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Monday, 14 March 2011 20:46

System Design in Diamond Manufacturing

Written By: Gilad, Issachar
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The author acknowledges the assistance of Mr. E. Messer and Prof. W. Laurig for their contribution to the biomechanical and design aspects, and to Prof. H. Stein and Dr. R. Langer for their help with the physiological aspects of the polishing process. The research was supported by a grant from the Committee for Research and Prevention in Occupational Safety and Health, Ministry of Labor and Social Affairs, Israel.

The design of manually operated work benches and working methods in the diamond polishing industry has not changed for hundreds of years. Occupational health studies of diamond polishers have identified high rates of musculoskeletal disorders of the hands and arms, specifically, ulnar neuropathy at the elbow. These are due to the high musculoskeletal demands placed on the upper body in the practice of this manually intensive profession. A study conducted at the Technion Israel Institute of Technology addressed itself to the investigation of the ergonomic aspects and occupational diseases relating to safety issues among craftsmen in the diamond polishing industry. The tasks in this industry, with its high demands for manipulative movements, include movements that require frequent, rapid hand exertions. An epidemiological review conducted during the years 1989-1992 in the Israeli diamond industry has pointed out that the manipulative movements experienced in diamond polishing very often cause serious health problems to the worker in the upper extremities and in the upper and lower back. When such occupational hazards affect workers, it produces a chain reaction that eventually affects the industry’s economy as well.

For thousands of years, diamonds have been objects of fascination, beauty, richness and capital value. Skillful craftsmen and artists have tried, through the ages, to create beauty by enhancing the shape and values of this unique form of hard carbon crystal formation. In contrast to the continuing achievements of artistic creation with the native stone and the emergence of a great international industry, very little has been done to improve some questionable working conditions. A survey of the diamond museums in England, South Africa and Israel allows one to draw the historical conclusion that the traditional polishing workplace has not changed for hundreds of years. The typical diamond polishing tools, working bench and work processes are described by Vleeschdrager (1986), and they have been found to be universally common to all polishing setups.

Ergonomic evaluation performed at diamond manufacturing setups points to a great lack of engineering design of the polishing workstation, which causes back pain and neck and arm stress due to working posture. A micromotion study and biomechanical analysis of motion patterns involved in the diamond polishing profession indicate extremely intense hand and arm movements that involve high acceleration, rapid movement and a great degree of repetitiveness in short-period cycles. A symptom survey of diamond polishers indicated that 45% of the polishers were younger than 40 years of age, and although they represent a young and healthy population, 64% reported pain in the shoulders, 36% pain in the upper arm and 27% pain in the lower arm. The act of polishing is performed under an extensive amount of “hand on tool” pressure which is applied to a vibrating polishing disk.

The first known description of a diamond polishing workstation was given in 1568 by the Italian goldsmith, Benvenuto Cellini, who wrote: “One diamond is rubbed against another until by mutual abrasion both take a form which the skilled polisher wishes to achieve.” Cellini’s description could have been written today: the role of the human operator has not changed over these 400 years. If one examines the working routines, hand tools and the nature of the decisions involved in the process one can see that the user-machine relationship has also hardly changed. This situation is unique among most industries where enormous changes have occurred with the entry of automation, robotics and computer systems; these have completely changed the role of the worker in the world today. Yet the polishing work cycle has been found to be very similar, not only in Europe where the polishing craft started, but in most industries all over the globe, whether in advanced facilities in the United States, Belgium or Israel—which specialize in fancy geometry and higher-value diamond products—or the facilities in India, China and Thailand, which generally produce popular shapes and mid-value products.

