CHIPPER KNIFE AND HOLDER THEREFOR The present invention relates to a chipper knife of an indexable insert type for chippers of the kind which comprises a rotatable tool in the form of a drum, disc or truncated conical body, which supports a plu- rality of such chipper knives to ensure, during rota- tion of the tool, chipping of pieces of wood which are fed to the same, the chipper knife comprising two oppo- site main surfaces which are adapted to abut and be retained against corresponding seat surfaces of a holder on the tool, two cutting edges which are formed between the main surfaces by single or double bevelling from one main surface and from both of the main surfaces, respec- tively, and which, by turning of the chipper knife, are locatable in turn as a machining cutting edge and an inactive cutting edge, respectively, and means for pre- venting the chipper knife from being pressed into and extracted from the holder. The invention also relates to a holder for such chipper knives.

Background of the Invention Chipper knives for chippers are relatively rapidly worn and must therefore be either regrindable and/or replaceable. Regrindable chipper knives suffer from the drawback that they have to be thoroughly positioned after regrinding, i. e. set in a correct position in relation to the rotatable drum, disc or truncated conical body on which they are mounted. Generally, chipper knives and cutting edges should move along an exactly defined path of motion in relation to a counter tool fixedly mounted in the machine body. A normal measure of the distance between the cutting edges of the chipper knives and the counter tool is about 0.4 mm as they pass each other.

Such exact positioning is, of course, very time-con- suming.

For this reason, it is generally preferable only to exchange the chipper knives without regrinding them, since such chipper knives can have a position which is fixed in advance and do not have to be positioned.

In particular in the case of non-regrindable chipper knives, it is advantageous if they are as cheap as pos- sible. They should, on the one hand, contain as little material as possible and, on the other, be easy to manu- facture while requiring as little machining as possible.

As a rule, the chipper knives are preferably made in the form of indexable inserts having two separate cutting edges which, by turning of the chipper knife, can be located in turn in a chipping position. As a result, the service life of one and the same chipper knife may be doubled.

To ensure safe retaining of the chipper knife in the holder and to define a fixed position for the chipper knife, it is generally provided with some kind of posi- tioning and retaining means which engages corresponding positioning and retaining means in the holder.

US 5,501,256 discloses a chipper knife of the type mentioned by way of introduction. It has a generally pla- nar body and its thickness is small in comparison with its width. For safe retaining of the chipper knife in the holder, each side of the chipper knife is provided with a ridge of triangular cross-section. The ridges are formed such that one side extends perpendicularly to the main surface of the chipper knife whereas the other side forms an angle of about 45° with the main surface of the chip- per knife. The ridge located on the chip side of the chipper knife abuts with its transverse side edge against a front edge of the inner clamping member of the knife holder. At the same time the inclined side edge of the ridge serves as a chip guiding surface which prevents pieces of fibre from being wedged between the chipper knife and the seat surface of the inner clamping member.

To prevent the chipper knife from being extracted from

For this reason, it is generally preferable only to exchange the chipper knives without regrinding them, since such chipper knives can have a position which is fixed in advance and do not have to be positioned.

In particular in the case of non-regrindable chipper knives, it is advantageous if they are as cheap as pos- sible. They should, on the one hand, contain as little material as possible and, on the other, be easy to manu- facture while requiring as little machining as possible.

As a rule, the chipper knives are preferably made in the form of indexable inserts having two separate cutting edges which, by turning of the chipper knife, can be located in turn in a chipping position. As a result, the service life of one and the same chipper knife may be doubled.

To ensure safe retaining of the chipper knife in the holder and to define a fixed position for the chipper knife, it is generally provided with some kind of posi- tioning and retaining means which engages corresponding positioning and retaining means in the holder.

