CNC Milling Application Case: Manufacturing Process of Aerospace 6061 Aluminum Alloy Lightweight Bracket-Case Studies

As a core load-bearing component of airborne equipment, aerospace lightweight brackets must simultaneously meet the requirements of lightweight, high strength, and high precision. Their structure includes special-shaped grooves, precision positioning holes, and inclined step surfaces, making it difficult for traditional processing to ensure dimensional consistency. This case uses 6061 aluminum alloy as the raw material and adopts vertical CNC milling technology to achieve batch precision processing. The core control indicators are dimensional tolerance ±0.008mm and surface roughness Ra≤0.6μm. Through scientific process planning and precise multi-process coordination, the problems of aluminum alloy's easy chatter and tool adhesion are effectively solved. The following elaborates on the complete manufacturing process and technical key points in detail.

I. Product Specifications and Raw Material Preparation

The aerospace lightweight bracket processed this time is an irregular plate structure with an overall size of 120mm×80mm×15mm. The main body is designed with 3 symmetrical special-shaped grooves (width 8mm, depth 10mm), 4 M6 threaded positioning holes distributed at both ends, and a 30° inclined step surface on one side, which must withstand a load of ≥200N without plastic deformation. 6061-T6 aluminum alloy plate is selected as the raw material. After solution aging treatment, this material has a tensile strength of 310MPa and a density of only 2.7g/cm³, featuring both lightweight and corrosion resistance, complying with the aerospace industry standard AMS-QQ-A-200/11. The plate size is 130mm×90mm×18mm, with 3mm machining allowance reserved.

Raw material pretreatment must take both flatness and cleanliness into account: first, precision grind both sides of the plate with a surface grinder to ensure flatness ≤0.01mm, avoiding the impact of plate warpage on subsequent milling reference precision; then ultrasonically clean with acetone for 30 minutes to remove surface oxide film and oil stains, followed by low-temperature drying (60℃, 30 minutes) to prevent surface defects caused by mixing of cutting fluid and impurities during processing, and improve the cutting adaptability between the tool and the workpiece.

II. CNC Milling Process Planning and Equipment Selection

Combined with the product structure complexity and precision requirements, a VMC850 vertical machining center is selected, equipped with the FANUC 0i-MF system, supporting three-axis linkage control, with a spindle speed range of 0-8000r/min and a repeat positioning accuracy of ±0.003mm. It is equipped with an automatic tool change system (tool magazine capacity of 24 tools) to realize continuous multi-process processing. Cemented carbide coated tools are selected, with specific configurations as follows: face mill (diameter 50mm, 4 teeth), end mill (diameter 8mm, 3 teeth), thread mill (M6 specification). The coating material is AlTiN, which can improve tool wear resistance and adapt to the high-speed cutting needs of aluminum alloy.

The process route is planned as a step-by-step flow of "rough milling - semi-finish milling - finish milling - thread processing - deburring - cleaning", and processing stress is controlled through layered cutting and path optimization. The rough milling stage removes 70% of the allowance, leaving 0.3mm for semi-finish milling; the semi-finish milling optimizes contour precision, leaving 0.05mm for finish milling; the finish milling stage ensures the final dimension and surface quality; the subsequent thread processing and deburring processes improve detailed structures. Water-soluble cutting fluid (8% concentration) is used for cooling and lubrication throughout the process to avoid aluminum alloy tool adhesion and thermal deformation.

III. Step-by-Step Milling Process and Parameter Control

(I) Clamping, Positioning and Rough Milling

Clamping adopts a vacuum suction cup combined with positioning pins for positioning. The suction force of the suction cup is adjusted to 0.6MPa to ensure the plate is closely attached to the workbench. The fit clearance between the positioning pins and the plate reference holes is ≤0.005mm to ensure no displacement during processing. Before clamping, a thin silica gel pad is laid between the workbench and the plate to prevent surface damage caused by excessive suction cup pressure. Meanwhile, the X and Y axis references of the plate are calibrated to make the parallelism error between the reference surface and the machine tool coordinate axis ≤0.006mm.

