Maintenance planning for condition monitoring in a high-speed machining shop

Transcription

1 Maintenance planning for condition monitoring in a high-speed machining shop S. Goodfellow,(i) A. Starr,(2) D.G. Ford(3) (i) Micro Metalsmiths Ltd, Ings Lane, Kirkbymoorside, N. Yorkshire,YO62 6DW, UK (2) Manchester School ofengineering, University of Manchester, Oxford Road, Manchester, Ml 3 9PL, UK (3) The School of Engineering, The University of Huddersfield, Queensgate, Huddersfield, HD1 SDH, UK Abstract In manufacturing operations the availability of machine tools is critical to plant efficiency. A maintenance audit has been conducted at a facility operating high accuracy non-ferrous investment casting and high speed CNC milling. Maintenance has been planned using RCM derived methods. Novel condition monitoring techniques have been developed to prevent axis and spindle faults which rapidly affect finish quality. This paper reports progress in the integration of on-line error correction and condition monitoring. 1 Introduction The operation of the machine tools is critical to the production processes. The machine tools are high capital plant items, with a high production rate, but the economic justification for such machines is sensitive to availability. Following typical teething problems after purchase, which produce poor reliability and poor quality work, different machine types settle into patterns of good operation, interrupted by periods of poor performance and occasional system failures, which are only beginning to be understood. Previous work has shown that the limited amount of published failure data supports the findings at the collaborating industrial sites [1]. Empirical and statistical studies agree that the most commonly occurring faults in CNC machine tools are [2]:

2 Spindles and router heads (bearing failures); Tool changers; Pallet loaders; Controllers. Much research has concentrated on tool wear monitoring, and application of this area of work is currently progressing at the partners' sites. At the host site an extensive maintenance audit revealed shortcomings in the treatment of the most critical faults. These were addressed with a top-down structured planning exercise based on the RCM philosophy. The most critical faults affect machine availability and product quality. The product quality is particularly sensitive to degradation of several machine tool sub-systems, including axes, spindle and tool tips. In this application error compensation of the machine tools is central to the strategy. The measurements for the error compensation area also a readily available input for condition monitoring. This paper describes the maintenance audit, the formalisation of procedures and choice of a software package to support the maintenance management, including condition based maintenance. The techniques used are described, and some sample results are illustrated. 2 Description of the application 2.1 Introduction With a change in company direction it was necessary that the latest methods of preventative maintenance were addressed to ensure that the level of production provided could compete on the world market. Recent graduates were introduced at the company using a Teaching Company Scheme grant to conduct feasibility studies into which methods could be successfully introduced into the company. Any method deemed appropriate was to be integrated during the duration of the project. 2.2 Micro Metalsmiths Ltd Micro Metalsmiths is an experienced manufacturer of intricate non-ferrous components for a wide selection of customers. The company established over 30 years ago, with the now Chairman Mr Christopher Shaw, developing a successful high precision investment casting technique. This technique combined with his expertise in microwave products led to the successful entrance into the microwave casting market. The casting process uses injected wax components and a very high grade ceramic investment to achieve the high tolerance castings. The machine department has developed many special techniques over the years in the mastering of high tolerance machining of castings. To fully utilise the extensive machining knowledge at the company the decision to offer machined from solid components has developed in recent years. This has meant a culture change from a company that has been able to determine

3 Laser Metrology and Machine Performance 53 it's own -delivery schedules to a company that needs to compete on the established 'Just in Time' market. 2.3 High Speed Machines To ensure that the company could compete on the market level new machine tools were introduced. The company took delivery of four machines that have spindle speeds of up to 15,000 RPM. The machines are all manufactured by Cincinnati Machine Tools and offer adequate specification for the level of precision needed. Two configurations of machine exist: A four axis horizontal machining centre with three axes associated with the tool motion and a rotary table with the work piece motion. A three axis vertical bridge type machining centre with two associated axes with the tool and the remaining axis being linear movement of the work piece. Both machine types share common construction modules, with spindle and axes drjye units all governed by Cincinnati Milacron 850 controllers. 2.4 Maintenance planning With the change in machining needs it quickly became apparent that the current style of maintenance was unable to supply the required service. It was necessary to use a structured approach to the analysis of maintenance requirements, since the existing procedures were informal, and a high level of on-failure maintenance occurred. Maintenance history was sparse. A thorough audit of existing procedures and plant was followed by a formalised criticality analysis, as an input to a modified RCM approach. A maintenance plan was devised which incorporated a mixture of preventive strategies to optimise life cycle costs. A computerised maintenance management system with vibration monitoring and analysis capabilities was installed. 3 Techniques The selection of condition monitoring techniques must address the fault modes. Here we have identified product quality as a symptomatic effect of a number of potential faults and it is a natural key performance parameter. Spindle faults, as is characteristic of a rotating system, manifest themselves clearly in classical vibration analysis [3,4]. Tool wear is shown in a variety of vibration, acoustic and power signals [5]. Axis faults, however, are shown in product quality as dimensional faults and surface quality defects and the purpose of calibration is to prevent the faults reaching the product. Calibration is traditionally an occasional procedure but high tolerance applications use laser interferometry followed by periodic checking with a ball bar [6]. These are primarily intended for error correction either as a one-off measure or as an on going strategy, but ball bar checks are rapid enough to consider as a condition monitoring technique.

