The present invention relates to an arrangement in hand-held grinding machines intended for grinding button drill-bits and comprising a driven spindle which has a grinding disk mounted thereon.
Because button bits are able to remove more material than chisel-edge bits or other straight cutting-edge bits, button bits have been used to a much greater extent in all types of rock drilling operations. Initially, button drill-bits were used as disposable products and were scrapped as soon as the buttons were completely worn down. One reason for scrapping the bits was because of the lack of suitable grinding equipment by means of which such drill bits could be resharpened or reshaped. Attempts were made to use high speed hand-held grinding machines provided with a simple grinding disk, with which the buttons of the stationary drill bit were machined in a punctiform manner. It was quickly discovered, however, that it was practically impossible to restore the buttons of the drill bit to their original, hemispherical tip-configuration with the use of such primitive grinding equipment, not least because of the considerable amount of time taken, and it was quickly realized that it was more viable economically to scrap the drill bits than to attempt to grind the buttons thereof with the aid of the grinding equipment available at that time.
As in the case of all other machines, grinding equipment intended for grinding button drill-bits has, however, been developed and improved considerably, and the most significant step in this development is considered to be the realization that the button to be ground should rotate about its own longitudinal axis. The second important step in this development is considered by many to be the configuration of the actual grinding disk itself, with the provision of a profiled groove which corresponds substantially to the hemispherical profiled shape of the button tips . The most effective button-bit grinding machines known at present also include such a grinding disk and a rotable holder or fixture intended for holding firmly the drill bit whose buttons are to be ground and by means of which the buttons of the drill bit can be adjusted to and fixated in a grinding position and rotated about their own longitudinal axis in said grinding position. The worn buttons of a button drill-bit can be restored to their original shape very quickly and effectively with the aid of these machines, even in those instances where it is necessary to remove material from around the button, this material being considerably softer than the material from which the button is made, e.g. cemented carbide material.
These modern grinding machines for grinding the buttons of button drill-bits are highly effective and demand a relatively high price, which prevents such machines from being used as standard ancillary equipment with transportable rock drilling equipment, but must normally be mounted statically at fixed locations so that button drill-bits which require sharpening can be sent to the machine and thereby provide effective and remunerative use of the modern grinding machines.
Because the use of button drill-bits has been increased in all kinds of rock drilling operation, and particularly in rock drilling operations which are not directly relatable to mining operations, there is an increasing desire and demand for a simple, but nevertheless effective grinding machine which can be used in the immediate location of a rock drilling unit out in the field for grinding the worn or damaged buttons of a button drill-bit to the intended tip-profile shape in a relatively quick and effective manner, preferably without needing to detach the drill bit from the drill equipment. Attempts have been made to fulfill this desideratum, by using conventional hand-held grinding machines equipped with grinding disks of the aforesaid kind, i.e. a grinding disk provided with a profiled groove having, in cross-section, longitudinally 1 the disk a substantially semi-circular shape. In order to be able to grind a button to its intended tip-profile shape, while being able to rotate the drilling machine around the button at the grinding point at the same time, it is necessary to hold the grinding disk centred on one single point in relation to the button. This cannot be achieved with a conventional hand-held grinding machine, and hence the aforesaid attempts to use conventional hand-held grinding machines have not been successful.