The Battle of Manufacturing: Additive vs Subtractive
Ian Wright posted on August 31, 2016 |
In the manufacturing neighborhood, 3D printing is still the new kid on the block. Although additive manufacturing has been making some impressive inroads in the aerospace and automotive industries, CNC machining remains the go-to for the vast majority of manufacturers.

However, the emergence of on-demand manufacturing is leading more and more manufacturers to give 3D printing some serious consideration. When it comes to on-demand manufacturing, is there a clear winner between 3D printing and CNC? 

ENGINEERING.com had the chance to get some insight on this topic from Gordon Styles, founder and president of Star Rapid, a provider of rapid prototyping, rapid tooling and low-volume production services.


How does one decide whether to use 3D printing or CNC for plastic parts?

For prototyping in plastic, I almost always recommend 3D printing. The materials for technologies such as stereolithography (SL) and selective laser sintering (SLS) are very advanced and offer a huge variety of mechanical properties and fillers. SL tends to be more accurate than SLS and is easier to paint and sand, but SLS has the advantage on mechanical properties.

The problem with CNC machining for the prototyping of plastic injection molding designs is that CNC milling uses cylindrical cutters. Therefore, it is difficult to cut all of the internal square corners that are prevalent on these kinds of designs. When CNC machining a prototype of an injection molding, for example, some of the features cannot be machined per the design because the CNC cutters are cylinders and the feature might be a square internal corner, so sometimes the CNC programmer must try and decide how best to make the part.

Generally, CNC-machined plastic components are much more efficient by CNC machining. 3D printing suffers from the fact that it is a part volume-driven process. Fat parts have been slow to produce by 3D printing until recently. However, the new HP MJF machine may change this.


What about metal parts?

For metals, it is a different issue.  There is no point in 3D printing anything that is designed for CNC machining; it will naturally be a lot slower to print because the printing time is relative to the volume of material. Most metal parts designed for CNC are voluminous and therefore take a long time on a 3D printer.

(Image courtesy of Star Prototype.)
(Image courtesy of Star Rapid.)
If the part geometry is designed with 3D printing in mind and has impossible features, then one simply cannot make a comparison to CNC. One must think of metal 3D printing as a liberating technology. You can now design seemingly impossible parts and still get them made, but the key is to keep the volume of material to a minimum.

Most metal 3D printing is for production and it is vital that the metal is of a very high integrity. It is also critical to be able to know the metallurgy of every component and test its density and strength. With that in mind, we have X-ray fluorescence (XRF) and optical emissions spectrometry (OES) in-house for checking metallurgy as well as hardness and density testing.


Is there a way to compare the cost of 3D-printed parts vs. CNC including finishing?

The best way is to simply get a quote for the two processes from your rapid prototyping service provider. One rule of thumb, though, is this: if the part can be CNC machined, then it is probably cheaper to CNC machine it—and in 90 percent of cases, this is true.


How often do you need to machine a 3D-printed part?

For plastics, it is extremely rare.  For 3D-printed metal components, it is probably about one in five, but I would say that four in five need some kind of manual finishing in addition to removing the supports.

It also depends on the customer’s requirements. If the parts need to be in very high tolerance, then we’ll implement a CNC post-machine process. Some fine features—such as thread—that can’t be done by metal 3D printing will also need to be post-machined to ensure functionality. Furthermore, if the 3D-printed part’s surface finish is vital for aesthetic purposes, CNC post-machining will be needed as well.


Does material availability constrain 3D printing compared to CNC machining?

Yes, today there is a constraint on the range of materials available for metal 3D printing, but we are seeing a huge expansion in this area. We are doing work for a number of powder manufacturers who want to develop parameters for new materials. While we are limited today, 10 years from now we will see hundreds of available materials. 

(Image courtesy of Star Prototype.)
(Image courtesy of Star Rapid.)
The problem is not really the availability of materials, it is the time and effort required to switch materials. Today it takes about 24 hours to turnaround a machine for a new material. Most manufacturers claim it takes only a matter of a few hours but this is simply not true. You have to wait for almost an entire day for all of the internal components to dry off completely. With CNC, you just spend 20 minutes sweeping/washing out the machine and you can go to the next material. Most metal 3D printing companies like to keep one machine for one material.

 

How do the component size constraints for your 3D printers compare to your CNC machines?

The biggest single constraint on size for metal 3D printers is the stress that builds up in the component during building. If you are making a large part and the temperature of the bed is about 100 °C (cold build), which is common for most machines in use today, then the stresses can be such that the part will literally tear itself from the bed. We have even seen the bed getting bent and breaking the screws holding it down, which is a disaster.

Most manufacturers of metal 3D printing machines are now designing machines with a high temperature bed that keeps the entire build above 500 °C. This makes the machines more expensive to buy and use, but results in less stress. So, the bigger you get, the disproportionately more expensive the machines become.

CNC doesn’t really suffer from these kinds of problems.  When CNC machining something, you do get stress relief in the material, but compared to metal 3D printing it is a minor problem.


Do you get parts designed specifically for 3D printing, or do customers leave it to you to decide which process to use?

We do get parts that have been specifically designed for metal 3D printing, but that is rare at this time. We are spending a lot of time and effort educating our customers on how to design for metal 3D printing. Over the next decade, this will be the big push: getting all customers to understand how to design for the process. Right now we have too many customers asking for metal 3D printing quotes when the parts should just be CNC machined. Design is the key to the future of metal 3D printing.


Are hybrid 3D printing/CNC machines the future of manufacturing, or are we more likely to see the machines stay separate but connected via a pallet-changing system?

(Image courtesy of Star Prototype.)
(Image courtesy of Star Rapid.)
It is like CNC milling and CNC turning.  You can still buy ‘milling only’ machines and ‘turning only’ machines but there is a market for hybrid mill/turns and turn/mills. The same will be true for the metal 3D printing and CNC hybrids. They will find their niche over time. Turbine manufacturers and the aerospace, motor sport and nuclear industries should be going crazy for the DMG Mori powder fusion/mill/turn machines. This makes impossible precision tubular construction a real possibility. But again, there has to be a revolution in the ability to design specifically for the processes.

 

3D Printing vs CNC Machining

It seems that on-demand manufacturing is much like manufacturing in general: what works best depends on your application. However, given the importance of design when it comes to deciding between CNC machining and 3D printing, we may see more manufacturers turn to the latter technology thanks to the next generation of design engineers.

These professionals will have grown up with 3D printing, so the phrase ‘impossible design’ won’t hold the same significance for them that it does for us. CNC machining may be the dominant technology now, but 3D printing will only become more prominent in the years to come.

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