In aircraft manufacturing difficult-to-cut-materials are used with much greater frequency. Unfortunately, these difficult-to-cut-materials significantly shorten tool life. Responding to market demand for more innovative machining methods that drastically prolong tool life when working with these special materials, Mitsubishi Materials kept its focus on the development of next-generation rotary cutting tools. In this feature, we focus on two of these, driven rotary cutting tools employed in multi-task machines and passive rotary cutting tools used on general machining centres
It was about 20 years ago that Mitsubishi Materials first developed rotary cutting tools for lathes that rotated the inserts during machining. At the time, an innovative mechanism was applied that drove the rotation by using the cutting resistance. This significantly reduced boundary wear, a major cause of reduced tool life during the machining of difficult-to-cut materials. While this first-generation rotary cutting tool was highly regarded, the complicated mechanism limited rigidity; and they were relatively expensive compared with standard tool holders. Some customers continued to use them, but demand gradually dropped.
However during that time, new rotary cutting tools were under development. This new development leveraged the know-how accumulated through the company’s experience with its first tools. In the design of the new rotation mechanism, the appearance of multi-task machines provided a big hint. The first rotary turning tools rotated inserts by utilising resistance generated during the cutting process, which, depending on cutting conditions, caused unevenness in the rotating force and made it difficult to realize stable performance. It was thought that if a stable, predetermined rotating force could be generated regardless of cutting conditions, there could be successful way to develop a new type of rotary tool. It was around 10 years ago that thoughts started about new rotary cutting tools.
About that time a study on driven rotary cutting tools was conducted by Professor Sasahara at Tokyo University of Agriculture and Technology. A consultation period was undertaken with them for a while; and then three years ago, a full-scale joint research was started. Using multi-task machines made it possible to realise voluntary control of tool rotation, which paved the way to achieving driven rotary cutting tools.
The multi-task machines not only allowed control of the tool rotation, but it also allowed contact angles to be freely set. This prompted research for the best combination of cutting conditions and tool contact angles.
In addition to rotational frequency (rotation speed of tools), it is important to identify the best contact angle. Chip thickness, which has a significant influence on tool life and flow direction, varies depending on basic conditions such as speed, feed and cut. Along with these considerations, the new design employed different tilt angles, which created the potential for difficulty in finding the best combination of cutting conditions. To address this, Professor Sasahara was asked for help in looking at the figures from a theoretical viewpoint to investigate the best conditions.
Meanwhile, the greatest challenge in the development of tool forms is minimising the misalignment of the centres when fixing the insert to the tool. Larger misalignments cause eccentric rotation against the rotation axis of the tool, which changes the amount of cut and causes a mismatch between the predetermined and actual size of the part being machined. In addition, changes in amount of cut causes instability in cutting resistance, which generates chattering and damages the inserts.
After repeated trials, it was possible to reduce the degree of concentricity between the insert and cutting tool to 0.01 mm or less.
Another major feature of the new cutting tool is found in the internal coolant. The tool was designed to supply coolant from the space between the insertion hole and clamp screw. This mechanism tends to lower clamp force when the insert is installed onto the cutting tool; however, this unique design maintains the necessary clamping force. The tool itself rotates consistently, which evenly disperses the heat generated during the cutting process over the entire circumference of the cutter. Supplying the coolant from inside the cutting tool makes it possible to effectively cool the entire insert and to discharge chips smoothly.
The newly developed driven rotary cutting tools offer the following features:
1. Use of the entire circumference of the insert evenly disperses tool abrasion to promote longer tool life.
2. Stable rotation of the tool itself effectively disperses cutting heat; and the internal coolant design significantly reduces insert abrasion.
3. The uniquely developed high-precision and high-rigidity clamp mechanism realises stable, high-performance machining.
Looking at the general compression ratio (CR) of chips, it was thought that approximately one-third of the machining speed, which is equivalent to the chip discharge speed, may be ideal as the rotation speed of the inserts to reduce the flank wear that is often a problem in machining difficult-to-cut materials. The first rotary cutting tools were rotated by cutting resistance, which did not allow control of the rotation speed. Therefore, a detailed examination of this hypothesis was not done at that time.
The new rotary cutting tools have several parameters, which makes it difficult to identify the optimal cutting conditions. Although recommended conditions have been identified for general use, it is very interesting to know that the optimal rotation speed of the tool against the machining speed of the workpiece is now one-third of the speed assumed for the first rotary cutting tools. Driven rotary cutting tools are now under development with a view to market introduction in 2017.(Left): Yuji Takada, Tsukuba Aero Group, Aerospace Dept. who was involved in the development of passive rotary cutting tools
The new passive rotary cutter was developed as a milling tool utilising know-how gained through experience with the first rotary cutting tool.
Since the launch of the first rotary cutting tool, Mitsubishi Materials has applied a mechanism which rotates the insert with cutting resistance to end mills and face milling cutters. However, it was very difficult to install the rotational mechanism of the first rotary cutting tool to the milling tool due to its size, making it seem an impossible goal.
However, advancement of difficult-to-cut materials over a wide range of industries required further improvement of machining efficiency as well as prolongation of tool life. About 10 years ago, realizing the potential of inserts that rotated during milling, Mitsubishi Materials started joint development of rotary cutters with Nagoya University and Mitsubishi Heavy Industries, Ltd.
The first challenge was to identify the ideal angle to drive insert rotation utilising cutting resistance and to ensure an optimal rotating force. If the cutting resistance is too low, it won’t generate sufficient drive to rotate the insert. If it is too high, it causes chattering during machining and leads to tool or insert damage. We needed to identify the angle that would generate enough cutting resistance to rotate the insert reliably to allow a broader range of cutting conditions.
Nagoya University overcame this difficult challenge. Applying complex formulas, the engineers successfully identified the optimal angle for insert placement for effective rotation. Compared with the trial-and-error method employed in the development of the first rotary cutting tools, being able to calculate the optimal values theoretically, significantly reduced the time needed for development.
The next challenge was fitting the insert into an extremely narrow space, a particularly difficult challenge. It was necessary to design a rotational mechanism that could be installed in such a narrow space. This required optimisation of the clearance of the insert hole and clamp screw to allow smooth rotation of the insert during machining. If the clearance is too small, it will stick; too large and it causes chattering. In addition, to achieve sufficient rigidity it is important to have the best clamp screw thickness for the size of the insert. After repeated examination and analysis, several prototypes and lots of experimentation, a spring above the clamp screw was successfully installed, which made it possible to develop a rotational mechanism that had both the ideal clearance and strength required. Just as the end of the development was in sight with the rotational mechanism, another challenge needed to be faced. The bottom of the insert came into contact with the cemented carbide shim on the tool body during rotation, which caused uneven wear. The rotation of the insert could even out cutter wear; but the cemented carbide shim that received the cutting resistance faced an uneven load, plus the load on the section below the cutter was intense. Because the insert and shim were both cemented carbide, contact and continued rotation under a local load would definitely create uneven wear. To address this problem, a movable metal plate was placed between the insert and cemented carbide shim as a buffer.
The biggest merit of rotary tools is long-time unmanned machining that does not require corner change; and as the graph below shows, we were able to achieve tool life that extended eight to ten times longer than our existing cutters.
This passive rotary cutter is scheduled to be on the market in 2017. It is planned to expand this success to the development of cutters for end mills, face milling and turning. Along with the expansion of the sizes of the insert, it is also planned to develop cutters for ramping.