How to combine metal 3D printing and CNC machining in the most reasonable way?

一, Technological complementarity: a logical change from "opposition" to "symbiosis"
Metal 3D printing (using SLM/DMLS technology as an example) uses a laser to melt metal powder layer by layer, making it possible to build complicated interior structures all at once. Its main benefits are:
Breakthrough in structural degrees of freedom: able to make lattice structures, conformal cooling channels, uneven surfaces, and other things that typical CNC machines can't. For instance, a certain hydraulic valve body gets staggered oil circuits through 3D printing, which makes the flow channel 300% more complicated. CNC machining needs many clamps and is hard to make sure it seals.
Additive manufacturing doesn't waste any material, and the material utilization rate can be above 90%, which is far greater than the 50% to 70% rate for CNC machining.
The capacity to quickly iterate: After changing the digital model, it can be printed right away without having to be remolded. This has cut the time it takes to develop new goods from months to days.
But the initial accuracy (± 0.04mm) and surface roughness (Ra12.5 μ m) of 3D printing make it hard to meet the needs of high-precision assembly. This is when CNC machining becomes very important:
Size correction: To make up for shrinkage deformation during printing, you should mill the machine tool guide surface to an accuracy of ± 0.02mm.
Surface finishing: Precision milling raises the surface roughness from Ra12.5 μ m in the as-cast state to Ra1.6 μ m, and mirror polishing can even raise it to Ra0.2 μ m.
Key feature machining: CNC is great at doing all kinds of local machining, like making end faces with high precision and threaded holes with high precision.
二, A common use case is when you need to meet both complicated structure and precision requirements.
1. In the aerospace business, there needs to be a balance between being light and being able to carry a lot of weight.
One aerospace business uses the "3D printing+CNC" method to make engine combustion chambers:
3D printing process: Printing complicated shapes with conformal cooling channels out of Inconel 718, a nickel-based high-temperature alloy. This makes the structures 35% lighter and able to handle temperatures up to 1200 °C.
CNC process: Ultra-precise machining of the sealing surface to a flatness of 0.01mm to make sure it works well in high-pressure situations.
Effect verification: The production cycle is 60% shorter than with standard casting and welding methods, and the fatigue life is twice as long.
2. Medical implants: a mix of personalization and biocompatibility
How titanium alloy orthopedic implants are made:
3D printing: Using CT data from the patient, print a porous femoral stem with a porosity of 60% to 80% and a pore size of 200 to 500 μm. This will mimic the shape of natural bone trabeculae.
CNC machining: precise milling of the conical mating surface that touches the bone marrow cavity to make sure it meets H7 level tolerance and achieves biological fixation.
Surface treatment: Sandblasting and anodizing make the surface rougher, which helps bone cells stick to it.
3. Industrial molds: a good balance between complicated flow channels and good cooling
A certain mold company uses a blended manufacturing solution:
3D printing makes a mold core with three layers of internal cooling channels all at once. This makes cooling 30% more effective and fixes the problem of leaks that happens with standard block splicing.
CNC machining: polish the parting surface to Ra0.4 μm to make it easier to remove plastic parts.
Cost comparison: The cost per piece has gone down by 42%, and there is no need to worry about mold scrap from welding distortion.
三, Path of process integration: improving the whole process from design to post-processing
1. Design phase: Optimize the topology depending on the limits of the manufacturing process.
DFAM (Design for Additive Manufacturing): Using a lattice structure generating method to cut the weight in half while keeping the strength.
Reserved machining allowance: Set aside 0.3–0.5 mm for elements that need CNC finishing, like assembly surfaces and hole placements. This will keep printing layer patterns from influencing accuracy.
Optimization of the support structure: Use simulation analysis to cut down on the quantity of support while making sure that CNC tools are still easy to get to. For instance, the support for a certain aviation bracket is placed on the non-machined surface, which cuts the CNC machining time by 30%.
2. Printing stage: Working together to regulate settings and do post-processing
Choose spherical powder (flowability>30s/50g) to make the powder distribute more evenly and lower the porosity to less than 0.5%.
The heat treatment technique includes stress relief annealing at 650 °C for 2 hours and hot isostatic pressing (HIP) to raise the density to above 99.9%.
Direction control: Use Magics software to find the best angle for placing elements to cut down on the amount of support needed for hanging constructions.
3. The CNC machining stage: five-axis linkage and smart compensation
Five-axis machining center: The Siemens 840D system is used to clamp and machine complex surfaces in one go, which prevents mistakes in positioning.
Digital twin technology: using Vericut simulation to predict how machining will change and making adjustments to the model ahead of time. For instance, simulation improved the contour accuracy of a given turbine blade from ± 0.05mm to ± 0.02mm.
On machine inspection: Using Renishaw probes to keep an eye on machining dimensions in real time and fix mistakes that happen because of tool wear.
4. Stage of surface treatment: combining functionalization and ornamentation
Sandblasting treatment: Use 120 mesh glass beads to make the surface roughness Ra3.2 μm to help the coating stick better.
Micro arc oxidation: Make a ceramic coating 10 μm thick on the surface of titanium alloy. The film is 1000HV hard and five times more resistant to wear.
PVD coating: Putting on a TiN coating makes the surface harder (2200HV) and gives it a golden look.