Comparative occupational risk assessment in a CNC machine tool operator position

Abstract Law requires from employers to assess occupational risk in workplaces. A well-executed occupational risk assessment promotes the improve-ment of working conditions and reduces a negative impacts on employees’ health and life. Employers by conducting occupational risk assess-ment not only meet legal requirements but also benefit from it in two ways. They eliminate loss of an enterprise connected with employees’ absence, and they also decrease the probability of high compensation pay due to situations that have a destructive influence on the employees’ health. However, in order to benefit from such actions, firstly, it is necessary to select a risk assessment method properly, then to apply it and implement preventive measures. The paper presents the results of the occupational risk assessment carried out on the example of a CNC ma-chine operator by means three-stage methods in accordance with the PN-N-18001, PHA and Risc Score. Despite the subjectivity of the assess-ment, there were differences in the results. Thus, it is advisable to choose a risk assessment method skilfully, for example by comparing several similar methods.


Introduction
Occupational risk assessment is a primary prevention and health task in every workplace. This assessment is mainly intended to ensure the safety of employees and to eliminate or partially reduce existing threats by taking appropriate remedies. Employers, by carrying out occupational risk assessments, in their workplaces not only meet legal requirements, but also they benefit in two ways: they eliminate companies' loss related to absence of employees and to the need to pay high compensations in cases that are destructive for employees' health. Therefore, identifying hazards at every workplace is an extremely important part of an appropriate health and safety policy. Correctly estimated likelihood and risk tolerance statement of a particular risk, make employers and workers aware of how important it is to use health and safety regulations in everyday work circumstances and to use appropriate protection measures to prevent or minimize possible consequences of unforeseen dangerous situations. (ULEWICZ R., ŻYWIOŁEK J. 2016, KABER D., ZAHABI M. 2017 Occupational risk assessment is not a one-time process but a repetitive one. If changes have been made to the organiza-tion of work, requirements, protective measures using or new equipment or materials have been introduced, a risk assessment process should be repeated. An occupational risk assessment process should also be repeated if it is necessary to reduce the risk after the assessment was carried out, of course, after a prior selection and the application of appropriate safety measures. An occupational risk assessment should be performed by a team of at least two people, and, also, with appropriate knowledge and experience in this area. (KRAUSE M. 2011, WOŹNY A., PACANA A. 2014, NOWICKA-SKOWRON M., ULEWICZ R. 2016 The methodology of conducting occupational risk assessment includes 4 main steps: collecting information, identification of hazard, occupational risk evaluation and determination of occupational risk admissibility (PN-N 18002:2011).
There are many methods of occupational risk assessment, therefore, subject literature divides the methods of occupational risk assessment. Current division of methods include dividing into inductive and deductive, and quantitative and qualitative methods. It is important to select an adequate method out of such a variety to properly assess occupational risk in a particular workplace. Employees responsible for safety and health, sometimes try to use one risk assessment methodology for all job positions. It is not the most advantageous solution, as there are various risks in different workplaces (ROMANOWSKA-SŁOMKA I., SŁOMKA A. 2104) What is more, for example, there are different types of dangers that occur in locations where conventional cutting machines and CNC machines are situated. Hence, it seems advisable to diversify methods of risk assessments according to the risk in different positions and, at the same time, to discover as adequate as possible for a specific position. An attempt to do this is presented in the paper in relation to the position of the operator of numerically controlled machines.

