Ball nose end mills

The present invention relates to ball nose tools, preferably ball nose end mills, with improved properties.

Ball nose end mills are advanced tools which have to manage the most changing working conditions at the same time. The most distinguishing feature of ball nose end mills are good cutting properties which are required in the form of wear resistance and the ability to resist high temperatures at the periphery, i.e. at full nominal measure, and simultaneously as the tool is required to generate chips and function as a cutting tool in the centre, where the cutting speed approaches zero. Between the periphery and the center there is a continuous change of the cutting speed through all possible built up edge areas etc.

Ball nose end mills are often used in difficult operations, where demands are very high in terms of surface finish, for example, the aerospace industry when milling wing spars etc. In this application, no unevenness and notches whatsoever may be tolerated which later may be able to cause failure. Another large area of application is the finishing of moulding tools where demands on high surface finish, accuracy to shape with simultaneous high productivity and predictable long tool life are especially great. Further, the tool may not be exchanged during the machining operation, which could result in worse precision of the manufactured part.

The geometries which normally exist on today's conventional tools, whether they are manufactured in tough materials such as high speed steel or brittle more wear resistant cutting materials such as cemented carbide, often have a negative rake angle and a small chip room in and near the centre of the ball nose end mill. The drawbacks with this tool are primarily high cutting forces and insufficient space for the chip in the centre which results in uneven wear and risk of edge damages.

Through Swedish patent SE 392 482 a material is known containing 30-70 volume% submicron hard constituents in a metallic binder phase. This material has superior wear resistance compared to advanced high speed steel and can therefore be considered as having properties between cemented carbide and high speed steel. Further, Swedish patent SE 440 753 discloses superior compound tools made with i.a. the above material placed in the areas subjected to high cutting speed and with high speed steel in the centre for drilling applications where there is the zero speed problem. The purpose with this later invention was with the aid of a tougher core partly to obtain a tool with better macro toughness and partly to achieve better grinding economy since hard constituent rich material is experienced as considerably more difficult to grind than, for example, high speed steel. In addition, the zero speed problem and built up edge formation areas were experienced as problematic for the submicron hard material with 30-70 volume% hard constituents.

It has now been shown possible partly to improve the toughness behaviour of the submicron hard material discussed above, such that long, slender, solid tools with good macro toughness and high rigidity have been made, partly also to develop the grinding technique to an economically reasonable level to be able to grind advanced edges even in the centre of a tool, in this case a ball nose end mill. In combination with advanced geometries remarkably advanced tools with outstanding ability to chip formation without built up edge formation over a very broad cutting speed area have been possible to be produced in this way. This results in low wear and a maintained sharp cutting edge which generates good accuracy to shape and very good surfaces of the work piece. This also applies to work piece materials which of tradition are regarded as very difficult to machine. The tools are used preferably in coated condition and it is then the so called PVD-method which is most actual. The coating is performed mainly with titanium based hard material such as TiN, Ti(C,N), (Ti,Al)N etc.

According to the present invention there now exists long, slender, solid tools, preferably ball nose end mills, of a hard material with 30-70 volume% submicron hard constituents in the form of carbides, nitrides and/or carbonitrides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and/or W in a metallic matrix based on Fe, Co and/or Ni. Preferably, the hard material consists of 30-70 volume% hard constituents of mainly TiN in a matrix of high speed steel type where the enriched hard constituents have a grain size of <1 µm, preferably <0.5 µm. By means of a well- balanced combination between tool material and tool geometry unique tools with superior performance can be obtained. The material also shows through its fine grain size and good dispersion between hard constituents and binder phase a unique good adhesion ability for a PVD-applied pure hard constituent layer. This method gives otherwise often less good adhesion compared to the more metallurgical bond which arises at so called CVD-method. The reason is foremost that CVD takes place at higher temperature. The layers that have been applied are primarily titanium based and specially good properties have/has been obtained with Ti(C,N) but even (Ti,Al)N shows great advantages. Particularly excellent properties have been obtained with 2-4 µm Ti(C,N).

