Can you use subtractive processes to make parts out of metal material? Of course. Can you use additive processes to make parts out of metal material? Yes, but . . .

The ability to use additive processes to create metal parts is growing. Both additive and subtractive companies have made recent announcements in their ability to offer metals as a material selection. Proto Labs, Inc. is the latest to announce metals materials for its subtractive FirstCut™ processes, 7075-T651 Aluminum and 260 Brass.

3D Systems and EOS also have metals available. 3D Systems offers LaserForm™ A6 metal for use on its Sinterstation® HiQ™ system. EOS offers 17-4 and 15-5 stainless steel, cobalt chromium, inconel 625 and 718, and titanium Ti6Alv4, among others.

Selective Laser Sintering (SLS), or direct metal laser sintering (DMLS), are the common terms used for this additive metal material process, but it can also be termed Selective Laser Melting (SLM). Both the 3D Systems and EOS machines use a laser to locally melt and then additively fuse these fine metal powders layer by layer to form shapes defined in 3D CAD programs. Layer thicknesses can be as little as 20 microns.

Parts made from these metals tend to be dense end-use parts with very good to excellent surface finish, good feature detail and tight tolerances. You can test them for function.

So what are the criteria for choosing between SLM and CNC subtractive processes? One criterion is market. Presently, these SLM processes are used for making parts for medical and aerospace applications; medical implants, dental caps and crowns, and lightweight parts.

Cost and time to prototype or manufacture a part will be other factors to examine. Traditional subtractive processes may be more suitable when you need the parts in a few hours to a day or two. Fast turnaround with CNC has been (and sometimes still is) an issue. Proto Labs, however, has lead the way in automating the creation of machine code directly from a 3D CAD model, a process that the Proto Lab’s FirstCut CNC service has perfected. Through this process, you can obtain a quote on the produceability of your part within hours, and see the final result often within a day.

Other factors to examine are geometric complexity and surface finish. Certain complex designs simply cannot be made on a CNC. “But up to 95% of all metals-based parts are machineable,” said Brad Cleveland, CEO Proto Labs. “One of the main benefits of a process like FirstCut’s is the final surface finish. It is not a separate step as might be needed with additive processes.”

In addition, some subtractive services can deliver a part more quickly than additive, depending on the part geometry, of course.

Both additive and subtractive prototyping/manufacturing can produce metal parts. The strength of additive is in its ability to handle almost any imaginable geometry. The strength of subtractive is in the final materials properties and surface finish.