US 5,501,256 discloses a chipper knife of the type mentioned by way of introduction. It has a generally pla- nar body and its thickness is small in comparison with its width. For safe retaining of the chipper knife in the holder, each side of the chipper knife is provided with a ridge of triangular cross-section. The ridges are formed such that one side extends perpendicularly to the main surface of the chipper knife whereas the other side forms an angle of about 45° with the main surface of the chip- per knife. The ridge located on the chip side of the chipper knife abuts with its transverse side edge against a front edge of the inner clamping member of the knife holder. At the same time the inclined side edge of the ridge serves as a chip guiding surface which prevents pieces of fibre from being wedged between the chipper knife and the seat surface of the inner clamping member.

To prevent the chipper knife from being extracted from

the knife holder, shallow holes or recesses are made in the inner clamping member as well as in the chipper knife and pins are arranged in these holes, which prevents the chipper knife from being displaced in relation to the knife holder. The ridge located on the wood piece side of the chipper knife adjacent to the flank of the cutting edge is inactive and accommodated in an elongated groove in the seat surface of the outer clamping member of the holder.

The chipper knife which is shown in said US 5,501,256 suffers from a number of drawbacks. It is, among other things, relatively expensive to manufacture as many ma- chining steps are required, on the one hand, to form the two ridges on the main surfaces of the chipper knife and, on the other, to form the holes in said main surfaces. The use of loose pins to fix the chipper knife in relation to the holder also makes it difficult to replace and turn chipper knives since the pins may easily get lost. Fur- thermore, the elongated groove in the outer clamping member of the holder constitutes a weakened area along which the clamping member may break.

Summarv of the Invention One object of the present invention is to obviate the problems and drawbacks of chipper knives and holders of the type mentioned by way of introduction. More par- ticularly, it is an object of the invention to provide a chipper knife which has a simple geometrical shape with a flat and thin body and large plane surfaces and which is cheap to manufacture, on the one hand, because it con- tains little material and, on the other, because it does not require much machining. A further object of the in- vention is to provide a chipper knife having means for retaining and chip guiding which are easy to manufacture and ensure distinct and safe retaining in the correct po- sition in the holder while preventing wood fibre from penetrating between the chipper knife and the inner

clamping member. At least these objects are achieved by means of a chipper knife according to claim 1.

The invention also relates to a holder having a de- sign which makes it stronger than prior-art holders of an equivalent kind and being able to hold the chipper knife in a firm grip. At least these objects are achieved by means of a holder according to claim 10.

The invention is thus based on the understanding that the above-mentioned objects and advantages can be achieved by means of a chipper knife comprising two oppo- site abutment surfaces or main surfaces which are each composed of two partial surfaces the planes of which are relatively offset to form a stepped edge which extends along each main surface and parallel to the cutting edges. In a preferred embodiment, the partial surfaces on each main surface are plane parallel to each other, and also the main surfaces are mutually parallel. How- ever, the partial surfaces on each main surface could be located in planes which are not parallel, optionally while being parallel or without being parallel in pairs to the partial surfaces on the opposite main surface. The stepped edge which is located on the main surface facing away from the rotational direction of the tool, i. e. lo- cated on the wood piece side of the chipper knife, is directed forwards to the machining cutting edge, whereas the stepped edge which is located on the main surface facing the rotational direction of the tool, i. e. located on the chip side of the chipper knife, is directed back- wards to the inactive cutting edge.

This results in a chipper knife whose shape is very simple and which is cheap to manufacture since it is easy to machine. Such a chipper knife can also be manufactured with a very small and material-saving thickness.

The two oppositely directed stepped edges form well- defined end positions for the position of the chipper knife in the holder. In addition, the stepped edge which is directed backwards and which is located on the main

clamping member. At least these objects are achieved by means of a chipper knife according to claim 1.

The invention also relates to a holder having a de- sign which makes it stronger than prior-art holders of an equivalent kind and being able to hold the chipper knife in a firm grip. At least these objects are achieved by means of a holder according to claim 10.

