Tools and conditions of machining of high speed cutting

Tools and coatings manufacturers have been forced over the years to adapt to the increasingly more "extreme conditions" to which are subjected the tools, especially due to the popularization of high speed machining. This study, carried out at national school of engineers of Metz (ENIM), in France, analyses the Court conditions and requirements which must meet a tool, to then focus for teeth grinding.

The national school of engineers of Metz (ENIM) is part of a network of four national school of engineers. This way in five years generalist engineers in the field of mechanics. With the arrival of Pierre Padilla to the address of the school in 1994, a mobilization of human and physical resources around the theme of the machining of high-speed (AVM) with the aim of creating a centre of excellence in this sector of activities in the ENIM is generated. Today the ENIM has physical resources of first level and develops research activities in the areas of the milling and the rectification of high speed. Actions find their application in the courses taken in the school (option industrial methods and innovative technologies), the transfer of technology to utilizadoras industries of the AVM (Techspace Aero, Airbus France Nantes, SMAE) and of course in the research sector. The researchers of the school are linked to the laboratory of physics and mechanics of materials (LPMM, UMR CNRS), at the laboratory of study of textures and materials LETAM (CNRS UMR) or applications to the laboratory of Industrial Engineering and mechanical production (LGIPM), establishments in close cooperation with the ENIM. The covered topics are summarized in Figure 1.

The ENIM has two machines AVM: a milling machine Gambin 120 CR capable of working with different types of electric spindles, including the spindles of magnetic bearings developed by the company S2M; and a cylindrical grinding machine ROUCHAUD GENDRON completely instrumented. These machine tools are associated with modern systems of control and measuring (microscopy interferometric, force sensors, built-in temperature sensors, inductive sensors, accelerometers, etc.).

High speed machining presents multiple consequences on the processes of manufacturing parts by chip startup. The notion of high speed is relative to every material and every process. The most common consequences are a net improvement of surface finish, the reduction of the efforts of court, the evacuation of most of the heat in the chip and the augmentation of the amount of material removed. This contributes to machined parts to a higher speed to improve its quality at the same time. The set of parameters that characterize the mechanical and physical properties of the produced piece is summed up by the notion of surface integrity. This term includes waste efforts and texture. The integrity of the surface produced is sensitive to many factors, among which is the cutting tool. It is for this reason that numerous works relating to the development of new cutting tools adapted to the conditions of machining of high speed (speed of court and progress in particular) are currently developed. The materials used in the manufacture of these tools are carbides, the cermets, CBN (cubic Boron Nitride), the PCD (polycrystalline diamond) and ceramics. The carbides are usually used with a coating: TiN, TiCN, Tiain, CrN, etc. The cutting tool should meet, in a general way, to the following criteria:

Figure 1. Research topics developed in the ENIM.

Cylindrical grinding machine for grinding high speed

Rouchaud-Gendron

Features:

* Cutting speed: 314 m/s

* Screw porta-muelas 29 kW / 15000 rpm

* Screw porta-piezas 9.5 kW / 7500 rpm

* Based on granite

* Diamond Sharpener

* Positioning accuracy: 0.1 µm

* Oil tank: 3000 l (oil)

Grinding wheel of rectification

* Diameter: 400 mm

* The abrasive band thickness: 5 mm

Milling of high

speed Gambin 120 CR

Features:

* 5-axis

* Advance: 18 m/min

* Carrereas X, Y, Z: 1400 / 700 / 840 mm

* Electric spindle with bearings Fisher 30000 rpm, 12 kW, HSK 50

* 45000 S2M magnetic bearing electric spindle rpm, 40 kW, HSK50

* 35000 S2M magnetic bearing electric spindle rpm, 70 kW, HSK63A

* Thermal expansion adjustment system

Figure 2. (Grinding and milling machine of high speed of the ENIM.)

• good resistance to friction (wear),

• good resistance to impact (resilience),

• good resistance to penetration (hardness),

• good resistance to heat,

• good resistance to pressure,

• a good chemical stability.