Tools according to the invention function very well in cutting normal steel and in hard work piece material, having a hardness of 250-450 HB, such as tool steel, sticking material such as stainless steel, aluminium alloys or titanium alloys.

The invention is described in attached drawings wherein:

Fig 1 shows the tool in a side view.

Fig 2 shows a section through the tool according to line B - B in Fig 1.

Fig 3 shows an end view of the tool.

Fig 4 shows a section through the tool according to line A - A in Fig 1.

Fig 5 shows a side view of the front end of the tool.

The unique tough and wear resistant properties of the material have made it possible to develop a ball nose end mill with light cutting and durable geometry along the whole radius. This has been obtained by shaping the tool with constant positive rake angle (3) with size + 8° + 2° along the whole radius (4) and a continuous, dimension depending decreasing clearance angle (1) and (5) towards the centre where the clearance angle is 10° ± 2°. In the diameter range 4 - 20 mm the clearance angle varies on the periphery between 17 + 2° for 4 mm diameter and 12 + 2° for 20 mm diameter, respectively. The said angles are measured in a plane perpendicular to the cutting edge. The tool further contains substantial chip room in the centre of the tool and a small distance (2) between the opposite ground ball nose edges (4), namely 0,10 + 0,05 mm to 0,25 ± 0,05 mm for the dimensional area 4 - 20 mm respectively. The above mentioned chip room includes an extra recess, 0,35 - 0,75 mm ground on both sides of the centre (6).

Further the cutting edge of the tool is given an edge rounding treatment which is continuously decreasing towards the centre of the tool from 10 - 30 µm on the periphery to less than 10 µm in its centre, preferably 10 - 20 µm on the periphery and less than 5 µm in the centre. This results in the tools cutting to the very centre despite the fact that the chip there becomes extremely thin due to the shape of the edge radius.

What above has been said about solid tools applies also to solid tools with welded shaft in other material. The essential is that all cutting edges consist of solid hard material.

The dimensional area has above been described as ø4-20 mm but can of course be increased to for example ø3-25 mm.

Example

The example below shows in detail how material and geometry have been joined to products with superior properties. A solid rod in dimension 9.6 mm including capsule was extruded with the method according to earlier mentioned patent SE 440 753, example 3. From this rod 8 mm ball nose end mills with different geometries were ground after heat-treatment. The tools were covered with a 2 - 4 µ m thick TiCN-coating by in itself known PVD-technique. A finishing test in SS2541 was performed subsequently with conventional high speed steel and cemented carbide tool as well as in-house made tools.

Tool A: Ball nose end mill according to the present invention with from the periphery to the centre decreasing clearance angle from 15° to 10° and decreasing edge rounding from 20 µm to 5 µm.

Tool B: Ball nose end mill to a great extent according to the invention and tool A, but with constant clearance angle 15° and decreasing edge rounding from 20 µm to 5 µm.

Tool C: Ball nose end mill, manufactured in solid fine-grained cemented carbide, coated with 2 - 4 µm PVD-coating with constant clearance angle 10° and sharp edge.

Tool D: Ball nose end mill manufactured in solid highly alloyed high speed steel, coated with 2 - 4 µm TiCN, PVD-coating with constant clearance angle 11° and sharp edge.

The tools were tested in finishing of a moulding tool with axial cutting depth 1 mm and radial cutting depth 1 mm.

Cutting data and result are shown in the table below:

Test Tool

Cutting Speed (m/min)

Table Feed (mm/min)

Tool life (m)

A

80

425

129.5

B

80

425

92.5

C

80

425

91.8

D

40

165

24.0

The cutting data have followed the manufacturer's recommendation.

The test showed that test tool A according to the invention gave clearly the best results in the sense of long tool life, balanced wear and good surface finish of the work piece.

Test tool B gave worse result than tool A according to the invention with more unbalanced wear due to small chippings.