GB2047589A - A tool for metal cutting, more particularly a face milling cutter - Google Patents

Abstract

A face milling cutter (10) has circular sintered hard metal throwaway cutting inserts (13) mounted at a setting angle of 0 DEG , and having a sintered-in clearance angle ( alpha ) of 11 DEG and a sintered-in chip-guiding formation (16) in the form of an annular groove, in the endface (15) presented to the workpiece. This formation has, from its deepest point (19), a linearly bevelled region (17) running radially outwards and a circularly rounded region (18) running radially inwards. A front rake ( gamma ) of 11 DEG or 25 DEG corresponds to the angle of the bevelled region (17) relative to the plane of the endface (15). The rounded region has a radius of about 3.1 mm. The cutting edge (22) has a stabilising land with a width (a) of only 0.08 mm. <IMAGE>

Description

SPECIFICATION A tool for metal cutting, more particularly a face milling cutter This invention relates to a tool for metal cutting, more particularly a face milling cutter, having circular sintered hard metal cutting inserts detachably secured to a cutter body.

It is known to machine metal components by cutting processes, for example turning or milling, with the aid of cutting heads to which the actual cutting tools are detachably secured, for example with the aid of screws.

Circular toolbits of the sintered hard metal throwaway cutting insert type have proved outstandingly advantageous for this purpose, since when worn in one region they can be restored for immediate further use by simply rotating them through a certain angle (stepless rotation) until the entire cutting edge has worn out over the entire 360 circumference.

At this stage, the worn-out cutting inserts can be thrown away and replaced by a new set. It is also known to produce these hard metal throwaway cutting inserts by sintering. Because of the extremely high loads on the compacting tools (die and punch), it has hitherto been customary to keep the clearance angle of the hard metal throwaway cutting inserts, which gives them their truncated conical shape, as small as possible, so as to minimize the load on the compacting tools and thus prolong their useful life. The absolute minimum load on the compacting tools would obviously be reached by reducing the clearance angle to zero and the shape of the inserts to perfectly cylindrical, since this would eliminate the taper on the outer surfaces. On the other hand, however, cutting becomes impossible when the tool clearnace angle is 0'.

For these reasons, the known circular sintered hard metal throwaway cutting inserts have a relatively small clearance angle of 7 .

Experience then demonstrates that such a small clearane angle of only 7" is not the optimum for a large and even predominant range of machining processes. Hitherto, it has only been possible to obtain the required clearance angle for tools smaller than 100 mm in diameter by mounting the hard metal throwaway cutting inserts at an angle in the cutter body. However, the advantage of deliberately and artificially increasing the clearance angle in this way is offset by the serious disadvantage that because the throwaway cutting inserts are mounted at an angle the front rake simultaneously required for optimum chip removal is simultaneously and seriously reduced. It often leads to high cutting forces and associated undesirable vibrations in the machine tool, for example a milling machine.

The consequences are machining errors on the workpiece and rapid cutting tool wear. In order to match the tool geometry to the machining requirements, i.e., to adjust the clearance angle and/or the front rake, the known sintered hard metal throwaway cutting inserts must be subjected to a difficult and costly final machining process, by grinding to shape.

The object of the invention is to modify a cutting tool of the type described above so that it can be used for the widest variety of machining operations with optimum results and without encountering the problems discussed above, yet does not require any difficult or costly final preparations.

According to the present invention the circular sintered hard metal throwaway cutting inserts are mounted in the tool body at a setting angle of 0 , and have a sintered-in clearance angle of 11" and a sintered-in chipguiding formation, in the form of an annular groove, in the endface presented to the workpiece.

In one preferred embodiment of the invention, the chip-guiding step-starting always from its deepest point-has a linearly bevelled region running radially outwards and a circularly or substantially circularly rounded region running radially inwards. It is preferred to give the circular sintered hard metal throwaway cutting insert a front rake of 11", corresponding to the angle of the linearly bevelled region of the chip-guiding step.

