Challenge! CNC Turning of Ultra-Thin-Wall Parts at 0.018mm!

Ultra-thin-walled metal component machining serves as a key indicator of precision manufacturing capability.

Although current internationally leading roll forming processes can produce “tearable steel” sheets as thin as 0.03 mm, they have limitations.

These include restricted processing versatility and high dependence on specialized equipment.

This paper focuses on the turning process of 0.018mm-thick aluminum alloy cylindrical components.

The key technical challenges include several critical aspects. 

First, achieving wall thickness tolerances at IT2 to IT3 precision levels.

Second, controlling cylindricity over an 80 mm machining length. 

Third, mitigating the risk of thin-wall tearing caused by micro-Newton-level cutting forces.

No existing literature reports thin-walled turning processes exceeding the 0.018 mm critical threshold.

This paper achieves stable machining of aluminum alloy parts with a wall thickness of 0.018 mm for the first time.

It accomplishes this through innovative fixture design and optimized cutting parameters.

This approach offers new insights for manufacturing complex ultra-thin components.

Machining Characteristics

The turning process for 0.018mm ultra-thin wall components (see Figure 1) represents an extremely challenging and high-precision machining operation.

To date, no publicly documented cases of thin-wall turning below 0.018mm thickness exist either domestically or internationally.

This case exemplifies the cutting edge of advanced manufacturing technology.

Internationally, hand-tearable steel currently ranges from 0.03 to 0.04mm thick, while domestically it measures 0.015 to 0.03mm.

However, manufacturers produce hand-tearable steel via roll forming—a process limited to sheet materials—whereas our approach uses CNC turning, providing greater flexibility.

Machining parts with a 0.018 mm wall thickness imposes stringent demands on equipment precision and process control.

It also requires careful consideration of materials, fixtures, cutting tools, and operator skills.

This paper provides a detailed analysis of the machining process for 0.018 mm wall thickness parts, addressing each of these critical aspects.

Machining Precision Analysis and Machine Tool Selection

0.018mm corresponds to the IT6 precision grade (0.009–0.0185mm) in mechanical machining, representing the tolerance level for CNC turning events in national and international competitions.

An ultra-thin wall thickness of 0.018mm demands machining precision controlled at an extremely high level, with tolerances within 0.005mm or even smaller.

Additionally, with a machining length exceeding 80mm, stringent requirements for part coaxiality, straightness, and other parameters further increase processing difficulty.

This far exceeds the precision scope of ordinary machining, falling within the micrometer-level machining accuracy category.

For machining with a base dimension of 0.018mm wall thickness, conventional lathes cannot meet the required precision.

Taking the Shenyang CA6140 conventional lathe as an example, its longitudinal feed accuracy typically ranges from 0.028 to 0.054mm, making the task unfeasible.

Operators must select a CNC lathe with superior rigidity, stability, spindle accuracy, and X/Z-axis positioning and repeatability precision for the job.

Table 1 presents a comparison of key precision parameters for various machine tools.

Considering machine tool accuracy and stability, the team ultimately selected a turning-milling composite machining center to complete the machining.

Process Innovation and Optimization

• Design of an Expansion-Adsorption Clamping System

Ultra-thin-walled components are highly susceptible to deformation during clamping.

Conventional clamping methods often cause distortion due to uneven or excessive clamping forces, compromising subsequent machining accuracy.

However, for components with wall thicknesses as thin as 0.018mm, any form of clamping force will induce deformation.

The optimal solution is to employ vacuum-adsorption fixtures.

Utilizing specialized materials with differing thermal expansion coefficients, we developed an expansion-adhesion fixture based on vacuum-adhesion principles.

Part clamping relies entirely on expansion and contraction to achieve mounting, disassembly, and interference fit effects.

This securely adheres the workpiece to the fixture, ensuring it remains unaffected by clamping forces beyond cutting forces during machining to prevent deformation.

  √ Expansion-Adhesion Fixture Design Based on Thermal Coefficients

Based on the expansion-adhesion fixture principle, the thermal expansion coefficients of the part and the fixture must differ significantly.

Specifically, the fixture’s coefficient should exceed that of the part.

Given the thermal expansion coefficient of the machined material—6061 aluminum alloy—is 23.6×10⁻⁶/℃, a material with a higher thermal expansion coefficient than 6061 aluminum alloy must be selected to meet the requirements of the expansion-adhesion fixture.

PA6 nylon has a thermal expansion coefficient of 75×10⁻⁶/℃, meeting the fixture material requirement. Therefore, PA6/polyamide 6 (nylon) material was selected for the fixture to construct a staged clamping system.

The fixture mates with the part’s internal bore. Through variations in temperature, the part and fixture achieve an interference fit (see Figure 2).

Friction ensures the part remains stationary during machining, eliminating clamping forces and preventing radial and axial deformation.

