Mirror Finishing Technology for Ultra-Precision CNC Machining of Multi-Materials

As high-end manufacturing evolves toward higher precision, superior surface quality, and greater reliability, mirror finishing has become one of the key technological indicators of a nation’s manufacturing capabilities.

Mirror-finished surfaces feature extremely low surface roughness, high flatness, high gloss, and high dimensional accuracy.

They effectively reduce component friction loss, enhance fatigue strength and sealing performance, improve corrosion resistance, and ensure precise fit, making them widely used in critical fields such as aerospace, precision molds, optical instruments, and electronic chips.

Mechanical mirror finishing is achieved through removal processes such as cutting, milling, turning, rolling, and grinding.

Unlike traditional manual polishing and electrochemical polishing, it offers advantages such as high efficiency, good consistency, suitability for complex surfaces, and the ability to implement digital control.

Building on existing precision vertical machining centers and the achievement of Ra 0.04 μm standard mirror-finish machining, this paper focuses on multi-material mirror-finish milling technology, conducting systematic research on staged processes, parameter optimization, and quality control.

Currently, the main challenges in mirror finishing lie in equipment precision stability, the preparation of ultra-hard tool edges, and the coordinated optimization of process parameters.

Based on this, this paper focuses on milling and rolling mirror finishing technologies to develop a systematic technical solution addressing the machining needs of multiple materials and precision grades, which holds significant engineering application value.

Research Scope and Technical Approach

• Main Research Focus

Existing research has not established a tiered machining scheme tailored to different roughness requirements.

There is a lack of comparative machining studies on red copper and die steel, and process parameters are insufficiently adapted to actual equipment.

Based on actual machining results, this paper establishes a multi-material and multi-grade mirror finishing system;

Optimizes process parameters and machining strategies to address the challenges of insufficient mirror finishing accuracy and non-standardized processes;

Formulates a tiered machining strategy and optimizes process parameters for various materials and roughness grades;

Analyzes factors affecting mirror finishing quality and error control methods to validate the feasibility of the process scheme;

And outlines future trends in mirror finishing technology while proposing directions for technological breakthroughs.

• Approccio tecnico

This paper adopts a technical approach consisting of “theoretical analysis—classification system—process design—application validation—future trends.”

It establishes a classification system based on national standards, develops process solutions in conjunction with existing equipment, analyzes machining errors and key quality control points, and ultimately establishes a comprehensive technical system for mirror-finish machining.

Theoretical Foundations of Mirror Finishing and the Development of a Grading System

• Concepts and Technical Characteristics of Mirror Finishing

Mirror finishing is an ultra-precision machining technique that removes minute amounts of material from the workpiece surface through processes such as precision turning, milling, grinding, rolling, and polishing.

It achieves a surface roughness of Ra ≤ 0.1 μm while ensuring high flatness, a minimal surface-altered layer, and the absence of microscopic scratches and pits.

Compared to traditional finishing processes, mirror finishing possesses three key technical characteristics:

First, the finishing allowance is extremely small, with the cutting depth during the finishing stage typically at the micrometer or even nanometer level;

Second, the equipment and cutting tools must meet extremely high precision requirements; spindle speed, positioning accuracy, tool dimensional accuracy, and tool edge radius must all reach precision or ultra-precision levels;

Third, a highly controllable machining environment, where environmental factors such as temperature, vibration, and dust must be strictly managed to prevent any adverse effects on surface quality.

• Analysis of Mirror Finishing Properties  

The physical and mechanical properties of different materials—including hardness, ductility, and thermal conductivity—directly determine the difficulty of mirror milling and the corresponding process strategies.

This paper selects four common materials—red brass, die steel (SKD61), aluminum alloy, and stainless steel—to analyze their mirror finishing characteristics, thereby providing a basis for subsequent optimization of process parameters.

Red Copper: With good ductility, low hardness (HV 55–70), and high thermal conductivity, this material is prone to chip buildup during machining, leading to increased surface roughness.

It is considered a material that is “easy to cut but difficult to finish” in mirror milling.

Die Steel (SKD61): After heat treatment, it exhibits high hardness (HRC 48–52), good thermal stability, and resistance to thermal fatigue.

However, the cutting process involves high cutting forces and rapid tool wear.

