F. J. Campa

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.

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.

Toolpath dependent stability lobes for the milling of thin-walled parts

The milling of thin-walled parts can become a seriously complex problem because the parts have variable dynamics. Firstly, the dynamics evolution of the part has been calculated through Finite Element Method (FEM) analysis. Then, the 3D stability lobes have been calculated for the thin walls and the thin floor. Finally, several milling tests have been performed in order to validate the predictions made by the model.

Ultrasonic Assisted Turning of mild steels

In this work, the advantages and drawbacks of Ultrasonic Assisted Turning (UAT) have been investigated focusing on the effect of tool vibration on surface quality. Several experiments have been performed on mild steels changing the cutting speed, feed and depth of cut, to study how the influence of the ultrasonic vibration on the surface roughness varies depending on the cutting conditions. The results obtained show that the ultrasonic vibration can improve the surface quality. The authors also propose a new booster design based on the theory of longitudinal vibration of a bar with varying cross-sectional area for a higher amplification of the ultrasonic vibration.

Productivity improvement through chatter-free milling in workshops

High-speed machining has improved performance in manufacturing sectors such as the aeronautical, automotive, and mould and die industries. However, one problem of reaching such high speeds is the self-excitation of the spindle-tool system during the machining process. This so-called chatter vibration produces several negative effects: a reduction in surface quality and accuracy, and in tool and spindle life. Nowadays, it is still the most common limitation to increased productivity and quality of metal-removal operations. Although theoretically calculated stability-lobe diagrams can be used to calculate stable cutting conditions, some uncertainties affect the accuracy of this solution. This paper presents a method based on experimental tests where an inclined plane in the workpiece is machined at several spindle speeds. The gradual increase of the axial depth of cut allows the milling to be interrupted when chatter is detected. The final result is that the stability lobes are machined in the workpiece. This method can be used for process optimization and avoiding trial and error tests to determine optimal depths of cut and spindle speeds. The creation of a database with the optimal cutting conditions for a given spindle-tool holder-tool system can help the computer aided manufacturing (CAM) programmer select the cutting conditions that maximize the material removal rate and avoid vibration problems. The method presented is especially suitable for small- and medium-sized workshops where operators are unfamiliar with analytical methods or modelling techniques. It is an interesting way of saving money that does not require complex knowledge, and can easily be applied without additional costs owing to its simplicity.

New Concepts for Structural Components

This chapter is focused on analysing new concepts and trends related to the structure of machine tools. In fact, the structure of the machine has a decisive influence on the three main parameters that define the capabilities of a machine, which are: motion accuracy, the productivity of the machine and the quality of machining. In this respect, this study on structural components will add a new basic parameter, eco-efficiency, because the structure of the machine also has a decisive influence on the whole life cycle of the machine and especially on the materials and energy resources consumed: an issue of increasing concern among machine tool builders.

Definition of Cutting Conditions for Thin-to-Thin Milling of Aerospace Low Rigidity Parts

The main drawback of the high speed milling of monolithic parts for the aerospace industry is the high buy-to-fly ratio that leads to a huge material waste. This problem is caused by the need to stiffen the part during the machining in order to avoid chatter, excessive vibration and residual stresses. The present work proposes a methodology for the milling of compliant parts based on the selection of cutting conditions free of chatter. First, the modal parameters of the part in the most problematic stages of the machining are calculated by means of the finite elements method. Secondly, a three-dimensional stability model is used in each stage to calculate a three-dimensional stability lobes diagram dependent on the tool position along the whole tool path. Given the fact that the depth of cut is defined by the bulk of material, the three-dimensional stability diagram can be reduced to a two-dimensional one, which relates tool position during the machining and spindle speed, and indicates how to change the spindle speed in order to avoid the unstable areas. What is more, the proposed methodology can also be used to dimension the bulk of material, select the proper tool or improve the fixturing of the part. Finally, the methodology is validated experimentally on a test part.Copyright

Automatic Identification of the Inertia and Friction of an Electromechanical Actuator

The aim of the present work is to present a fast identification method to estimate the inertial and friction parameters of a rigid body dynamic model of an electromechanical actuator. These are the dominant effects that determine the dynamics of machines and although the inertia is easily predictable, the friction can only be accurately known performing experimental measurements. The work provides the mathematical aspects of the identification method as well as several practical aspects for its implementation. The procedure is applied to an actuator based on a motor and a gearbox, and a comparison of the torque estimated and the real one are provided.

Critical thickness and dynamic stiffness for chatter avoidance in thin floors milling

A common problem in the aeronautical industry is the chatter vibration due to the lack of dynamic stiffness in the milling of thin walls and thin floors. The present work proposes a method for chatter avoidance in the milling of flexible thin floors with a bull nose end mill. It allows the calculation of the thickness previous to finish milling or the minimum dynamic stiffness that the floor must have to avoid the chatter vibration appearance. To obtain these values, the stability model algorithm has been inverted to estimate the thickness or the dynamic stiffness required in a floor to allow a stable milling. This methodology has been validated satisfactorily in several experimental tests.

The Effects of Ultrasonic Vibration Parameters on Machining Performance in Turning of Mild Steels

The present work presents the results obtained when an ultrasonic vibration is applied to tool on the turning of mild steels. Currently high‐end materials, usually present low machinability so it is necessary to employ new machining technologies. Ultrasonic assisted turning (UAT) is presented as an effective process for machining these difficult‐to‐cut materials. In order to achieve a better understanding of the process, a complete study of the influence of the vibration parameters is presented when machining mild steels. Several parameter of UAT were monitored, including surface roughness, tool wear, chip formation by a high‐speed imaging camera, and tool temperature by means of an infrared camera: surface roughness and tool temperature decrease and an tool life increase as compared to conventional turning (CT).