Patent Application
20080298913
This invention related generally to the metals machining and more particularly to a bore relieved milling tool having reduced mass for process damping and high performance milling.
Finish Machining of deep pocket aircraft structural components is limited by deflection and chatter. Modern designers are consistently pursuing weight reduction opportunities in metallic structure. Machined parts with deep pockets and small corner radii require long slender end mills ti cut the corners. Long slender cutting tools are more susceptivle to chatter and vibration than shorter more rigid tools. Long cutting tools exhibit lower natural frequencies, which reduces the process damping effects which can stabilize chatter. This requires small cuts and slower cutting speeds to avoid chatter, which can increase manufacturing costs. Current methods to increase machining rates include using higher speeds and tools with more cutting edges. Both of these techniques can result in more chatter for longer cutting tools.
Current methods exist to reduce cutting tool vibration and chatter. These include using an eccentric relief on the cutting tool to enhance the rubbing of the cutter on the machined part. This rubbing will also stabilize the cutting tool. The use of an eccentric relief is a benefit for shorter cutting tools, but the effect is not useful for longer tools, when the resonant frequency of the cutting tool creates a wavelength that is longer than the eccentric relief.
It is therefore desirable to provide modified cutting tools which retain or increase process damping effects to stabilize chatter.
The embodiments disclosed herein provide a cutting tool incorporating a body terminating in cutting edges distal from a chuck mount and having an axial bore. In certain of the embodiments, the body is preformed from a steel or carbide blank a cylindrical pipe forming the hollow bore.
In alternative embodiments, the avial bore is filled with a vibration absorbing material. A light weight polymer is used in exemplary embodiments.
These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein:
The embodiments of the tool disclosed herein are applicable to rotating milling cutter and stationary boring cutters where the work piece rotates instead of the tool. As shown in
The cutting tool mass is reduced by pre-forming the carbide or steel blank into a cylindrical pipe before grinding the cutting edges. For the embodiment shown, a reduction of over half the mass of a conventional tool is achieved. The mass reduced form increases the resonant frequency of a milling cutter, as the example embodiment, without significantly reducing the tool stiffness. This allows the tool to cut with approximately the same static deflection, but with significantly reduced dynamic deflection and chatter, as will be discussed in greater detail subsequently. In alternative embodiments, boring of the center hole in the completed tool or prior to heat treating or sintering and grinding of cutting edges is accomphished.
In alternative embodiments, the large hole in the center of the cutting tool is filled with a vibration absorbing material such as light weight polymer 24 as shown in
Testing of embodiments shown herein has shown a significant reduction in cutter vibration. The cutting tool with less mass vibrated at a higher frequency. The natural frequency, Wn, of the resulting mechanical system is given by Wn=sqrt(k/m), where k is the stiffness and m is the mass. As mass is reduced, the natural frequency is increased by the square root of the mass. Dynamic stiffness of the cutter is measured using impact testing with an accelerometer attached to the tool. By striking the tool with a mallet, the dynamic stiffness of the cutter is reported by a displacement Frequency Response Function (FRF) monitored on an oscilloscope output from the accelerometer. Tuning of resonant frequency by modifying the central hole diameter in the cutting tool can be accomplished for specific machining requirements such as tool rotational speed as desired. However, for most embodiments, achieving the highest frequency while maintaining necessary tool stiffness is desirable.
Creating higher frequency response on the tool allows smearing by an eccentric relief or clearance ramp 34 of the tool which is not possible at lower frequency. As shown in
Similarly, a stability zone prior to onset of chatter of the tool is achieved for cuts of greater depth as shown in
The embodiments disclosed have been tested and provide the ability for use for pockets up to 4 inches in depth. At this depth, the new hollow reduced mass cutting tool is more than twice as productive as a prior art solid counterpart. Pockets of up to 8 inches in depth are anticipated to be within the capability of the tool. The embodiments disclosed herein allow more productive use of long, slender end mills, which are traditionally problematic.
Having now described exemplary embodiments for the invention in detail as required by the patent statutes, those skilled in the art will recognize modifications and substitutions to the specific embodiments disclosed herein. Such modifications are within the scope and intent of the present invention as defined in the following claims.
