RISK ANALYSIS OF ABRASIVE WATERJET TECHNOLOGY BY MEANS OF FMEA METHOD

АНАЛИЗ РИСКОВ В ТЕХНОЛОГИИ ОБРАБОТКИ

ВЫСОKОСКОРОСТНЫМ ГИДРОАБРАЗИВНЫМ ЛУЧОМ С ПОМОЩЬЮ МЕТОДА FMEA

Ing. Agáta Radvanská, PhD 1., Ing. Sergej Hloch, PhD 2.

Faculty of Manufacturing Technologies 1,2,, Technical University, Slovak Republic Abstract: The paper deals with the evaluation of safety and noise level of technological system with abrasive waterjet cutting technology. The safety has been evaluated by means of Failure Modes and Effects Assessment Method. Noise as a negative phenomenon of abrasive waterjet cutting has been evaluated by design of the experiment. Further, the manufacturing system with abrasive waterjet machining and the cutting process was evaluated. Results show the abrasive waterjet factors significance and their effect to the noise environment. It has been found that the most significant is the focusing tube diameter. KEYWORDS: ABRASIVE WATERJET, RISK ANALYSIS.

1. Introduction

With the development of technology, the scientists and the technologists in the field of manufacturing are facing more and more challenging problems. The demand for the highest accuracy and surface finish, the challenge to produce critical surfaces and complex shapes has necessitated the use of non-traditional machining techniques. The use of such non-traditional machining techniques is found to be the best option for manufacturing complex dies and aerospace components with the required high precision and accuracy. Competition and scientific progress requires introduction of technologies that perform challenging claims of modern production in automation field, from economy, environmental and energy efficiency point of view. Abrasive waterjet cutting represents all of these claims. Nowadays represents cold precise, computer controlled shape cutting without any strain. These attributes poses this technology to the position of permanent use in the future, that represents excellent perspective for expansion in volume sectors, especially there, where the materials with excellent utility properties are used. Abrasive waterjet technology is for to-date high requirements on quality and productivity applied in full-automated workplaces with automatic CNC control. Flexible and smart automated technique application does not exclude the human being from the manufacturing process; just moves his working activities from strenuous jobs and jobs in malign environment to the areas of control, maintenance and operation management.

2. AWJ Production system factor analysis The following Figure 1 shows the manufacturing systems and subsystems of abrasive waterjet machining technology. Manufacturing system of abrasive waterjet cutting consists of subsystems, through which the cutting tool is created.

Technology operation system

Material flow

high-pressurewater

programme control run

Material

programme transferPC CAD/CAM

system

Material flow

CNC operation workplace

Water treatment

Water System Preparation

filtrate, permeate Pressure

generation

CNC production equipment

Waste

Product

Abrasive flowAttenuatorAbrasive hopper

1

Abrasive feeding system

Fig. 1 Division of assessed subsystems of AJW production system

Waterjet with the purity of redistilled water flows from treatment system and enters the system where pressure is generated and flows through the attenuator to the cutting head. In cutting process high pressurized water is transformed by orifice to the high-speed water.

3. Risk analysis method

Failure Mode and Effects Analysis is a method to assess production processes weaknesses and potential effects of process failure on the product being produced. Failure Mode and Effects Analysis emphasizes the importance of actions that can be taken to eliminate or reduce the potential causes leading to the process failures. However, it has been observed that manufacturing engineers are too occupied with how to make things work and thus fail to consider the potential pitfalls. Thus, it is imperative that Failure Mode and Effects Analysis is conducted throughout the process and should be revised whenever a change has been made to it. Failure Mode and Effects Analysis ensures that the manufactured products are met with the engineered product specifications and that the process defects do not result in product safety problems in the field. As a risk assessment method the Failure Mode and Effects Assessment method was used to evaluate the risk size. The method is semi-quantitative and evaluates the accident occurrence probability (O) and effects (S) seriousness and can be expressed as a combination of both factors as follows: R = OxS (1) The total risk (R) is expressed in risk matrix listed in (Sinay, 1997). Tab. 1. Risk matrix

