How to achieve high-precision tolerance through post-processing?

一, Technical principle: A tolerance correction mechanism that works with different physical fields to work together.
Post-processing technology uses the combined impacts of mechanical, chemical, thermodynamic, and other physical fields to change the microstructure and improve the performance of parts that have been processed. There are three main groups of its key principles:
Fixing mechanical stress alleviation
When you machine metal pieces, they will create residual stress, which will cause them to change shape. For instance, following laser melting, the internal tension of titanium alloy parts made with 3D printing might be as high as 200 to 300 MPa. The tolerance deviation could be more than 0.05 mm if stress reduction is not done. By applying vibration at a certain frequency (typically between 15 and 100 Hz), vibration aging technology rearranges the microscopic grains and speeds up the stress release rate to over 85%. A German aerospace manufacturer used this method, and the qualification rate for satellite parts went from 85% to 95%. The tolerance fluctuation range was also cut down to ± 0.003mm.
Selective correction of chemical dissolution
By adjusting the rate of anodic dissolution, electrolytic polishing technique makes the surface micro geometric morphology more uniform. For example, treating the inner cavity of 316L stainless steel in a phosphoric acid and sulfuric acid mixed electrolyte with a voltage of 15V for 3 minutes can lower the surface roughness from Ra2.5 μ m to Ra0.4 μ m and fix the tolerance deviation from ± 0.02mm to ± 0.005mm. This approach works best on complicated inner cavity structures, like the micro hole treatment of car fuel injectors, which can get rid of leftover burrs from machining and make sure that fuel injection is even.
Correction of thermodynamic phase transitions
The heat treatment technique changes the material's crystal structure by managing the heating and cooling curve, which fixes dimensional tolerances. For instance, T6 thermal treatment (540 °C solution + 175 °C aging) can cut the coefficient of linear expansion of aluminum alloy parts by 12% and enhance their dimensional stability by 30%. This procedure is used by a US engine maker to treat turbine discs. It cuts the tolerance fluctuation range from ± 0.03mm to ± 0.01mm and extends the fatigue life to 2.5 times the original.
二, Process implementation: exact answers for each situation
1. Processing of metal items that were 3D printed
Metal 3D printing technologies like SLM and EBM may make complicated structures, but the surface roughness is normally Ra10-20 μ m, and there are problems like unfused powder. To regulate tolerances after processing, you need to do three things:
To remove the support structure, utilize waterjet cutting or electrical discharge machining (EDM). This will keep the shape from changing because of mechanical clamping. For instance, GE Aviation employs EDM to precisely remove supports and keep tolerance errors within ± 0.008mm when making fuel nozzles for LEAP engines.
Treatment for surface densification: Hot isostatic pressing (HIP) is used on porous materials. After 4 hours of treatment at 1200 °C and 150 MPa, the porosity may be decreased from 5% to 0.1%, and the rate of dimensional shrinkage can be kept at 0.3% to 0.5%, which ensures accuracy in tolerances.
Precision polishing: By using abrasive flow polishing technology, the roughness of the inner cavity can be decreased from Ra12 μ m to Ra0.8 μ m by treating it with silicon carbide abrasive at a pressure of 0.5MPa for 10 minutes. The tolerance fluctuation must stay below ± 0.005mm.
2. After processing CNC machined components
Even though CNC machining can be very precise, things like tool wear and thermal distortion can still cause tolerance errors. Post-processing must be integrated with the subsequent technologies:
Intelligent tool compensation: Sensors keep an eye on variations in tool diameter in real time and automatically adapt the cutting routes. For instance, the Fanuc CNC system can automatically fix the coordinate values when the tool wears down by 0.03mm, making sure that the aperture tolerance stays at ± 0.005mm.
Low-temperature cooling treatment: During processing, spray liquid nitrogen at -40 °C continuously to keep the workpiece's temperature from changing by more than 2 °C. This keeps the workpiece from expanding too much and causing dimensional variation. The tolerance certification rate of thin-walled parts went up from 78% to 95% after a Japanese company that makes precision parts used this method.
Calibration with a laser interferometer: Use a laser interferometer on a regular basis to check how accurately the machine tool is positioned and fix any geometric faults using compensation algorithms. After calibration, for instance, the accuracy of the spatial placement of a five-axis machining center might go from 0.015mm/1000mm to 0.005mm/1000mm.
3. After treatment of composite material parts
After processing, composite materials (such plastics reinforced with carbon fiber) are likely to have faults like delamination and burrs. Tolerance control must be done by post-processing.
Ultrasonic cleaning: Cleaning using ultrasonic waves at a frequency of 40 kHz for 10 minutes can get rid of more than 90% of processing residues. This stops particle embedding during assembly from causing tolerance deviation.
Laser polishing: Using a nanosecond laser (pulse width 100ns) to micro-process the edges, taking off between 0.001 and 0.005mm of material, and fixing the tolerance variation from ± 0.05mm to ± 0.01mm.
Vacuum hot pressing treatment: Hot pressing for 30 minutes at 180 °C and 5 MPa in a vacuum can get rid of stress concentration in the composite material and make it 40% more stable in terms of size.
三, Industry application: common examples in the high-end manufacturing sector
1. The field of aerospace
After the SLM technology is used to make the engine blades of the Boeing 787 Dreamliner, the following post-processing steps are used to regulate the tolerances:
For HIP treatment, heat the material to 1250 °C and 170 MPa for 6 hours to get rid of internal pores and keep the size shrinkage rate at 0.4%.
Electrolytic polishing: Use a phosphate-based electrolyte with 12V of voltage for 5 minutes to make the surface smoother, going from Ra15 μm to Ra0.2 μm, and fix the tolerance deviation from ± 0.03mm to ± 0.005mm.
Laser measurement: A three-coordinate measuring machine (CMM) is used to check blades in full size, and reverse engineering is performed to fix the machining route so that the tolerances are right.
2. In the business of making cars
In the making of hybrid transmission valve bodies, Toyota uses the following post-processing methods:
Electrolytic deburring: Use a current density of 10A/cm² in NaCl electrolyte for 2 minutes to get rid of burrs at the cross holes and make sure the hydraulic system is sealed.
Abrasive flow polishing: Use 800 mesh silicon carbide abrasive at a pressure of 0.3MPa for 3 minutes to make the inner cavity less rough, going from Ra3.2 μ m to Ra0.4 μ m, with a tolerance range of less than ± 0.008mm.
Online detection: Adding a laser scanner to the processing line to keep an eye on the aperture size in real time, changing the processing parameters based on feedback control, and raising the tolerance pass rate to 99.2%.
3. Field of medical devices
The following post-processing steps help Johnson&Johnson DePuy Synthes make acetabular cups that are both biocompatible and accurate in terms of tolerance:
Electrolytic polishing: Lower the surface roughness of the Ti6Al4V substrate from Ra3.2 μm to Ra0.2 μm and get rid of the particles that weren't fused during SLM molding.
Micro arc oxidation: Use a 300V voltage in a silicate electrolyte for 5 minutes to make a 20 μ m thick oxide coating that contains hydroxyapatite. This makes the bone bonding strength 40% stronger and keeps the tolerance deviation within ± 0.005mm.
Aseptic packaging: Parts are sterilized with ethylene oxide to make sure they meet ISO 13485 standards before they are put together. This keeps contamination from changing the size of the parts.