TECHNICAL FIELD The present invention relates to an apparatus for sharpening ma- chines intended for sharpening of circular saw blades preferably of the type intended for brush saws which sharpening machines com- prise a grinding disc driven by an electrical motor arranged verti- cally adjustable on a support and suspension means for the saw blade by means of which the saw blade could be brought into alter- native grinding positions for grinding of the cross-set teeth of the saw blade.

BACKGROUND ART Known grinding machines for circular saw blades are generally de- signed so that the saw blade is arranged to be rotated around a fixed shaft pivot and the grinding head of the machine is adjustable for grinding of a tooth set to the right or the left located in grinding position. These machines have almost without exception automatic feeding of the teeth to the grinding position whereby the grinding position is defined by the end position of the ratchet pawl. The ma- chines have also automatically controlled movement of the grinding head which means that the tooth to be sharpened during the feeding movement is also ground at the back side and then in the grinding position at the front side whereby the cut will become straight. Even if saw blades for brush saws when they are new have straight cuts the best results are achieved with hollow-ground cuts which type of

cut is also recommended by the manufacturer of saw blades. As this type of cut can not be achieved by means of the known grinding machines described above one will be reduced to sharpening the cuts manually by means of round files and special guiding means arranged on those files. Such sharpening is time-consuming and re- quires a certain skill.

BRIEF DESCRIPTION OF THE INVENTION An object of the present invention is to provide an apparatus ena- bling hollow grinding of teeth on a brush saw by means of a ma- chine. A further object is to provide an apparatus which defines the grinding position for a tooth in a way so that the saw blade keeps the original tooth pitch. These and other objects will be attained by means of an apparatus the characteristics of which appear from the characterising features of the appended claims.

DESCRIPTION OF THE DRAWINGS An embodiment of the invention will be described more in detail with reference to the enclose drawing on which figure 1 shows the ap- paratus according to the invention in front view with the grinding machine in rest position, figure 2 shows the same apparatus with the grinding machine in grinding position, figure 3 shows the appa- ratus except the grinding machine in plane view, figure 4 shows the means for adjusting cutting angles when grinding right and left cuts respectively and figures 5 and 6 show the position of the saw blade in relation to the grinding disc when grinding right and left teeth re- spectively.

PREFERRED EMBODIMENT Figure 1 shows in a front view the apparatus generally designated 1 and the associated grinding machine 2 in rest position. The appa- ratus comprises a cradle constituted by a longitudinal bar 3 having forward pointing end portions 4,5 being arranged pivoting in a frame constituted by a square tube 6 having end portions 8,9 ar- ranged on a plate 7. The axial line 24 defined by the bearings 25, 26 is arranged perpendicular to the plane defined by the grinding disc 20 of the grinding machine 2 which axial line in the figure partly is marked by means of a dotted line. By means of tilting the cradle as will be described below a tooth in grinding position could be given right or left grinding. In the bar 3 a longitudinal slot 10 is ar- ranged in which slot a shaft pivot 11 (not visible in the figure) is sli- dably arranged on which shaft pivot 11 a saw blade 12 is mounted having its centre in the axial line and being held by means of a strong helical spring 28 (not visible in the figure) arranged under the locking control 13. The saw blade will be brought into the horisontal position for grinding of the respective tooth by means of the control 14 and the associated screw 15 part of which has been removed in the figure in view of showing the slot 10. In the vertical direction the saw blade will be brought into position for grinding of the respective tooth by means of the control 16 and the associated screw 17 which acts on one end of a double-armed lever 18 at the other end of which the lock 19 is arranged to bear with a spring force against the saw blade at the rear part of the tooth the front part of which is to be ground. The front part of the tooth should be positioned at or close to the axial line 24 during the grinding.

In figure 2 the apparatus according to figure 1 is shown now with the grinding machine in grinding position to which position the ma-

chine has been brought by means of a lever arranged on the ma- chine. During the movement to the grinding position the grinding disc 20 has plane-ground the back side of the tooth 22 in front of the grinding position and thereafter ground the front side of the tooth 23 in the grinding position which has become hollow-ground due to the rounded form of the periphery of the grinding disc.

In figure 3 the apparatus 1 except the associated grinding machine is shown in a plane view mounted on a plate 7. As already men- tioned the frame comprised in the apparatus inclues a square tube 6 having end portions 8,9. Between these end portions the cradle is arranged pivoting in the bearings 25,26. In the figure the earlier mentioned shaft pivot 11 and the spring 28 which presses the saw blade 12 against a thrust collar on the shaft pivot 11 are shown. In the figure the screw 15 is also shown in full by means of which screw the shaft pivot 11 can be displaced in the earlier mentioned slot 10 (not shown) for adjusting the grinding position of the saw blade 12 horizontally. The shaft pivot 11 is locked in the adjusted position by means of the screw 30 which can be reached on the back side of the bar 3. In order to avoid vibrations in the saw blade 12 during the grinding procedure an adjustable support screw 29 is arranged on the bar 3 which is adjusted to bear against the back side of the saw blade.

Figure 4 shows the end portion 9 and behind this the end portion 5 of the cradle. This end portion is provided with a scale by means of which the grinding angle for right as well as left teeth could be ad- justed to a desired value. The end portion is locked in the desired position by means of the control 27. The adjacent control 14 is the one by means of which the horisontal position of the saw blade is adjusted.

In figures 5 and 6 the position of the saw blade in relation to the grinding disc is shown when grinding right and left teeth respec- tively.

The grinding of a saw blade is carried out in the following way. The saw blade is mounted on the shaft pivot 11 and the locking control 13 is tightened. By means of the control 14 the grinding position is adjusted in the horisontal direction and by means of the control 16 the grinding position of the saw blade 12 is adjusted vertically. Then it is checked that the correct cut angle for the present type of set- ting has been adjusted, if not it is adjusted. The grinding machine 2 is provided with an arrangement (not shown) for setting the desired grinding depth which has to be regularly adjusted in view of the wear of the grinding disc. The grinding takes place when the grind- ing machine is lowered by means of the lever 21 to its grinding po- sition, whereby at first the back side of the tooth 22 in front of the tooth 23 in the grinding position is plane-ground and thereafter the front side of the tooth 23 in the grinding position is hollow-ground.

After that the grinding machine is returned to its rest position. The spring mounted lock 19 is released and the saw blade is moved in counter clockwise direction to the next tooth with the same setting, i. e. the movement corresponds each time to two teeth, and the grinding and feeding procedures are then repeated until all the teeth having the same setting has been ground. Then the adjustment of the cradle is changed to a position corresponding to grinding of teeth having the opposite setting type and the whole procedure is repeated.

The design of the cradle may of course be varied within the scope of the invention without leaving the inventive concept.