The clearance angle of 11" and a large front rake can only be realized by the stabilised cutting edge of the circular sintered hard metal throwaway cutting insert. Thus the Applicant has discovered that the 11" setting for both the clearance angle and the front rake provides outstanding performance in the majority of machining operations. The chatter and vibration experienced in machine tools fitted with the known cutting tools of the type in question are eliminated, while chip removal proceeds cleanly.A further particular advantage is gained in that the Applicant has overcome the preconception of those versed in the art that throwaway cutting inserts of this type could only be produced by sintering if the clearnace angle is kept relatively small, i.e., about 7 . In other words, the Applicant has made the surprising discovery that the hard metal throwaway cutting inserts of the invention can be produced by sintering without incurring the feared premature wear in the compacting tools, even though the clearance angle is 11".

Although the invention can in principle be applied to any type of machining operation involving chip removal, including turning operations for example, it has been primarily conceived with reference to face milling cutters. In this case, the specific advantages of the invention are particularly valuable. Thus it has been found possible to replace the expensive and uneconomic solid high-speed steel milling cutters of DIN 1880, which have hitherto been used for smaller workpieces, by economical face milling cutters with throwaway hard metal cutting inserts, which have only hitherto given satisfactory performance at diameters exceeding 100 mm.

It is possible with a face milling cutter of the invention to shorten the machining times for depths of cut up to 8 mm by 85% and reduce the tool costs by up to 75%.

The circular sintered hard metal throwaway cutting inserts of the invention have the following further and outstandingly advantageous characteristics: 1. They have robust, i.e., thick cutting edges.

2. The circular insert matches the depth of cut, i.e., the insert is optimally utilized.

3. The optimum cutting geometry in conjunction with the favourable setting position gives high stock removal rates.

4. The positive cutting angle and the favourable shape of the sintered-in chip-guiding step prevent chip build-up and promote good chip formation.

5. The vibration amplitudes in the machine tool are reduced by about 25-32% compared with using the conventional hard metal throwaway inserts.

6. The more favourable vibration behaviour of the machine tool leads to longer insert life and better surface finishes on the workpieces.

7. The increased clearance angle and the positive front rake, combined with using an odd number of inserts, give a lower driving load on the machine tool and hence a lower power consumption.

Circular sintered hard metal throwaway cutting inserts in accordance with the invention can be used over the entire machining spectrum from steel and cast iron to aluminium, with equally optimum performances.

A face milling cutter embodying the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a face milling cutter; Figure 2 is a plan view of one of the circular sintered hard metal throwaway cutting inserts of the cutter of Fig. 1; Figure 3 is a half-sectional side view, the section being taken on the line Ill-Ill in Fig.

2; Figure 4 is a greatly enlarged sectional view of the area A indicated in Fig. 3; and Figure 5 is a diagrammatic front elevation of the milling cutter of Fig. 1 showing one insert only.

The milling cutter shown in its entirety in Fig. 1 consists in detail of a cutter body 10 having a tapered central bore 11 by which the cutter body 10 can be mounted on a miller spindle. Round the cutter body 10 there are five recesses 1 2 shaped as determined by trial and error to provide unhindered chip escape and in which are mounted a corresponding number of cutting inserts 13, each of which is detachably secured to the cutter body 10 by means of a screw 14.

As shown more particularly in Figs. 2 and 3, the cutting inserts 1 3 are circular, in the shape of truncated cones. They consists of sintered hard metal, and constitute what are known as throwaway cutting inserts. The machined surface finish and stock removal rate attained by cutting tools of this type depend critically on the tool geometry. Two angles in particular are decisive, viz., the clearance angle a shown in Fig. 3 and the front rake y shown in Fig. 4. The clerance angle a gives the cutting insert 1 3 its truncated conical shape. The clearance angle a shown in Fig. 3 is 11", and is in fact obtained at the end of the sintering process, so that no subsequent finishing of the cutting insert 1 3 by grinding is needed.

