Geometrical Challenges for Finishing Operations
AM processes allow unprecedented design freedom in the build phase of component manufacture (see Chapter 2). This freedom can lead to geometries and part shapes that it has previously not been possible to manufacture. In contrast, the large majority of traditional finishing techniques are not able to access these types of complex geometries. This can often lead to a situation where the finishing process (which is required to meet the final product requirements) limits the design freedom within the additive platform. For example, Figure 5.6 demonstrates geometries often seen in three common case-study geometries (as noted elsewhere Wohlers Associates 2018]), which each highlight a challenge for traditional finishing technology with AM products.
A key benefit of AM technology is the ability to optimise the topology of a part such that its load-bearing requirements are met, whilst the amount of material required to make the component is minimised. This can result in significant mass savings for applications such as aerospace, for example in load-bearing brackets. This topology optimisation, however, often results in complex freeform geometries (struts) within a part, which leads to the presence of tight corners and small gaps in the part that restrict access for methods using hard tools (blue features in Figure 5.6). However, there may be a need for these geometries to be finished for fatigue applications (Section 5.3.1). It can be common practice for these kinds of geometries to be processed manually, where operator dexterity aids with finishing these complex freeform topologies; however, there still may be challenges with tool access. Loose media finishing processes may also face challenges with these features, as
Example of common features seen in AM parts. Blue highlights those common in a topology-optimised structures, red an example of lattice structures and purple an area of no line-of-sight.
the broad range of feature radii (both convex and concave curvature) result in significant variation on the finished surface topographies and localised material removal rates.
The use of lattice structures within AM is another common way to reduce component mass (Hanzl et al. 2017). The internal structures within a lattice may be impossible to access with a hard tool or small media due to the convoluted tool path required to reach the internal struts, leading to no line-of-sight access (red features on Figure 5.6). Chemical methods are often applied, but these are dependent on the material properties (Dong et al. 2019). Difficulties in accessing surfaces for finishing leads to challenges in the exploitation of lattices, especially in applications such as space or medicine, where the final surface finish is required to demonstrate cleanliness from semi-sintered AM particles. The lattice geometry also presents a fixturing and datum feature challenge, which is applicable to a number of complex geometries, especially in precision applications. The complexity of the parts often causes specialist fixtures to be required to both hold the part in order to withstand machining forces and to provide datum features for the planned toolpath. This is often overcome by additional features being added to the part at the design stage.
Another common way to exploit the benefit of AM is to combine several individual parts from one assembly into one fully functional part. This can result in a number of challenges for the secondary operations. In some cases, part consolidation can result in enclosed features or passages with no line-of-sight access (purple features on Figure 5.6). These no- line-of-sight passage features are common in fluid channels or cooling applications and, therefore, commonly have surface requirements much improved from the as-built AM surfaces (Section 5.3.1). Depending on how much material needs to be removed and the size of the passage, there may be a limitation on the number of appropriate finishing techniques from tens of processes to just one or two. Within these internal passages, it may then be difficult to control the material removal, especially where significant variations on the internal geometry are concerned (such as changes in internal channel cross-sectional area).
As the AM process has combined a number of individually functioning parts into just one, this can also lead to a number of different surface requirements on the same part. The requirement on the secondary operation, therefore, is to remove different levels of material from different areas. This can be a challenge for certain finishing techniques, in particular for loose media-based processes where masking will be required in order to control surface finishes in specific, targeted areas.