J.A. Sánchez

Advanced cutting conditions for the milling of aeronautical alloys

Abstract This paper deals with possible improvement aspects on the chip cutting milling of two alloys that are used frequently in the aerospace industry, in particular the titanium alpha–beta-based alloy Ti6Al4V and the nickel alloy usually known as type 718. Both alloys are used widely in the manufacture of different turbo-engine parts, considering their excellent mechanic features, and their resistance to high temperatures. These alloys, however, are extremely difficult to be milled, due to different factors, which are analysed later in this paper. For of this reason, their milling, drilling, and turning are carried out at very low speeds and feeds. This paper studies the tool influence as to its geometry and coating, and as to the parameters of the process (i.e. cutting speed, tooth feed, and depth of radial cut), looking for an increase in the productivity of the milling process. The cutting conditions thus searched for are successful in increasing the efficiency in the milling of actual parts in this field.

The milling of airframe components with low rigidity: A general approach to avoid static and dynamic problems

Abstract At present, airframes are mainly composed of monolithic components, instead of small parts joined using welding or riveting. Ribs, stringers, spars, and bulkheads can be included in this category. After milling, they are assembled and joined to the aircraft skins, which have also been milled. The aim of these parts is to obtain a good strength-weight ratio, owing to their homogeneity. The milling of a monolithic structural part implies removing up to 95 per cent of the weight from the raw block material. Therefore, the main objective is to achieve the highest removal rate possible. However, conditions required to achieve this (high feed, large depth of cut) in milling imply high cutting forces, which in turn induce part deflection or vibrations in those zones (thin walls and floors) where stiffness is not sufficiently high. These static and dynamic problems often lead to inaccuracy of geometry, roughness, and possible damage to the machine spindle. This paper proposes a working methodology for efficient process planning, based on previous analysis of the static and dynamic phenomena that can occur during high-speed cutting. This methodology provides several steps that can be taken in order to minimize the bending and vibration effects; suggests optimal monitoring methods to detect process instability; and describes the best way to tune the cutting conditions and chip load, by means of simulation at different machining stages. In this way, the reliability of aeronautical production significantly increases. The global approach presented in this paper has been applied to two test pieces and two real parts, which were milled without suffering either static or dynamic problems.

Plasma Assisted Milling of Heat-Resistant Superalloys

The term Thermal Enhanced Machining refers to a conventional cutting process in which an external energy source is used to enhance the chip-generation mechanism. The work presented here analyzes the basic aspects and the experimental results obtained when applying an assisting plasma jet to the milling process. This process, known as PAM (Plasma Assisted Milling) has been applied to the machining of three very low machinability materials: a Ni-base alloy (Inconel 718), a Co-base alloy (Haynes 25), (both belonging to the group of the heat-resistant alloys) and the Ti-base alloy Ti6Al4V. The study focuses on two major topics. First, the efficiency of the milling operation in terms of cutting speed, feed, axial and radial depths of cut and the plasma operating parameters has been addressed. Second, a study on the alterations of the metallurgical structure and the properties of materials after the PAM has also been performed. The process conditions for the above-mentioned Ni-base and Co-base alloys are detailed. The study under these conditions has shown an excellent performance of the whisker reinforced ceramic tools. In fact, cutting speeds as high as 970 m/min and large radial and axial depths of cuts are possible, driving to a cost-effective machining process. The absence of changes in the metallurgical structure of the alloys after applying the PAM process is also addressed. Therefore, it can be stated that this is a feasible approach to the optimization of the machining process of heat-resistant alloys. Finally, the results obtained in the PAM of Ti6Al4V are detailed. In this experimentation, a certain level of degradation was observed in the microstructure of the alloy when undergoing the PAM process, therefore the use of this technique is not recommended for this material.

