Noise from vibration usually indicates some compromising of the machining operation, and can lead to component-quality issues, poor tool-life and even scrapped components and tools. These are dramatic shortcomings in machining.
In some cases, operations may seem impossible to perform, but as such they also provide a potential for being transformed to processes that are efficient and secure. This potential has been the driving force behind the development of vibration-dampened tooling – from initially being a problem solver to today’s position as a recognised productivity booster.
Research into the causes and possible remedies of vibration tendencies was started as early as in the 1960s. With vibrations having been a problem in machining as long as cutting metal has existed, a closer look was warranted into the nature of the problem, originating at the cutting edge – especially when related to tool overhang. It was established that a vibration could be seen as a variable deflection of the cutting tool and that with no or minute deflection there was no vibration that would affect machining to any consequence.
In cutting tools, vibrations are triggered and maintained by dynamic cutting forces. Even during continuous cuts, forces will have minute, rapid changes that affect the harmony of the cut. The main ways to eliminate vibrations from this source are to increase the static-stiffness of the set-up, to reduce cutting forces acting on the tool, and to increase dynamic stiffness.
Thus, once it was correctly established how chatter in machining comes from vibrations that originate in the dynamic interaction of the cutting process, cutting-tool, holding-tool and machine research could be commenced using devices for passive dynamic vibration-absorption. With a cutting force generated between the tool and workpiece, the magnitude will depend largely on the extent of engagement of the cutting edge.
The force strains the structure elastically and causes displacement of tool and workpiece, thus altering the engagement of tool and work: the undeformed chip-thickness. Any disturbance in the flow in the cutting process, such as that caused by irregularity, such as a hard spot, causes a deflection that, however small, alters the undeformed chip-thickness, with variation of the cutting force leading to vibrations. If left unchecked, the initial vibration can be self-sustaining and build up with the machine to oscillate in its natural mode of vibration.
Countering vibrations
The basic idea to counter vibrations originating from the cutting process was to have a mass suspended by springs in a fluid inside the tool’s shank. At the beginning, the tool was in most cases a boring bar but it was later found to be fully applicable in other types of tools that may be susceptible to vibrations. The extra mass was designed to vibrate at a different frequency to that generated in the cutting process, causing a neutralising (dampening) effect. Vibration tendencies in machining cannot, of course, be entirely eradicated, but may be minimised to a safe level with the right dampening facilities in the tool.
When the pioneering TNS-type dampened boring bars were introduced in the 1970s, it was a revolution for internal turning of deep holes. The bars were made available in three diameters, designed for overhangs of up to 10 times the diameter. These bars had exchangeable cutting heads that were radially adjustable and allowed different holder-types and indexable inserts to be used. In accordance with the original idea, the design included a slug of heavy moderating-mass, spring-suspended in a special type of oil. The oil took up the energy from any vibrations generated during machining and turned this into heat, which was absorbed by the oil. The inertness of the slug, and thus the frequency of vibration tackled, could be set with an adjustment screw on the bar. This setting altered the tension of the suspension to achieve optimum dampening. The boring bars also had a built-in system for coolant flow to help evacuate chips.
While rather primitive by today’s standards, the improvement in internal turning with this design was dramatic: material removal capacity could be doubled. Machining not possible before could be performed and operations that were painstaking bottlenecks were made more efficient. The surface finish at a bar-overhang of eight times the diameter, with the higher cutting data, was reduced to Ra 1.3 microns with the tuned bar as opposed to Ra 8.8 microns with a conventional solid boring-bar – a breakthrough in finishing holes.
In time, a short, standard boring-bar – for use up to overhangs of seven times the diameter – was introduced, having the advantage of not needing any setting to tune the bar to optimise dampening. This was a substantial improvement in the development of tuned boring bars, in that the vibration frequencies within the overhang area of the bar could be completely covered by the design of the dampening arrangement. It was an innovation that made the application and use of dampened tools easier and cut down-time in machines.
The next step was to make use of the tool material properties: that of cemented carbide. It was not only to be used as the material for the best cutting edge but also to increase the static stiffness of the tool. Cemented carbide has an average stiffness 2.5 times that of steel and was employed in the form of sleeves secured round the boring bar. In this way, the overhang of dampened boring bars could be increased to 12 times the diameter. It also meant that there was scope for tools to be used as rotating tools and for using the full potential of the cutting data possible with cemented carbide indexable inserts. With this development, dampened tools took on a much wider role as an important problem solver.
