Challenges in machining thin-walled work pices
Due to its precision and flexibility multi-axis milling is a key technology for realising the above described market potential for the manufacturing of thin-walled work pieces. However, since thin-walled work pieces show lower stiffness than solid bodies, there are process-related static displacements and dynamic vibrations of the work piece during the milling process which currently limit the efficiency of milling (see Figure 2: 1 and 2).
For example, form errors of the work piece and resonance vibrations with large amplitudes lead to poor surface quality and significant tool and work piece loads. In extreme cases resonance vibrations destroy the tool or the work piece. Particularly thin-walled work pieces made of hard-to-machine materials cause reduced tool life, poor process stability and therefore reduced process performance.
Challenges in machining thin-walled work pieces (Figure 2)
Furthermore, work piece deformations and displacements due to released residual stresses and clamping device induced forces are crucial factors which reduce the process stability (see Figure 2: 3 and 4). The consequences of the mentioned challenges are increased process time, manual refinishing, iterative process adaptation and scrap.
An exemplary application of manufacturing thin-walled work pieces is given by 5-axis milling of long and thin-walled turbine blades (see Figure 3). In this application problems are static work-piece deflection and dynamic vibrations. The static work piece deflection is caused by the milling forces which lead to form errors of the turbine blade. The dynamic vibrations are induced by the excitation of the tool engagement which results in remaining chatter marks and surface defects.
Due to these special challenges associated with the milling of thin-walled work pieces, process set-up is a very laborious. Process parameters such as feed rate and spindle speed are determined based on operator experience and numerous trial and error loops. Even if sufficiently stable processes for the manufacturing of thin-walled work pieces can be achieved in the end, they usually do not represent the most efficient set-up possible. In many cases intuitive measures taken during process setup result in suboptimal parameters, e.g. the feed rate is reduced where a variation of spindle speed would be a lot more effective to stabilise the work piece during machining. Therefore, milling processes are often operated at inefficient production parameters such as very low speeds to avoid work piece excitation during milling, which leads to a reduction of process efficiency in terms of time and production resources.
Taken together, the current situation leads to a number of severe challenges, which negatively impact competitiveness:
- Difficulty to achieve high work piece quality, due to vibration and processes forces, in industries that have very high safety and quality requirements such as aviation and space. Therefore, in many cases manual finishing of the work pieces is required.
- Achievement of acceptable work piece quality only through time and resource consuming trial and error identification of process parameters that circumvent critical process states. This often results in slow and inefficient processes
An adequate solution for these challenges is highly needed by industry. To overcome the above described limitations and to foster the competitiveness of European industry, multi-axis milling processes have to be improved to achieve the required accuracy and efficiency. Therefore, static forces, vibration, deformation and displacement of thin-walled work pieces during machining have to be minimised. To be effective, these improvements have to be combined with better process planning and setup methodologies to reduce the reliance on trial and error. Taken together, these measures could ensure accurate and efficient high-performance processes for thin-walled work pieces.