L.N. López de Lacalle

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.

Design and Test of a Multitooth Tool for CFRP Milling

This article deals with the new development of a family of router milling tools for the high-performance milling of carbon fiber reinforced plastics. The new milling tools are shaped by multiple left-hand and right-hand helical edges, which form small pyramidal edges along the cutting length. Several substrates and coatings have been tested including AlTiN and the new naCO with nanocrystalline structure. After the analysis of tests and modifications on the tool prototypes, the final result is a series of routing endmills optimized for carbon fiber composites defining the influence of each of milling tool features on tool performance, which was not clearly established till date. The specific cutting forces, tool wear, and others aspects are discussed in detail.

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.

Mechanistic modelling of the micro end milling operation

The presented research work has been done during the development of a mechanistic model to predict micromilling cutting forces. The aim of such a prediction is to be able to estimate the tool deflection and the real tool-path during the micromilling process. The first step to achieve a robust model is to develop a procedure to identify the specific cutting force coefficients. Beginning with the conventional end-milling cutting force model, based on six coefficients (three specific cutting force coefficients and three edge coefficients), several modifications are proposed to adapt it to the prediction of the micromilling cutting force. A variety of end mill shapes (cylindrical, ball, and bull-nose) are considered in the geometric part of the model. The paper presents these modifications and their experimental validation by the micromilling of tool steel (H13 hardened up to 60 HRC) using two-flute carbide micro end mills with diameters from 0.1 to 0.4 mm. Finally, the consistency between the simulated and measured cutting force is shown as the main conclusion. Weak points suitable for further research are also exposed.

Process planning for reliable high-speed machining of moulds

A method of generating NC programs for the high-speed milling of moulds is investigated. Forging dies and injection moulds, whether plastic or aluminium, have a complex surface geometry. In addition they are made of steels of hardness as much as 30 or even 50 HRC. Since 1995, high-speed machining has been much adopted by the die-making industry, which with this technology can reduce its use of Sinking Electrodischarge Machining (SEDM). EDM, in general, calls for longer machining times. The use of high-speed machining makes it necessary to redefine the preliminary stages of the process. In addition, it affects the methodology employed in the generation of NC programs, which requires the use of high-level CAM software. The aim is to generate error-free programs that make use of optimum cutting strategies in the interest of productivity and surface quality. The final result is a more reliable manufacturing process. There are two risks in the use of high-speed milling on hardened steels. One of these is tool breakage, which may be very costly and may furthermore entail marks on the workpiece. The other is collisions between the tool and the workpiece or fixtures, the result of which may be damage to the ceramic bearings in the spindles. In order to minimize these risks it is necessary that new control and optimization steps be included in the CAM methodology. There are three things that the firm adopting high-speed methods should do. It should redefine its process engineering, it should systematize access by its CAM programmers to high-speed knowhow, and it should take up the use of process simulation tools. In the latter case, it will be very advantageous to use tools for the estimation of cutting forces. The new work methods proposed in this article have made it possible to introduce high speed milling (HSM) into the die industry. Examples are given of how the technique has been applied with CAM programming re-engineered as here proposed, with an explanation of the novel features and the results.

Five-Axis Machining and Burnishing of Complex Parts for the Improvement of Surface Roughness

In this article, the ball burnishing is applied on sculptured surfaces, aiming at enhance surface roughness. Different strategies are possible for burnishing, the continuous burnishing (CB) which uses a five-axis interpolation of the machine tool, and the patch burnishing (PB) using a more simple 3 + 2 axis interpolation. Using both techniques complex parts are burnished and a big improvement in surface roughness achieved, but some differences between both approaches appear. Two parts have been previously machined in a five-axis milling center and finished using the ball burnishing approaches. The first one is a steel AISI 1045 with a hemisphere shape, whose geometry is simple. The second one is a steel DIN 1.2379 part (64 HRC), with more complex features. Surface quality was evaluated for both burnishing approaches, obtaining significant improvements on surface roughness and hardness. The main general conclusion is that ball burnishing reduces roughness without penalizing the manufacturing time or surface integrity and, therefore, is suitable for complex surfaces.