The invention is thus based on the understanding that the above-mentioned objects and advantages can be achieved by means of a chipper knife comprising two oppo- site abutment surfaces or main surfaces which are each composed of two partial surfaces the planes of which are relatively offset to form a stepped edge which extends along each main surface and parallel to the cutting edges. In a preferred embodiment, the partial surfaces on each main surface are plane parallel to each other, and also the main surfaces are mutually parallel. How- ever, the partial surfaces on each main surface could be located in planes which are not parallel, optionally while being parallel or without being parallel in pairs to the partial surfaces on the opposite main surface. The stepped edge which is located on the main surface facing away from the rotational direction of the tool, i. e. lo- cated on the wood piece side of the chipper knife, is directed forwards to the machining cutting edge, whereas the stepped edge which is located on the main surface facing the rotational direction of the tool, i. e. located on the chip side of the chipper knife, is directed back- wards to the inactive cutting edge.

This results in a chipper knife whose shape is very simple and which is cheap to manufacture since it is easy to machine. Such a chipper knife can also be manufactured with a very small and material-saving thickness.

The two oppositely directed stepped edges form well- defined end positions for the position of the chipper knife in the holder. In addition, the stepped edge which is directed backwards and which is located on the main

surface facing the rotational direction protects the front edge of the inner clamping member of the holder and thus prevents wood fibre from penetrating between the chipper knife and the inner clamping member.

The chipper knife according to the invention is gen- erally usable in all types of chippers, on the one hand, in conventional chippers which are used to cut chips for further use in the cellulose industry and, on the other, in chippers which are used to cut thin chips or pieces of veneer for use as raw material in the manufacture of wood fibre boards, so-called waferboards or OSB boards, in which the pieces of veneer are mixed with adhesive and pressed into compact boards. In the first-mentioned type of chippers, the chips are cut essentially from the end faces of the wood piece whereas in the second type the chips are cut from the lateral surfaces of the wood piece. Both types of chips are cut with disc chippers as well as with drum chippers with that difference that the chipper knives on the drum chippers for the first type of chips are arranged on the outer periphery of the drum whereas they are usually arranged along the inner periph- ery of the drum for the second type of chips and the wood is thus fed into the drum during chipping. The first type of chippers also exists in the form of conical discs.

In the description below and in the drawings, the chipper knife is shown when used in a drum chipper, i. e. a chipper in which the cutting edges of the chipper knives are located along the periphery of a rotating drum and parallel to its axis of rotation. It has been found that thin chipper knives are particularly advantageous for use in drum chippers, since the outer surface of the outer clamping member of the holder must follow a pre- determined cam curve against which the pieces of wood abut during chipping and which ensures correct feeding of the pieces of wood up to the following chipper knife.

The thinner the chipper knife, the more space is avail- able for the outer clamping member which can thus have a

surface facing the rotational direction protects the front edge of the inner clamping member of the holder and thus prevents wood fibre from penetrating between the chipper knife and the inner clamping member.

The chipper knife according to the invention is gen- erally usable in all types of chippers, on the one hand, in conventional chippers which are used to cut chips for further use in the cellulose industry and, on the other, in chippers which are used to cut thin chips or pieces of veneer for use as raw material in the manufacture of wood fibre boards, so-called waferboards or OSB boards, in which the pieces of veneer are mixed with adhesive and pressed into compact boards. In the first-mentioned type of chippers, the chips are cut essentially from the end faces of the wood piece whereas in the second type the chips are cut from the lateral surfaces of the wood piece. Both types of chips are cut with disc chippers as well as with drum chippers with that difference that the chipper knives on the drum chippers for the first type of chips are arranged on the outer periphery of the drum whereas they are usually arranged along the inner periph- ery of the drum for the second type of chips and the wood is thus fed into the drum during chipping. The first type of chippers also exists in the form of conical discs.

In the description below and in the drawings, the chipper knife is shown when used in a drum chipper, i. e. a chipper in which the cutting edges of the chipper knives are located along the periphery of a rotating drum and parallel to its axis of rotation. It has been found that thin chipper knives are particularly advantageous for use in drum chippers, since the outer surface of the outer clamping member of the holder must follow a pre- determined cam curve against which the pieces of wood abut during chipping and which ensures correct feeding of the pieces of wood up to the following chipper knife.

