The present invention concerns a process for making metal articles with complex geometry, such as, for example, nozzle plates, diaphragms, bladed rings, multi- blades (double, triple, etc.), rotors or similar for the construction of turbines.

Prior Art

The mass-production of geometrically complex metal articles is currently carried out according to various techniques. In particular, with regards to turbine manufacture, various components are prepared, for example, bladed rings and diaphragms, each composed of various elements that are then manually assembled by welding.

In particular, each blade is individually milled and then assembled by welding it between an inner and an outer ring. The thus obtained assembly is then reworked to perform the finishing.

Traditionally, nozzle plates and multi-blades are manufactured from forged bodies that are then processed using plunge electromachining. This technique contemplates the utilization of traditionally slow machines, which use copper or graphite electrodes to remove material.

However, the resulting surface of components obtained in this manner can have micro-craters that could give rise to crack formation, which would compromise the integrity of the machined component. This drawback can be circumvented by stress- relieving heat treatment and subsequent manual polishing to remove the first layer of material that could give rise to the negative phenomena mentioned.

It is consequently evident that all these manufacturing steps to obtain the finished article require extremely long times, hi addition, the manual assembly of components can entail assembly errors and the numerous welding areas can also be susceptible to the formation of further cracks.

One possible alternative manufacturing technique could be that of making the desired article from a solid piece, namely starting from a single block of the same material, from which excess material is gradually removed until the shape of the final article is obtained. However, this solution can be unfeasible in cases where the geometry of the final article is particularly complex, such as in the case of rotors or nozzle plates for turbines, h fact, even if particularly advanced numerical control machines are used, most of the areas from where material must be removed are inaccessible to the tool due the bulk of the spindles and the tools themselves.

Summary of Invention

That having been said, one object of the present invention is that of proposing a process for making shaped metal articles with complex geometry that allows manufacturing times to be considerably reduced.

Another object of the present invention is that of proposing a process of the above- mentioned type that allows one-piece shaped metal articles to be made with high precision, even if characterized by particularly complex geometry.

A further object of the present invention is that of proposing a process of the above- mentioned type that allows shaped metal articles of quality and high reliability to be obtained and, in particular, metal articles in which the possibility of crack formation is reduced or even cancelled.

These objects are achieved by the present invention thanks to a process according to claim 1 for making a shaped metal article, in one piece, by the removal of material from a starting blank. Further distinguishing characteristics of the present invention are detailed in the respective dependent claims.

In particular, the process according to the present invention comprises the steps of: a) providing a numerical control machine having at least one spindle to support a rotating tool;

b) defining a mathematical model of the shaped article to be made;

c) programming a work path for the tool to make the shaped article;

d) simulating, via software, the process of removing material according to the path established in step (c) to identify the points of collision between the shaped article undergoing machining and the tool's shank;

e) modifying the geometry of the tool's shank to avoid the collisions identified in step (d); and

f) carrying out machining to remove material from the starting blank by using a numerical control machine on which the tool as modified in step (e) is mounted. In particular, the geometry of the tool is modified by removing material from a portion of the shank located close to its working end.

A shrink-fitting spindle, i.e. a spindle having particularly compact dimensions with respect to those with mechanical clamping and a reliable hold on the tool's shank, is preferably used to support the tool.

By simulating beforehand the tool paths and the possible collisions with the piece in the machining phase, it is possible to establish which are the standard tools suitable for being used in the various machining phases and the possible modifications to the geometry of the shank that allow each tool to access all the points where material must be removed. The preventive simulation also allows the optimization of each single working and rapid extraction movement that the numerical control machine must perform during each step of the machining process.

In particular, the programming of the tool path provides for the definition of a reference axis with respect to which an orbital work path for the tool is followed.

The manufacturing process is totally revolutionized with respect to the known art, first of all by eliminating the electromachining of individual components, which is substituted by machining with removal of material. The subsequent stress-relieving heat treatments for removing any micro-craters from the individual components are also eliminated, as is the subsequent phase of component assembly via welding, which can entail assembly errors and/or the formation of areas subject to cracking. The process according to the invention permits a significant reduction in manufacturing times and, in consequence, high productivity. From the economic viewpoint, this also reflects considerably on the construction of new manufacturing plant and on the reduction of idle time for the maintenance of already installed plant. A significant advantage of the present invention is given by the fact that it is possible to make components for turbines that have never been made in one piece before, in particular bladed rings and diaphragms.

Brief Description of Drawings

Further characteristics and advantages of the present invention shall become evident from the description that follows, made with reference to the enclosed non-limitative drawings provided for illustrative purposes, where:

- Figure 1 is a- simplified schematic view that shows a shaped article during machining for the removal of material using a standard type of tool normally available on the market;

- Figures 2A, 2B and 2C show the various steps of modifying a standard tool to adapt it for machining the shaped article without collisions occurring;

- Figure 3 is a similar view to that of Figure 1, in which a comparison between the machining performed with a standard tool and a tool with modified geometry is also shown;

- Figure 4 is a partially sectional schematic view that reveals the path of the tool during the machining of a shaped article with complex geometry;

- Figure 5 is a schematic view that shows the tool's mode of operation during the removal of material;

- Figures 6A and 6B are perspective views of the respective opposite sides of a one-piece bladed ring made according to the process of the present invention;

- Figures 7 A and 7B are perspective views of the respective opposite sides of a one-piece diaphragm made according to the process of the present invention;

- Figures 8A and 8B are perspective views of the respective opposite sides of a one-piece multi-blade made according to the process of the present invention; and - Figures 9 A and 9B are perspective views of the respective opposite sides of a one-piece nozzle plate made according to the process of the present invention.