Additionally, due to the material’s high toughness, burrs are prone to form, making it a difficult-to-machine material for mirror milling.

Aluminum Alloy: With low density, good thermal conductivity, and low hardness, it generates low cutting temperatures during machining and is less prone to chip buildup.

Mirror finishing is less challenging than with red copper or tool steel, making it suitable for high-speed mirror milling;

However, due to impurities or silicon crystallization, its machinability limits are lower than those of red copper.

Stainless Steel: High hardness, high toughness, and low thermal conductivity result in high cutting temperatures during machining.

Tools are prone to adhesive wear, and the surface is susceptible to work hardening.

The difficulty of mirror finishing is second only to that of die steel.

The fundamental differences between mirror milling, rolling, and traditional polishing are shown in the table below.

• Establishment of a Grading System 

This paper takes GB/T 1031-2009, “Geometric Product Specifications (GPS):

Surface Roughness Parameters and Values Based on the Surface Profile Method” (replacing GB 1031-83), as its primary basis.

The national standard does not include specific standards for mirror finishes. It only provides equivalent descriptions.

This study classifies mirror finishes into four grades based on Ra values. The classification follows equivalent standards and engineering applications.

This paper focuses on mirror finishing methods achieved using machining center equipment.

Development of Mirror Finishing Solutions

• Analysis of the Current State of Mirror Finishing 

1. Mirror Finishing of Red Copper

Red copper is soft and highly ductile, making it prone to tool adhesion and burr formation during machining;

It is a material that is notoriously difficult to control in mirror finishing.

Internationally, Japanese companies use natural diamond cutting tools and ultra-precision milling processes to achieve an ultimate mirror finish precision of Ra 0.01 μm, with strong process stability and high consistency in batch production;

Domestic enterprises and research institutions face constraints from tooling and process limitations. These limitations restrict machining performance.

They typically achieve stable machining accuracies of Ra 0.02–0.03 μm. Surface scratches and roughness fluctuations frequently occur during mass production.

This research proposal uses only carbide milling cutters and optimizes process parameters to stably achieve “standard mirror finish” machining with Ra 0.025–0.05 μm, demonstrating feasibility for engineering implementation.

2. Mirror Finishing of Die Steel

Die steel has high hardness (HRC 48–52) and excellent wear resistance, requiring the use of super-hard cutting tools for machining.

German companies have achieved a mirror finish of Ra 0.02 μm using fine-grain PCBN tools and five-axis machining centers, and are capable of machining complex curved surfaces;

In China, mirror finishing of mold steel primarily relies on grinding and polishing, with direct milling achieving a surface roughness of only Ra 0.08–0.1 μm and low machining efficiency;

This study focuses on PCD tools while balancing machining efficiency, aiming to achieve a “standard mirror finish” of Ra 0.04 μm, thereby improving direct machining accuracy and reducing the need for subsequent polishing processes.

3. Aluminum Alloys and Stainless Steel

Internationally, mirror finishing of aluminum alloys can consistently achieve a surface roughness of Ra 0.02 μm, while mirror finishing of stainless steel can reach Ra 0.03 μm;

In China, the machining precision for aluminum alloys ranges from Ra 0.02 to 0.05 μm, while stainless steel is prone to work hardening, resulting in a precision of only Ra 0.05 to 0.1 μm;

This proposal focuses on red copper and die steel as core materials, with aluminum alloys and stainless steel serving only as points of comparison; they will not be studied in depth at this time.

• Core Requirements and Selection Criteria

Mirror-finish machining places stringent demands on a machine’s static accuracy, dynamic performance, and environmental adaptability.

The precision machining center selected in this article must meet the following core specifications:

Motion Accuracy: 5-axis simultaneous control, positioning accuracy ≤ 0.004 mm, repeatability ≤ 0.002 mm, linear axis feed resolution ≤ 1 nm;

Spindle System: Air-bearing spindle / fluid-bearing spindle, rotational speed ≥ 30,000 r/min, radial runout ≤ 0.5 μm, vibration-free and free of radial play;

Environmental Control: Ambient temperature 18–22 °C, equipped with a system-wide temperature control system to prevent dimensional deviations caused by thermal deformation; equipped with an anti-vibration foundation to isolate external vibration interference.