In the rough milling stage, the spindle speed is set to 4500r/min, the feed rate is 1200mm/min, and the cutting depth is 1.5mm. First, use a face mill to mill the upper and lower surfaces of the plate to flatten the reference surface; then use an 8mm end mill for layered rough milling along the contour of the special-shaped groove, adopting a spiral plunge cutting method (plunge angle 15°) to avoid tool impact caused by vertical plunge cutting; at the same time, mill the prototype of the step surface, reserving allowance for subsequent finish processing. During cutting, control the spray angle of the cutting fluid to ensure coverage of the cutting area, and keep the cutting temperature below 150℃ to reduce workpiece thermal deformation.

(II) Semi-Finish Milling and Finish Milling

In the semi-finish milling stage, the spindle speed is adjusted to 6000r/min, the feed rate is 800mm/min, and the cutting depth is 0.25mm. The tool compensation function of the machine tool is used to correct rough milling deviations, and the side surfaces of the special-shaped groove and the step surface are refined. Focus on controlling the groove symmetry error ≤0.008mm and the step surface inclination angle deviation ≤0.01°. This link adopts an arc transition cutting path to reduce tool vibration. For every 20 workpieces processed, a dial indicator is used to detect tool runout. If the runout exceeds 0.003mm, tool calibration is performed immediately.

Finish milling is the core process. The spindle speed is increased to 7500r/min, the feed rate is reduced to 500mm/min, the cutting depth is 0.05mm, and climb milling is adopted. The contour of the special-shaped groove is accurately fitted through the interpolation function of the CNC system to ensure the perpendicularity error of the groove side surface ≤0.006mm; angle compensation parameters are used during step surface processing to ensure the 30° inclination angle precision meets the standard. During finish milling, the cutting fluid flow rate is optimized (8L/min) to avoid chip residue affecting surface finish. After processing, the built-in probe of the machine tool is used for preliminary inspection of key dimensions, and workpieces with excessive deviations are marked in time for secondary finish milling.

(III) Subsequent Process Processing

M6 thread mill is used for thread processing, with a spindle speed of 3000r/min and a feed rate of 600mm/min. Threads of positioning holes are processed by spiral interpolation, with a thread depth of 12mm, ensuring thread precision reaches grade 6H. For every 5 workpieces completed, 1 piece is sampled for thread go-no-go gauge inspection to avoid thread slipping or precision non-compliance.

Deburring is performed by manual operation combined with ultrasonic cleaning. A special diamond file is used to polish burrs at the edges of threaded holes and groove corners (burr height ≤0.002mm) to avoid sharp edges scratching subsequent assembly components; then ultrasonic cleaning with pure water is conducted for 20 minutes to remove residual cutting fluid and chips on the surface, followed by hot air drying (80℃, 20 minutes) to ensure no stains on the workpiece surface.

IV. Quality Inspection and Finished Product Acceptance

Finished product inspection adopts the mode of "100% inspection of key dimensions + sampling inspection of surface quality and mechanical properties". Key dimensions are inspected by a coordinate measuring machine. 60 pieces are randomly sampled from each batch to inspect 15 indicators including special-shaped groove size, step surface angle, and thread precision, with a qualification rate of over 99.8% required. Surface quality is inspected by a roughness meter, requiring Ra value ≤0.6μm, and the surface is observed with a 20x magnifying glass to ensure no scratches, tool adhesion marks, or oxidative discoloration.

For mechanical performance sampling inspection, 10 finished products are selected for load testing, applying a 200N load for 10 minutes without plastic deformation; no corrosion spots appear after a 72-hour salt spray test, complying with the usage standards of aerospace airborne equipment. Qualified finished products are individually packaged, marked with batch number, production date, and inspection results to realize full-process quality traceability.

V. Case Summary

In this case, the aerospace 6061 aluminum alloy lightweight bracket is processed by CNC milling technology. With scientific process planning, precise parameter control, and reasonable tool selection, the problems of aluminum alloy high-speed cutting chatter and tool adhesion are effectively solved. The processing efficiency reaches 50 pieces per hour, which is more than 50% higher than traditional milling. The dimensional accuracy, surface quality, and mechanical properties of the finished product all meet the stringent requirements of the aerospace industry, verifying the core advantages of CNC milling in batch processing of precision aluminum alloy parts. The process plan of this case can be replicated for processing similar aerospace lightweight parts, providing technical reference for efficient and precision production in the aerospace manufacturing industry.