GB2354728A - Milling cutter - Google Patents

Abstract

A milling cutter 1 comprises a ball nose that blends tangentially into a barrel flank. The nose radius R<SB>N</SB> may be in the range 2mm to 8mm, and the barrel flank radius R<SB>B</SB> may be in the range 12mm to 50mm.

Description

2354728

Title of the Invention Milling Cutter

Field of the Invention

This invention relates to a milling cutter particularly, but not exclusively for the machining of difficult materials, such as titanium or nickel super alloys as are used as aircraft jet engine components.

Background of the Invention

Aeroengine manufacturers are increasingly using a new type of engine component called a bladed disk or blisk, or, in US terminology "integrated bladed rotors" or IBR's. Unlike conventional jet engine designs which use an array of individual blade airfoils fitted into a rotor, blisks are one monolithic structure. That is, blisks are one solid unit with no blade-to-rotor connection.

The elimination of the blade-to-rotor connection has many benefits such as:

1. 30% weight reduction compared to a blade and rotor assembly, which means a higher thrust to weight ratio.

2 A smaller hub diameter may be employed and therefore a larger airfoil for the same diameter, resulting in greater thrust for the same engine dimensions.

3. the blade-to-rotor connection was a weakness and its elimination means the blisk can be 25 rated for higher rpm use - again increasing thrust.

2 These performance benefits mean that blisk technology is being implemented in all new military engine programmes. It is also a relatively easy upgrade route for existing engine designs and it is only a matter of time before this technology enters civil engine designs.

The biggest current disadvantage with blisks is their high cost which is mainly due to their very long manufacturing cycle times using conventional ball nose milling cutters. The cycle times are long because the airfoils are of course complex compound curved surfaces and they are made, as indicated above, from low machinability materials such as titanium or nickel superalloys. Depending on their dimensions and material, blisk cycle times can vary from 400 to 1000 hours of continuous 5-axis machining, and as 5-axis machines are typically rated at f,100/hour, the machining cost for blisks is currently in the range of E40,000 to f 100,000.

The complex compound curved surfaces of blisk referred to above, which need to be machined to a specified surface texture, are currently machined by point milling using a ball nose milling cutter, which is very time consuming as an airfoil may need hundreds of point milling passes.

The measure of productivity for these surfaces is called the 'surface generation rate'or SGR. Its units are either mm'/min or cm'/min and comprises two variables:

Feed rate (mmJmm) Step over (mm) and as illustrated in Figure I of the accompanying drawings, which shows, to an enlarged scale, the surface imparted to a blisk by the point milling process. Step over is the distance W between each tool pass, R is the cutting radius of the tool and 'h' is the cusp height, and the following 3 equation: h)' shows the maximum distance'w'that a ball nose milling cutter can step over for a given surface texture.

With conventional ball nose milling cutters, illustrated by way of example in Figure 3 of the accompanying drawings, the two main problems are:

I. low surface generation rates (CM2/Min) because:

a) the cutting radius is constrained to be half the cutter nose diameter. 10 b) small ball diameters mean small cutting radii and low SGR's, and 2. low shear cutting action in point milling of materials such as titanium and inconel.

Object of the Invention A basic object of the present invention is the provision of an improved milling cutter having a geometry that can result in minimised machining time for low machinability materials.

Summary of the Invention

According to the present invention there is provided a milling cutter having a ball nose that tangentially blends into a barrel flank.

Advantages of the Invention The milling cutter in accordance with the invention thus ensures no discontinuity, but a blend, at the point where the nose radius and the barrel radius meet, the ball nose extending 4 contiguously or merging into the barrel flank, and vice versa.

Thus, the cutter in accordance with the invention addresses the shortcomings of the existing ball nose milling cutter and consequently facilitates reduced cycle time, high SGR manufacturing of blisk airfoils as, compared to current blisk machining practices, can triple productivity in semi-finishing operations which typically constitute about 60% of the total machining cycle time.

Further advantages of a ball nose barrel milling cutter in accordance with the invention over a conventional ball nose milling cutter are that:

(i) the cutting radius is independent of the nose diameter of the milling cutter which 10 allows large cutting radii on relatively small diameter milling cutters; (ii) large cutting radii facilitate higher SGR's for a given surface texture; (iii) cutting on the barrel radius (RB) as opposed to the nose radius (R. ) utilises the tools helix angle for a higher shear cutting action, which reduces cutting force when machining titanium and inconel materials for example; and (iv) the ball nose on the milling cutter protects, what would otherwise be vulnerable end teeth from chipping.

A typical range of values for RB is 12,50mm and for RNis 2, 8mm.

Figure 2 of the accompanying drawings is a graph showing a plot of equation I for a constant cusp height of 0.005 mm. It shows that, for a given surface texture, step overs can be increased if the cutting radius is increased.

Currently, blisk milling cutters use ball nose designs, as exemplified in Figure 3 of the accompanying drawings, that is designs in which, by definition, the cutting radius is half the nose diameter.

The disadvantage of a Figure 3 ball nose milling cutter in terms of surface generation rates is that, if for access reasons the nose diameter of the tool is small, then SGR's will be commensurately low.

A conventional barrel milling cutter is illustrated in Figure 4 of the accompanying drawings.

One example of ball nose barrel mill in accordance with the invention as shown in Figure 5 of the accompanying drawings.

A milling cutter I is rotatable about an axis (not shown) orthogonal to its mid plane 2 and terminates in a peripheral ball nose RN having a typical radius in the range 2,8mm. From the ball nose RN blends tangentially with a barrel flank R. having a typical radius in the range 12,50mm, the barrel flank RB extending toward the axis of rotation until side faces 3 of the cutter I are met at line 4.

6

Claims (4)

1. A milling cutter having a ball nose that tangentially blends into a barrel flank.

2. A milling cutter as claimed in Claim 1, wherein said ball nose (R.) has a radius in the range 2mm to 8mm.

3. A milling cutter as claimed in Claim 1, wherein said barrel flank (R,, ) has a radius in the range 12mm to 50mm.

4. A milling cutter substantially as herein before described with reference to Figure 5 of the accompanying drawings.

Amendments to the claims have been filed as follows CLAIMS. -7/ 1. A rotary cutting tool, particularly a milling cutter for increasing the CNC productivity of aeroengine components with three-dimensional surfaces, wherein 5 the tool comprises a ball nose of constant radius RN, that tangentially blends into a barrel flank of constant radius RB. 2. A tool as claimed in Claim 1, wherein said ball nose (RN) has a radius in the range 2mm to 8mm. 10 3. A tool as claimed in Claim 1, wherein said barrel flank (RB) has a radius in the range 12mm to 50mm. 4. A tool substantially as herein before described with reference to Figure 5 of the 15 accompanying drawings.