• Selection of Cutting Tool Materials and Determination of Geometric Parameters

Cutting tools are the core components in mirror finishing and must be selected based on the workpiece material’s hardness, ductility, and machinability.

This paper establishes a tool selection system categorized by material and grade:

1. Cutting Tools for Red Copper and Aluminum Alloys

Red copper and aluminum alloys have high plasticity and high viscosity, making them prone to sticking to the tool.

Therefore, cutting tools with smooth chip evacuation and sharp edges should be selected: Mist-Finish Mirror \ Standard Mirror: Select YG6 or YG8 tools with a large front angle (5°–10°) and a large back angle, ensuring the cutting edge is free of dullness and the radius does not exceed 0.003 mm to enhance sharpness.

Use high-flow extreme-pressure coolant to prevent tool sticking and wear, ensuring machining efficiency;

Pursuing Perfection: 

Further Finishing: Select polycrystalline diamond (PCD) tools with a cutting edge radius < 100 nm.

Combined with high spindle speeds and micro-lubrication, this enables ultra-precise mirror-finish machining.

Table 1. Chemical composition test results (mass fraction) of the fractured bolts.

Table 2 Vickers hardness test results of fractured bolts

2. Cutting Tools for Die Steel

Die steel is characterized by high hardness and high cutting temperatures, while stainless steel has a strong tendency toward work hardening and is prone to chip buildup and tool sticking.

Therefore, the selection of cutting tools differs for each material. Based on the focus of this study, stainless steel cutting tools are not recommended at this time.

Near-mirror finish: Select nano-coated carbide tools. The coating features high hardness and high-temperature resistance, with carbide matrix particles no larger than 1 μm.

Use a negative rake angle of -5° to 0°, and apply edge strengthening to enhance the cutting edge’s impact resistance.

Matte Mirror Finish: Select fine-grain PCBN tools with a grain size of ≤ 1 μm.

A negative rake angle design enhances edge strength, making them suitable for machining high-hardness materials and preventing tool chipping.

The clearance angle is typically 12˚ to 15˚ to reduce friction between the tool and the workpiece surface.

Standard Mirror Finish: Utilizes edge-free diamond-coated tools that have undergone precision polishing, with a substrate made of ultra-fine-grained cemented carbide containing low cobalt content.

3. Tool Clamping and Dynamic Balancing Control

In mirror finishing, tool clamping errors and insufficient dynamic balancing can cause vibrations, leading to surface scratches and increased surface roughness.

Therefore, the following control measures must be implemented: First, use hydraulic tool holders or heat-shrink tool holders with a clamping repeatability of ≤ 0.001 mm to prevent radial runout;

Second, perform high-speed dynamic balancing on the tool-holder assembly, achieving a balance grade of ≥ G1 (with a minimum of G2.5) to eliminate vibration sources during high-speed rotation;

Third, control tool overhang to no more than three times the tool diameter, minimizing overhang length as much as possible to enhance tool rigidity.

• Staged Machining Strategy and Process Parameter Optimization

1. Development of a Stepwise Progressive Machining Strategy

High-precision mirror-finish machining cannot be achieved directly; instead, a stepwise machining strategy involving progressive removal and refinement must be adopted.

Four processes—roughing, semi-finishing, finishing, and superfinishing—form the machining sequence. Each process gradually reduces cutting depth, feed rate, and pitch.

Each process eliminates surface defects from the previous stage. The sequence achieves the target surface roughness.

The specific division of processes is as follows:

Lavorazione grezza:

Removes most of the workpiece’s stock, machining to near-final dimensions with a single-side allowance of 0.2–0.5 mm.

Utilizes a combination of deep cutting depth, high feed rate, and high spindle speed, prioritizing machining efficiency to establish the dimensional foundation for subsequent processes.

The surface roughness of mold steel is generally machined to Ra 1.6–3.2 μm, while that of red copper is machined to Ra 0.8 μm.

Semi-finishing Process:

Eliminates rough machining marks and surface deformation, ensuring uniform stock allowance, especially at corners.

The tool radius used is no larger than that of the finishing tool.

The single-side allowance is 0.05–0.1 mm.

Parameters balance efficiency and surface quality, providing a flat reference surface for finishing.