The thinner the chipper knife, the more space is avail- able for the outer clamping member which can thus have a

greater thickness with higher strength. In addition, the thinner the knife, the shorter the bevel or the so-called flank forming the cutting edge. Since it is advantageous in terms of manufacture and from an economic point of view to make a plane bevel, the bevel will extend, seen in a cross-section of the chipper knife, in a straight line between the front edge of the bevel, i. e. the cut- ting edge, and its rear edge in the transition to the main surface. The rear edge of the bevel must not be located outside the predetermined cam curve since this would interfere with the smooth feeding of the pieces of wood and have a negative effect on the chip quality. This means that the thicker the knife, the more the knife has to be angled down at a more obtuse cutting angle with the pieces of wood, which requires more driving power and which disadvantageously results in thicker pieces of chips. Inversely, a thin chipper knife can be angled up at a more acute cutting angle with the pieces of wood, which requires less driving power and which advantage- ously results in thinner pieces of chips.

As already mentioned, the cutting edges can be formed by single or double bevelling from one main sur- face and from both of the main surfaces, respectively. In single bevelling, the bevelled surface will serve as the flank of the cutting edge. Single bevelling is preferred primarily when using the chipper knife in a drum chipper since in that case the chipper knife can have a large angle in relation to the outer surface of the outer clamping member of the holder and this yields space so that the outer clamping member can have great dimensions.

The accompanying drawings show two primary embodi- ments of the chipper knife according to the invention.

On the one hand, a first embodiment is shown, in which the stepped edges are facing each other, i. e. the partial surfaces, which form a stepped edge while being inwardly offset and located closer to the centre line of the chip- per knife, overlap each other. On the other hand, a sec-

ond embodiment is shown, in which the stepped edges are facing away from each other, i. e. the partial surfaces, which form a stepped edge while being inwardly offset and located closer to the centre line of the chipper knife, do not overlap each other. In the first embodiment, the portion of the chipper knife which protrudes from the inner clamping member of the holder is relatively short, which is advantageous from the point of view of strength if the chipper knife is exposed to forces directed to its flank.

In a preferred embodiment, the forwardly directed stepped edge abuts against a front edge portion of an elongated recess in the outer clamping member of the holder, said recess being rounded or arcuate in cross- section and extending parallel to the cutting edge. Such a design reduces the strength-reducing effect of the re- cess since the concentration of stress is reduced. A similar rear recess is arranged in the outer clamping member in the area of the rear inactive cutting edge of the chipper knife. It serves to provide sufficient space for the rear cutting edge, even if the latter has previ- ously been directed forwards in the active chip cutting position, which may have caused damage or burrs.

The dimensions of the chipper knife may vary within wide limits depending on the size of the machine, the de- sired result and the like. In a preferred embodiment, it is generally preferred that the width of the chipper knife should be at least five times its thickness. In practice, the thickness can be less than 10 mm, prefer- ably less than 8 mm and most preferred less than 6 mm.

Furthermore, the cross-sectional length of the stepped edge is less than 50 % of the smallest material thickness between the two main surfaces, preferably less than 35 % and most preferred less than 20 %.

In this description, certain positioning and direc- tional words and expressions are used. For instance, outer and inner refer to something that is located out-

ond embodiment is shown, in which the stepped edges are facing away from each other, i. e. the partial surfaces, which form a stepped edge while being inwardly offset and located closer to the centre line of the chipper knife, do not overlap each other. In the first embodiment, the portion of the chipper knife which protrudes from the inner clamping member of the holder is relatively short, which is advantageous from the point of view of strength if the chipper knife is exposed to forces directed to its flank.

In a preferred embodiment, the forwardly directed stepped edge abuts against a front edge portion of an elongated recess in the outer clamping member of the holder, said recess being rounded or arcuate in cross- section and extending parallel to the cutting edge. Such a design reduces the strength-reducing effect of the re- cess since the concentration of stress is reduced. A similar rear recess is arranged in the outer clamping member in the area of the rear inactive cutting edge of the chipper knife. It serves to provide sufficient space for the rear cutting edge, even if the latter has previ- ously been directed forwards in the active chip cutting position, which may have caused damage or burrs.