Improving the surface finish in high speed milling of stamping dies

Abstract The high speed milling (HSM) of GG25 grey iron castings and GGG70L ductile iron casting stamping dies has proved its feasibility when it comes to finishing operations, giving an important cost reduction when compared with traditional manual polishing associated with conventional milling. However, a number of problems still remain unresolved, improvements in these subjects will undoubtedly lead to an optimum machining situation. In this paper, a systematic description of the main industrial problems is given, as a major step towards a stable and optimum industrial application of this technique. Thus, an important part of the study is devoted to the optimisation of tools to be used, from the point of view of their geometry, base material and coatings. Testing has been carried out using coated carbide tools and PCBN (polycrystalline cubic boron nitride) tools. The importance of the use of optimum machining strategies for roughing and finishing operations of stamping dies is then analysed. Finally, the problem of tool deflection when machining deep cavities is studied.

Development of optimum electrodischarge machining technology for advanced ceramics

In recent years, ceramic materials with improved properties have been developed to meet a large number of industrial applications. However, in most cases, the cost of the ceramic components is very high. On some occasions, the final machining of the component (especially if complex geometries are to be obtained) accounts for an important percentage of the final cost. The electrodischarge machining process can be a good choice if the material has at least a minimum electrical conductivity, since it can produce very complex shapes and it is not dependent on the hardness or abrasiveness of the material itself. In this paper, the development of sinking and wire electrodischarge machining technology for two ceramics with a promising future (boron carbide and silicon infiltrated silicon carbide) is described. The high removal rates, as well as the possibility of obtaining an excellent surface finish, prove the feasibility of the industrial application of this production method.

CALCULATION OF THE SPECIFIC CUTTING COEFFICIENTS AND GEOMETRICAL ASPECTS IN SCULPTURED SURFACE MACHINING

ABSTRACT This article presents a method for obtaining the shear and ploughing specific cutting coefficients for a ball-end milling cutting force model. Thus, by using the proposed calculation method, the need for introducing variable shear cutting coefficients has been identified. This fact is due to the dependency among the specific cutting coefficients and the cutting edge inclination angle, which is variable in ball-end mills. Linear, quadratic and cubic polynomial shear cutting coefficients have been calculated, and the degree of adjustment obtained in each approach has been analyzed. At the same time, the expressions of the ploughing specific coefficients have been analyzed. The proposed calculation method has been applied to the following materials: a 7075-T6 aluminum alloy and a 52HRC AISI H13 tool steel. The results obtained from the validation demonstrate how the obtained coefficients are capable of predicting cutting forces over a wide range of cutting conditions. Finally, the results from applying the coefficients calculated in horizontal slot milling tests have been introduced in a model capable of calculating cutting forces in slope milling cases, which validates the calculation method proposed as a generic method for estimation of cutting coefficients.

Cutting conditions and tool optimization in the high-speed milling of aluminium alloys:

Abstract The high-speed milling (HSM) of aluminium alloys was one of the first fields of application of this technology. It might be thought that this is the field where the technology has been most commonly used, with the largest number of users, and is therefore best known. However, the continuous improvements in modern machine tools on the one hand, together with the development of new tool materials and tool geometries on the other, have produced a constant increase in the cutting conditions (i.e. speed and feed rate). During the past 5 years the use of high-silicon (12–21 per cent) castings has grown considerably owing to the fact that a high silicon content results in excellent properties, mainly wear and high-temperature resistance. However, the machinability of the material becomes too poor on account of its abrasiveness. Aluminium alloys for structural elements in the aeronautical industry are nowadays high-speed milled (HSMed) at very high removal rates, which makes possible the use of monolithic structures instead of sheet metal formed structures. Complete removal of coolant fluids has not been possible so far, but their use can be effectively minimized if techniques such as micropulverization are applied. In this paper, the problems in the machining of the previously mentioned alloys are analysed, along with the possibilities of the so-called minimum quantity of lubricant (MQL) technique.