The polishing process is based on grinding the fixed rough diamond over diamond dust bonded to the polishing disk’s surface. Owing to its hardness, only grinding by friction against similar carbon material is effective in manipulating the diamond’s shape to its geometric and brilliant finish. The workstation hardware is composed of two basic groups of elements: workstation mechanisms and hand-held tools. The first group includes an electric motor, which rotates a polishing disk on a vertical cylindrical shaft, perhaps by a single direct drive; a solid flat table which surrounds the polishing disk; a bench seat and a source of light. The hand-held operating tools consist of a diamond holder (or tang) which houses the rough stone during all polishing phases and is usually held in the left palm. The work is magnified with a convex lens which is held between the first, second and third fingers of the right hand and viewed with the left eye. This method of operation is imposed by a strict training process which in most cases does not take handedness into account. During work the polisher assumes a reclining posture, pressing the holder to the grinding disk. This posture requires the support of the arms on the working table in order to stabilize the hands. As a result, the ulnar nerve is vulnerable to external lesions due to its anatomical position. Such an injury is common among diamond polishers and has been accepted as an occupational disease since the 1950s. The number of polishers worldwide today is around 450,000, of whom approximately 75% are located in the Far East, primarily India, which has dramatically expanded its diamond industry in the last two decades. The act of polishing is done manually, with each of the diamond facets being produced by polishers who are trained and skilled with respect to a certain part of the stone’s geometry. The polishers are a clear majority of the diamond craft force, composing about 80% of the overall industry’s workforce. Therefore, most of the occupational risks of this industry can be addressed through improving the operation of the diamond polishing workstation.

Analysis of the motion patterns involved in polishing shows that the polishing routine consists of two subroutines: a simpler routine called the polish cycle, which represents the basic diamond polishing operation, and a more important one called the facet cycle, which involves a final inspection and a change of the stone’s position in the holder. The overall procedure includes four basic work elements:

    1. Polishing. This is simply the actual polishing operation.
    2. Inspection. Every few seconds the operator, using a magnifying glass, visually inspects the progress made on the polished facet.
    3. Dop adjustment. An angular adjustment is made to the diamond holder’s head (dop).
    4. Stone change. The act of changing facets, which is done by turning the diamond through a predetermined angle. It takes about 25 repetitions of these four elements to polish a diamond’s facet. The number of such repetitions depends upon such aspects as operator’s age, stone hardness and characteristics, time of day (owing to operator fatigue), and so on. On average, each repetition takes about four seconds. A micromotion study as performed on the polishing process and the methodology used is given by Gilad (1993).

           

          Two of the elements—polishing and inspection—are performed in relatively static working postures while so-called “hand to polish” (H to P) and “hand to inspect” (H to I) actions require short and fast movements of the shoulder, elbow and wrist. Most of the actual movements of both hands are performed by flexion and extension of the elbow and pronation and supination of the elbow. Body posture (back and neck) and all other movements except wrist deviation are relatively unchanged during normal work. The stone holder, which is constructed of a square cross-sectional steel rod, is held so that it presses on blood vessels and bone, which can result in a reduction of blood flow to the ring and little fingers. The right hand holds the magnifying glass all during the polishing cycle, exerting isometric pressure on the three first fingers. For most of the time the right and left hands follow parallel movement patterns, while in the “hand to grind” movement the left hand leads and the right hand starts moving after a short delay, and in the “hand to inspect” movement the order is reversed. Right-hand tasks involve either holding the magnifying glass to the inspecting left eye while supporting the left hand (elbow flexion), or by putting pressure on the diamond holder head for better grinding (elbow extension). These fast movements result in rapid accelerations and decelerations that end up in a very precise placing of the stone on the grinding disk, which requires a high level of manual dexterity. It should be noted that it takes long years to become proficient to the point where work movements are almost embedded reflexes executed automatically.

          On the face of it, diamond polishing is a simple straightforward task, and in a way it is, but it requires much skill and experience. In contrast to all other industries, where raw and processed material is controlled and manufactured according to exact specifications, the diamond in the rough is not homogeneous and each diamond crystal, large or small, has to be checked, categorized and treated individually. Apart from the needed manual skill, the polisher has to make operational decisions at every polishing phase. As a result of the visual inspection, decisions must be made on such factors as angular spatial correction—a three-dimensional judgment—amount and duration of pressure to be applied, angular positioning of the stone, contact point on the grinding disk, among others. Many points of significance have to be considered, all in the average time of four seconds. it is important to understand this decision-making process when improvements are designed.