The dimensions of the chipper knife may vary within wide limits depending on the size of the machine, the de- sired result and the like. In a preferred embodiment, it is generally preferred that the width of the chipper knife should be at least five times its thickness. In practice, the thickness can be less than 10 mm, prefer- ably less than 8 mm and most preferred less than 6 mm.

Furthermore, the cross-sectional length of the stepped edge is less than 50 % of the smallest material thickness between the two main surfaces, preferably less than 35 % and most preferred less than 20 %.

In this description, certain positioning and direc- tional words and expressions are used. For instance, outer and inner refer to something that is located out-

wards and inwards, respectively, in relation to the wood- machining side of the tool. Front and rear, respectively, refer to a position forward towards the machining cutting edge of the chipper knife and backward towards the in- active cutting edge of the chipper knife. Moreover, the wood piece side of the chipper knife is directed outwards to the side from which the wood is fed into the tool and the flank or bevelled surface of the machining cutting edge is located on this side. The chip side of the chip- per knife faces inwards away from the side from which the wood is fed into the tool. Preferably, the stepped edge makes an angle of 90° with the respective partial surface on the same side. It is, however, fully possible for the stepped edge to make a different angle of, for instance, 80-100°, 70-110° or 60-120° with the partial surface which is located below the stepped edge. Therefore the feature in the description and the claims that the stepped edge faces the respective cutting edge is to be interpreted such that the cutting edge in question is located in front of an imaginary extension line in the cross-sec- tional extent of the respective stepped edge. To enable the stepped edge to serve as a retaining means and as protection against the penetration of wood fibre between the chipper knife and the inner clamping member of the holder, the stepped edge should not make such a flat angle with the partial surfaces as to be parallel or almost parallel to the bevelled surface which forms the cutting edge and which it faces. The angle between the bevelled surface and the stepped edge should be at least 30°, preferably at least 40° and most preferred at least 50°. The feature that the partial surfaces are substan- tially plane should be interpreted extensively and means that the partial surfaces may have small recesses or bulges without departing from the scope of the invention.

wards and inwards, respectively, in relation to the wood- machining side of the tool. Front and rear, respectively, refer to a position forward towards the machining cutting edge of the chipper knife and backward towards the in- active cutting edge of the chipper knife. Moreover, the wood piece side of the chipper knife is directed outwards to the side from which the wood is fed into the tool and the flank or bevelled surface of the machining cutting edge is located on this side. The chip side of the chip- per knife faces inwards away from the side from which the wood is fed into the tool. Preferably, the stepped edge makes an angle of 90° with the respective partial surface on the same side. It is, however, fully possible for the stepped edge to make a different angle of, for instance, 80-100°, 70-110° or 60-120° with the partial surface which is located below the stepped edge. Therefore the feature in the description and the claims that the stepped edge faces the respective cutting edge is to be interpreted such that the cutting edge in question is located in front of an imaginary extension line in the cross-sec- tional extent of the respective stepped edge. To enable the stepped edge to serve as a retaining means and as protection against the penetration of wood fibre between the chipper knife and the inner clamping member of the holder, the stepped edge should not make such a flat angle with the partial surfaces as to be parallel or almost parallel to the bevelled surface which forms the cutting edge and which it faces. The angle between the bevelled surface and the stepped edge should be at least 30°, preferably at least 40° and most preferred at least 50°. The feature that the partial surfaces are substan- tially plane should be interpreted extensively and means that the partial surfaces may have small recesses or bulges without departing from the scope of the invention.

Brief Description of the Accompanying Drawings In the drawings FIG. 1 is a cross-section of a portion of a rotat- able drum of a drum chipper which comprises a holder and a chipper knife according to a first embodiment of the present invention and which is illustrated when chipping a piece of wood, FIG. 2 is a perspective view of the chipper knife according to Fig. 1, FIG. 3 is a cross-section according to Fig. 1 but shows a holder and a chipper knife according to a second embodiment of the present invention, and FIG. 4 is a perspective view of the chipper knife according to Fig. 3.

