The present invention relate-, to a device for sharpening the teeth of motor chain saws. Users of motor saws are familiar with the necessity of sharpening the teeth on the chain at relatively frequent intervals to maintain satisfactory cutting, and are well aware that this procedure is difficult and time-consuming, or is a -source of irritation. By its nature, the problem is particularly familiar to forestry workers.

Many devices have been developed to facilitate the sharpening operation, including control of the file alignment, which must be set at a certain angle relative to the lateral alignment of the teeth. In general, however, these known devices for controlling the forward and backward movement of the file, or even developing a rotary movement of the file, have not achieved widespread practical use.

The invention is intended not merely as an 'ancillary' device, but as a genuine device for more or less automatic performance of the sharpening operation.

The invention demonstrates the possibility of using a new sharpening technique, in which the file is neither traversed forward and backward nor rotated against a stationary tooth, but is traversed unidirectionally against a tooth moving along the saw chain. Thus, if the said file is held against the tooth in the desired angular position at the start of its (the tooth's) travel along the guide bar of the saw, with the fully retracted end of the file in initial engagement with the tooth, forward movement of the file in a direction perpendicular to its (the file's) longitudinal axis, in other words, a movement at an angle with respect to the guide bar, will cause the leading end of the Ale to move increasingly further from the guide bar, while the opposite end of the

file will move increasingly closer. In other words, the file will execute a progressive longitudinal movement relative to the tooth; that is, it will automatically perform a unidirectional sharpening action as the saw chain is driven forward by the file or as the file is traversed by movement of the chain along the guide bar.

While it has formerly been the practice to perform sharpening on stationary teeth, the invention simply utilises the fact that the teeth can move along the guide bar, whereby a type of dynamic sharpening is performed by the angular movement of the laterally mounted file relative to the guide bar, the file being mounted movably in a separate driving device attached to the guide bar, while the guide bar is mounted in a file drive support frame. This ensures that the file movement itself is generated automatically and that the file is maintained at the desired sharpening angle relative to the tooth throughout the operation.

Furthermore, it is not a condition that the file be moved in a direction perpendicular to its own direction, but may be positioned more or less at an angle. This means that the angle between the direction of travel of the file and the file angle should not be identical; in other words, the direction of travel of the file for a given tooth sharpening angle may be more or less at an angle to the guide bar. The greater this angle, the faster the file movement, and the shorter the file which can be used for a given sharpening 'distance'. Ideally, the width of the fle path should be such that, on one side, it intersects the start of the tooth path on the guide bar and, on the other side, it intersects the end of the tooth path, ensuring that the file interacts with the tooth over its (the file's) full length and over the full length of the guide bar.

Since the attraction of this sharpening principle, which is considered to be unique, at least in the present context, may possibly be lessened if only a

single file is used to sharpen a single tooth per operating cycle, it should be emphasised that the system is extremely adaptable to an equally unique method employing a number of files to sharpen a number of teeth simultaneously in the dynamic sharpening process.

Application of these files parallel to each other in the same direction of travel, at a pitch corresponding to that of the teeth on the chain, enables several teeth to be sharpened at the same time, greatly increasing the sharpening capacity compared with the basic version.

In the preferred embodiment, the files are carried on an endless belt, on which they are mounted at a pitch corresponding to that of the teeth on the chain, the belt being of sufficient operational length to extend along the motor saw guide bar, but at an angle to the latter, so that the files on the said belt continuously engage with and sharpen the successive teeth on the chain. Provided that the system is correctly adjusted and mounted, sharpening may be carried out at a high speed, enabling all of the teeth on the chain to be sharpened in a fraction of the time normally required,

However, it should be noted that this solution may also be generally attractive if it is not wished to prematurely abandon the system employing only one or a relatively small number of files, given that the purpose of using a special sharpening device is to ensure that the said file or files is or are engaged automatically with successive teeth or groups of teeth on the chain, and that sharpening can be carried out even at extremely high operating speeds, provided that complete sharpening of all of the teeth is performed automatically and in a significantly shorter time than hitherto.

A very significant advantage of the invention is that the sharpening device can be attached to the motor saw itself as an accessory, normally in a passive mode, from which it may be deployed in an operational mode above or below the guide bar of the saw to interact with the chain-mounted teeth extending from the latter. The motor saw may, thereby, be equipped with an integral sharpening unit which may be used to sharpen the teeth in an extremely short time, without the least difficulty to the user, who is required only to engage and disengage the device at intervals in a relatively short time.

The files may be driven at an extremely high speed by the chain saw's own motor or at a normal, slower and probably safer speed by driving the file- carrying belt by motor or manual power, for example if the resistance to movement of the toothed chain is increased.

The sharpening device may also be installed inside the motor casing, in which case it may either be engaged and disengaged externally or even left in permanent engagement; sharpening may be sufficiently effective if the teeth are sharpened briefly, successively and individually.

The invention is described in further detail below with reference to the appended drawings, of which

Fig. 1 is a plan view of an automatic sharpening device in accordance with the invention and

Fig. 2 is an elevation of the same device.