Figure 4. Example of an insert for milling of coated carbide

Figure 5. Evolution of the relationship between the force of progress and the force cut depending on the speed of court for three different plates. Machining material: TA6V.

The mechanisms of wear of cutting tools also evolve with the cutting speed. In the following figure can be observed diffusion becomes the predominant wear mechanism at high cutting speeds. For this reason, it is necessary to create barriers of dissemination tools (coatings).

Figures 5 and 6 show the influence of the material and the lining of the tool on the efforts of machining (effort of progress and effort of court), and also on the length of life of the tool. These figures correspond to trials of turning in an alloy of titanium TA6V. Can be observed that the Triefus insert of carbide with inset soldier PCD produces the best results: a life span of higher, even at high cutting speeds and therefore an increase in productivity, further efforts of machining are kept at reasonable levels and that practically do not evolve with the cutting speed. The two other plates, Tizit and Sandvik, present an answer to the quite similar cutting speed, but with lower levels of effort for the Insert Tizit, possessing a broken chips.

Figure 6. Evolution of the wear depending on the speed of court time for three different plates (machining Material: TA6V, f = 0, 06 mm/rev, w = 3, 4 mm).

The rectification is used to perform precision machining as well as also for the machining of materials of high hardness. Typically used to perform the finishing of surfaces previously produced through conventional operations, rectification of high speed can carry a piece of State gross directly to its final State.

Cycle times are short with regard to those of conventional rectification, the length of life of the teeth increases and the amount of removed material becomes significant, comparable to that obtained in the turning of materials of high hardness. However, the specific nature of the rectification are kept: formation of micro-viruta, under court, friction/heat production performance. The energies released game considerably increase with the speed of court. The grinding wheel must be then designed to withstand high speeds, as well as major energy sources put at stake.

A grinding wheel in grinding of high speed this constituted of a body and an abrasive band on its periphery, as shown in Figure 7.

Figure 7. Grinding wheel for rectification of high speed.

The body must submit a minimum of deformation under the effect of force centrifuges and thermal expansion. Its mass must be low in order to facilitate its balance. You must also enable the Union with the spindle porta-muelas of the machine and a movement of rotation to high speeds implementation. In most cases, the bodies are metallic, steel or aluminum, but the best solution is a body developed in composite material. The characteristics of the three materials are presented below:

The band abrasive made of abrasive grain and a binder. As a result of important energy put into play and high friction, it is necessary to use abrasive materials and a binder of high thermal conductivity. It is also important that the Binder has spaces (pores) between the grains in order to take fluid cutting to the area of interaction between the grinding wheel and the material. Some binders have a more or less high porosity. The following tables show the characteristics of the different types of grains and binders.

(CBN) cubic Boron Nitride grains are used for the machining of ferrous materials, which do not have chemical affinity, in contrast to the diamond. Diamond grains are used for machining nonferrous materials like carbides.

The binder resin and metallic Binder do not produce porosities in the abrasive band. Consequently, its employment needs a sharpening operation to renew the edges of the grains. These types of binders are used when the profile of the grinding wheel has fine detail and their resistance must be raised. This type of binding agent is used especially in finishing teeth. The vitrified cement presents a good porosity and is an optimal balance between la muela of roughing and finishing wisdom teeth. The galvanic binder is obtained by applying a layer of abrasive on the metal body through a process of electroplating. The abrasive grains are very prominent, so this type of teeth are engaged in roughing operations.

The rapid development of high speed machining associated with a run rampant toward the gain in productivity, quality and cost-cutting has led to an important development of cutting tools. Either in turning, drilling, milling or rectification, the tools materials and coatings are developed specifically to satisfy increasingly extreme cutting conditions. A new component comes now to complicate the task of the manufacturers of tools and coatings: ecological machining (machining dry). The removal of lubrication is effect to increase the stress suffered by the machined piece thermomechanical and threatens to significantly modify its properties. Various studies are carried out in this area now