* unitary = made of one single piece BACKGROUND OF THE INVENTION Blanks of the above mentioned kind are extensively used, particularly for the manufacturing of shaft cutters. The teeth of a shaft cutter are conventionally made by grinding the hardened and tempered blank by means of grinding disks. Normally the ground portion of the cutter extends over not more than half of the whole length of the blank. The rest, the unground shank portion, is used for fastening the tool in a chuck or other tool holder on the milling machine. Depending on the kind of work which the shank cutter shall be used for, e. g. for roughing or for finishing, a blank having a hardness that is adapted to the work in question is selected. In order that the toolmaker shall be able to manufacture tools with different hardness in the working part of the tool, the manufacturer therefor must have a store of blanks with different hardnesses. It is desirable to be able to reduce the store without at the same time reducing the possibilities in every situation to be able to manufacture tools having desired hardness in the working part of the tool, but instead to increase that possibility.

DISCLOSURE OF THE INVENTION It is the purpose of the invention to solve the above problem. According to one aspect of the invention this can be achieved therein that a first end portion of the blank, i. e. of the rod, extending over 30-60 % of the length of the rod measured from said first end of the rod, has a hardness which is 1-10 HRC units (Hardness Rockwell C units) higher than the hardness of the second end portion of the rod, said second end portion extending over 20-60%, preferably 25-45 %, of the length of the rod measured from the second end of the rod. Preferably, however, none of said end portions extends over more than half of the length of the rod. Preferably there is a continuous transition of hardness in a

region between said end portions from the higher hardness in the first end portion to the lower hardness in the second end portion. A high speed steel is a suitable blank material, preferably a high speed steel which contains in weight-% 0.5-2.7 C, 0.1-0.9 Mn, 0.1-0.9 Si, 3-6 Cr, 5-15 (Mo + 2), 0-20 Co, totally max. 5 % of other alloying elements which possibly may exist in the steel, balance iron and unavoidable impurities.

In the method for the manufacturing of the blank according to the invention, the two ends of the rod are heat treated differently for the achievement of the above mentioned hardness difference. Preferably a first end portion is hardened to a hardness of 68-75 HRC, preferably to 69-73 HRC, while the second end portion is hardened to a hardness of 64-71 HRC, preferably to between 65-69 HRC. The difference in hardness can be set to e. g. 2-6 HRC units. The difference in hardness can be achieved therein that the first end portion is hardened from a higher hardening temperature than the second end portion before the rod is tempered, the tempering being performed of the entire rod, i. e. so that the two end portions are tempered simultaneously. For example the first end portion can be heated to its hardening temperature by being submerged in a bath of e. g. molten salt having the hardening temperature in question for that end portion, whereafter it is cooled to below 60°C, whereupon the second end portion is submerged in another salt bath having the temperature which is intended for the second end portion, whereupon also that end portion is cooled to a temperature below 60°C, whereafter the whole rod is tempered. For example the first end portion is hardened by cooling from a first hardening temperature which is above 1000°C, while the second end portion is cooled to below 60°C from a second hardening temperature which is 30-140°C higher than the first hardening temperature, whereafter the whole rod is tempered at a temperature between 450 and 650°C. In order not to mix up the two ends, one ore both ends are marked through punching or in any other way prior to the heat treatment. The finished blanks suitably may be further marked, e. g. through different colour markings of the two end portions in order that the user readily shall see which end that is the hardest and which one that is the softest.

After hardening, the matrix of the rod material substantially consists of tempered martensite containing 10-50 vol-% of one or more of those hard particles which belong to the group consisting of carbides, nitrides, and oxides.

The blanks according to the invention may have the shape of straight rods with circular cross section, normally in the dimensional range 0 5-50 mm. The length may be 50-330

mm, the blanks having the shortest lengths typically also having the smallest diameters and vice versa.

Further features and aspects of the invention will be apparent from the appending patent claims and from the following examples.

