What you need to know about Subtractive Prototyping

Correction roller opt 300x154Not everyone thinks that subtractive prototyping has a place in the Additive Manufacturing arena. After all, subtractive is really CNC machining—a process that removes material from a larger piece of material through standard machining processes such as milling, turning/lathing or drilling until the prototype part is created.

Subtractive prototyping gives you the opportunity to design, prototype, and manufacture in end-use materials. It is an appropriate choice for parts used for small and large volume production runs, to obtain specific finishes, or to obtain specific mechanical properties.

Should you wish to handle your subtractive prototyping and manufacturing needs in-house, you can purchase desktop and larger sized machines from Roland DGA, an OEM of subtractive prototyping machines. Should you wish to use outside services for your subtractive needs, you can choose from a number of vendors, such as Proto Labs. These subtractive prototyping and manufacturing providers are skilled in CNC machining and can deliver prototypes and finished parts very quickly, often in a day or two.

Subtractive prototyping typically works with resins or metal materials, including ABS, Acetal Coplymer, acrylic, aluminum, brass, Delrin, HDPE, HMW, LDPE, Lexan, Lucite, Nylon, PEEK, PVC, Phenolic, plexiglass, polycarbonate, polypropylene, Rulon, Teflon, UHMW, Ultem, and wood. Depending on your application, you can match your design to a material for strength, flexibility, chemical resistance, dielectric properties, and other critical characteristics.

The advantages of subtractive prototyping and manufacturing

Besides eliminating the layering of additive processes, subtractive processes have other advantages. They offer a variety of surface finishes, doing away with the “stepped” surfaces often found in many additive processes. The finish can be functionally important if parts must slide and cosmetically important if the prototypes are to be used in market testing.

However, because the process is removing material instead of adding it, milling undercuts can sometimes be difficult. Machining also tends to be somewhat more expensive than the additive processes.

One long viewed disadvantage of subtractive has been the programming required to turn CAD instructions and information on an object into machine executable code. This disadvantage no longer exists. The automation of tool path generation is the key that makes subtractive prototyping competitive with the additive methods. Today’s vendors have found ways to make CNC tool-path generation easy and efficient. Proto Labs, for example, has found a way to automatically translate a 3D CAD design into instructions for high-speed CNC milling and uses a large cluster of computers to quickly generate code. With the Roland machines, you export your CAD model as an .STL, .DXF, .3DM or .IGS/IGES file. Through Roland’s SRP Player software, you use its wizard to process your data. Based on data you provide the wizard, it will handle feeds and speeds, cut depth, tool selection and G-Code programming.

Subtractive RP closely mimics the surface finishes and tolerances of injection-molded parts, enabling you to test the design for function. Another development in subtractive RP is the geometry it can now support. The tool path software can handle more complex shapes, often on par with additive processes. Additive is still the choice for certain geometries, such as a sphere within a sphere or parts that interlock in unusual ways.

Thus, the advantages of subtractive prototyping include a wide selection of end-use materials, good dimensional control and surface finish, and a high degree of repeatability suitable for end-use manufacture. The drawbacks are that there is some material waste, and geometry limitations.

Rapid injection molding

Sometimes viewed as a subset of subtractive prototyping, Rapid Injection Molding (RIM) is done by injecting thermoplastic resins into a mold, just as is done in production injection molding. What makes the process “rapid” is the technology used to produce the mold, which is often made from aluminum instead of the traditional steel used in production molds.

Molded parts are strong and can have excellent finishes. It is also the industry standard production process for plastic parts, so there are inherent advantages to prototyping in the same process if the situation allows. Almost any engineering grade resin can be used, so you are not constrained by the material limitations of the prototyping process.

There is an initial tooling cost with RIM that does not occur with any of the additive processes or with CNC machining. In most cases, though, it makes sense to do one or two rounds of rapid prototypes (subtractive or additive) to check fit and function before moving to injection molding. ProtoMold is one of the suppliers of RIM technology.

A number of service bureaus can meet your subtractive and RIM needs. Some do the work in their facilities. Some contract the work out to others.

Subtractive examples

By starting with a solid object and removing unwanted material, Roland DGA Corp.’s Subtractive Rapid Prototyping (SRP) machines have a low cost of ownership. They work with a range of materials – woods, brass, aluminum, ABS, Acetal, Nylon, and FDA approved plastics; deliver smooth surface finish, and meet tight tolerances.

 

MDX 15+20 medRes optMDX-15 MDX-20

These machines suit benchtop prototyping for small objects and jewelry. They fit on a desktop and machine mill-tooling board, wax, and plastics for prototypes, parts and precision models. You can test form, fit, and function. The MDX-15 and MDX-20 models feature scanning and milling in one at a resolution of up to 0.002 in. scanning and 0.001 in. milling. Bundled with MODELA Player4,™ MODELA 3D Design,™ Dr. PICZA™ and Virtual MODELA,™ Dr. Engrave,™ and 3D Engrave software.

MDX-15 $3,145 US

MDX-20 $4,695 US

 

MDX 40A PC MecSoft medR opt 300x245MDX-40A

The MDX-40A is a desktop subtractive rapid prototyping (SRP) system with a large 12 in. x 12 in. x 4.1 in. maximum work volume without rotary axis, and an optional 4th rotary axis that allows unattended 360° milling and supports materials up to 10.6 in. long by 4.7 in. in diameter. An optional 3D scanning head that uses Roland’s innovative Active Piezo Sensor technology for reverse engineering is also available.

MDX-40A $7,9995 US
Rotary 4th axis, $3,595 US

 

MDX 540 Angle medRes opt 250x300MDX 540 SRP System

The MDX-540 SRP® System combines precision desktop milling with CAM software, making it easy to produce prototypes with quickly and accurately. It mills a variety of nonproprietary materials and popular engineered plastics such as ABS, Delrin®, and nylon. The benchtop device produces functional prototypes for structural, thermal, and electrical testing. It also produces non-ferrous metal rapid injection molds. MDX “S” models deliver very smooth surfaces and have a high repeat accuracy for optics, metal molds, and other high tolerance applications.

MDX-540 $20,995 US

MDX0540A $31,495 US (automatic tool changer)

MDX-540S $26,295 US (high Precision)

MDX-540SA $36,795 US (high precision with automatic tool changer)

 

DWX 50 Front medRes opt 300x238DWX-50

Roland also offers SRP machines specific to dental applications, like the DWX-50.

  • TechPro

    Great article, however I disagree that subtractive or CNC machining is more expensive than additive processes.  With additives you must use that machines proprietary material, with SRP or CNC I can buy my material from anywhere I want.

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