Titanium alloy thin-walled parts – how to tackle this notoriously difficult machining challenge

When titanium alloys are used for thin-walled parts, machining a 2mm thin wall on a material generally considered difficult to machine doubles the difficulty. However, after specific analysis, there are still corresponding solutions: Analysis of Thin-Walled Cavity Parts of Titanium Alloys.

1.1 Part Size Analysis
The minimum rectangular outline dimensions of the cover plate part are 567mm x 426mm x 56.5mm. The overall appearance is a large, thin-walled cavity structure, with a wall thickness of 2mm on all arc surfaces and a shoulder thickness of 6mm at both ends. To ensure the sealing performance of the cover plate after installation with the housing, the flatness of the cover plate mounting surface must be 0.1mm.

Titanium alloy thin-walled parts – how to tackle this notoriously difficult machining challenge (1)

 

Figure 1

1.2 Machining Process Analysis
The key machining part of the cover plate is the cavity. The difficulty lies in addressing chatter during machining, given the thin-walled structural characteristics of the cover plate. Its overall structure is simple. The cover plate and the housing’s mounting surface must meet high requirements for flatness and surface quality. In contrast, the dimensional accuracy and surface quality requirements for other parts are not high. All machining processes can be completed on a CNC machining center.

Here, a 570mm x 430mm x 60mm blank plate is selected. The machining of the titanium alloy thin-walled cavity cover plate part requires three steps: ① Rough and finish milling of the convex cavity (i.e., machining of the mounting holes); ② Rough and finish milling of the concave cavity; ③ Finish machining of the mounting surface.

Machining Process and Tooling Design

2.1 Rough Milling, Finish Milling, and Mounting Hole Machining of the Convex Cavity
The outer dimensions of the cover plate part are 567mm ± 0.1mm x 426mm ± 0.1mm x 57mm. The surface roughness of all machined surfaces is Ra 3.2μm, with a 0.5mm machining allowance in the thickness direction. Four M16 threaded holes, each 40mm deep, are machined on the large flat surface as fixing holes for the convex cavity machining process. These four threaded holes are positioned with the symmetrical center point of the large flat surface as a reference, and their dimensions correspond to the fixing holes of the 6061 aluminum-magnesium alloy tooling fixed on the worktable. This allows the cover plate part to be positioned using a “one-sided, two-pin” positioning method, with screw clamping for positioning and clamping.

Rough Machining

Figure 2: Rough Machining

Next, the convex cavity and the mounting steps on both sides of the cover plate part are machined. For rough machining of the convex cavity and rough and finish machining of the mounting steps on both sides, a four-flute end mill with a high cobalt content is used. The parallel milling surface roughing method is shown in Figure 2. During cutting, the high-speed tool steel tool must maintain sufficient cutting fluid to extend its service life.

Then, for the final machining of the convex cavity, a polycrystalline cubic boron nitride ball end mill is used. The streamlined surface-finish machining method is shown in Figure 3, with the overall error controlled to within 0.012 mm. Finally, the 16 mounting holes on the mounting surface (all threaded) are machined. Direct drilling with a 0.5 mm carbide drill bit can meet the machining requirements. After this process, burrs should be removed, sharp edges blunted, and the surface of the convex cavity polished to remove any noticeable tool marks that could affect the positioning and clamping in the following process.

Finish Machining Simulation of Convex Cavity on Part

Figure 3: Simulation of the finish machining of the convex cavity

2.2 Cavity Rough and Finish Milling and Milling Fixture

Figure 4 shows the milling fixture for rough and finish milling of the concave cavity. The fixture, previously fixed to the worktable, is machined with a concave surface to mate with the convex cavity of the cover plate. The two sides of the fixture mate with the mounting steps at both ends of the cover plate part to achieve precise positioning. The intersections of the concave surface with the plane and between planes are cleaned to avoid burrs or sharp edges that affect positioning accuracy. The cover plate part is then clamped and fixed to the fixture using the 10 M10 threaded holes on both sides.

The rough machining of the concave cavity of the cover plate part uses a grooving method. The cutting tools, coolant, and cutting parameters are basically the same as for the convex cavity. The four M16 clamping holes from the previous process are prone to vibration and tool damage during milling; care must be taken during cutting. The cutting tools and milling parameters are the same as for the finishing machining of the convex cavity.

Titanium alloy thin-walled parts – how to tackle this notoriously difficult machining challenge (4)

Figure 4

Points to note during machining:
① A natural failure treatment must be arranged between the roughing and finishing of the concave cavity of the cover plate part to completely release the internal stress generated by the cutting process and prevent deformation.
② During finishing clamping, the mating surfaces of the cover plate part and the tooling should be rotated multiple times, and all black grinding marks on the mating surfaces within the tooling should be repeatedly repaired to ensure that there are no obvious gaps between the positioned cover plate part and the mating surface of the tooling.
③ Clean the chips from the corners and edges of the tooling and the mating surfaces to avoid damaging the cover plate part and affecting surface quality.
④ When tightening the screws on both sides, they should be tightened symmetrically, alternating between left and right, to minimize deformation caused by clamping force.

2.3 Finishing the Mounting Surface
After inspection, the main reasons for the flatness and surface roughness exceeding the tolerances of the finished mounting surface are tool marks or machining deformation, necessitating re-finishing of the mounting surface.

Titanium alloy thin-walled parts – how to tackle this notoriously difficult machining challenge (5)

Figure 5

The positioning datum and clamping fixing surface should both be selected from the parallel mounting step surfaces on both sides of the cover plate part. First, choose the precision milling method, then the precision grinding method, and finally the lapping method, with continuous monitoring throughout the machining process. A machining allowance of no more than 0.5 mm should be reserved during the machining of the convex cavity to meet the accuracy requirements. The finished cover plate part is inspected and approved.

Titanium alloy TC4 is a difficult-to-machine material. The cover plate part has a wall and cavity structure. In actual CNC machining, the same tooling is used, and the design can conform to the overall process of the cover plate part. Technically, this not only solves the clamping and positioning problem, but also cleverly overcomes the vibration problem during the machining of a 2 mm thin wall, avoids excessive clamping force, and prevents deformation. In machining practice, effective tooling can reduce costs, streamline operations, and save time, thereby increasing efficiency.