Demanding dampening
Components within several manufacturing industries have evolved to include deep and sometimes complicated bores that need machining. Aerospace, energy and die-and-mould industries are examples of some with very demanding holes or compartments. But also many manufacturers in general engineering, automotive and machine-making have scope to apply dampened tools to solve problems. The unique anti-vibration concept has been available in standard tools as well as lending itself well to engineered tools, designed to suit specific machines, components and operations. Examples included cylindrical, tapered, bent and elliptical boring-bar sections – the aim being to maximise rigidity to dampen vibrations when deep tool-accessibility is needed.
The expansion of the oil and gas industry means a number of deep-bore components have to be machined efficiently and securely, with no incorrect cuts. Flat-bed lathes have for some time now experienced a rebirth partly as oil-country lathes, being more rigid and better equipped to machine the long bores needed in the oil-exploration parts. A range of dedicated dampened boring-bars were developed early in a diameter range of 80-300 mm, ideal for roughing and finishing when bars capable of 10 times the diameter are needed – with specials that can cope with tool-overhangs of fifteen times the diameter. The first of these were manually tuned, through a radial tuning screw and had exchangeable cutting-heads. Examples of carbide-reinforced bars that have been applied to cope with the largest diameters weigh almost three tons. These bars provide a solution for many demanding operations and save delivery times of parts needed at short notice in oil fields.
For the masses of slant-bed CNC-lathes, used widely in all walks of manufacturing, a standard range of pre-tuned dampened bars was also an early development, typically for a diameter range of 16-100mm. Equipped with small, light cutting units, mounted through a serration-type coupling, set-ups are easy, quick and secure to change. A short and long version of boring bars was initially available for up to seven and 10 times the diameter, respectively. These tools pioneered an efficient, easy-to-apply solution to many varied internal-turning operations for many different applications in different industries.
This range can be said to be the forerunners to the Silent Tools of today, where cylindrical clamping, Coromant Capto, CoroTurn SL and QC are employed for internal turning, grooving and threading. Quick changing of tools in turning machines have increasingly been seen as an important route to better machining economy and dampened boring bars have been a vital part of today’s options in turning centres where quick change of tools is a priority for green-light machining. Tuned tooling has moved on from being just a problem solver to being a qualified, proven operational optimiser.
Tools with vibration-dampening facilities, available in different size and reach, are also an important part of tooling in multi-task and mill-turn machines. Separately today, dampening units have been developed to be incorporated in Serration Locking blades, engineered particularly for the aerospace and energy industry to make deep and complex grooves, often in demanding materials.
Today’s programme of Silent Tools includes standard tools with diameters up to 250mm. The largest boring bar delivered up to now is with a 450mm diameter intended for an overhang of 10 times the diameter and weighing seven tons. Dampened tools are also on the increase for rotating-tool applications within the boring tool range for machining centres.
In large machines, for example when ISO-standard Coromant Capto C10 couplings are used, the quick-change facility is an advantage when boring bars stick out so far that it is hard for other operations to be carried out with the bar in the turret. Instead of setting-up the large boring bar every time, which takes an average of 40 minutes, the quick-change option takes only five minutes. Automatic tool change has also been installed for some machines, such as when a robot, which also changes components, typically changes a 100mm boring bar capable of machining with an overhang of 14 times the diameter.
Regarding examples of solutions, the application of large dampened boring bars was recently developed with a well-known maker of large mill-turn machines for the internal turning of long titanium-titanium components. The engineered bars have to have a reach of 13 times the diameter – 176mm diameter with an unsupported reach of 2300mm. They also have to have automatic tool change at the front of the bar. Furthermore, to provide easily evacuated chips for operational security, the bar was equipped with an ultra-high-pressure coolant supply of 350 bars, with modern nozzle-technology for jet-assisted chipbreaking at the cutting edge to form satisfactory chips for evacuation, ensure the right surface finish, and improve tool-life and process security.
This form of state-of-the art vibration-dampening tooling with high-pressure coolant assisted machining has meant a lot more than problem-solving – it took its place as part of the planned solution where it was instrumental in boosting the productivity and material removal rate, along with the process security to eliminate any scrapped parts.