Detailed Description of Preferred Embodiments of the Invention Reference is first made to Fig. 1 which schemati- cally shows a cross-section of a portion of a rotatable drum 1 of a drum chipper. Along the periphery of the drum, a number of chipper knives 2 are supported, of which only one is shown in the Figure and which are posi- tioned with their respective cutting edges parallel to the axis of rotation of the drum. Each chipper knife is mounted in a holder comprising an inner clamping member 3, which is fixed in a recess or a seat in the drum, and an outer clamping member 4 which can be pressed, by means of a bolt joint 5, towards the inner clamping member when clamping the chipper knife.

Fig. 1 also illustrates a piece of wood 6 which during the rotation of the drum is cut into chips by the chipper knives 2. The pieces of wood are fed to the chip- per knives at an angle of about 30° in relation to the vertical and abuts against the outer periphery of the drum by the action of gravity during chipping. This outer periphery follows a stepped cam curve of which the outer surface 7 of the outer clamping member 4 forms part and which extends from the cutting edge of the respective

chipper knives at a successively reduced radial distance from the axis of rotation and ends essentially in front of the cutting edge of the subsequent chipper edge and radially closer to the axis of rotation than this chipper edge. As a result, the pieces of wood are fed succes- sively towards the drum and a predetermined length of wood is cut by each chipper knife. In Fig. 1, the piece of wood 6 has been cut along a surface 8 by the immedi- ately preceding chipper knife and the surface 8 abuts and slides against the peripheral portion 9 of the drum, which portion follows the predetermined cam curve. As apparent, the peripheral portion 9 ends essentially in front of the cutting edge of the chipper knife and at a radial distance from the same. A chip passage 10 is de- fined between the peripheral portion 9 and the chipper knife/holder through which the chips are passed after being severed from the piece of wood for further trans- port to a storage or the like and in the Figure this is illustrated by a severed piece of chip 11. The radial distance between the end of the peripheral portion 9 and the cutting edge of the chipper knife determines the length of the cut chips whereas the thickness of the chips depends on the angle between the underside or chip- guiding surface of the chipper knife and the fibre direc- tion of the wood. The greater the angle, the thinner the chips, which in most cases is advantageous to the subse- quent processing in the manufacture of paper pulp and the like.

Reference is then made to Fig. 2, in which the chip- per knife in Fig. 1 is shown in perspective view on a larger scale. The chipper knife comprises two opposed cutting edges 12,12'and two main surfaces 13,13'which are each composed of two substantially plane, parallel partial surfaces 14,14'whose planes are relatively off- set to form a stepped edge 15 which extends on each main surface and parallel to the cutting edges. In the shown embodiment, the cutting edges 12,12'are formed by

chipper knives at a successively reduced radial distance from the axis of rotation and ends essentially in front of the cutting edge of the subsequent chipper edge and radially closer to the axis of rotation than this chipper edge. As a result, the pieces of wood are fed succes- sively towards the drum and a predetermined length of wood is cut by each chipper knife. In Fig. 1, the piece of wood 6 has been cut along a surface 8 by the immedi- ately preceding chipper knife and the surface 8 abuts and slides against the peripheral portion 9 of the drum, which portion follows the predetermined cam curve. As apparent, the peripheral portion 9 ends essentially in front of the cutting edge of the chipper knife and at a radial distance from the same. A chip passage 10 is de- fined between the peripheral portion 9 and the chipper knife/holder through which the chips are passed after being severed from the piece of wood for further trans- port to a storage or the like and in the Figure this is illustrated by a severed piece of chip 11. The radial distance between the end of the peripheral portion 9 and the cutting edge of the chipper knife determines the length of the cut chips whereas the thickness of the chips depends on the angle between the underside or chip- guiding surface of the chipper knife and the fibre direc- tion of the wood. The greater the angle, the thinner the chips, which in most cases is advantageous to the subse- quent processing in the manufacture of paper pulp and the like.

