Additive vs. Subtractive Manufacturing

Additive manufacturing is a process that builds parts from the base up by adding successive layers to manufacture a product. 3D printing is the technology most associated with additive manufacturing. Subtractive manufacturing removes material to manufacture a part. This process traditionally uses Computer Numerical Control (CNC) machining.

Both technologies can utilize computer-aided design (CAD) software models to produce products. These manufacturing technologies have tremendously impacted prototypes and production and continue to make advancements.

This article discusses Additive vs. Subtractive Manufacturing, their difference, expenses, applications, and the hybrid process.

The differences between additive manufacturing and subtractive manufacturing are significant. Additive manufacturing, often referred to as 3D printing, adds successive layers of material to create an object. Subtractive manufacturing removes material to create an object.

Additive Manufacturing

Both technologies utilize CAD drawings to create parts; additive manufacturing melts or fuses powder or cures liquid polymer materials to form parts based on the CAD drawings. Additive processes are slower to manufacture, and several technologies require post-manufacturing methods to cure, clean, or finish the product. The surface finish is not as smooth as subtractive manufacturing, and the tolerances aren’t as precise. These processes are ideal for lighter parts, material efficiencies, rapid prototyping, and small to medium-batch manufacturing.

Complex geometries, including the printing of articulating joints with additive manufacturing, are available. The geometries are more complicated, and set-up is quick and easy, with no operator required during the printing process. The most common materials used in additive manufacturing are plastics and metals. The equipment cost is less than subtractive manufacturing, and various material colors are available for most 3D printing operations.

The additive technologies include:

• Binder Jetting — The process uses liquid bonding agents selectively deposited to join powder materials. Materials include metal, plastic, ceramic, and sand.
• Directed Energy Deposition (DED)—DED utilizes focused thermal energy to fuse materials as they are deposited. Materials include Wire and powder.
• Material Extrusion—Extrusion selectively dispenses material through a nozzle or orifice in layers. Materials include plastics, nylons, FDM, FF, and sand.
• Material Jetting—Droplets are selectively deposited to form products. Material includes photopolymers, wax, sand, and poly-jetting material
• Powder Bed Fusion—Thermal energy selectively fuses regions of a powder bed. Materials include metals, polymers, and fibers.
• Sheet Lamination—Sheets of material are bonded to form an object. Materials include metal, paper, wood, and plastics.
• Vat Photopolymerization—Pre-deposited photopolymer is selectively cured by light-activated cross-linking of adjoining polymer chains. Materials include photopolymers.

Subtractive Manufacturing

Subtractive manufacturing involves material removal with turning, milling, drilling, grinding, cutting, and boring. The material is typically metals or plastics, and the end product has a smooth finish with tight dimensional tolerances. A wide variety of materials are available. Change-overs are longer, but automatic tool changers help reduce time-consuming delays. The processes can be fully automated, although an attendant may oversee two or more machines.

The equipment costs are higher and usually require additional jigs, fixtures, and tooling. It is best suited for large production with reasonably fast manufacturing time but lengthy changeovers. Material handling equipment helps both processes with material loading and removal. Geometries are not as complex as additive manufacturing processes.

Subtractive manufacturing technologies include:

• Abrading—This manufacturing process uses friction to wear the material surface by grinding and polishing products.
• CNC Machining Centers — The computerized manufacturing process to control complex machinery via pre-programmed software to regulate the machining equipment to cut and shape parts. This equipment includes milling, turning, drilling, boring, grinders, and 5-axis CNC machining centers.
• Electrical Discharge Machining (EDM)—Also known as “spark machining,” “wire burning,” or “wire erosion,” EDM is a non-traditional machining process that utilizes electrical discharges ranging from 80.00ºC to 12,000ºC to remove material from a workpiece placed in a dielectric liquid. 
• Laser Cutting—This manufacturing method utilizes a gas laser, often CO2, for energy. The laser beam is guided by mirrors and directed toward the workpiece to remove material. The laser beam output is between 1,500 and 2,600 watts.
• Waterjet Cutting—Waterjet cutting is an accelerated erosion process using a high-pressure water stream. This CNC method of cutting objects uses energy from ultra-high-pressure water that becomes hypersonic at speeds up to 2,500 mph (Mach 3).

Materials for the subtractive manufacturing processes include hard metals, soft metals, thermoset plastics, acrylic, wood, plastics, foam, composites, glass, and stone.

Additive and subtractive technologies have a variety of different processes. The costs and capabilities range from desktop machines to large industrial equipment. Prices have dropped significantly in recent years, especially for additive technologies. Compact, easy-to-use desktop additive and subtractive manufacturing tools are available today for professional workspaces, machine shops, and workshops.

Entry-level 3D printers start at several hundred dollars, and the cost of suitable desktop printers for enthusiasts begins at approximately $3,500 up to $20,000. Industrial printers start at $10,000 and cost over $400,000.

Hobbist mills and lathes start at $2,000, and an entry-level CNC machining center can begin at $60,000. Industrial 5-axis machining centers cost $500,000 plus.

Additive and subtractive manufacturing produce a wide range of products across several industries. Additive manufacturing is preferred for rapid prototyping, small batch production, and on-demand production. Subtractive manufacturing has long produced prototypes but is best suited for large production runs.

Almost every industry uses additive manufacturing products, including aerospace, architecture, automotive, aviation, consumer goods, lifestyle, industrial manufacturing, material handling, robotics, medical and dental, and sports. Products include medical implants, aircraft components, automotive parts, investment casting patterns, tooling, brackets, turbine blades, jewelry, entertainment, biking, and customizable products.

Subtractive manufacturing products are in the same industries and more, including aerospace, architecture, automotive, aviation, consumer goods, food service, lifestyle, industrial manufacturing, material handling, oil and gas, robotics, medical and dental, and sports. These products are often used in harsh environments where robust products are critical. Subtractive manufacturing parts are used everywhere, from pistons to fan blades to steel drums to turbines.

Subtractive and additive components often complement each other in producing tooling, jigs, and fixtures.

Hybrid approaches combine additive and subtractive processes, taking the best of both worlds and incorporating them into one machine. These hybrids capitalize on both manufacturing systems’ strengths and require specialized equipment. Hybrid manufacturing makes producing complex parts quicker and easier.

An example would be polishing the surface of a 3D-printed part or drilling a hole that requires a tight tolerance.