In machining, boring is the process of enlarging a hole that has already been drilled (or cast), by means of a single-point cutting tool (or of a boring head containing several such tools). Boring machines come in a large variety of sizes and styles. Boring operations on small workpieces can be carried out on a lathe while larger workpieces are machined on boring mills. Cooling of the bores is done through a hollow passageway through the boring bar where coolant can flow freely. Tungsten-alloy disks are sealed in the bar to counteract vibration and chatter during boring. The control systems can be computer-based, allowing for Automation and increased consistency. A typical boring mill price is in the 20-100k Euros range.
The boring bar is the critical tool part of the boring machine and one of the application areas for the damping material, but other types of tools will be studied, such as grooving / parting off tools and threading tools.
Depending on its size, a standard steel boring bar can cost between 40 and 100 Euros, while a tungsten carbide reinforced bar can cost up to 300 Euros High performance damped boring bars (such as Sandvik Silent Tools that integrate an internal vibration damping mechanism) can cost up to 2000 Euros a piece.
The basic cause of chatter is the dynamic interaction of the cutting process and the machine tool structure. During cutting, a force is generated between the tool and workpiece, which acts at an angle to the surface. The magnitude of this cutting force depends largely on the tool-work engagement and depth of cut. The cutting force strains the structure elastically and can cause a relative displacement of the tool and workpiece, which alters the tool-work engagement (undeformed chip thickness). A disturbance in the cutting process (for ex. because of a hard spot in the work material) will cause a deflection of the structure, which may alter the undeformed chip thickness, in turn altering the cutting force. There is a possibility for the initial vibration to be self-sustaining (unstable) and build up, with the machine oscillating in one of its natural modes of vibration.
To make the increased capability of modern cutting edges, the cutting-tool materials available, powerful and stable machine tools, holding tools and cutting tools are required. The shape accuracy of the produced parts is
determined by the kinematic machine tool behavior and the static, dynamic and thermal stiffness of the machine tool system. The surface quality that can be achieved depends on the geometry of the cutting edge, the machining parameters and the dynamic behavior of the system: machine tool - cutting tool - workpiece.
High levels of the boring bar vibrations result in a lower metal removing rate, poor surface finish, reduced tool life and severe acoustic noise in operator’s working environment. Active vibration damping mechanisms are very
expensive and require machine “tuning”. The global market for metal-cutting machine tools is forecasted to expand 8.6% annually to 60Bn Euro in 2014, with rotational tools representing 25%