Characteristic of the position of CNC machine operator
Machine toll-controlled with a computer (CNC) performs automated processing with CNC computer control. The computer integrated with a system takes over the whole control process as well as the control functions of the machine. This computer supervises all the movements in the machining process, machining parameters and auxiliary operations so as to obtain the product with the de-sired features of shape, dimension and specific surface. These facts affect the specific hazards of this workplace. Work at the operator's station is an individual work, typically 8 hours, sometimes shifted. This work requires a long period of focus, both visual and auditory concentration. During work, there may be increased energy expenditure, monotony of activity, monotonous position of the body. It is forbidden to wear loose clothing while working at the operator station. The microclimate of work can most often be assumed to be stable, which is beneficial for the tasks performed. The work should be performed in a closed, heated, well exposed and ventilated room.
Working as a CNC machine operator's connected with the risk of both dangerous and harmful or disruptive factors. Their occurrence is likely to result in an accident at work and it is imperative that an operator must follow the required health and safety measures every day.
Performing work in the operator's CNC ma-chine position is associated with the occurrence of dangerous mechanical factors causing injuries. Examples include moving parts of used machines, falling machine parts or hot surfaces. Hazardous substances also include the dangers of electric shock. The harmful factors occurring during the work in the operator position of CNC machines are, for example, noise and vibration, oil mists or grinding dusts, and temperature and humidity of the air. The onerous factors affecting the operator of a numerically controlled machine tool may include many hours of standing work, stress and physical exertion. Negative effects of such threats entail, but are not limited to, tendons and backbone injuries, limb injuries, but also psychological stress.

Risks at the position of CNC machine tool operator
Risks in a particular workstation after being identified should be classified. Danger that was identified in CNC machine tool operator's work-place is presented in the Tab. 1. It was supported with the Polish standard PN-N-18002 and the guidelines of the Polish Labor Inspection (PIP) (GÓRSKA E. 2012., PN-N 18002:2011, WOŹNY A., PACANA A. 2014. Protection measures are omitted in the table since the information is available in the literature.

Occupational risk assessment of CNC machine operator
As a method proposed for occupational risk assessment on a CNC machine operator position, a three-stage method according to PN-N-18002, a Risc Score method and a PHA method were chosen. The methodologies are described in the literature of the subject, for example (GÓRSKA E. 2012, KRAUSE M. 2011, ROMANOWSKA-SŁOMKA I., SŁOMKA A. 2104, WOŹNY A., PACANA A. 2014. In case of assessment based on the PN-N-18002 standard all risks have been classified into five groups, followed by probability and severity of consequences. Then, using the (WOŹNY A., PACANA A. 2014), the value of a risk and its admissibility in acceptable and unacceptable categories were determined (Tab. 2). The average value of risk occurring at the CNC machine operator's position, estimated with the PN-N-18002 standard is 1.25. It means that it is defined as small, so the risk is admissible. Table 3 shows the results of the conducted occupational risk assessment of a CNC machine tool operator using the Risc Score method. The average value of the risk in position of CNC operator calculated with use of the Risc Score method is 29.2, that according to the categories of this method qualifies it as acceptable, in a scale of: very big, big, significant, small and acceptable. Table 4 presents the occupational risk assessment of CNC machine tool operator, performed with use of the PHA method. The average value of risk at the workplace of numerically controlled machine tools, calculated by the PHA method, is 5.4. It is defined as permissible in a scale of acceptable, permissible and unacceptable.
In the next step, the obtained results of the occupational risk assessment were compared and presented in Tab. 5.
Tabela 5. Comparison of occupational risk assessment.

No
Identified threat The conducted with three methods occupational risk assessment determines the operator's occupational risk as acceptable, but there are differences in the results that may indicate the choice of an adequate risk assessment method.

Summary and conclusion
As a result of the occupational risk assessment, it can be observed that, according to all risk assessment methods that were used, the risk of injuries, burns, infections, diseases and pollination is permissible and acceptable. Differences occur in the case of dangers related to the occurrence of stressful situations. In accordance with the PN-N-18002 standard and with the PHA method a risk is permissible, while the Risc Score method identifies it as small, that indicates the need to periodically controlling the intensification of negative effects. Another difference occurs in case of threat of injuries and contusions. The Risc Score method, as the only one of the analyzed methods, classified these threats as significant, and therefore suggests actions to minimize the impact of this harmful factor. Noise has been classified similarly in each of the three methods analyzed. Nevertheless, it is worth to note that the most radical method for this occupational risk assessment at CNC operator position is the Risc Score method. In spite of the simplifications and assumptions that were made in hereby paper, the Risk Score method can be proposed to assess occupational risk for this workplace.