To Print or Machine - How the Process Impacts your Prototype

Written by George Turvey

Prototyping is a crucial step in any development process and today we have more options than ever before. When choosing the avenue in which to develop your prototype, it’s critical to validate your design outside of a CAD environment, as there are some things that can’t be checked in a computer or are just easier to evaluate with parts in hand. For most human scale items, this leads to a decision between two major technologies - machining or 3D printing. Historically, this has been an easy decision, as 3D printing materials were limited by being brittle, expensive, and fairly rough. However, as the technology and materials improve, it becomes more tempting to use 3D printing in place of a machined part.

When deciding, it is important to evaluate the strengths and weakness of each process as they pertain to cost, timeline, geometry, and material properties. Let’s look at five instances when 3D printing is a great option and five times it might be better to choose a machined part. 

When to Use 3D Printing

Highly featured part.
A great benefit of 3D printing is that it usually costs nothing to add extra details. Asking a machine shop to produce a complex array of ribs will invariably lead to higher cost, however in 3D printing, those ribs will actually reduce the amount of material used, thereby reducing cost. Features like internal sharps and extruded logos and text can also be added without the extra cost that would be incurred in a machined part. 

Figure 1 Adding The Ribs In The Upper Image Would Be Very Costly For Machined Parts But Is Actually Cheaper For Printing

Visual model.
To sell an idea to investors or future clients, it is beneficial to have parts that look just like the final product, even if it doesn’t function. 3D printing can be a great way to achieve this, as the part can be made exactly as designed without the compromises necessary for a machined part. 3D prints can also be easily finished in a variety of colors and textures using paint techniques to match a final tooled part. 

Figure 2 Surface Finish Options For A 3 D Printed Part This Is A Painted Texture Similar To One That Would Be Applied In Mold

Guess and Check.
It’s not uncommon to make a part that fits with another already existing piece, often without CAD data to reference. In these instances, 3D printing can be a great to tool the check that the fit between the designed part and the existing part will match as intended and iteration can occur very quickly to check geometric tweaks. 

Ergonomic evaluation.
3D printing is great for ergonomic evaluation as it is fast and inexpensive to test multiple configurations and minor iterations of an idea until the correct feel is achieved. For items like the contoured handle of a kitchen utensil, machining may be extensive and costly as curved surfaces require many finishing passes. Conversely, printing a curved surface takes just as much time as a straight one. 

 

Figure 3 Multiple Variations Of Parts Can Be Printed For Ergonomic Evaluation Without Adding Significant Cost

Assembly mock-up. 
In complex systems, it’s one thing to design in a computer and another entirely to be able to assemble in the real world. 3D printing can be very effective at mocking up the volumes of different components to make sure fasteners are accessible when putting together a dense assembly. 

When to Use Machine Printing

Tight Tolerance.
The most important reason to choose machining over 3D printing is to get parts accurate to the design. Even on high end printers, you cannot expect much greater than 0.05mm (.002”) accuracy, and that is often not sufficient to check fitment for precise assemblies. Most competent machine shops should be able to easily produce prototypes to within +-.025mm (.001”) tolerance and even tighter where required.

Material Properties
Strength. 3D printing materials have come a long way from the brittle gypsum Binder Jet printers, but it is still difficult to find material that will match a processed material as well as machining parts from a block of the native material. 

 

Figure 4 Brittle Failure Of A 3 D Print Garyhodgson Com

Elastic Modulus. One of the best cases for prototyping is to evaluate the “feel” of a product, as things like buttons and catches can be particularly personal. One can calculate the opening forces for a latch easily, but without parts in hand it is difficult to directly correlate that force to a “feel.” However, you can’t simply print the part, as snaps & buttons rely on the modulus of the material to provide the spring force, so they must be prototyped out of materials with the same modulus as the production material.

Isotropic Properties. Most 3D printed parts have distinct lateral lines that result from the layer-by-layer nature of the process. While these are often largely cosmetic, there is  a difference in material properties depending on the orientation of these lines, so if you need equal strength in all directions it is best to use a machined part, as the base material will be homogeneous.

Figure 5 Lateral Striations In The Direction Of Print Are Natural Starting Points For Print Failure

Process evaluation. If your prototype relies on the properties of a thermoplastic material, like those typically used in injection molding, then some 3D printing technologies will let you down. SLA and other resin-based print technologies result in a thermoset material (meaning they don’t melt). If you are trying to ensure accurate pullout strength for a thermal threaded insert, then it would be best to machine a prototype from the intended material.

Figure 6 Thermal Insert Installation In Thermoplastic Material Assembly Magazine Com

Large volume. Not only is 3D printing material costly, but the machines that can print large parts are even more expensive, which usually means a higher per volume cost as vendors need to amortize their investment. If you have large, simple parts to make, it’s often cheaper to machine, particularly if you can use a low-cost prototyping material such as 40lb machinable foam.

There are obviously exceptions to all these cases, but in the current landscape of 3D printing technology, as it applies to prototyping, keeping these strengths and limitations in mind should lead to a prototype that provides all the validation necessary to move forward with production investment. It should be noted that no singular process is the perfect answer and mixing both processes can work to optimize cost. For example, use a 3D printed model for ergonomics and show, but combine with a machined piece to evaluate the snap feel. Be sure to let your prototyping vendor know the use case for the final part, as most companies will have both processes available and will be able to steer you in the right direction based on your needs. 

About The Author

George Turvey

George Turvey

Mechanical Engineer

George is a mechanical engineer at StudioRed, an award-winning product development consultancy in Silicon Valley providing brand research; UX/UI and industrial design; mechanical, structural and optical engineering; prototyping; and production services. He turns clients’ design vision into reality, applying his broad engineering expertise and experience in consumer, enterprise, medical and biomedical products. To see his work and learn more about StudioRed, visit www.studiored.com.

Share Article:

Sign up to the Hardware Massive newsletter

Related Guest Articles