Prototypes: 3D Printing or Machining
We love the expansion of technology in rapid prototyping (like 3D Printing) with new materials almost daily. While technology is ever improving, the boundaries for choosing the best methods become ever more blurry. Should I use 3D Printing or Machining?
That answer was easier a few years ago. Now, with improvements in process and materials, the 3D Printing or Machining question is harder.
In a previous post about rapid prototyping we gave just a little history and some added info about some of the processes. We’ve also written about The Purpose of Prototypes. Both of these articles talk about advantages of different types of prototypes.
Our purpose here is to look at fundamental differences for functional prototypes and compare 3D Printing or Machining with Production.
3D Printing Advantages
The big advantages for 3D Printing are speed, complexity, and sometimes material. Let’s throw cost in there too.
Compared to machining, 3D Printing offers the ability to get complex shape without the need to figure out how to machine them. In fact, you can 3D print things that are very hard to make any other way. You can print things, for instance, with tiny deep wavy slots that you can’t mold or machine. You can print intersecting parts that would be impractical to machine. Then there’s the really fun stuff like parts inside of parts.
The cool aspect of 3D printing is the speed to create complex parts. Yes, it’s a real snoozer to watch 3D Printing, and not as exciting as watching chips fly in a CNC. However, the printing processes are like the tortoise, and it just keeps building until it’s done. For the most part, 3D printed parts don’t require multiple setups, so just let it build.
And the parts you can create are unlimited — well, almost. Limited, perhaps only by your ability to create the parts in 3D CAD. For more, check out AMFG’s tips to working with FDM (one type of 3D Printing).
Rapid prototyping also allows you to create parts in materials that are near impossible to machine — elastomers, for instance. The white part above is a soft rubbery compound via FDM. Though it’s not very complex, this part is not easy for CNC machining.
As it turns out, the part also highlights some of the hiccups that can happen. The little nub in the center of the cylindrical surface faded up with printing, so it is not quite round. You can see some weirdness in the corners as well.
Finally, choosing the build direction makes a big difference in quality for features like the white nub and for round areas. Certainly a different build direction would make the white nub more round, but the larger cylindrical surface may suffer. Often it’s a trade-off.
Now look deeper at using materials, build direction and other techniquest to do even more. Things like our experimentation with 3D printing in rapid prototyping foam proved both successful and enlightening.
We should note, however, that other 3D Printing technologies do better with resolution, but perhaps don’t have the rubbery materials. When we think about 3D Printing or Machining, the resolution and/or accuracy is a pretty important consideration.
The big advantages of Machining are precision, finish, and strength.
Going back to the white part above, the little white nub is a hiccup with 3D Printing. True, that would be a little hard to machine, but it would be round. (Though CNC machining soft rubber poses a challenge of its own.) Also, things like the threads of this little brass part would be near impossible to make well in 3D printing. Of course, the ability to accomplish it does depend on the orientation of construction and the process.
Basically if it can be machined, the features and precision are better with CNC.
Rapid prototyping is coming right along with precision, but still does not compare with machining. Holding tolerances in the 0.005 or below range is not so practical via printing. Yes, for some technologies in some circumstances, but not globally. On the other hand, machining with tolerances at 0.005 and below is common.
Along with precision, machining can deliver amazing finishes. We often talk about finish as the look or feel of a surface. Well, there’s not a lot that looks more beautiful than a sparkling precision CNC part. However, there’s more to finish than just look.
For instance, making treads that fit snug and feel smooth is nearly impossible with 3D Printing. Even big course ACME threads don’t feel great when sliding together. Yet, like in the two photos here, threads and threads in threads are common and easy to machine. And, they fit and function perfect. The threads in threads parts with the yellow background are an excellent example.
Finally, there’s not much that can compare with the strength of machined prototype parts. As discussed in this article on CNC Parts To Take The Heat, when prototyping, machining is the choice when strength or heat are involved.
That said, strength with machine parts is not limited to metals. Even in plastic — yes, machining plastic prototype parts is common. The prototype shown (in the image being cut) was machined because strength is a main characteristic of the needed part. Yes, it would be easier and cheaper with 3D Printing, but the strength is not there. This part is 25% glass filled nylon.
For plastics, 3D printing can approach 80% or so the strength of a similar injection molded or machine part. In metal, things are improving quickly, so I won’t quote values, but the goal is to approach base material strength. Perhaps some day they’ll even find ways so 3D printed metal parts are stronger than the base material. That will be really cool.
3D Printing or Machining Prototype?
When comparing prototype processes like 3D Printing or Machining, it helps to know what you need. What Is Your Prototype For? Perhaps asking Why make a prototype will lead to the right answer.
Can you make beautifully finished printed parts? Though many images of 3D Printing parts are sort of rough in appearance, they don’t have to. This little unlimited class derby car is certainly an example with a gloss painted exterior. 3D Printing is the body, with machining for the wheels and some chassis parts. This prototype car is a combination based on the goals. It’s not 3D Printing OR Machining . . . it’s 3D Printing AND Machining. It’s even machining the 3D Printing parts! (Yes, there is some machining on the body interior.) That’s how we meet the project goals.
The body did require a lot of sanding and primping, but the result is an auto body level paint job. See the Derby Article for more images.
Often we talk about 3D Printing with respect to complex shapes. While that is easy with rapid prototyping, you can also do it with parts for machining.
So if you can do everything (as we have for years) with machining, why use 3D Printing?
What Is Your Prototype For?
Going back to our key question, the processes you choose should meet the reasons you are building a prototype in the first place. The best reasons for 3D printing are SPEED, MATERIALS, and the ability to get COMPLEX shapes for much less cost than similar machine parts.
The reasons to stick with the old fashioned machine parts are first and foremost, STRENGTH, then PRECISION. Finish and Size can enter in there too.
Many things are decisions between what works best, and what costs less. Just don’t forget the purpose of the prototype — because the best function might be a combination of manufacturing methods. Three good examples of mixed prototyping methods are: 1) The image here that has CNC parts, etched parts, 3D printing parts, purchased parts, as well as some fabricated parts. 2) Read about this rack invention which is also a combined prototype. 3) The derby car above.
Use 3D Printing or Machining or Etching or Fabrication or any other methods — at the right times — for the purpose (function or look or testing) of your prototype. Combining many is often the best way to get there. (Here’s another example of mixed technology prototypes.) Oh, and don’t forget the wonders of carving wood, playdough, and the great old Duct Tape and Bailing Wire.
Make the prototype function for what the prototype needs to do. Let that drive the decisions about which prototyping method to use.