A number of current trends in manufacturing are magnifying the difficulty of creating precision bores and performing turning operations with extended-length tools. Demand for tighter tolerances and unfailing repeatability grows continuously. New high-performance workpiece materials are more difficult to machine, boosting stress within the machining system. To save time and money, manufacturers are consolidating multiple parts into single monolithic workpieces that require machining of deep bores and turning of complex components on multitasking machine tools.
Manufacturers seeking to overcome these challenges must study all elements of their machining systems and apply techniques and tools that will assure success. Among the key elements are machine stability, tool holding, workpiece clamping and cutting tool geometry. In general, solid fixturing, rigid tooling and careful tool application make up the basic foundation for accurate, productive boring and long-reach turning processes.
Producers of oil and gas, power generation and aerospace components are prime candidates for updated tooling and techniques because they regularly deal with large, complex parts with features that require the use of extended-length tools. Many of the parts are made from tough alloys that are difficult to machine and thereby produce high, vibration-generating cutting forces. In general, nearly any manufacturer can benefit from improving productivity and reducing costs in long-reach boring operations.
DEFLECTION AND VIBRATION
Deep boring is distinguished from other cutting operations in that the cutting edge operates in the bore at an extended distance from the connection to the machine. Long-reach internal turning operations feature similar conditions, and both these boring and turning operations can involve holes with interrupted cuts, as is the case on workpieces like pump or compressor housings. The amount of resulting tool overhang is dictated by the depth of the hole and can result in deflection of the boring bar or extended-length turning tool.
Deflection magnifies the changing forces in a cutting process and can cause vibration and chatter that degrade part surface quality, quickly wear or break cutting tools and damage machine tool components, such as spindles, and cause the need for expensive repairs and long periods of downtime. The varying forces result from machine component imbalances, lack of system rigidity or sympathetic vibration of elements of the machining system. Cutting pressures also change as the tool is periodically loaded and unloaded while chips form and break. Negative effects of machining vibrations include poor surface finish, inaccurate bore dimensions, rapid tool wear, reduced material rates, increased production cost and damage to tool holders and machine tools.
MACHINE RIGIDITY AND WORKPIECE FIXTURING
The basic approach to controlling vibration in machining operations involves maximizing the rigidity of the elements of the machining system. To restrict unwanted movement, a machine tool should be built with rigid, heavy structural elements reinforced with concrete or other vibration-absorbing material. Machine bearings and bushings must be tight and solid.
Workpieces must be accurately located and securely held within the machine tool. Fixtures should be designed with simplicity and rigidity as primary concerns, and clamps should be located as close as possible to the cutting operations. From a workpiece perspective, thin-walled parts or welded parts and those with unsupported sections are prone to vibration when machined. Parts can be redesigned to improve rigidity, but such design changes can add weight and compromise performance of the machined product.
TOOLHOLDING
To maximize rigidity, a boring bar or turning bar must be as short as possible but remain long enough to machine the entire length of the bore or component. Boring bar diameter should be the largest possible that will fit the bore and still permit efficient evacuation of cut chips.
As chips form and break, cutting forces rise and fall. The variations in force become an additional source of vibration that may interact in sympathy with the tool holder’s or machine’s natural mode of vibration and become self-sustaining or even increase. Other sources of such vibrations include worn tools or those not taking a deep enough pass. These cause process instability, or resonance that also synchronize with the natural frequency of a machine’s spindle or the tool to then generate unwanted vibrations.
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