Uncontrolled vibration creates multiple problems in metalcutting operations. Varying forces in the cutting process cause vibration and tool chatter that degrade part surface quality, quickly wear or break cutting tools and damage machine tool components. By Pierre Zunino, Product Manager, and Yannick Groll, R&D Engineer, at Seco Tools.
Trends in product design can also incite vibration. To enhance product strength and reduce assembly costs, manufacturers increasingly machine parts from monolithic workpieces. Producing internal features of the parts requires tools to reach into deep cavities, and the extended tool length exaggerates vibration.
Attempting to minimise vibration by reducing cutting parameters decreases productivity and increases manufacturing cost. Prime among the various approaches to vibration control are passive/dynamic systems that utilise tuned-mass damper concepts to absorb vibration before it progresses and disrupts the machining process.
Vibration – A common issue
All sectors of industry recognise excessive vibration as a destructive condition. Repetitive operating and/or external forces generate sympathetic motion that can resonate within a machine, building, or bridge and grow to a dangerous magnitude. Manufacturers and builders often apply tuned mass damper concepts to overcome vibration. A tuned mass damper is a component that is suspended within a machine or structure and is designed to resonate out of phase with the unwanted vibration, absorb its energy, and minimise the vibratory motion.
In metalcutting, vibration is generated by the changing forces that occur when making chips. The intermittent forces are apparent in the interrupted cutting process of milling and also appear in turning operations when the toolholder bar is periodically loaded and unloaded as chips form and break.
A passive approach to vibration control in metalcutting involves maximising the rigidity of the elements of the machining system. To restrict unwanted movement, a machine tool can be built with rigid structural elements, made larger and heavier, and filled with concrete or other vibration-absorbing material. From a workpiece perspective, thin-walled parts and those with unsupported sections are prone to vibration when machined. To a limited degree, parts can be redesigned to improve rigidity. However, such design changes can add weight and compromise product performance.
For cutting tools, a passive approach to vibration control includes use of short, rigid tools and replacement of steel toolholders with those made of stiff tungsten carbide.
A passive/dynamic approach to vibration control for tools involves application of the tuned mass damper concept. The Steadyline system from Seco features a pre-tuned vibration damper consisting of a damper mass made of high-density material (to minimise its overall dimensions) suspended inside the toolholder bar via radial absorbing elements. The damper mass absorbs vibration immediately when it is transmitted by the cutting tool to the body of the bar.
The Steadyline system can enable typical long-overhang operations to be performed at least twice as fast as with non-damped tools while enhancing part surface finish, extending tool life, and reducing stress on the machine tool. Passive/dynamic vibration damping technology can make it possible to accomplish certain applications, such as some uses of tool lengths of up to ten times bar diameter, that would not otherwise be possible even at minimal machining parameters.
In the Steadyline dynamic/passive vibration control system, the vibration-absorbing mass is positioned at the front of the bar, where potential for deflection is highest and the mass can damp vibration immediately when it is transmitted from the cutting edge to the body of the bar. The Steadyline system also includes short, compact GL cutting tool heads that place the cutting edge close to the damping mass to maximise the vibration-absorption effect. The system is adaptable to a wide range of applications and is most useful in milling (contouring, pocketing and slotting), turning and both rough and fine boring operations.
Application comparisons
A good example of the Steadyline system’s effectiveness involved a difficult boring operation in 42CrMo4 steel where a cylindrical 105.8mm bore was enlarged to a conical 129mm bore in five roughing passes at a 3mm depth of cut decreasing to 0mm depth. With an 80mm diameter bar, the initial cutting length was 600mm, representing an extended tool length-to-diameter ratio of 7.5. Roughing was accomplished at a feed rate of 0.3mm per revolution and cutting speed of 157m per minute. Pre-finishing to a final 130mm diameter took place at 0.5mm depth of cut, 0.2mm per revolution feed rate, and a cutting speed of 200m per minute. Even though the bulk of the workpiece prevented use of the full rotational speed capability of the Steadyline bar, machining time in the operation was reduced from 12 hours to two hours (more than 80%) with use of the Steadyline passive/dynamic vibration-control system.
Demonstrating the Steadyline system’s benefits in a side milling operation, a Combimaster milling cutter holder without passive/dynamic vibration control was applied with a 20mm diameter cutter at 312m per minute and 0.3mm/tooth feed rate at a cutting depth of 0.9mm in 1.1206 CK50 steel. When a version of the same tool employing the Steadyline system was applied at the same cutting speed and feed as the undamped version, it was possible to increase cutting depth to 2.2mm (an increase of 2.3 times) without unwanted vibration.
Producers of equipment for oil and gas, power generation and aerospace customers are prime candidates for use of passive/dynamic vibration control systems because each of these industry segments regularly deals with large, complex parts with features that require the use of extended-length tools. Additionally, such parts usually are made from tough alloys that are difficult to machine and thereby produce high, vibration-generating cutting forces. However, it is clear that nearly every manufacturer faces applications where the vibration-absorbing properties of Steadyline tooling can expand their capabilities, improve their productivity and reduce their costs.