Probability Catastrophic Cricital Marginal Negligible Very high 1 3 7 13 High 2 5 9 16 Probable 4 6 11 18 Not probable 8 10 14 19

Very low 12 15 17 20 Abrasive waterjet machining contributes to the manufacturing process by number of incipient dangers, which mostly contribute to the risk origin of the AWJ technology system operation. Effects of possible personal accidents are in general considered to be most serious; therefore the assessment of the possible risk is necessary. For general assessment of the defect cause it is necessary to nominate the systems that can significantly influence the work

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safety at technology of abrasive waterjet by their operational properties and performance function. Each estimated system is assessed independently (Tab. 1). At the selection of assessed systems not only common working process is taken into account, but also some exceptional activities. Very important part of the method is the estimation of assessed systems parameters. Tab. 2. Overview of possible dangers and their effects

Low pressure circuit Danger source Danger Effect

Electrical system, Pressure accessories, Hydraulic fluid.

Electric isolation damage, Fluid outflow, Fluid outflow in form of aerosol, Part breakage.

- falling, - killing, - bruising, - poisoning, - lungs damage.

High pressure circuit Danger source Danger Effect

High pressure accessories, High pressure fluid (permeate)

Damage of tubes, Outflow of permeate, Hit of operator by high pressure tubes breakage

- killing, - falling, - cutting, - amputation, - bruising.

Filtration system Danger source Danger Effect Deficient of water treatment, Impurities sedimentation,

Mechanical danger, Damage of device

- killing, - cutting, - bruising.

Abrasive feeding system Danger source Danger Effect Abrasive hopper, Abrasive flow, Recycling system

Impurities penetration, damage of abrasive cutting head.

- Production losses

Material machining Danger source Danger Effect

Abrasive waterjet Workpiece

Abrasive particle rebounding Abrasive waterjet rebounding Noise Workpiece fall, sharp edges, Physical and chemical composition of workpiece

- amputation, - hearing damage,- cutting, - lungs damage, - poisoning.

Operation system Danger source Danger Effect

Operation system, Manipulation system, Motion system, Abrasive waterjet.

Collision with manipulation and motion system, Affection of abrasive waterjet to operator, Dispersion of abrasive waterjet.

- amputation, - hearing damage,- cutting, - lungs damage, - poisoning.

The two elements, high speed waterjet and workpiece take part at the cutting process. In manufacturing automation of the AWJ cutting technology a number of factors influences the creation of defects and risks. In the Table 1 the risks for each subsystem of AWJ technology are assessed.

Fig. 2 Malfunction of the staff wit the material at the abrasive waterjet machining

According table 1 there are a number of risks associated with waterjet cutting, like most industrial processes designed to cut through materials much denser than the human body. Extreme care and proper training on the part of the operator are required to prevent injuries. Waterjet cutting should never be undertaken by someone who is intoxicated, taking judgment impairing prescription medications, tired or sick, or under the influence of other controlled substances. Another major risk associated with waterjet cutting is severe damage, especially to extremities, associated with inadvertent contact with the jet. Waterjet cutting is used to cut through extremely hard materials and is fully capable of removing a limb from an inattentive operator.

Fig. 3 Rebound of abrasive waterjet from material AISI 304 20 mm thick.

Fig. 4. Sharp edges, bottom of 25 mm thick stainless steel AISI 304 More commonly, exposure to the jet results in deep puncture wounds and internal bruising or bleeding. A common danger associated with waterjet cutting is eye damage. Should the human eye be exposed to a high pressure jet of water, it may suffer corneal scratches, detachment of the retina, or complete dislocation. Damage to the cornea can heal, but detachment of the retina or displacement of the eye can lead to blindness. Waterjet cutting is also associated with a high volume of noise, which can damage hearing at sustained levels. Tab. 3. Qualitative risk expression of the assessed system