Operators must control the fixture’s dimensional accuracy within (68 ± 0.003) mm to prevent uncontrollable wall thickness variations during machining.

    √ Stage-Specific Fixturing for Roughing and Finishing

Due to substantial machining allowances and high material removal rates during roughing, coupled with significant fluctuations in cutting forces and heat generated, the custom-made expansion-adhesion fixture cannot complete the entire process in a single setup.

Separate fixtures are used for roughing and finishing operations.

Rough Machining Stage: Heat the aluminum alloy blank to 90°C to form a self-locking structure with the nylon inner support.

Finishing Stage: In addition to controlling fixture dimensions (68±0.003) mm, the fixture’s cylindricity must be ≤0.005 mm and straightness ≤0.005 mm.

Failure to meet these tolerances compromises the machining of the 0.018 mm wall thickness.

Post-machining, a cryogenic disassembly process (utilizing nylon’s higher contraction rate than aluminum alloy at 0°C) enables damage-free separation of the workpiece.

• Tool and Cutting Parameter Optimization

1.　Tool Optimization

For machining 0.018mm ultra-thin-walled parts, tool selection must prioritize cutting edge sharpness and chip evacuation performance.

This paper analyzes the impact of cutting force and heat generation on factors such as tool material, radius, and friction coefficient.

Tool sharpness, material, and parameters critically determine machining quality.

Tool sharpness determines the magnitude of cutting force.

For example, 0.03 mm hand-tearable steel requires only 2–5 N to tear, while 0.018 mm aluminum tears with even less force, making extremely sharp tools essential.

During machining, minimal cutting forces are essential to prevent part tearing. Tool material determines both cutting force magnitude and tool friction coefficient.

Different tool materials and workpiece materials exhibit varying friction coefficients; a lower coefficient results in reduced cutting forces during machining.

We select diamond tools and metal ceramic tools.

Diamond tools are exceptionally suited for machining non-ferrous metals, offering a very low friction coefficient that generates minimal cutting forces.

They maintain excellent cutting performance at high cutting speeds, ensuring machining accuracy and surface quality.

Although cemented carbide tools have a higher coefficient of friction than diamond tools, they are regrindable.

Operators can achieve smooth chip evacuation and reduced cutting forces by adjusting the tool angles and sharpening the cutting edges.

Among tool parameters, the tip radius exerts the most significant influence.

In thin-wall machining, a larger tip radius increases cutting forces and adversely affects surface quality.

Selecting a tip radius between 0.03 and 0.10 mm minimizes cutting forces while ensuring a surface roughness value Ra below 1.6 μm.

2.　Cutting Heat Control

During turning operations, cutting heat causes thermal deformation and expansion in parts.

For components with ultra-thin wall thicknesses of 0.018mm, the effects of thermal deformation and expansion become significantly more pronounced.

If the part expands due to cutting heat, the vacuum-type fixture may fail to secure the workpiece or create slight gaps.

This alters the friction force generated by the interference fit, potentially causing thin-wall tearing and machining failure.

Therefore, establishing the relationship between cutting heat and friction force is essential to ensure successful machining.

Developing a Cutting Force-Heat Coupling Model

F_c=K_ca_pf（1+0.5T_rise/100）　（1）

In the formula, K_c is the specific cutting force coefficient; a_p is the depth of cut (mm); f is the feed rate (mm/r); T_rise is the cutting temperature rise (℃).

Researchers determine the optimal parameter combinations through orthogonal experiments.

K_c is related to the coefficient of friction. The coefficient of friction between diamond tools and aluminum alloys typically ranges from 0.05 to 0.15.

The friction coefficient between cermet tools and aluminum alloys ranges from 0.25 to 0.40, depending on the coating material.

Engineers can derive the cutting force range from the formula to determine the appropriate cutting parameters.

3.　Cutting Parameter Optimization

The three primary cutting parameters—cutting speed, feed rate, and depth of cut—significantly impact machining quality.

For ultra-thin-walled parts as thin as 0.018 mm, excessively high cutting speeds generate substantial heat, causing thermal deformation.

Conversely, excessively low speeds may increase cutting forces, also leading to part deformation or vibration.

Engineers must precisely calibrate the feed rate and depth of cut to extremely small values and fine-tune them based on factors such as the part material and tool material.

Diamond tool: tip radius 0.1mm, γ_o=15°, λ_s=5°. Cermet tool: tip radius 0.03mm, γ_o=15°, λ_s=5°.

Cutting parameters: n=1200 r/min, f=0.01 mm/r, a_p=0.1 mm. Under these parameters, cutting force < 0.5 N, surface roughness Ra=0.76 μm.

Based on the above analysis, the team selected diamond tools and cemented carbide tools to complete the machining.

• Optimization of Turning Process

1) Optimizing Roughing and Finishing Techniques

During rough machining, cutting forces and heat generation surpass those in finishing operations.

Therefore, operators must create substantial interference fits between the workpiece and fixture.

