customer can be provided.
A process that used to take a few hours of work from an engineer to complete (with considerable investment from the side of the manufacturer), now is almost instant, automated and free. This advancement alone has considerably reduced the cost of CNC.
A Practical Example
To illustrate the cost of modern CNC machining, I ran a small experiment. Using the test bracket of the image below as an example, I request quotes for different manufacturing processes, materials and quantities from the online network of manufacturing service providers of 3D Hubs. Since 3D Hubs operates at a global scale and provide automated pricing, this a good representation of the 'real-time' market price for manufacturing this particular geometry.
The table below summarizes the results of this short experiment, showing the cost of producing this part through CNC machining in aluminum and stainless steel, or through 3D printing in SLS Nylon or in PLA with FDM.
Notice that the difference of CNC machining the bracket out of aluminum versus 3D printing printing it out of nylon is relatively small even at low volumes (about $55 for a one-off part). CNC machining actually becomes more economical for quantities above 100 units.
For the record, if you wanted to produce the same bracket through metal 3D printing with SLM, it would cost you $1,500 upwards for an one-off part.
Why would you choose 3D printing then?
Both CNC machining and 3D printing are exceptional tools with unique benefits that make each more suitable for different applications.
For example, the cost of creating a single prototype of this bracket with FDM 3D printing was less than $8, while the cost of manufacturing it out of plastic (Delrin, ABS or Nylon) with CNC was about the same as aluminum (at around $100).
Or when topology optimization for weight reduction is critical (for example, in aerospace applications) and traditional methods cannot produce the designed part geometry, then metal 3D printing with DMLS/SLM is still the best option available today.
As a rule of thumb, parts with relatively simple geometries that can be manufactured with limited effort through a subtractive process should generally be CNC machined, especially when producing metal parts.
The next table summarizes the most important use cases of 3D printing and CNC machining today:
Lately, there has been a lot of interest in the area of metal 3D printing and additive manufacturing. Even though the benefits of this technology for high-end applications are really exciting, the cost greatly outweighs the rewards for the majority of today’s manufacturing needs.
Today, CNC machining is still the most accessible solution for on-demand manufacturing of custom parts, and this is not probably going to change in the near future.
Of course, the new metal 3D printing systems (based on Binder Jetting and Material Extrusion technologies) that are due to be released this year may change this, but we will have to wait and see if they will have indeed a significant impact on the industry.
Alkaios Bournias Varotsis, PhD is a technical marketing engineer at 3D Hubs.