The surface roughness of die steel is generally machined to Ra 0.8 μm, and that of red copper to Ra 0.4 μm.

Processo di finitura:

The core forming process. Machining uses tools with a radius smaller than the corner radius. The process applies minimal cutting parameters.

It removes the semi-finishing allowance. It achieves a near-mirror finish.

The equipment used should be a high-speed machining center, with tool dynamic balance at G2.5.

Subsequent mirror finishing requires a single-side allowance of 0.003–0.02 mm. The process emphasizes surface precision control.

Machining generally achieves a surface roughness of Ra 0.2–0.4 μm for die steel. Machining achieves Ra 0.1 μm for red copper under the same requirements.

Ultra-finishing Process: Eliminates microscopic tool marks and defects from the finishing process to achieve an ultra-precise mirror finish.

Machining requirements determine the selection of different cutting tools for the material. Machining requirements also determine the use of different machining methods.

2. Optimization of Process Parameters by Material

Mirror finishing follows the basic principles of shallow cutting depth, high spindle speed, small feed rate, climb milling, narrow pass spacing, and intensive cooling and lubrication.

These principles aim to minimize residual height. They suppress vibration. They reduce thermal damage.

Integration of tool selection, machine performance, and staged machining objectives supports the optimization of core process parameters for each material and grade.

Core process parameters include cutting speed, feed per tooth, cutting depth, and tool path spacing. Extensive process trials determine the optimized parameter values.

This resulted in standardized process parameter tables; for example, when using a bullnose cutter, refer to Tables 3 and 4.

Table 3: Process Parameters for Mirror-Finishing of Pure Copper

Table 4: Process Parameters for Mirror Finishing of Die Steel

3. Optimization of Cooling and Lubrication Processes

Pure copper and aluminum alloys have high viscosity, making chip buildup likely during machining;

Mold steel and stainless steel generate high cutting temperatures, which can lead to tool wear and surface thermal deformation.

Therefore, cooling and lubrication methods must be optimized: For rough machining, use high-pressure, high-flow coolant to reduce cutting temperatures;

Finishing and superfinishing operations use minimum quantity lubrication (MQL), oil-air lubrication, or high-precision filtered coolant (filtration precision ≤ 2 μm).

These lubrication methods minimize coolant residue and surface contamination. They improve tool lubrication performance. They prevent tool sticking and surface scratches.

4. Ottimizzazione del percorso utensile

Toolpaths directly affect surface finish and flatness. For mirror-finish machining, the following toolpath strategies should be considered:

(1) Use down-cut milling to reduce cutting forces and workpiece deformation, and avoid burrs caused by up-cut milling;

(2) Use circular or unidirectional toolpaths instead of reciprocating toolpaths to eliminate feed pause marks caused by direction changes;

(3) For complex surfaces: Use 5-axis RTCP (Rotary Tool Path Control) to ensure stable normal orientation;

(4) Calculate the pass spacing based on the ball-nose radius and surface requirements; smaller pass spacing results in finer surface texture and lower residual height;

(5) When machining flat surfaces with a ball-nose cutter, ensure the pass spacing matches the cutter tip radius (pass spacing ≤ 1/5 of the cutter tip radius) to prevent visible tool marks on the surface.

Machining Validation Analysis

• Test Conditions and Inspection Methods

1. Test Conditions

(1) Test Equipment: High-speed five-axis precision machining center (positioning accuracy 0.004 mm (VDI), maximum spindle speed 36,000 rpm);

(2) Test Materials: T2 red copper, SKD61 die steel (HRC 48–52)

(3) Test Tools: Carbide tools, edge-less PCD tools, fine-grain CBN tools, nano-coated carbide tools;

(4) Machining Environment: Constant temperature of 20±2°C, vibration-free workshop.

2. Metodi di prova

A contact-type roughness tester (measuring range: Ra 0.01–12.5 μm) measures surface roughness. The process takes the average of three measurement points.

A coordinate measuring machine measures dimensional accuracy.

• Risultati e analisi dei test

1. Results of Red Copper Machining Tests

Ultra-finishing parameters described in this paper consistently maintain the surface roughness of T2 red copper at Ra 0.03–0.04 μm.

The surface was free of scratches, burrs, and built-up edges, meeting the standard for a mirror finish.

