A VIBRATION-FREE GRINDING MACHINE.

SPECIFICATION.

This invention relates to a hand-held power tool to be used primarily for grinding and similar operations.

The static and dynamic vibration generated by the machine are dampened by means of shock and vibration-absorbing elements positioned in a special geometrical arrangement, between the relatively lightweight motor and spindle unit and the outer case, which is equipped with handles. This enables the motor and spindle unit to move in three dimensions relative to the outer case. The working movement of the shock and vibration- -absorbing elements is never exceeded by the amplitude of the vibration and the working speed is much higher than the cri¬ tical resonance speed of the machine.

BACKGROUND.

The vibration generated by hand-held power tools e.g., chain saws, grinding machines, impact wrenches etc. is liable in time to injure the operator. Injuries caused by vibration are known to result in circulatory changes in the fingers and hands. These symptoms, known as "white fingers", include numbness (parasthesia) and loss of temperature and tactile sensations. When using high-powered tools, vibration is more easily transferred to the hands since the operator has to hold such tools very firmly. This applies particularly to large grinding machines which sometimes have a power output of many horsepower/kilowatt and are used together with dep¬ ressed centre grinding wheels, cutting wheels or cup wheels. The machine has to be held differently for different grinding operations depending on which grinding wheels are used. A cutting wheel has a radial contact with the workpiece whilst

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other grinding wheels have an almost axial contact. The angle of contact also varies according to the type of work to be carried out. The machine has to be held in the best way to balance the forces resulting from the weight of the machine, the operating pressure on the work piece, and the grinding friction force.

Machines that vibrate with a constant frequency and amplitude are easier to devibrate. For example, chain saws have succes¬ sfully been equipped with devibrated handles. However, the situation for grinding machines is much more complicated. A devibrated handle on a grinding machine usually consists of a rubber block mounted between the machine and the handle/s. The improvement is marginal since the handles are light in weight relative to that of the machine. The rubber blocks need to be rather stiff to accomodate the large bending moments the handles are exposed to during certain grinding operations. Such a machine is inconvenient and tiring to hold. This results from the operators' hands being more widely spaced due to the presence of the rubber blocks. This results in higher bending moments on the wrists which leads to the operator having to hold the machine even more firmly.

The frequency and amplitude of the vibration changes during the grinding operation due to the wear of the grinding/cutting wheel. Differences in the wear make the wheels deviate from their original rotational symmetrical shape which may become eccentric, oval or polygonal. These differences in shape com¬ bined with different angles of contact and operating pressures, result in grinding machines with devibrated handles similar to those of e.g., chain saws, the machine will easily enter a critical resonance phase. The same resonance phase easily occurs in grinding machines where the vibration-damping ele¬ ments consist of rubber rings such as O-rings which compress far less than the amplitude. With a machine running at a criti¬ cal resonance speed, the vibrations become amplified and are transferred to the hands of the operator.

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THE PURPOSE OF THE INVENTION AND ITS MOST IMPORTANT CHARACTERISTICS.

The intention is to isolate the vibration of a hand-held com¬ pressed air-driven grinding machine as closely as possible to the source that is at the grinding wheel, and to prevent as efficiently as possible the vibration from spreading to the handles and to the hands of the operator. The machine, which is equipped with a speed governor, works at a speed above that of the critical resonance speed. To keep the critical resonance speed as low as possible, the drive unit i.e., the pneumatic motor with its integral drive spindle, is made as light as possible. A low weight has been achieved through manufacturing certain parts, such as end plates, of aluminium instead of steel. At the same time, numerous holes have been drilled in the rotor to lighten it. The motor housing is supported by soft shock and vibration-absorbing elements which are positioned in such a way that the vibration caused by the imbalance of the grinding wheel and by torsional vibration of the motor are absorbed in three dimensions. When the grinding wheel is not balanced, it automatically becomes centred around its outer periphery while the rotary centre moves eccentrically or like a pendulum, depending on the size and the shape of the grinding wheel. However, the motor housing^ support in the outer case is sufficiently rigid to resist the operating pressure against the workpiece without the the motor housing touching the outer case. Side forces which occur at different angles of contact between the grinding wheel and the workpiece are absorbed by the upper shock and vibration-absorbing elements, each of which is mounted with its compression/elongation axis intersecting approximately the contact point/periphery of the grinding wheel. The trigger handle and the support handle are mounted rigidly on the outer case without any resilient elements while the whole drive unit remains "floating".

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GENERAL DESCRIPTION OF FIGURES 1-6.

Figure 1 shows a vertical section through the grinding machine fitted with a depressed centre grinding wheel. The drive unit with its integrated drive spindle is shown as well as the upper and lower shock and vibration-absorbing elements. Figure 1 also shows the compressed air regulating valve together with a flexible membrane which seals the motor housing and the outer case.

Figure 2 shows a vertical section of the lower part of the machine with a cutting wheel installed. In Figures 1 and 2, the machine is fitted with fixed wheel guards.

Figure 3 shows a vertical section of the lower part of the machine fitted with a cup grinding wheel with an integral rotary wheel guard.

Figure 4 shows a vertical section of the machine where the lower positioned shock and vibration-absorbing element con¬ sists of a resilient ring. Figure 4 is another embodiment of the innovation.

Figure 5 shows a section along the A-A line in Figure 1 with the positioning of the lower shock and vibration-absorbing elements. Two of these elements are mounted radially and one tangentially. With this arrangement, all vibration and shock forces from any direction are absorbed.

Figure 6 shows a section along the B-B line in Figure 1 with the arrangement of the upper shock and vibration-absorbing elements. This section also applies to Figure 4.

DESCRIPTION OF PARTS 1-16 IN FIGURES 1-6.

Part 1 designates the outer case, and 2 the housing of the drive unit with its integral spindle, 13. The upper shock and vibration-absorbing elements are designatet 3 and the lower ones 4 and 4'. The grinding machine can be fitted with

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different types of grinding wheel; 5 shows a depressed centre grinding wheel, 6 a cutting wheel and 7 a cup wheel. When work is carried out with a depressed centre grinding wheel or with a cutting wheel, the machine must be provided with a fixed wheel guard, 14. The trigger handle, 9, consists of a pneumatic valve, 15, which is actuated by the trigger, 11. Compressed air flows from the pneumatic valve to the air motor through a flexible hose, 10. When operating the grinding machine, the operator has to hold the trigger handle, 9, and the support handle, 8. A flexible membrane, 12, prevents the exhaust air from escaping through the gap between the outer case, 1, and the housing of the drive unit, 2. Instead, an expansion chamber is formed between the outer case and the housing of the drive unit and it acts as a silencer. The lower shock and vibration-absorbing elements, 4 and 4', as well as the membrane, 12, can be replaced by a resilient ring, 16.

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