Metal 3D Printing vs. CNC Machining: How to Choose the Best Method for Your Part?
With the boom of Industry 4.0, more and more product developers are opening their doors to manufacturing components through Metal 3D Printing. While Metal 3D Printing involves different technologies, we will be concentrating on DMLS, the most widely used Metal additive technique.
Although CNC Machining has always been the preferred choice for manufacturing precision components, using DMLS has been able to provide unique advantages such as weight reduction, part consolidation and improved functional performances.
You might have often faced the question “Which method among the two should I choose to manufacture my part?” The answer to this question depends on a number of factors such as part size, quantity and complexity, which influence the selection of the right manufacturing method. Let us first understand the process first and then go through the influencing factors for process selection.
CNC Machining is a subtractive process. You take a metal block and remove unwanted material with cutting tools to get the final part. DMLS/Metal 3D Printing/Metal Additive Manufacturing as the name suggests, is an additive process. Metal powder is spread on a bed and laser selectively melts the powder layer by layer to get the final part.
There are however Hybrid machines which can simultaneously print and machine each layer, but we won’t be discussing them due to their high cost and limited availability.
Influencing Factors for Process Selection
Metal 3d printers start from a bed size of 100mm X 100mm X 100mm and go upto 400mm X 400mm X 400mm . The most common size among them is the 250mm x 250mm x 300mm machine. So, if your part lies within any of the above size, 3D printing can be considered. If the part is bigger, the CAD model has to be cut into sections, printed individually and then welded together to form a single part. This will increase the cost significantly and should be preferred only when the part is too complex for machining. On the other hand, most CNC machines can easily handle parts up to a metre in length.
In most cases, DMLS is best suited for prototypes and low batch production. Whereas Machining works well for medium to high volume production. For manufacturing multiple prototypes with minimal design changes, CNC machining has no advantage as it needs custom fixturing and tooling for each prototype design, while DMLS can print multiple designs together in a single build. The batch quantity DMLS can handle is dependent entirely on the part size. Let us assume 2 scenarios here.
1. Assuming a printer bed size of 250mm X 250mm and part size of 15x15x15mm with a requirement of 50 nos.
With DMLS, the entire 250 numbers can be printed in a single build within a day. Whereas machining the same would need atleast a day for just fixturing, tooling, programming and proving out the 1st part. In this case, DMLS is the best choice.
2. Assuming a printer bed size of 250mm X 250mm and part size of 200x150x200mm with a requirement of 20 nos.
Even though the quantity is much lesser in this case, the printer can print just 1 part in a single build. It could take a couple of months to print the 20 nos and the cost would be high due to the part weight. Whereas, machining the same would be much faster and economical. Once the initial fixturing, tooling and part proveout is done, the CNC machine can produce parts within weeks. In this case, CNC machining is the best choice.
Since DMLS process happens layer by layer, it can manage to print any complex shape. Machining complex shapes is difficult for reasons such as tool inaccessibility and increased machining time.
For example, consider a waveguide having internal hollow channels throughout. Due to tool inaccessibility, machining approach would require a design change by cutting the model into sections, machining these sections and then fastening them together.
Whereas 3D Printing can easily print the whole waveguide as a single part. Lattice structures that are used for weight reduction also eliminate the possibility of CNC machining. On the other hand, if the part is a simple shaft with an I.D, or a rectangular casing with pockets and holes, it is better to machine the part as it is much faster and economical.
To help you select the best process for manufacturing your part, we have classified parts on the basis of size, quantity required and design complexity.
Depending on the size, quantity and complexity of your part, the best manufacturing process can be selected from the table below. The different combinations are created to ensure that the parts are available in shorter lead times and at the most economical price.
Example 1) If your part size is small, quantity required is low and the part complexity is high, i.e (S1,Q1,C3), these kind of parts are the first choice for DMLS.
Example 2) If your part size is small, quantity required is large and the part is simple in design, i.e (S1,Q3,C1), these kind of parts are the first choice for CNC machining.
*These parts can be manufactured through any of the two processes. The part has to be further studied to determine which is the most economical and faster option among the two.
Part Tolerances & Surface Finish
During Metal Printing, powder particles gets partially sintered to the surface of the printed part. This results in a surface finish of around 10Ra (microns). Also, due to continuous heating and cooling of each layer, the thermal stresses in the part is high. Due to the above reasons, the dimensional tolerances of DMLS parts is around +/- 0.1mm. On the other hand, machining can yield surface finish below 1Ra (microns) and dimensional tolerances up to +/- 5 microns.
If a part needs tight tolerances on majority of its dimensions, it doesn’t make sense to get it 3d printed. On the contrary, if a complex part needs tight tolerances only in a few areas, the part can be 3d printed and the high tolerated zone are can be machined later. Also, the surface finish of a 3d printed part can be improved by different post processing methods.
Apart from Stainless Steels, Nickel Alloys, Tools Steels, Titanium and Aluminum, not many alloys are presently available in DMLS due to the time and cost involved in qualifying a material. Whereas with machining, most of the alloys are available in bar stock form.
If a part needs to be printed in a certain alloy unavailable in DMLS, an easier option is to select an alternative material used in DMLS and prove the material for the required application, or develop the particular alloy which will turn out to be a costly affair.
DMLS can however print difficult to machine alloys such as Inconel and Cobalt Chrome without any difficulties.
Improved Functional Performances
Parts for machining are designed based on manufacturability while parts for 3D printing and designed based on functionality. Hence, the design of a machining part is mostly frozen and does not aid much towards improving the functional performance. Whereas in metal 3D printing, the existing part or the to-be-designed part can be designed with DfaM rules to reap the benefits of Metal AM.
Design alterations such as part consolidation for a compact design, topology optimization for a reduced part weight or conformal cooled channels for improved productivity can be incorporated to enhance the functional performances of a part.
It is to be noted that 3d printing is not a challenging technology or replacement to CNC machining. It can never replace machining. They need to be used together to make use of the benefits of both the processes.
At Rapid DMLS, we help you design your parts for Metal Additive Manufacturing and manufacture them making use of both Metal 3d printing and CNC Machining.