As shown in Fig. 4, the front rake y is also 11". This angle is particularly adapted to the milling of steel and cast iron workpieces.

Other front rakes can also be adopted when machining other materials, for example 25 for aluminium workpieces. The front rake y--like the clearance angle a-is also formed during the sintering process, but a stabilising land of width only a = 0.08 mm is retained, since a final grinding operation would otherwise be required.

Figs. 2 and 3 further show that the cutting insert 1 3 has a chip-guiding formation 16, in the form of an annular groove, in the endface 1 5 presented to the workpiece. As shown more particularly in the greatly enlarged view of Fig. 4, the chip-guiding formation 16 consists of two regions, viz., a linearly bevelled region 1 7 running radially outwards and a circularly or substantially circularly rounded region 1 8 running radially inwards. The two regions 1 7 and 1 8 both start from the lowest point 19 in the chip-guiding formation 16.

The linearly bevelled outer region 1 7 produces the front rake y already referred to. The rounded inner region 1 8 has a radius of R = ca. 3.1 mm. Like the clearance angle a already referred to, the chip-guiding formation 1 6 is produced directly during the sintering process, so that again no subsequent finishing by grinding is required.

The circular sintered hard metal throwaway cutting inserts 1 3 are secured to the cutter body 10, as already indicated by means of the screws 1 4. For this purpose, the throwaway cutting inserts 1 3 each have a central bore 20, while the cutter body 10 has corresponding tapped holes (not shown). A bevelled shoulder 21 in the bore 20 causes tight fitting of the screw 14 which ensures that no undesired rotation of the hard metal throwaway cutting insert 1 3 can occur during machining operations. The hard metal throwaway cutting insert 1 3 can only be rotated after first lossening the securing screw 14. However, this operation is only necessary after the part of the cutting edge on the throwaway cutting insert 1 3 in current use has worn out.

Fig. 5 shows a circular sintered throwaway cutting insert mounted in a normal position on the cutter body 10. In other words, the cutting edge 22 lies exactly on a radial plane 23 of the cutter body 1 0. This obviously applies also to the remainder of the throwaway cutting inserts 1 3 on the cutter body 10 (as shown in Fig. 1 for example.) The substantial advantages of the invention, as enumerated above, thus arise on the one hand from the true positioning of the throwaway cutting inserts 1 3 just referred to and on the other hand from the large angles (clearance a and front rake y).

Claims (8)

1. A tool for metal cutting having circular sintered hard metal throwaway cutting inserts detachably secured to a cutter body, the hard metal throwaway cutting inserts being mounted in the cutter body at a setting angle of 0 , and having a sintered-in clearance angle of 11" and a sintered-in chip-guiding formation, in the form of an annular groove, in the endface presented to the workpiece.

2. A tool as in Claim 1, wherein the chipguiding formation has, from its deepest point, a linearly bevelled region running radially outwards and a circularly or substantially circularly rounded region running radially inwards.

3. A tool as in Claim 1 or Claim 2, wherein each circular sintered hard metal throwaway cutting insert has a front rake of 11", corresponding to the angle of the linearly bevelled region of the chip-guiding formation relative to the plane of the endface.

4. A tool as in Claim 2 or Claim 3, wherein the rounded region of the chip-guiding formation has a radius of exactly or approximately 3.1 mm.

5. A tool as in Claim 1 or Claim 2, wherein each circular sintered hard metal throwaway cutting insert has a front rake of 25'.

6. A tool as in any one of the preceding Claims, wherein the cutting edge of each hard metal throwaway cutting insert has a stabilising land with a width of only 0.08 mm.

7. A tool for metal cutting substantially as hereinbefore described with reference to the accompanying drawings.

8. A circular sintered hard metal throwaway cutting insert substantially as herein before described with reference to Figs. 2 to 4 of the accompanying drawings.