Hybrid process of machining of high speed/machining laser for the manufacture of moulds

The introduction of the technology of machining at high speed in the lines of production of moulds has allowed to obtain a considerable reduction in the time of manufacture and improvement in surface finish. But there are still problems with regard to the limitation of the minimum diameter of tool and the need to control wear and breakage of tool that can occur during the operation of milling. In this sense, the mechanized laser presents the opportunity to improve the current mold manufacturing process to be able to have a very small diameter tool and which does not suffer wear. Therefore, the combined process of AV milling and machining laser could be the solution, as this new hybrid process suitable for the production of molds with complex details.

This paper presents the study of the process of laser machining done on a steel mould (X153CrMoV12) in order to determine the most appropriate process parameters. Surface finish and the time of machining are considered as process requirements. Likewise, the methodology of the design of experiments can identify most influential process parameters and their relationship with the results.

The mould makers suffer increasingly greater demands by the end customer, quality, accuracy, and cost of the final piece. This implies that they are constantly searching for solutions for the improvement of the manufacturing process. At the same time, machine tool manufacturers are introducing new technologies to obtain machines hybrid, able to carry out operations of various kinds in the same machine.

Although the current cutting tools far outweigh the limitations that had a few years ago, and can readily available tools for milling of diameters of up to 0.5mm, the requirements of machines for these tools are very high. In addition, may wear and/or tool breakage may occur, or does not maximize production rates.

The laser beam has the great advantage of being a tool of work without contact, of very small diameter, which does not suffer breakage or wear during the machining process.

Combining the machining and laser technologies on a single machine, you get a hybrid machine able to carry out supplementary operations among themselves. The DMU60L machine Deckel Maho, available in Tekniker is a clear example of hybrid machine milling / laser.

This article contains the study carried out in the search and obtain the most appropriate process parameters of laser machining. Identify these parameters and relate to the results. The presented study is a study that was carried out specifically for the DMU60l machine.

Planning a comprehensive battery of experiments, which are intended to identify the parameters that have the best result in their finish piece (less roughness), and machining time (less time machining).

The equipment available in Tekniker, as shown in Figure 1, is a milling machine model DMU60L of Deckel Maho, which has two heads: head of machining at high speed and laser head.

Head of 28kW of power that can carry out machining at high speed, and can work up to 18. 000rpm, with a speed of progress of work of 25 m/min.

He is available a pulsed laser in the solid state of ND: YAG, with a wavelength of 1,064 ° m, pumped by lamp, whose average power is 100W, and that by adjusting the frequency of the Q-Switch can get up to 20kW power.

The laser head has:

• Transducer: it allows to measure machined depth.

• House of vision: allow the precise placement of the piece.

• Scanner mirror: creates a workspace of approximately 70 mm 2.

When you need a larger work area, the machine performs displacement in its X-Y axes, according to the needs of the laser beam.

You have the option of choosing the size of the diameter of the beam:

• Small: 40 ° m.

• Large: 100th m.

Figure 1. Hybrid machine DMU60I

The laser head is controlled by the application LaserSoft 3D, is also carrying out the search for the parameters using this control.

At the same time, machining programs are created using the LPSWin application.

This application allows, as shown in Figure 2, on the basis of a CAD (3D volume) file, create "tpf" files containing machining strategies and process parameters assigned, by layers. Finally, some "l4d" programs, which create strokes compatible with machine laser are created.

The studied material is a steel of tools for working in cold, of quality X153CrMoV12, which presents a State of quenching and tempering, with a hardness of HRc 46.

The test tubes is rectangular, with dimensions 80 x 70 x 20 mm, enabling his later characterization in the electronic scanning microscope (SEM) and in the confocal microscope.

Experimental procedure

The procedure followed in the conduct of the experimentation is in the definition of initial parameters of process, provided by the manufacturer, that will be optimized according to the results obtained.

The aim is that the time of machining and roughness values are as small as possible.

Of all process parameters, are chosen as independent variables to consider:

• Thickness of layer.

• Distance between beams.

• Speed of the beam.