BRIEF DESCRIPTION OF DRAWINGS In the following examples, reference will be made to the accompanying drawings, in which Fig. 1 is a side elevation of a blank of the invention according to a first embodiment ; Fig. 2 shows how the hardness varies along the length of the blank; Fig. 3 shows an example of a shank cutter made from the blank according to the invention, wherein the hardest end portion of the blank is used for the working part of the cutter; Fig. 4 shows a shaft cutter made from an equal blank, wherein the softer end portion of the blank is used for the working part of the cutter; Fig. 5 is a side elevation of a blank of the invention according to a second embodiment; and Fig. 6 shows how the hardness varies along the length of the blank according to said second embodiment.

EXAMPLE 1 Two round rods 1,0 12 mm and length 110 mm, of a known high speed steel with the trade name ASPE 2060 with the following nominal composition in weight-%: 2.30 C, 0.3 Si, 0.4 Mn, 7.0 Mo, 6.5 V, 10.5 Co, balance iron and unavoidable impurities, was heat treated in the following way. A first end 2 of the rods was submerged to a depth of 45 mm, corresponding to 40 % of the whole length of the rod, in a bath of molten steel- hardening salt having the temperature 1180°C. The rods were kept with the end 2 submerged in the bath until the end had been through-heated, which took 100 s.

Thereafter the rods were cooled in a step quenching salt bath to about 500°C, whereafter the rods were allowed to cool freely in air to a temperature between 40 and 60°C. The same procedure then was carried out with the second rod end 3 of both the two rods 1, which were submerged to the same depth but in a salt bath with the temperature 1100°C, whereafter the rods 1 again were cooled in a step quenching salt bath to about 500°C, whereafter they were allowed to cool freely in air to between 60 and 40°C.

Finally the whole rods 1 were tempered three times at 560°C, each time for 1 h, and between the tempering operations the rods were cooled to room temperature, 25°C, as

well as after the third tempering operation. Through hardness measurements it was noticed that the first end portion 2 of the two rods 1 had a hardness of 68 HRC along a length of 40 mm from the end 4, while the second end portion 3 had a hardness of about 66 HRC along a length of 40 mm from the second, opposite end 5. The hardness difference, V HRC, in other words was about 2 HRC according to the example. In the region 6 between the two end portions 2,3, there was a hardness transition from about 68 HRC to about 66 HRC from the first 2 to the second end portion 3, including a hardness minimum, Fig. 2, within the unhardened central section.

Of the two rods there were manufactured shaft cutters 8,9 of the type shown in Fig. 3.

In the first case, the end portion 2, which had a hardness of about 68 HRC, was ground, and in the second case the second end portion, which had a hardness of about 66 HRC, was ground. In both cases the non-ground end portions, which had a hardness of about 66 HRC and about 68 HRC, respectively, were used as mounting members for the tool when mounting the tool in a chuck or other tool holder in a milling machine.

A shaft cutter 8 for finish working operations, Fig. 3, was made of the first rod, wherein the first end portion 2 with the hardness of about 68 HRC, Fig. 1 and 2, was ground to the shape of a conventional working part 10, Fig. 3, while the rest was worked to form the shaft portion 11 of the tool. From the second rod there was made a shaft cutter 9 for roughing down working, Fig. 4. The working part, which was made of the softer portion 3 with a hardness of about 66 HRC has been designated 12, while the mounting portion 13 was made of the harder, first portion 2.

EXAMPLE 2 Two more rods 1'of an identically same type as in Example 1 were heat treated in the following way. The first end 2'of the rods 1'were submerged to a depth of 75 mm, corresponding to 68 % of the whole length of the rod, in a bath of molten salt for steel- hardening with the temperature 1180°C, and were kept submerged for 100 s in the same way as in Example 1. Then the rods 1'were cooled in the same way as in Example 1.

The same procedure then was carried out with the second rod end 3', i. e. the rods also in this case were submerged to a depth of 75 mm in a bath of molten salt for steel- hardening with the temperature 1100°C, whereupon the rods again were allowed to cool in the same way as in Example 1. A central overlapping portion 6'in other words were hardened twice, first from a higher hardening temperature, 1180°C, and then from a lower hardening temperature, 1100°C. Finally the rods 1'were tempered in the same way as in Example 1. As a result, the approximate hardnesses 68 HRC and 66 HRC,

respectively, were achieved in the outer portions, while the central portion 6'exhibited a more successive transition between the two hardness levels, Fig. 6. The same type of shaft cutters or other tools can be made of the hardened and tempered blank as in Example 1.