FIELD OF THE INVENTION This invention relates to a wood chipper segmental knife holder (hereinafter termed knife holder) by which term is meant replaceable knife holders carried on a rotor that is employed to subdivide timber into chips having predetermined size ranges with a view to the chips being used for a variety of purposes including the production of reconstituted wood or paper products.

Such wood chips are often used as a convenient form of transportation, as a bulk product, of wood to be used for such purposes.

BACKGROUND TO THE INVENTION The production of wood chips from timber is, as indicated above, carried out by chipping up pieces of tree trunk and branches utilizing rotors fitted with knife holders each of which supports a knife with a cutting edge to form what will be termed herein a chipper segment. The wood chips that are destined for specific purposes are often required to have a size falling within a predetermined range of sizes and the size of the individual chips is dependent, at least some extent, on the condition of the knives and knife holders. As the chipper segments become worn so the size of chips cut by them changes and, accordingly, it is important that the wear on the chipper segments be monitored and the knives and knife holders be changed just as soon as is necessary.

It is accordingly desirable that, inter alia, the knife holders be produced in a manner ensuring that wear be limited as much as is reasonably possible taking into consideration relevant cost factors.

OBJECT OF THE INVENTION It is, accordingly, an object of this invention to provide a knife holder and process for manufacturing same that enables the life of the chipper segments to be extended, at least to some extent, relative to that being achieved using equivalent knife holders presently available.

SUMMARY OF THE INVENTION In accordance with this invention there is provided a process for the production of a chipper segmental knife holder comprising the steps of:- i. forming a segment blank from a suitable steel ; ii. forming, by an arc welding process, a layer of hard faced material on a surface of the blank that is to extend to a leading edge of the knife holder ; iii. forming attachment and/or locating formations on the knife holder ; and, iv. machining the knife holder in order to form a leading edge extending along one edge thereof such that the surface of the hard faced layer extends to a leading edge of the holder ; the process being characterized in that; v. preparatory to forming the layer of hard faced material a profile is applied as may be necessary to the surface of the blank that is to be hard faced such that the surface is substantially flat after the hard facing operation;

vi. the hard facing is carried out using anvil powder selected according to the said suitable steel ; and in that vii. the welding process is a plasma transferred arc welding process.

Further features of the invention provide for the profile to be applied utilizing a CNC mill, the profile being configured such that the relevant hard faced surface is substantially flat following on the hard facing operation; for the knife holder to be preheated, typically to a temperature of from 350 degrees to 450 degrees centigrade preparatory to the hard facing plasma transferred arc procedure; for the hard faced layer to have a thickness of from 1.0 to 1.5 mm; and for the attachment and or locating formations to be formed after the hard facing operation and to comprise a plurality of screw threaded bolt holes and locating holes for cooperation with locating pins, said holes extending into a face opposite the hard faced surface.

The invention also provides wood chipper segmental knife holders made by a process as defined above.

In order that the invention may be more fully understood one embodiment thereof will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:- Figure 1 is a plan view of a segment blank cut from stock steel material; Figure 2 is an orthographic projection illustrating the contoured surface of the chipper segment;

Figure 3 is a view similar to Figure 1 and showing the passes of a welding head forming the hard faced surface; Figure 4 is a cross-section taken through a completed knife holder ; and, Figure 5 is an orthographic projection of the completed knife holder.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS In this embodiment of the invention given simply as an example of the production of knife holders according to the invention, blanks (1), as illustrated in Figure 1, are cut from suitable stock steel, conveniently in the form of plate of a suitable thickness. The steel is chosen to provide an advantageous hardness when hard faced utilizing the plasma transfer arc welding procedure described below. One suitable steel has been found to be that sold as grade 50B by Dillinger Hutte of Germany, the steel having a composition of 0.190 % carbon; 0.439% silicon; 1.52 % manganese; 0.014% phosphorus; 0.0152% sulphur ; 0.034 percent aluminium; and 0. 001 % vanadium.

This blank is then subjected to milling on the one major face thereof that is to be hard faced using a numerically controlled mill in a number of passes to provide a contour that is illustrated in Figure 2 as being slightly convex as indicated by numeral (2). The contour is selected using techniques known to hose skilled in the art and calculated to provide, subsequent to the hard facing process, a substantially flat face.

The blank is then preheated to a temperature above 350 degrees centigrade, and preferably from 425 to 450 degrees centigrade, preparatory to applying to the contoured surface anvil powder and subjecting it to plasma transferred

arc welding to form a single layer of hard faced material having a thickness of from 1 to 1,5 mm. The welding is achieved in a number of passes and each pass is from 25 to 30 mm wide with flow in the direction of the straight edge that is to be the leading edge described below, as indicated by numeral (3) in Figure 3.

One anvil powder that has been found to be effective, at least when used with the steel identified above, is that sold under the trade reference ANVAL B-60 by Dynamet Anval of Torshålla, Sweden. This anvil powder has a composition of 0.76% carbon; for 0.05% silicon ; 0.006% sulphur ; 14.6% chromium; 72.3% nickel ; 0.36% molybdenum; 0. 01% titanium; 0.30% copper; 0.02% cobalt ; 4.40% iron; and 3.00% boron. The anvil powder had a particle size of 99 percent passing a 140 mesh; 90 percent passing a 170 mesh; 43 percent passing a 230 mesh and 8 percent passing a 325 mesh.

The hard faced blanks are allowed to cool in still air for approximately twelve hours to enable them to achieve their predetermined form and the hard faced surface is then dressed with an angle grinder to remove any high spots.

Excess material on the two major straight edges is machined to the finished size using a conventional milling machine.

The partly formed knife holders are thereafter placed on a numerically controlled milling machine with the hard faced surface (indicated by numeral (4) in Figures 4 and 5) facing downwards. The top face (5), that is the one opposite the hard faced surface, is then machined to provide the knife holder with its finished thickness and a knife supporting face (6) is rough machined at this stage.

Bolt holes (7) and locating holes (8) are then drilled in the top face and the bolt holes are tapped. These bolt holes include a row of bolt holes (7a) for clamping a knife to the knife supporting face. The locating faces are then machined to size and checked with a template. The knife holders are then

de-burred and finished as may be required such as by providing undercuts (9) at the ends of the leading edge (10).

It has been found that knife holders made as described above have the ability to outlast at least some existing commercially available knife holders by almost 100% and in any event by a significant amount.

It will be understood that numerous variations may be made to the embodiment of the invention described above without departing from the scope hereof.

In particular, the type of steel used can be varied widely and the anvil powder can be chosen to provide desired results. The selection of the anvil powder may depend on the exact composition of the steel. The conditions under which the plasma arc welding takes place may be varied as will be apparent to those skilled in the art.