In the actual processing of titanium alloys, several factors must be taken into account. As a result, the milling process for titanium alloys differs from the main machining methods used for a long time.
Thanks to the continuous development of applications and two new milling tool solutions, new possibilities for titanium alloy milling have emerged.
In contrast to many other materials, the potential for successful processing of titanium alloys is much smaller, and it has different processing properties. Since titanium alloys are more variable in terms of machinability, the choice of machining methods and tools, as well as the machining process, are affected.
However, as with any other alloy, more careful planning is necessary – from selecting the appropriate machine to programming the cutting details.
While parts in the aviation manufacturing industry share similar characteristics, their size and shape can vary significantly. Consequently, the choice of machine, fixture, coolant, tool, machining method, and cutting parameters will also differ.
Due to limited space, flexibility is a primary condition for the process. The shank type and tool mounting adjustment are key factors in terms of productivity and capacity.
For indexable and solid carbide tools, roughing and finishing operations need to be planned according to different parameters, and titanium alloy milling requires different application areas.
The size and shape of the part to be machined, along with the appropriate tool type, are the first determining factors. Indexable tools are the preferred choice for roughing as they have the highest efficiency of material removal. They are also unmatched in finishing large and flat surfaces. Integral carbide tools are commonly used in semi-finishing and finishing processes, especially when radii, cavities, and grooves are too small for indexable blade tools.
The programming data of the parameters of the part to be machined is the basis for selecting a dedicated milling tool. The maximum metal removal rate should be balanced with an economical tool life to determine other tool variables.
In the case of titanium alloys, the tool must have a carbide material with a sharp and strong cutting edge and a relatively large positive front angle. These features meet the special thermal and chemical requirements of titanium alloys.
Indexable insert technology has undergone significant development in terms of grooves and tool materials, and it is becoming a more cost-effective solution to replace a large number of existing carbide tools, even for medium and large-sized tools.
Radial milling of titanium alloy materials
Titanium alloy machining is well-suited to radial milling, but a large radial depth of cut can significantly reduce tool life. However, a large axial depth of cut has little effect on cutting temperature and therefore won’t affect tool life in the same way.
To efficiently remove titanium alloys, it’s best to use a long-flute milling cutter with a tight tooth pitch, a radial depth of cut of 30%, and the maximum axial depth of cut allowed for the specific application. This type of cutter is suitable for both roughing and finishing the sidewalls of many titanium alloy parts.
The long spiral flute of the long-flute milling cutter is ideal for performing large amounts of radial milling in titanium alloy machining. Indexable inserts in long-edge tools consist of multiple rows of inserts that are the same as the continuously sharpened integral carbide tool cutting edges.
However, the current indexable inserts that rise up from the bottom of the cutter and are arranged along the outer periphery have reached their limit in terms of achieving good machining performance and safety in titanium alloy.
For excellent machining performance, a large chip flute for efficient chip evacuation is necessary, along with an efficient positive rake angle and a sharp blade. These elements are combined into an indexable long-edge tool, which can achieve exceptional machining performance.
In titanium alloy milling, stable clamping of the cutting insert is crucial. Even during roughing, any movement of the cutting insert can result in uneven wear and jeopardize the cutting edge. Even minor signs of wear can lead to dulling of the cutting edge during the titanium alloy cutting process, which can accelerate wear and cause tool breakage.
Obtaining outstanding performance when using long-edged milling cutters requires a strong interface between the blade and the cutter body. A row of tightly fixed continuous blades poses a challenge for axial support of the blade, leading to excessive dependence on the blade screw.
To overcome this challenge, the blade holder must have a definite support and locking device, with special consideration given to the axial force and rotation force.
Coolant is critical.
The effectiveness of titanium alloy milling is dependent on the quality of the coolant used. Higher quality coolants lead to better machining results.
Research indicates that high-pressure cooling applications (with a pressure range of 70 × 105 to 100 × 105Pa, depending on the system used) have proven to be advantageous.
Given that high-pressure cooling has become standard on many modern machine tools, it represents a potential resource for optimizing titanium alloy milling.
High-pressure cooling affects various aspects of the machining process such as heat distribution, chip formation, cutting edge bonding tendency, tool wear, and surface integrity, all of which significantly impact the processing results of titanium alloys.
Because titanium alloys are prone to chemical reactions, they tend to weld the workpiece material to the cutting-edge during processing, which affects tool life and leads to secondary cutting of chips and blockage of hard chips.
