Narrow channels may be difficult to finish due to the limitations they place on tool access. Straight channels may be suitable for drilling (mechanical or EDM) or electrochemical processes, provided that a thin enough tool can be obtained. Longer, narrow channels (channels with a high aspect ratio between length and width) may only be appropriate for non-conventional methods, such as EDM drilling.
Complex Internal Channels
Internal channels which have non-straight paths (for example, those which could not be drilled) have line-of-sight restrictions on which processes may be applicable. Neither physical delivery of a tool nor optical processes which require line-of-sight are applicable to these features. Flexible tooling or liquid-based techniques such as abrasive-flow machining can be used instead.
Internal Cavities (Surface Connected)
Surface connected internal cavities (i.e. features where the major dimensions are significantly greater than those of access channels) have similar restrictions in terms of tool access to narrow channels, as mechanical tooling cannot be delivered to the salient surface. As such, chemical methods are likely to be most appropriate for finishing features of this type. However, closed-ended features may also cause stagnation of finishing media in mass finishing techniques or inhibit process control in chemical or electrochemical techniques.
Variable Cross-Section Internal Channels
Processes that can finish internal channels often require localised control to achieve consistent finishes throughout the length of the channel. Electrochemical or abrasive flow techniques may require localised process control, for example by use of a shaped electrode for electrochemical processes, to maintain surface texture consistency.
Outer Lattice Surfaces
Outer surfaces of a lattice may be suitable for secondary finishing by a range of techniques, but care must be taken not to cause damage to the feature when using mechanical techniques due to contact forces in material removal, or thermal damage when using laser- based processes. Low energy mass-finishing techniques or chemical processes may be most appropriate for finishing both outer and inner lattice surfaces.
Inner Lattice Surfaces
Secondary finishing processes that are suitable for use on outer lattice surfaces may not be appropriate for inner surfaces, such as mechanical techniques and their tool access requirements. Therefore, if an entire lattice surface needs to be finished, then different processes may be required. Localised control of chemical and electrical processes may also cause issues for finishing surfaces deep into a lattice feature.
As with lattice features, thin features will pose challenges to mechanical processes where contact forces could cause part deformation or loading. Thermal loading can also cause issues in terms of localised metallurgical changes, part deformation or inconsistent surface texture. Chemical and electrochemical processes, where there is no mechanical and no bulk thermal loading (there may be thin layers of high thermal loads in electric processes), can be most appropriate for these features.
Closed Internal Cavities
Surfaces that are part of completely closed internal cavities, where there is no access for physical, optical or chemical tooling, in general cannot be finished through a secondary operation. If a closed internal cavity is required as a feature of the part for its functionality, which must also be finished to a certain texture, then the part can be manufactured in multiple parts and then assembled into the final geometry Alternatively a hybrid AM machine combining a machining tool with powder-bed laser technology could be considered, in this case the part is machined every few layers, allowing internal cavities to be machined as they are built.