Producing near-net shape components with fine details and close tolerance in one step is new challenge in manufacturing. There exist two different approaches to facilitate such demand. In one approach, layer by layer material is added one over another to obtain 3-D components with all geometrical features; while in other approach, material is removed from a solid blank to obtain desired products. As the name suggests, additive manufacturing refers to all those processes where layers of materials are added to build a complete 3-D product; while the reverse one is called subtractive manufacturing.
What is additive manufacturing?
Basic principle of additive manufacturing approach is creating a 3-D CAD model of desired component and feeding it to additive manufacturing (AM) machine in STM file format. Computer software, inbuilt with such machines, interprets CAD data and virtually slices that model into a number of thin layers based on accuracy requirement. Then it instructs the nozzle holder of the AM machine to follow a specific path (as per first layer) to deposit a layer of material in semi-solid or powder form.
Various other operating parameters can be controlled and inserted in the software. Typical process parameters include thickness of layer, nozzle travel rate, volumetric density of deposited material, temperature, time interval between two depositions, etc. Successive deposition of material ultimately produces entire 3-D component. 3-D Printing, Rapid Prototyping (RP) and Direct Digital Manufacturing (DDM) are widely used additive manufacturing techniques.
Advantages of additive manufacturing processes
- Near-net shape production.
- Ability to control material density and thus component weight.
- Can make intricate features and internal closed cavities.
- No need of process planning.
Disadvantages of additive manufacturing processes
- Slow process.
- Not suitable for large size products.
- Limited working materials (low melting point like plastic, wax, etc.)
Applications of additive manufacturing processes
- Industrial model fabrication.
- Making jewelry.
- Medical and dental elements.
- Various solid freeform features.
What is subtractive manufacturing?
Although emergence of additive manufacturing approach is considered as third industrial revolution, traditional techniques including subtractive manufacturing and forming are still advantageously utilized in different fields of manufacturing. Subtractive manufacturing indicates all those processes where material is removed layer by layer from a solid block to obtain desired 3-D component. All CNC based conventional machining processes (like turning, milling, drilling, etc.), non-traditional machining processes (like CHM, EDM, LBM, etc.) as well as newly developed micro and precision machining processes (like micro-milling, diamond turning, etc.) follow subtractive manufacturing approach.
Now-a-days these processes utilize computerized system (CNC control) inbuilt with design software (CAD) and integrated manufacturing facility (CAM and CAPP). Initially a CAD drawing is created for the required component and an optimum process plan is generated considering a number of operating parameters including raw material, machine capability, tool availability, level of quality and tolerance requirement, etc. Based on this plan, programs are generated and it is fed to the machine. Machining is then carried out as per the program and excess material is removed in various ways (shearing, erosion, melting, sputtering, dissolution, etc.) in order to get desired object. In absence of CAPP, computerized process plan and automatic retrieval of part program is not feasible. In such cases, programs can be written manually.
Advantages of subtractive manufacturing
- Wide variety of materials can be processed.
- Large size objects can be fabricated.
- Processes are faster.
Disadvantages of subtractive manufacturing
- Cannot alter volumetric density of building material.
- Material wastage takes place.
- Process planning is mandatory (it is laborious task that needs a lot of data).
- Limited capability in feature form (enclosed features cannot be generated).
Applications of subtractive manufacturing
- Fabricating contoured profiles and textured surfaces.
- Cutting various features that are useful in household applications as well as automobile, aerospace, electronics, dental and medical requirements.