Heating the aluminum expands the bore, allowing it to adhere to the nylon fixture upon cooling. Rough machining reduces wall thickness to 0.5mm.

After removal, operators prepare the workpiece for finishing. They select 6061 aluminum alloy as the material.

The blank has a diameter of 100mm and a length of 110mm, with a process head length of 15mm.

First, the process head and locating end face undergo finishing.

Rough-machine the workpiece’s bore to a depth of 101mm and diameter of 65.96mm using a 20mm-diameter bore tool, leaving a 0.5mm machining allowance on the bore diameter and end face.

Internal bore finishing: Due to machine tool accuracy limitations and turning clearance issues, machining long journals or bores may introduce taper errors.

Therefore, during internal bore finishing, ensure bore cylindricity first.

By adjusting the machining program and parameters, control the taper error within 0.005mm over an 80mm length and the bore diameter error within ±0.005mm.

2) Thermal Expansion Installation of Nylon Inner Supports

Installation of roughing inner support. The roughing inner support is made of nylon material.

Nylon provides adequate support strength while possessing a high thermal expansion coefficient.

The outer diameter of the nylon rod is machined to (65.97 ± 0.005) mm, ensuring the outer diameter of the inner support exceeds the inner bore dimension of the ultra-thin-walled part by 0.02 mm, while maintaining taper error within 0.005 mm.

Heat the aluminum alloy blank with the finished bore to approximately 90°C. The heated blank’s bore diameter increases by 0.05–0.06 mm.

Insert the nylon inner support into the bore at this stage, then allow the entire assembly to cool to room temperature before machining.

3)  Outer Diameter Roughing and Support Removal

Rough turning of the outer diameter. Self-developed turning chuck jaws clamp the positioning journal of the process head.

The runout of the clamped positioning journal must be measured to be <0.01mm. Rough-machine the outer diameter with a 0.5mm finishing allowance.

After removal from the chuck, place it in a cold storage room to cool thoroughly to approximately 0°C.

Due to the differing thermal expansion coefficients of nylon and aluminum alloy, nylon experiences greater dimensional change under identical cooling conditions.

This design allows operators to easily remove the nylon inner support after it cools.

4) Precision Fixturing for Ultra-Thin-Wall Finishing

During the finishing stage, operators must use proprietary turning techniques to assemble the fixture.

This ensures that the fixture’s total runout aligns with the machine tool spindle’s precision, maintaining control within 0.005 mm.

At the same time, operators slightly reduce the interference fit between the workpiece and fixture to approximately 0.01 mm compared to rough turning.

It is crucial to note that operators must exercise extreme caution during clamping because the fixture material is soft.

Any external force could compromise fixture accuracy. For a wall thickness of only 0.02mm, even a 0.01mm loss in precision is critical.

5) Precision Machining of the Finished Nylon Inner Support

Turning the finished inner support. The finished inner support is also machined from nylon material.

The nylon bar is clamped onto the spindle, and its outer diameter is machined to (65.97 ± 0.003) mm.

The outer diameter of the inner support exceeds the inner hole dimension of the ultra-thin-walled part by 0.01 mm, while the taper error is maintained within 0.005 mm.

Without removing the finished inner support, the heated aluminum alloy blank (90°C) is fitted onto the inner support and machined after cooling to room temperature.

This step ensures the inner support maintains identical runout precision with the spindle.

Fitting the blank directly onto the inner support prevents secondary clamping-induced accuracy loss and achieves ultimate clamping precision.

6) External finishing. 

For finishing, use a diamond insert with a cutting edge radius of 0.1mm, spindle speed of 1200 rpm, feed rate of 0.01mm/r, and a diameter cutting depth of 0.1mm.

Perform external finishing in five passes.

The objective is to identify the error patterns between the theoretical compensation values and the actual machining results.

This allows for the accurate calculation of the final pass compensation value, ensuring that the last pass finishing error stays within 0.005 mm.

7) Final Precision After Cryogenic Disassembly

After machining, employ a cryogenic disassembly process (nylon shrinkage rate > aluminum alloy shrinkage rate at 0°C) to achieve non-destructive separation of the workpiece.

Machining is now complete. Measurement results are as follows: Using a ball-tip micrometer, the wall thickness is 0.018mm, cylindricity is 0.005mm, and straightness is 0.005mm.

Conclusion

The innovative expansion-adsorption clamping system achieves zero-force clamping by exploiting differences in material thermodynamic properties.

This design effectively solves the problem of deformation in ultra-thin-walled components during clamping.

This enables CNC turning of parts with walls as thin as 0.018mm.

The phased machining strategy reduces total cutting forces.

During the finishing stage, we apply a temperature compensation algorithm to control errors at the 0.005 mm level.

This process offers a scalable solution for micron-level thin-walled metal component machining and has been successfully implemented in transmission housing manufacturing and assembly.