The CAM as the centre of gravity of the five-axis high speed milling of complex parts

The use of multi-axis high-speed milling has increased in different industrial sectors such as automotive, aeronautical, and the manufacturing of complex moulds. This trend can be observed at the latest technical fairs and the catalogues of the main machine tool manufacturers. Furthermore, for machining impossible shapes, multi-axis machining introduces two main advantages. First it gives the option of performing all operations in only one set-up of the raw block, which can be a prismatic block or a near-to-net shape form. Second it offers the capability of setting the cutting speed, depth of cut and feed to optimize tool life and part quality. However, multi-axis milling is a very complex process that requires special care in the CNC program preparation in the CAM stage, which is critical for a successful process. Thus, the use of a virtual machining simulation utility is highly recommended. Collisions, over-cuts, interferences and dangerous machine movements can be predicted and avoided. On the other hand, continuous variation of the tool can be used to optimize cutting parameters such as cutting forces. Final result is the minimization of tool deflection due to the cutting forces and, in this way, the precision and roughness of finished parts are improved. In this paper a reliable method for multi-axis HSM is presented. This methodology is based on two aspects. First a cutting force estimation in order to get minimum cutting force tool-paths. Second a complete virtual simulation to ensure a collision-free tool-path. A final objective is to generate reliable CNC programs. In this manner, the CAM becomes the centre of gravity of the machining planning procedure. The methodology has been applied to the machining of two plastic moulds in hardened steel (32 HRC), a 7075-T6 aluminium honeycomb part for aeronautical purposes and a 65 HRC AISI 1.2379 part. Times, tolerances and surface roughness have been measured to check the success of the purposed methodology.

Simultaneous measurement of forces and machine tool position for diagnostic of machining tests

A diagnostic system of the milling process through the cutting forces and cutting tool position (coordinates X, Y, and Z) simultaneously collected is detailed in this paper. After the milling test, the cutting forces generated during the machining and the machine position where they occurred can be correlated. After collecting and recording of both the force and position signals, a graphic representation of force components can be applied. In this way, the machine user is able to check those events that happened during the milling; thus, a more useful diagnosis of machining problems is possible. The collecting system is composed by a dynamometric plate for the measurement of the three components of the cutting force, as well as an acquisition device connected to the analog output of the position control loops. After machining, the file containing position and force data is post-processed and chromatic and vector maps are generated. The analysis tool shows its capabilities being applied to three examples, referring to three research projects.

Cutting force integration at the CAM stage in the high-speed milling of complex surfaces

High-speed milling (HSM) technology has been rapidly absorbed by the die and mould manufacturing industry and by the aeronautical sector. The new cutting tools can withstand much higher machining conditions than 10 years ago. Last-generation, high-speed machining centres are equipped with high-frequency spindles with hybrid ball bearings, leading to rotational speeds over 18 000 r/min. Multiaxis machining can be effectively carried out in five-axis machining centres of different architecture. These new and advanced machining processes are much more complex and provide the final component with a higher added value. However, the reliability of the whole process must be reconsidered, since collisions, tool breakage and dynamic problems can result in expensive machine repairs and some parts may be impossible to recover.  In order both to minimize the above problems and increase machining performance, a new machining approach based on two ideas has been developed. First, virtual verification of the NC programs, avoiding collisions or tool – machine interferences that may arise during the machining of complex surfaces. Second, toolpath optimization in the machining of complex surfaces. For this purpose a utility to estimate the cutting forces before machining has been integrated in the computer-aided manufacturing (CAM) planning process stage.  The estimation of cutting force uses a semi-empirical approach, in which the pair tool/material is characterized by six specific cutting force coefficients. The force model introduces the effect of part slope in calculations, just with tool geometry, cutting conditions, and material. The value of cutting force is used as an estimator for selecting the best cutting toolpaths for a complex surface. In this way a more accurate, better-finished surface is machined, and a reduced tool wear is withstood.  The global CAM process is applied to three examples that are discussed. They are representative of a highly efficient high-speed process, without any risks of tool collisions, surface machined errors and low cutting forces.