Modes for Carrying out the Invention

Figure 1 shows, by way of example, a shaped metal article 10 that can represent, in a very simplified form, a nozzle plate having an inner ring 11, an outer ring 12 and a plurality of blades 13 between which spaces 14 are delimited that constitute the so- called "nozzles". The article 10 is made in one piece, starting from a blank that is preferable pre-formed by means of any of the known techniques in this sector, such as forging for example.

This view shows a simulation step of the machining of the shaped article 10 with a standard-type rotating tool 20 shrunk fit onto a spindle 30 of a numerical control machine. The latter preferably consists of a numerical control machine able to move the tool 20 simultaneously with respect to five axes.

First of all, the process according to the present invention provides for an initial analysis of the three-dimensional mathematical model that represents the article 10 to be obtained.

Using CAD or CAD/CAM software normally available on the market, a step of simulation is carried out on the paths that the tool must follow during the removal of material from the starting blank, at the same time examining each single working and rapid extraction movement that the machine must perform.

For simplicity, this description refers to a single tool, but it is opportune to underline that the work cycles to obtain the desired article can also be carried out with a number of tools of different types. In fact, various machining cycles are contemplated; for example, rough machining, roughing and finishing stages, with the best machining strategies being determined for each one.

The simulation allows the identification of points of collision between the shaped article 10 undergoing machining and the shank of the tool 20. Figure 1 shows, by way of example, at least two points of collision, identified by reference numerals 2 and 3.

To allow machining to be carried out, the present invention proposes to modify the geometry of the tool's shank so as to allow the nozzles 14 to be machined nevertheless, even in depth, whilst at the same time avoiding any collisions.

Figure 2A shows, by way of example, a standard type of tool 20 normally available on the market, in particular a cutter with a "lollipop" type of tool tip 21 supported by a shank 22 having a "relief portion 23, namely of slightly smaller diameter, close to the tool tip 21.

After the simulation via software has been completed and the points of collision identified, the modification to be made to the geometry of the shank 22 is calculated: as shown in Figure 2B, the extent of the relief on the shank 22 is more pronounced, further reducing the diameter of the shank with respect to the nominal diameter and axially extending the relieved portion 23' for a longer length from the tip 21.

In this way, a tool 20' (Figure 2C) is obtained with a shank 22 having more relief with respect to the nominal diameter and with a tip 21 able to work in undercut as well, also allowing the milling depth to be extended without collisions occurring. A comparison between- the modified tool 20' and the standard tool 20 with respect to the shaped article 10 is shown in Figure 3, where the two tools 20 and 20' are overlaid so that the advantages that can be achieved thanks to the present invention can be appreciated.

The machining of the shaped article 10 is then carried out by fitting the tool 20' on the spindle 30 of the numerical control machine. The spindle 30 is preferably of the shrink-fit type as this has less bulk compared to traditional spindles with mechanical fitting.

Figure 4 shows a sectional view of a portion of a nozzle plate 110 equipped with a centre ring 111 from which the blades 113 that delimit the nozzles 114 project.

Only the path followed by the tool is shown, for clarity schematically represented here by a broken line 50 that crosses one of the nozzles 114, but it should be underlined that, in reality, the tool is manoeuvred with a "sinuous" movement that follows the shape of the nozzle 114 delimited by the blades 113 so as to avoid collisions.

First of all, to define the path of the tool according to this sinuous movement, an axis 60 is manually generated via a CAD program and placed perfectly at the centre of the section of the nozzle 114.

This allows the axis of the tool to be positioned via the combination of angular movements of the numerical control machine and to perform three-axis milling for each section of the nozzle. As the tool advances inside the nozzle, the machine also controls the movement of the tool with respect to the two remaining axes, thereby using all five axes simultaneously to achieve the complete machining of the nozzle. As is also shown in Figure 5, the axis 60 is used as a reference (pivot) and allows the CAM software to keep the tool's shank at centre of the nozzle 114 to avoid collisions. As material is removed, the section of the nozzle changes and, with it, the direction of the reference axis 60. The removal of material therefore takes place with orbital movements of the tool's tool tip, represented in this case by the concentric dashed lines in Figure 5.

At the end of the machining, further finishing processes can be carried out such as, for example, light polishing to remove the signs of milling and then a phase of fine sandblasting.

As examples of articles with particularly complex geometry made in one piece by means of the process of the present invention, Figures 6A and 6B show the respective opposite sides of a bladed ring, Figures 7A and 7B show the respective opposite sides of a diaphragm, Figures 8A and 8B show the respective opposite sides of a multi-blade, and Figures 9A and 9B show the respective opposite sides of a nozzle plate.

Various modifications can be made to the embodiments represented here without leaving the scope of the present invention. For example, it would be evident to an expert in the field that besides the one with the "lollipop" tool tip that is described by way of example, other types of tools can likewise be used.

It is also implicit that the process according to the present invention can also be applied to other articles with complex geometry in addition to those described herein, for example, to components of other types of machine such as compressors or similar, as well as to individual elements in one piece that must then be assembled by welding, such as multi-blades for example, which are traditionally obtained by means of plunge electromacliining.