          Before one can advance to the stage at which motion analysis can be used for setting better ergonomic design and engineering criteria for a polishing workstation, one has to be aware of yet further aspects involved in this unique user-machine system. In this post-automation age, we still find the production part of the successful and expanding diamond industry almost untouched by the enormous technological advances made in the last few decades. While almost all other sectors of industry have undergone continuing technology change that defined not only production methods but the products themselves, the diamond industry has remained virtually static. A plausible reason for this stability may be the fact that neither the product nor the market have changed through the ages. The design and shapes of diamonds have in practice remained almost unchanged. From the business point of view, there was no reason to change the product or the methods. Furthermore, since most of the polishing work is done by subcontracting to individual workers, the industry had no problem in regulating the labour force, adjusting the flow of work and the supply of rough diamonds according to market fluctuations. As long as the production methods do not change, the product will not change either. Once the use of more advanced technology and automation are adopted by the diamond industry, the product will change, with a greater variety of forms available in the market. But a diamond still has a mystic quality that sets it apart from other products, a value that may well decrease when it comes to be regarded as merely another mass-produced item. Recently though, market pressures and the arrival of new production centres, mainly in the Far East, are challenging the old established European centres. These are forcing the industry to examine new methods and production systems and the role of the human operator.

          When considering improving the polishing workstation, one must look upon it as part of a user-machine system that is governed by three main factors: the human factor, the technology factor and the business factor. A new design that takes account of ergonomic principles will provide a springboard to a better production cell in the broad sense of the term, meaning comfort over long working hours, a better quality product and higher production rates. Two different design approaches have been considered. One involves a redesign of the existing workstation, with the worker given the same tasks to perform. The second approach is to look at the polishing task in an unbiased manner, aiming at an optimal, total station and task design. A total design should not be based on the present workstation as input but on the future polishing task, generating design solutions that integrate and optimize the needs of the three above-mentioned system factors.

          At present, the human operator performs most of the tasks involved in the polishing act. These human-performed tasks rely on “filling” and working experience. This is a complex psychophysiological process, only partially conscious, based on trial and error input which enables an operator to execute complex operations with a good prediction of the outcome. During periodic daily work cycles of thousands of identical movements, “filling” manifests itself in the human-automatic operation of motor memory executed with great precision. For each of these automatic motions, tiny corrections are made in response to feedback received from the human sensors, like the eyes, and the pressure sensors. In any future diamond polishing workstation these tasks will continue to be performed in a different way. As to the material itself, in the diamond industry, by contrast with most other industries, the relative value of the raw material is very high. This fact explains the importance of making maximal use of the rough diamond’s volume (or stone weight) in order to get the largest net stone possible after polishing. This emphasis is paramount throughout all the stages of diamond processing. Productivity and efficiency are not measured by reference to time only, but also by the size and precision achieved.

          The four repetitive work elements—“polish”, “hand to inspect”, “inspect” and “hand to polish”—as performed in the polishing act, can be classified under the three main task categories: motor tasks for motion elements, visual tasks as sensing elements, and control and management as decision-content elements. Gilad and Messer (1992) discuss design considerations for an ergonomic workstation. Figure 1 presents an outline of an advanced polishing-cell. Only the general construction is indicated, since the details of such a design are guarded as a professionally restricted “know-how”. The term polishing cell is used since this user-machine system includes a totally different approach to polishing diamonds. In addition to ergonomic improvements, the system consists of mechanical and optoelectronic devices that enable the manufacture of three to five stones at the same time. Parts of visual and control tasks have been transferred to technical operators and management of the production cell is mediated via a display unit that provides momentary information about geometry, weight and optional operation moves in order to support optimal operating acts. Such a design takes the polishing workstation a few steps ahead into modernization, incorporating an expert system and a visual control system to replace the human eye in all routine work. Operators will still be able to intervene at any point, set up data and make human judgements on machine performance. The mechanical manipulator and the expert system will form a closed-loop system capable of performing all polishing tasks. Material handling, quality control and final approval will still reside with the operator. At this stage of an advanced system, it would be appropriate to consider the employment of higher technology such as a laser polisher. At present, lasers are being used extensively to saw and cut diamonds. Using a technologically advanced system will radically change the human task description. The need for skilled polishers will diminish until they will deal only with polishing larger, top-valued diamonds, probably with supervision.

          Figure 1. Schematic presentation of a polishing-cell

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