The coolant sprayed from the nozzle under high pressure plays a crucial role in temperature control, which in turn affects the processing results and reliability.
The tool nozzle is aligned directly with the part of the blade in contact with the finishing surface, creating a “hydraulic wedge” between the chip and the rake face of the blade.
Since these nozzle holes are non-adjustable, they are optimized during assembly to eliminate any instability, resulting in a more consistent and safer processing process.
Due to practical considerations, the indexable milling cutter typically has a lower diameter of 12mm, while the carbide cutter has a maximum diameter of 25mm based on cost-effectiveness.
The selection of tool sizes in the intermediate and overlapping ranges depends on the particular application. For finishing, fine grinding of solid carbide end mills is generally the preferred solution, while for roughing, an indexable tool is the optimal choice.
Nonetheless, tools suitable for the mid-range are continuously being developed, as the modularity of the tool provides a distinct advantage through the use of interchangeable head milling cutters.
Applications of small diameter milling cutter
Long tool reach is necessary to machine deep and narrow cavities. However, machining these cavities can become a bottleneck, and a solution is needed to balance performance, small tool capabilities, and machining flexibility.
Using an extended chuck to clamp the solid carbide end mill for cavity penetration is not the best option, as it limits cutting parameters and poses a risk to part quality. Instead, interchangeable head tools offer the dual advantages of indexability and finishability of integral carbide tools.
Interchangeable head tooling systems have significant advantages in the 10 to 25 mm tool diameter range in terms of performance, machining results, tool cost, and flexibility requirements. This tooling system not only offers high flexibility but also reduces tool inventory.
The finishing capabilities of this tool surpass those of indexable insert tools, while its cost is significantly lower compared to traditional carbide tools. Additionally, there is no need to worry about the blade size being reduced after resharpening.
The ability to choose between various head and shank combinations provides a high degree of flexibility and optimization possibilities.
The interface between the cutter head and shank is a critical element of this tool. Its performance relies on strength, stability, accuracy, repeatability, and ease of clamping. To achieve the unique interface required between the tool head and tool holder, a sufficiently large axial support surface, conical radial support surface, specially developed thread profile, and screw support are combined.
This interface forms the foundation for excellent machining performance, even in large tool overhang conditions.
Successful milling of titanium alloys
When it comes to rough machining milling, the main factor to consider for obtaining the best metal removal rate is the axial depth of cut. On the other hand, in finishing milling, the selection of the best feed rate is crucial.
In titanium alloy machining, regardless of roughing or finishing, there are always limitations, though different levels of cutting speed may be possible.
Having a grasp of these fundamental principles in titanium alloy processing can go a long way in optimizing the process, making it more competitive, and achieving reliable outcomes.
There are four key factors to consider: machine capacity, coolant supply, cutting tool, and machining method.
In the case of radial milling at lower cutting speeds, the machine must have sufficient power and torque, as well as the appropriate spindle to attain a satisfactory metal removal rate.
If the machine employs small diameter tools, the spindle speed range must be high enough to achieve excellent machining results.
In general, the spindle interface must be evaluated, and its coupling stability must not be too weak.
Obtaining sufficient tool bending stiffness requires good end face and taper contact as a basic requirement. Additionally, adequate clamping pressure is essential to eliminate tension on the tool from spiral or radial milling tools.
The use of titanium alloys in machining workshops has increased, making the development of corresponding processing technology crucial for improving the performance and processing results of this material.
Milling dominates in titanium machining, especially for machining cavities, profiles, slots, and edges in aircraft fuselage parts, which is a challenging process. Most of the processing is two-dimensional, and the processing requirements are becoming more strict due to the cavity depth and radius requirements and other challenging factors.
The machines used for titanium alloy machining are outdated, and thus, it is necessary to select the best tooling systems and machining methods to achieve excellent machining and maximum machine utilization. The method and programming of titanium alloy milling is more advanced than normal machining. By machining fillets and contours according to the recommended method, distinctive results in the production cycle of the machine tool can be achieved while minimizing scrap.
To ensure the right toolpath is machined from the start, taking a little extra time to optimize programming before machining will do more good than choosing an off-the-shelf process. New developments in milling tools have resulted in performance improvements that directly enhance the performance of titanium alloy materials.
Special tools play a major role in overcoming the challenges of machining titanium alloys and selecting and applying the right tools for the process in the optimization section.
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