Danger Accident probability

Effect seriousness Risk size

Verbal expression of

risk size

Electric current D I 8 Undesirable

Falling of persons A III 7 Undesirable

Water pollution E II 15 Acceptable

with inspection Oil dispersion inhalation D II 10 Undesirable

Hit by AWJ C II 6 Undesirable

Noise A III 7 Undesirable

Sharp edges A III 7 Undesirable

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Ear protection should be worn at all times when operating a waterjet, and the user should try to remain aware of the decibel level. Prolonged exposure to high volumes of sound can lead to tinnitus, difficulty in hearing, and ultimately deafness. Basing on the properties of evaluated systems risk size has been estimated according to Failure Mode and Effects Assessment method (Tab. 2). This induces the elements oscillation and results from the energy change that forms the acoustic field. This is manifested by noise, predominantly of high frequency, which has a negative impact on central nervous system of the CNC motion operator. To reduce this negative phenomenon and enhance operators’ safety it is necessary to recognize the most potential noise sources in water jet machining system. Results show the abrasive waterjet machining factors significance and their effect to noise environment. Abrasive waterjet manufacturing system consists of subsystems, by which the initial tool is created. Technological cutting process by hydroabrazive erosion is performed by means of cutting tool – abrasive waterjet – properties of which are not reduced due to the operation unlike it is at the conventional cutting knife. Noise is initially produced by vibration of solid objects, by turbulent motion of fluids, by explosive expansion of gases, or by other means. The pressures, amplitudes, and velocities of the components of the sound wave within the range of hearing are quite small. With a proper catcher, the abrasive waterjet is as loud as 90 dB. In the open air the noise level can reach 110 dB or more. Another disadvantage is mixing nozzle wear - the mixing nozzle needs to be replaced every 2-6 hours depending on the abrasive type being used and abrasive flow rate. Machined material participates on danger origin by many ways. Here belongs target material weight, its chemical composition and physical properties, material handling. Danger may be caused at material handling before or after manufacturing operations. The next dangers arise during the cutting process itself, at improper material placing or work clamping. Attention has to be given to the chemical composition of the material. The impact of dispersed particles to organism is different in case of allergens. The effects of the particles to the skin is different too, if the material is brittle or ductile. The possible dangers are drop of the work piece, cuts at material handling, dispersed particles inhalation.

4. Conclusion

Abrasive waterjet technology risk analysis proved the existence of numerous dangers that expose the operators at workplace. Except of acceptable risk the Failure Mode and Effects Assessment has shown the existence of undesirable and unacceptable risks. According to risk assessment, the most severe are risks caused by personal protective equipment omission, workplace noise, waterjet and rebounded abrasive particles hitting. Undesirable and unacceptable residual risks have to be delimitated or eliminated in given working system to acceptable level and it is necessary to create acceptable working conditions for stuff operators and hence raise the effectiveness of AWJ technological devices, their reliability, working safety. At the experiment from the occupational safety and health point of view, the limits of acoustic sound pressure level were exceeded. For audio frequency noise the overrun was 7 dB, for high frequency noise up to 26 dB. The noise elimination at abrasive waterjet machining can be achieved by reduction of the sources of acoustic sound pressure level. Regular hearing tests are highly recommended for waterjet cutting operators. This paper presents example of applied Failure Mode and Effects Assessment method in at abrasive waterjet machining risk evaluation. Failure Mode and Effects Assessment method creates database that can be applied on similar products or manufacturing operations. That reduces possibility of same failures repetition in future. Failure Mode and Effects Assessment method gives good results because of team work, whose members are experts in all significant design phases i.e. manufacturing process. Failure Mode

and Effects Assessment method is newer finished. This means that Failure Mode and Effects Assessment method constantly supplements itself and changes. Problems during Failure Mode and Effects Assessment method implementation can be Failure Mode and Effects Assessment team coordination, slow implementation of suggested measures in construction and manufacturing process as well discrepancy between design and manufacturing departments.

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Acknowledgment The authors would like to acknowledge the support of Scientific Grant Agency of the Ministry of Education of Slovak Republic, Commission of mechanical engineering, metallurgy and material engineering, for their contribution to project VEGA 1/4157/07.

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