Is it necessary to perform CNC precision machining after metal 3D printing?

1. The core of technology: the ability of additive and subtractive materials to work together
The main distinction between CNC precision machining and metal 3D printing is what makes them work together in the manufacturing process:
Different Ways to Form
Metal 3D printing makes things by melting metal powder one layer at a time. The surface shows typical layer patterns and melt pool traces. There may be problems with the microstructure, such as powder that hasn't fused and micropores. CNC precision machining uses tool cutting to remove materials, creating a mirror effect with a Ra0.8 μm or below, and it can manage dimensional tolerances to within ± 0.01 mm.
Limit of process capabilities
3D printing has some benefits. For example, it can make complicated structures that are hard to make using older methods, like conformal cooling channels, lattice weight reduction structures, and cavities with many angles. For instance, a certain aviation engine blade uses 3D printing to make the inside hollow, which cuts the weight by 40% while keeping the strength of the structure.
Benefits of CNC machining: More efficient machining for common forms like planes and cylinders, and no need to cope with leftover supporting structures. For instance, CNC milling made the surface roughness of a certain car transmission shaft Ra0.4 μm, which met the wear resistance needs for high-speed rotation.
The trend of hybrid manufacturing
The hybrid method of "3D printing + CNC precision machining" is becoming standard in the industry. For instance, a company that makes precision molds utilizes 3D printing to make a mold core with three layers of interior cooling channels. Then, they employ CNC machining to make the cooling process 30% more efficient and cut the delivery time from 14 days to 5 days.
2. Industry demand: various criteria for surface roughness
distinct industries have distinct needs for surface quality, which directly affects the need for CNC precision machining:
Field of aerospace
Parts must be able to handle harsh conditions (high temperature, high pressure, and high stress), and surface flaws can lead to fatigue cracks.
A common example is that CNC machining reduces the roughness of the sealing surface from Ra12 μ m to Ra0.8 μ m when a certain type of rocket engine nozzle is 3D printed. This extends the sealing life at high temperatures from 50 times to 200 times.
Choosing a process: Key parts, such sealing surfaces and mating surfaces, must be CNC precision machined. Non-bearing surfaces can keep printed textures to save weight.
Field of medical implants
Essential requirement: Surface roughness influences the likelihood of bone cell attachment and bacterial proliferation.
A titanium alloy hip joint prosthesis needs to have a surface quality of Ra1.5–2.5 μ m to let the bone grow into it. A certain company makes prosthetic bodies using 3D printing. Then, to make the surface smoother to Ra0.8 μ m, they employ a combination of chemical polishing and CNC polishing. This keeps the microporous structure that was made by printing, which makes the body more compatible with living things.
Choosing the right process: The functional surface needs CNC precise cutting, whereas the structural surface can keep the printed texture.
In the world of consumer electronics
The smoothness of the surface is a key factor that impacts how the product looks and how well it works optically.
In a typical scenario, CNC machining lowered the surface roughness of a 3D-printed mobile phone camera mount from Ra3.2 μ m to Ra0.05 μ m. This made the mount reflect more visible light, from 85% to 92%, which is what laser communication systems need.
Choosing a process: The optical surface needs to be machined with CNC precision, whereas the structural surface can keep the printed texture.
Energy and Mold Industry: The surface quality must strike a balance between being resistant to corrosion and being easy to work with.
A 3D-printed injection mold core for a car was used to make a conformal cooling channel. Then, it was sandblasted to make the surface less rough, going from Ra15 μ m to Ra6.3 μ m. This increased the mold's lifespan from 100,000 uses to 500,000 uses.
Choosing a process: For surfaces that don't touch, you can utilize low-cost methods like sandblasting. For surfaces that do touch, you need CNC precision machining.
3. Cost-effectiveness: The Economic Logic of Choosing a Process
To decide whether to utilize CNC precision machining, you need to carefully look at the technical feasibility, delivery cycle, and cost of production.
Assessment of technical feasibility
Structural complexity: If the product has features that are hard to manufacture with CNC (such cross holes on the inside or thin-walled structures), 3D printing may be the sole choice. For instance, 3D printing makes the full combustion chamber of a given airplane engine, which avoids the stress concentration difficulties that might happen with traditional welding methods.
Requirement for accuracy: CNC precision machining is needed if the tolerance requirement is higher than what 3D printing can do (like ± 0.01mm). For instance, a blank for a high-precision gear is made via 3D printing, and CNC grinding raises the tooth profile accuracy from IT8 level to IT5 level.
Optimizing the delivery cycle
For urgent orders, 3D printing can skip the mold development step and quickly respond to "design print delivery." For instance, a new energy vehicle firm used 3D printing to make a prototype battery pack bracket. The whole procedure, from design to assembly, took only 48 hours.
If the batch size is big (more than 1000 pieces), CNC machining may be cheaper for batch production. A standard parts manufacturing company can use CNC batch processing of aluminum alloy connections to make a single item 60% cheaper than 3D printing.
Control of the total cost of delivery
When using CNC machining, you have to think about implicit expenses like programming, clamping, and mold testing. On the other hand, 3D printing can lower the cost of tools and the risk of design iterations. For instance, 3D printing can make a complicated structural part all at once, cutting down on the time needed to change tools and process paths by 70%.
Material utilization rate: CNC machining normally uses 50% to 70% of the material, while 3D printing can use more than 90%. For instance, 3D printing makes a particular titanium alloy item, which costs 40% less than CNC milling.