Describes a circle of 5 mm in diameter as a geometric shape machining.

Prior to the machining of the circle, carried out a search of technology, where the parameters (frequency, intensity, parameter First Pulse Killer (FPK)) are adjusted to the material object of study.

Analyzing trends, and results achieved, will have been adjusting parameters to obtain suitable roughness values.

As regards the characterisation of the test tubes, through a Perfilometer without contact of the PL ° brand have obtained images confocal that allow to measure the roughness.

At the same time, through the electronic scanning microscope JEOL JSM-5600LV, have been obtained photographs of the surface quality.

Figure 2. Creation of machining program.

The way to machining is a circle of 5 mm in diameter. For each group of parameters, there have been three repetitions, with a view to assessing the repetitibilidad of the results obtained. Two of the trials has been with a depth of machining of 0, 2 mm, while the third test has been carried out with a depth of 0, 4 mm.

Because the initially proposed objective was to improve the rugosida, to note that different value of depth of machining, the value of the obtained roughness varies, most of the experimentation has been done with a depth of 0, 2 mm.

The initial process parameters are indicated by the manufacturer of the machine for a common steel, and are contained in table 1.

Table 1. Initial process parameters

The value of the surface roughness Ra obtained with these parameters is approximately 2.2 microns. The image of Figure 4 shows an image of roughness obtained with the microscope confocal for these initial conditions.

Figure 3. Image of roughness

Figure 4. Image of roughness

Figure 5. Image of roughness

As at the time of machining, with initial process conditions are needed 12 minutes for the machining of the defined geometric shape.

After extensive experimentation, you get a set of parameters, which obtain a Ra of approximately 1 m. Figure 5 shows an image corresponding to this value of roughness.

There have been different groups of parameters of process give us a roughness of approximately 1 m, having the machining time difference between them. Machining time varies from 25 minutes to 42 minutes.

At the same time, the shorter machining time is achieved with a combination of parameters which presents a surface roughness of approximately 3.4 ° m, which is shown in the image of Figure 6.

Finally, have been compared the images obtained with the scanning electronic microscope, where you can see that the surface obtained with conditions that present the smallest roughness is more uniform than when the roughness has a larger value.

For machining in less time, there are a few holes on the machined surface.

Figure 6. ((Imágenes_obtenidas_con_SEM:_a) conditions initials, (b)) best roughness, c) less time

Finally, example a mold machining by hybrid technology. Mechanized material is the same steel as the studied using experimentation.

Part of the mold is machined using cutting tool, making a first roughing operation, and a later finishing. Complete machining by means of the laser beam.

The following tables and figures show the used process parameters, and images of the mold at different stages of manufacture.

Table 1. Roughing operation

Figure 7. Mold after roughing

Table 2. Finishing operation

Figure 8. Mould after the finish

Part of the mold is machined using cutting tool, making a first roughing operation, and a later finishing. Is completed by the laser beam machining

The smallest details, such as those shown in Figure 9, machining by laser.

Figure 9. Details machined by laser

As a general conclusion, mentioning that the hybrid process of machining by starting of chips and laser machining is feasible for obtaining moulds with small and complex geometric shapes.

The operation of high speed milling achieves a high startup rate, while the mechanized laser allows to get small complex figures.

Anyway, it is necessary to carry out a feasibility study for each of the pieces of interest, because the laser is a very slow process, and is not valid for machining depths from 5 mm.

As particular conclusions of the process are obtained, after analyzing the trends followed by the results depending on the parameters, they are:

• Reduced thickness of layer, lower is the value of the roughness.

• To lower value of distance between beams, lower is the value of the roughness.

• Speed not affect much the value of the roughness.

• Roughness obtained with larger diameter beam, is worse than with small diameter.

• To shallower depths of machining, minor is the roughness.

• The diameter of the beam does not influence the machining time for the same value of h.

• The machining time is less, when the thickness and the distance between beams is greater.

Thanks to the call Etortek of the Basque Government, the grant to the CIC Margune, which has allowed dealing new manufacturing processes, which include the hybrid machining milling / laser.