2. Results of Die Steel Machining Tests

After precision machining with a CBN milling cutter, SKD61 die steel (HRC 60) achieved a surface roughness of Ra 0.07–0.09 μm with no micro-scoring.

It meets the requirements for precision molds without the need for subsequent polishing, representing a 1–2-grade improvement in precision compared to traditional Chinese processes.

SKD61 die steel (HRC60) finished with a blade-less PCD tool achieved a surface roughness of Ra 0.04–0.05 μm with no micro-scoring.

It met the requirements for precision dies without the need for subsequent polishing, representing a 2–3-grade improvement in precision compared to traditional Chinese processes.

• Summary of the Experiment

The multi-stage machining strategy, process parameters, and quality control plan developed in this paper can reliably achieve mirror-finish machining targets for various materials and grades.

The process plan is feasible, stable, and has practical engineering value, contributing to the enhancement of mirror-finish machining capabilities.

Particularly in the machining of red copper, the use of carbide tools ensures controllable costs, efficiency, and quality.

Industry Outlook and Conclusions 

• Tendenze future 

1. Nanoscale Fabrication Technology 

Future cutting tools will evolve toward nanoscale precision at the cutting edge and the use of composite materials (such as diamond-PCBN composites).

The edge radius of natural diamond cutting tools will break through the 20 nm barrier, while the grain size of PCBN tools will be further refined.

Composite coating technology will enhance tool wear resistance, enabling higher-precision mirror finishing.

2. Integrated Application of Assisted Machining Technologies

Ultrasonic-assisted milling technology applies high-frequency vibrations to the cutting tool, significantly reducing cutting forces, minimizing tool wear, and suppressing work hardening.

It is particularly suitable for mirror-finish machining of difficult-to-machine materials such as die steel and stainless steel, and will eventually be fully integrated with ultra-precision machine tools.

3. Intelligent Systems and Closed-Loop Control

The machining equipment integrates AI algorithms, online inspection, and real-time error compensation systems.

It enables autonomous optimization of process parameters. It controls surface quality automatically. Consistency of mirror-finish machining is enhanced.

It improves the intelligence level for complex curved surfaces and irregularly shaped components.

4. Technological Self-Reliance and Control

Overcome bottlenecks in core components for ultra-precision equipment, high-end super-hard cutting tools, and specialized process software;

Reduce reliance on foreign technologies for high-end mirror-finishing; and establish a self-reliant and controllable full-industry-chain technology system.

5. Applications of Digital Twin Technology

AI is used to build a digital twin model for mirror milling, enabling virtual simulation of the machining process and parameter optimization.

This approach conducts preliminary debugging of process parameters in a virtual environment.

It reduces the costs and time associated with physical testing. It improves the efficiency of process optimization.

• Conclusioni della ricerca

This paper conducts a systematic study on mirror finishing techniques in mechanical machining and draws the following key conclusions:

(1) Mirror finishing must adopt GB/T 1031-2009 as a unified evaluation system and use Ra as a quantitative indicator to ensure standardization and repeatability.

(2) This study clarifies the application scenarios and technical requirements for each grade. It defines the fundamental differences between mirror milling, rolling, and traditional polishing.

Machining processes adapt to the precision requirements of various industrial scenarios. It achieves a balance between machining efficiency and surface quality.

(3) This study establishes a tool selection system suitable for red copper and die steel. It optimizes tool geometric parameters and clamping processes. It provides a core operational foundation for mirror finishing.

(4) This study formulates a graded, progressive machining strategy. Experimentation optimizes process parameters for various materials and grades.

The process establishes standardized process protocols.

Given the significant differences in machinability among materials, a precise match between material, tool, and parameters is essential to consistently achieve nanometer-level surface finishes.

(5) Experiments and research demonstrate that this process protocol can consistently achieve the target surface roughness.

It can improve machining yield and efficiency. It shows strong practical engineering value. This contributes to the enhancement of mirror finishing capabilities.

• Limitations of the Study and Future Prospects

This study focuses on the machining of flat and curved mirror surfaces; however, further research is needed on processi di lavorazione for straight cavities and microstructures.

Moving forward, we will integrate practical applications to further advance mirror machining technology toward greater cost-effectiveness, broader application scenarios, and greater autonomy.