4 54 Laser Metrology and Machine Performance 3.1 Ball bar tests In parallel to this, methods were applied which are gaining momentum in the machine tool sector for calibration and compensation. These included circular tests using a ball bar, laser interferometry, and artefact comparisons. All of these tests form inputs to the condition monitoring system implemented at the company. Fastened tm Machine Bed Figure 1: Ball bar The initial phase of the implementation was to conduct full laser interferometry tests of the machines so that their individual error maps could be plotted. The machines were adjusted and tuned to their optimum levels of specification before trending analysis commenced. By setting alert levels on measurements the technique can be used on a monthly basis to spot deterioration in machine tool performance. Figure 2: Typical ball bar output The monthly checks have been successful in highlighting a number of problems ranging from servo mismatch to machine construction faults.

5 Laser Metrology and Machine Performance Vibration Analysis Vibration techniques were developed so that reciprocating areas of the machines could be observed for condition. The main function of this analysis was to observe the condition of spindle bearings, with this being the fundamental concern of a machine tool. The techniques were extended to include monitoring of pumps, motors and carriers. Failure frequencies for bearing types within the machines could be calculated at input into the analysis software to flag up various faults. Again levels could be set to determine what remedial action was necessary. Problems discovered by vibration monitoring range from leaking seals in a gusher pump to dry bearings in a spindle. 3.3 Oil Comparison Oil comparison methods and monitoring were introduced to ensure that the hydraulics and lubrication units of the machine could be monitored to observe contamination or degradation. Wear or failure of part of the hydraulics system usually manifests itself as an increase in particles within the hydraulic reservoir. This highlights impending problems and also makes the oil unsuitable for purpose. Levels are monitored until they reach a pre-set level once this has been achieved the system is flushed and cleaned and any remedial action is taken. A major oil consideration with metal cutting is the functionality of the cutting fluid used during the machining cycle. Daily procedures were implemented to ensure that coolant life was maximised. Coolant condition is an ideal means for determining lubrication efficiency of a machine, as all machines in the company use total loss system. This tramp oil can be measured, when removed from the coolant using either skimming methods or coalescent filters. Impending Health and safety regulations brought the need for bacterial and fungal testing of the fluids to ensure correct working conditions. 3.4 Artefact Comparison With the majority of machines at the company having probing capability, the use of an artefact to check machine specification was able to be developed. The artefact being a collection of straight edges, steps and rings that can be measured within the machine envelope. Errors in machine positional accuracy and repeatability can be detected. This method is also useful for determining the transfer accuracy and repeatability of pallet change systems.

6 56 Laser Metrology and Machine Performance Results Figure 3: Artefact on machine Although at the time of this paper the project has not finished the benefits received have already started to reward the company. A full maintenance strategy has been implemented into the machining department that utilises the applicable elements of failure, planned and condition monitoring maintenance techniques. The techniques discussed in section 3 being the condition monitoring techniques used. The regular ball bar tests and analysis have proven to be a key part of the condition monitoring of the machine tools at the company. Problems have been able to be remedied to planned time-scales, ensuring breakdown failure is kept to a minimum and production output is not affected by maintenance. 5 Conclusions Whilst there are many methods that can be used to observe machine tool behaviour, certain methods lend themselves to particular applications. It has been proven during this project that the use of the ball bar as a condition monitoring tool is the most effective for determining machining capability in this particular application. Whilst the other methods tried are used to some degree they are used to find only certain faults where the ball bar spots a wider range of problems. Equipment and measurements obviously have their price but when compared to the expense of scrapping high value added components the return can be realised in a matter of months. The current work flow in the department has greatly improved with work in progress at an all time low thanks to the greater capability of planning.

7 Laser Metrology and Machine Performance 57 6 Acknowledgements The authors gratefully acknowledge the support of the University of Huddersfield, Micro Metalsmiths Ltd, Renishaw pic, and the Teaching Company Directorate (Grant no 1766). 7 References [1] Starr A G, Mounsey, R, 1997, Selection Of Condition Monitoring Techniques For Machine Tools, 10th International Congress on Condition Monitoring and Diagnostic Engineering Management (COMADEM 97), Espoo, Finland, Vol. 2 pp , ISBN X [2] Davies, A., et al, 1993, Review of research into machine tool breakdown diagnostics, Proc. Comadem 93, Bristol UK, pp , ISBN [3] Stein, J.L., Tu. J.F., 1995, On line preload monitoring for anti-friction spindle bearings of high speed machine tools, Trans. ASME, Journal of Dynamic Systems, Measurement and Control, vl 17 pp [4] Williams, J.H., et al, 1994, Condition based maintenance and machine diagnostics, Chapman & Hall, London ISBN [5] Smith, G.T., 1993, Trends in tool condition monitoring techniques wear and cutting forces on turning centres, Proc. Comadem 93, Bristol UK, pp , ISBN [6] Blackford, D.W., Smith, G.T., 1993, Correlation between a laser interferometer calibration of a turning centre and a ballbar check, Proc. Comadem 93, Bristol UK, pp , ISBN