Which parts usually require secondary machining?

一, Secondary machining needs that come from core functions
1. The sealing surface and the mating surface
Sealing surface: The sealing surface must be able to handle high-pressure fluids (such hydraulic oil and gas) in places like hydraulic valve bodies and gas turbine combustion chambers. To stop leaks, the surface roughness must be kept below Ra0.4 μm. For instance, the sealing surface of the 3D-printed titanium alloy valve body of an aircraft engine fuel pump needs CNC cutting to get rid of unmelted powder particles so that it fits well with the rubber sealing ring.
To get IT5-IT6 level accuracy, matching surfaces such gear meshing surfaces, bearing mounting holes, and so on need to be ground or honed. After 3D printing a certain kind of reducer planetary gear, the tooth surface roughness goes from Ra6.3 μ m to Ra0.8 μ m, and the noise goes down by 15dB thanks to severe turning and grinding.
2. System of threads and holes
Thread: 3D printed threads often have incomplete teeth profiles because of powder adhesion, therefore they need to be tapped or rolled. For instance, following 3D printing, the threads of bone screws in medical implants need to be fixed with a tap to make sure they fit tightly with bone tissue.
Hole system: To make sure that deep holes and holes that cross each other are coaxial, they need to be bored and reamed. For instance, the cooling holes on an aviation engine's turbine disk are controlled to within ± 0.02mm of the aperture deviation using a mix of 3D printing and electrical discharge machining (EDM) technologies.
3. Channels for light and fluids
Polishing optical surfaces like laser reflectors and infrared windows to a surface accuracy of λ/10 (632.8nm wavelength) requires ultra-precision. For instance, a certain kind of satellite optical bracket is made by 3D printing it and then using magneto rheological polishing to get rid of surface ripples so that it meets the needs of space optical systems.
Electrochemical polishing (ECP) is needed to get rid of burrs on the inside walls of microchannel heat exchangers, fuel nozzles, and other fluid channels. This makes the flow less resistant. The fuel nozzle of GE Aviation's LEAP engine, for instance, includes a 3D-printed internal flow route that has been treated with ECP. This has made the size of the fuel atomization particles 30% smaller and the combustion efficiency 5% higher.
二, The need for further machining because of process limits
1. The roughness of the surface is higher than normal.
Common places: the contact surface of the supporting structure, the overhanging surface, and a big plane. The contact surface of the support structure of a 3D-printed titanium alloy acetabular cup has a roughness of Ra12 μ m because powder sticks to it. To reduce wear on bone tissue, this needs to be sanded using an abrasive belt to Ra1.6 μ m.
Data support: The SLM process prints Inconel 718 alloy with a roughness of Ra8–15 μ m on the surface. After milling, this roughness is reduced to Ra0.8–1.6 μ m, and the fatigue life is extended by three times.
2. Not enough dimensional accuracy
Important measurements include the aperture, slot width, step height difference, and so on. For instance, a certain type of turbine blade tenon groove has a width tolerance of ± 0.05mm, but after 3D printing, the variation is ± 0.2mm, which has to be fixed by wire cutting (WEDM).
In the case of Siemens Energy's gas turbine guiding vanes, 3D printing and five-axis linkage milling technology are used to keep the blade shape's thickness deviation under ± 0.05mm, which improves airflow efficiency by 2%.
3. Fixing defects inside
There are different kinds of defects, such as porosity, lack of fusion, cracks, and so on. For instance, if X-ray examination shows faults that are worse than normal in important load-bearing parts of aviation structures, they need to be fixed by drilling, welding, and machining. After getting rid of faults through local milling, the 3D printed section of the landing gear outer cylinder of a certain type of aircraft is fixed by electron beam welding. Then, heat treatment gets rid of any residual stress.
三, Examples of how the industry is used and how it is used in real life
1. The field of aerospace
Parts of the engine: Rolls Royce UltraFan ® The engine fan frame is made of a 3D-printed titanium alloy and has installation holes that need to be bored to make sure they are in line with the bearings. This cuts vibration values by 40%.
Satellite structural components: 3D-printed aluminum alloy parts of a certain type of satellite bracket. The support residue was eliminated using CNC machining, which made the parts 15% lighter while still fulfilling space-grade vacuum sealing standards.
2. Implants for medical use
Personalized joint: To get a Ra0.2 μm smoothness on the femoral condyle surface of Johnson & Johnson DePuy Synthes' 3D printed knee joint implant, the surface must be ground with extreme precision. This makes the bone cement wear less quickly.
Dental implants: Nobel Biocare's 3D-printed titanium alloy implants need micro milling to get rid of powder that sticks to the root of the threads. This makes them 25% more stable at first.
3. Tools for energy
Nuclear power valves: The nickel-based alloy valves made by China National Nuclear Corporation need laser cladding and grinding to keep them from leaking at a high temperature of 650 °C. They last twice as long as regular castings.
Fuel cell bipolar plate: The 3D-printed stainless steel bipolar plate for the Toyota Mirai fuel cell needs chemical etching and polishing of the flow channel to lower the contact resistance from 10m Ω· cm ² to 1m Ω· cm ². This makes the system 8% more efficient.