The automatic sharpening device 2 used to sharpen the teeth on the chain 4 of a chain saw 6 consists of a support frame 8, the ends of which are provided

with two rollers 10 supported in bearings, which rollers carry an endless belt 12 to which are attached a number of round files 14 mounted at an angle to the longitudinal axis 16 of the belt. The perpendicular distance between the round files 14 is less than the distance between the edges 26 of two successive, similarly oriented teeth 18, the angle of the files relative to the direction of travel 16 of the belt being adjusted to suit the desired sharpening angle of the cutting edges on the teeth, the normal value of which on a chain saw is approx. 30°.

The support frame 8 is provided with a pivoted, adjustable bracket 20 provided with clamps 22 for clamping the guide bar 24 of a chain saw. The bracket 20 is adjustable to an angle V relative to the longitudinal axis 6 of the belt (which coincides with the direction of travel of the files). The value of the angle V is a function of the distance between the files and of the distance between two successive, similarly oriented teeth 18. The angle V, combined with the angular position of the file relative to the longitudinal axis 16 of the belt, ensures correct engagement of the files 14 with two successive, similarly oriented teeth 18 on the saw chain.

The combination of the angular position of the files relative to their direction of travel and the angle of the saw chain relative to the same direction, together with the length of the files, means that at least two similarly oriented teeth on the saw chain are in engagement with the files, while two further teeth are, respectively, on the point of engagement and on the point of disengagement with the files.

The files 14 used to sharpen the cutting edges 26 of the teeth 18 are round files, both ends 28 of which are tapered and smoothed to facilitate their entry in the cutter recess without damaging the tooth 18.

As shown by the typical embodiment illustrated in Fig. 2, the files 14 are attached to the belt 12 by fastening devices consisting of a swan-neck stalk 30, one end of which is attached to the belt 12 and the other to the file 14, close to the mid-point. Thus, since the files are attached to the belt at this point alone, they can travel around the curved rollers without problems.

The attachment between the belt or chain 12 and the stalks 30 may be of a pivoted, lockable type, enabling the files 14 to be locked in practically any random position relative to their direction of travel 16. This enables the tooth to be sharpened to a desired, optimum angle since only the angular position of the guide bar relative to the file path can be adjusted correspondingly to suit the tooth pitch.

The relative settings should be adjusted so that sharpening takes place with the tooth edges. However, this does not preclude the possibility of sharpening against the edges. When setting the file angles, it is self-evident that all of the files should be set at the same angle relative to their direction of travel 16; in other words, the longitudinal axes 42 of the files must be aligned in parallel during sharpening. The tooth sharpening files which, in the foregoing, have been described as round files 14, need not necessarily be of a round cross- section, provided that they are of an appropriate shape in the region of interaction with the teeth.

Similarly, each of the swan-neck stalks 30 may be provided with a flat file 32 aligned parallel to the round file 14 and attached to a spur-shaped stalk 34 projecting from the rear of the stalk 30 as illustrated in Figs. 1 and 2. The upper face of the flat fle is located at a suitable level and distance relative to the round file, in such manner that a chain saw drive dog 36 located between two successive teeth is dressed by the flat file as the teeth are sharpened by the

round files 14.

The side of the belt 12 facing the chain 4 is enclosed by a guide in the form of a rail 38, which is provided with a slot 40 in the direction of the belt to permit passage of the fle stalks 30 attached to the belt. The rail 38 is slightly curved at a curvature identical to that of the saw guide bar 24, ensuring that the files and teeth follow the same path within the sharpening range.

In another embodiment of the automatic sharpening device which is not illustrated, the belt and file arrangement consist of an endless belt of a special, flexible, synthetic material carried on two rollers in similar manner to the embodiment already described. The upper face of the belt is provided with a number of angular ribs corresponding to a desired tooth sharpening angle. The top surfaces of the ribs are coated with an abrasive, such as powdered diamond or carborundum, and are otherwise of similar shape to the round and flat files already described. In this embodiment, the flexibility of the synthetic material is utilised in combination with the facility of the ribs, which replace actual metal files, to travel over the rollers without damaging the abrasive coating since the ribs, being oblique, can flex during this passage. Since it is obviously impossible to alter the angle of the sharpening faces relative to their (i.e. the belt's) direction of travel, it must be accepted that at least one group of teeth will be sharpened against the edges, unless an arrangement similar to the file configuration is adopted, in which the ribs are aligned in the opposite direction, enabling die opposite facing group of teeth and associated drive dogs to be sharpened.

It is also possible to construct a device in accordance with the sharpening principle used in the invention, in which each group of laterally opposite teeth on a saw chain is sharpened simultaneously. An embodiment of this type

consists of two belts/chains 12 and files 14, the files 14 on each belt/chain being set at an individual angle relative to the direction of travel due to the addition of an upper sharpening system to sharpen the teeth at the upper side of the guide bar.

The new principle of sharpening the teeth of a chain saw in a quick, simple manner may naturally be employed in other embodiments. For example, the automatic sharpening device may be mounted directly on the chain saw to enable sharpening of the teeth to be initiated by one simple operation of a handle or pushbutton. Sharpening of the teeth may also be carried out continuously while the saw is in use.