Subtractive manufacturing (CNC machining) offers high quality geometric and material properties but is slow, costly, and not feasible in some cases. On the other hand, additive manufacturing (rapid prototyping) has total automation but may compromise quality. A hybrid, layered, manufacturing process combines the best features of both these approaches.
This hybrid approach uses arc weld deposition for building near-net shapes, which are subsequently machine finished. Time and cost savings of this process can be attributed to reduction in NC programming effort and elimination of rough machining. It is envisioned as a low-cost retrofit to existing CNC machines for making metallic objects without disturbing its original function. Near-net shape building and finish machining happening at the same station are unique features of this process. Custom software generates the NC program for near-net shape building. The details of integrating arc welding with a CNC milling machine, introduced here as Hybrid Layered Manufacturing or HLM, are presented in more detail in a new text by the authors.*
In HLM, the near-net shape of the part is obtained by alternately depositing metal using pulsed synergic Gas Metal Arc Welding (GMAW) and face milling. The same is then machine finished. Eliminating rough-machining as well as manual NC programming are two significant advantages of HLM. Availability as a retrofit to any 3-to-5-axis CNC machining center too is an attractive
feature of HLM; the same machine can be used for both subtractive and hybrid manufacture. HLM is cheaper, faster and better than the powder-based metallic RP processes as well as CNC machining for tool making.
The machine selected for HLM is a Fronius TPS4000, a sophisticated pulsed synergic GMAW. All the welding parameters are preset on this machine and only on/off control is done by the CNC through M08/M09 codes. Spatter-free stable deposition, low and uniform heat input and sharp feature definition are more important for this application than high accuracy or speed of deposition. Under synergic control, mean current and step-over increment are the only parameters that influence the slice thickness and the yield of the process.
Zero order uniform slicing is adopted for building the part in layers. After slicing, each layer is then offset by the required machining allowance of 1.0 to 1.5 mm for finish machining. For the first layer, deposition is made on a substrate that will become the base plate of the die later. Deposition is laid using a direction-parallel (or zigzag or raster) fill pattern followed by contouring. After deposition of filler metal, every layer is face milled. Face milling ensures Z accuracy and provides a scallop-free nascent surface for deposition of the next layer. By continuing this process the near-net shape of the part is formed. The process does not pose any restriction or loss of accuracy on the prototype with change in size and is limited only by the traverse available on the CNC. Stress relieving was not needed for mild steel wire. However, this may be essential for harder materials.
*“Retrofitment of a CNC machine for hybrid layered manufacturing”
Springer Link
http://www.springerlink.com/content/77223n514722640x/
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