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No matter how hard we try, it is virtually impossible to obtain perfect, flawless pieces of ware from the kiln every time. Kiln dirt, decal and decorating faults, and forming imperfections often appear on the surface of the ware, and these flaws must be corrected before the quality of the piece will be acceptable for sale.
In many conventional operations, highly skilled workers use grinding lathes and polishing stones to remove surface imperfections from ceramic and glass products. However, this process is time-consuming and costly, and it can be difficult to find workers with enough experience to carry out these tasks successfully. As a result, a number of companies have instead begun using a double-arm, bench-mounted abrasive belt grinder*—a machine that carries out both the grinding and polishing tasks in a single unit. With the use of this system, companies can save time and money while ensuring that the products shipped from their facility are of the highest possible quality.
Grinding and PolishingThe grinder is fitted with two quick-change arms, which carry wheels of various shapes and densities. An abrasive belt is used on one arm to grind out imperfections, while the second arm carries a cloth or felt belt that is used with suitable compounds to polish the ground area, thereby allowing grinding and polishing at one workstation. In many cases, only one type of belt is necessary on each arm for all types and sizes of products. This avoids the old method of changing wheels, depending on the article size and shape and on whether the article requires polishing or felting. The operator can carry out both grinding and finishing without putting the item down.
When the item to be worked is brought into contact with the belt, the arm rises slightly to give a cushioned contact. This prevents the machine from “grabbing” the item and makes the machine easy to use, even by inexperienced operators.
The double-arm grinder can be used to overcome all types of surface imperfections that occur throughout the production process, removing them without a trace at both “in glaze” and “on glaze” stages. It corrects faults on the ware such as kiln dirt and decal imperfections, simplifies gold flake removal on gold burnished pieces, and grinds underglaze faults prior to glazing. The system is specifically designed for bisque and glass grinding, glaze and finished polishing, pin mark removal, and finished mending. Both polishing and grinding can be performed prior to reworking. At the final inspection stage, selecting, polishing and grading can be undertaken in a single operation, prior to packing items for shipment.
Training Unskilled StaffConventional methods of grinding and polishing using lathes and polishing stones require high levels of skill, acquired from experience over a considerable amount of time from people employed as polishers. With the double-arm grinder, an unskilled operator can be trained and become fully proficient and productive in two weeks due to the simplicity of the machine. Operators who were previously trained in conventional grinding and polishing methods can easily switch to using the double-arm grinder and can substantially increase their throughput within one month of using the machine. One ceramic manufacturer even claimed an improvement of more than 100 percent for existing operators within two weeks.
Reliability and Safety FeaturesA number of features have been built into the double-arm grinder to ensure safe, reliable operation. A residual current device (RCD) protector plugged into a single-phase socket supplies power to a solid-state inverter, which is housed in the base of the double-arm grinder. The solid-state inverter converts the power into a three-phase supply that is used to power a reliable, double-ended, three-phase motor. The machine’s speed control is stepless between 1000 and 7500 rpm, and both the inverter and motor are fitted with overload and overheating protection.
Additionally, due to the RCD protection, a slurry composed of a polishing compound and water can safely be applied to the cloth or felt belt, enabling the system to be used for the final finishing of crystal, soda glass, optical glass, china, porcelain and other fine materials.
If any problems arise during the grinding or polishing operation, an emergency stop button can be pressed to manually cut the main electricity supply. The motor will not re-start when the supply is reconnected until the “start” button is pressed. A fan draws air from the rear of the machine to cool both the inverter and the motor.
Conventional methods of grinding and polishing are loud and incur high levels of vibration. Operators can suffer from “vibration white finger”—damage to circulation, usually to the fingers, hands and arms, arising from contact with a vibrating tool—if they do not true the wheels and mops on the spindles every time they are changed. These conditions can result in excessive downtime and costly workers’ compensation claims.
The vibration levels of the double-arm grinder are less than 2.5 m/s2, which is below the recommended safe limit of 2.8m/s2** and provides a much safer environment for operators. As a result, operators are far less tired at the end of their shift, even though their throughput has been greatly increased. Noise levels are largely determined by the type of item being ground and/or polished; however, these also tend to be much lower compared to conventional processes.
Manufacturing ImprovementsBecause the double-arm grinding machine is easier to use than conventional grinding and polishing methods, it can greatly reduce the amount of time required to remove flaws from defective ware, enabling manufacturers to significantly improve their production throughput. Additionally, highly skilled workers are no longer required to carry out the grinding and polishing operations; instead, unskilled or semi-skilled people can be recruited at lower labor rates.
Changing the machine’s arms and belts is also simple and does not require special mechanical skills. Finally, the grinder uses less than 4 amps of electricity to quickly achieve an excellent finish on all types of ceramic and glass products.
As a result of these benefits, manufacturers using the double-arm grinder have experienced significant cost reductions compared to using conventional grinding and polishing methods. One ceramic manufacturer reported that the output from each of its finishing stations increased by 150 percent, which resulted in a significant labor savings and a payback time of just three months on the machine.
The double-arm, bench-mounted abrasive belt grinder has been proven on a wide range of china, porcelain and earthenware tableware products and is especially beneficial on higher-value items. However, it is not just confined to tableware ceramics. The system has also demonstrated advantages on industrial ceramic products, glass tableware and sheet glass, as well as in unique processes for the engineering and turbine industries.
References:*The Double Bench Beltit, supplied by Seco Engineering, Fleet, Hampshire, UK
**Great Britain’s Health and Safety Executive (HSE) has set a recommended safe limit of 2.8m/s2 as the mean acceleration to which hands are subjected for an eight-hour period. The U.S. Occupational Safety and Health Administration has not established vibration limits but does recommend that vibration be kept as low as possible to avoid worker injury.