Reference is then made to Fig. 2, in which the chip- per knife in Fig. 1 is shown in perspective view on a larger scale. The chipper knife comprises two opposed cutting edges 12,12'and two main surfaces 13,13'which are each composed of two substantially plane, parallel partial surfaces 14,14'whose planes are relatively off- set to form a stepped edge 15 which extends on each main surface and parallel to the cutting edges. In the shown embodiment, the cutting edges 12,12'are formed by

single bevelling between the two main surfaces, more par- ticularly, in such manner that the stepped edge 15 faces a bevelled surface 16 forming the cutting edge. In the embodiment according to Figs 1 and 2, the partial sur- faces 14 of the main surfaces 13,13'nearest to the bevelled surfaces 16 are so large that they overlap and the stepped edges 15 on the opposed main surfaces are thus facing each other.

As appears from Fig. 1, the abutment or seat surface of the outer clamping member 4 is formed with two elon- gated recesses which are arcuate in cross-section. More particularly, it has a first recess 17 which is formed in the area of the rear inactive cutting edge and serves to accommodate the cutting edge, on the one hand, without damaging an unused cutting edge and, on the other, to ac- commodate also a damaged, used cutting edge having burrs or being upset. A second recess 18 is located adjacent to the stepped edge 15 of the chipper knife, more specifi- cally in such manner that the stepped edge abuts against a front portion of the recess 18, thus preventing the chipper knife from being extracted from the holder. The rounded shape of the cross-section of the recesses makes the recesses less inclined to act as fracture lines and strengthens the holder.

The stepped edge 15 on the inwardly directed main surface abuts against a front edge 19 of the inner clamp- ing member 3. As a result, the chipper knife is prevented from being pressed into the holder and the stepped edge also serves as a protection against the penetration of wood fibre between the chipper knife and the inner clamp- ing member.

A great advantage of making the chipper knife thin is that the bevelled surfaces 16 become short and thus the front edge of the outer clamping member of the holder can be located near the cutting edge and offer the chip- per knife the necessary support. A thin chipper knife also yields space to increase the thickness of the outer

clamping member in spite of the fact that its outer surface has to follow the predetermined cam surface.

As a result, the holder gets a corresponding enhanced strength.

Figs 3 and 4 show an alternative embodiment of the holder and the chipper knife. In this embodiment of the chipper knife, the partial surfaces 14 nearest to the bevelled surface 16 on the respective main surfaces are so short that they do not overlap each other and the stepped edges 15 on the opposed main surfaces 13,13' will not face each other but face away from each other.

The advantage of this embodiment is that the actual chip- per knife will be somewhat stronger but the front edge of the inner clamping member 3 will abut against and support the chipper knife further away from its machining cutting edge. This chipper knife and holder thus provide a some- what reduced strength in view of any possible forces act- ing on the chipper knife in the inward direction towards the axis of rotation of the drum. In addition, the recess 18 will be located further out towards the cutting edge, which may possibly increase its strength-reducing effect on the outer clamping member.

clamping member in spite of the fact that its outer surface has to follow the predetermined cam surface.

As a result, the holder gets a corresponding enhanced strength.

Figs 3 and 4 show an alternative embodiment of the holder and the chipper knife. In this embodiment of the chipper knife, the partial surfaces 14 nearest to the bevelled surface 16 on the respective main surfaces are so short that they do not overlap each other and the stepped edges 15 on the opposed main surfaces 13,13' will not face each other but face away from each other.

The advantage of this embodiment is that the actual chip- per knife will be somewhat stronger but the front edge of the inner clamping member 3 will abut against and support the chipper knife further away from its machining cutting edge. This chipper knife and holder thus provide a some- what reduced strength in view of any possible forces act- ing on the chipper knife in the inward direction towards the axis of rotation of the drum. In addition, the recess 18 will be located further out towards the cutting edge, which may possibly increase its strength-reducing effect on the outer clamping member.