EP2948272B1

EP2948272B1 - A method of grinding workpieces of aluminum alloy to achieve a high surface quality

Info

Publication number: EP2948272B1
Application number: EP14719198.5A
Authority: EP
Inventors: Martin Novak
Original assignee: Univerzita J E Purkyne V Usti Nad Labem
Current assignee: Univerzita J E Purkyne V Usti Nad Labem
Priority date: 2013-01-28
Filing date: 2014-01-27
Publication date: 2016-07-06
Anticipated expiration: 2034-01-27
Also published as: CZ201351A3; CZ304847B6; WO2014114276A1; EP2948272A1

Description

Technical field

The invention relates to a method of grinding workpieces of aluminium alloys to achieve high surface quality as well as the specified parameters of roughness, surface profile and material share of the surface profile.

State of the art

At present, the grinding technology is not used in the process of machining and processing aluminium alloys due to the mechanical and physical properties of aluminium alloys. When using the technology of grinding aluminium alloys, the abrasive grains of the tool become clogged, just like to pores of the tool, and the chips become stuck to the ground surface of the product, which has a negative impact on the roughness, surface profile and geometric accuracy. The problem is solved using high-speed milling or turning, or possibly it is impossible to achieve the required quality parameters of the machined surface, these technologies increase manufacturing costs since it is necessary to use high-precision or computer-controlled machines and expensive special tools.
The above-mentioned aspects of the need to achieve high-quality surfaces in terms of geometric accuracy and surface roughness requirements for the parameters Ra, Rz, Rt, etc., and in relation to the manufacturing costs, force the manufacturing companies to invest into advanced CNC machines which significantly weaken the company financially. Given the return on investment for the purchase of these machines, it is virtually necessary to operate the machines continuously, which may not always be achieved in practice, for example in case of small batches or series - several pieces of products. Consequently, the use of tools with a defined cutting geometry, such as lathe tools and milling cutters, carry the risk of build-up, i.e. adhesion of small particles of the machined material on the blade of the tool, which not only effects the geometry of the tool - and the area of primary plastic deformation - but also significantly affects the geometric accuracy of the final workpieces and their roughness. This negative phenomenon is also influences the lifetime of the blade; when cohesion forces are overcome, the build-up can be torn away and the entire blade could be subject to deformation and rupture.
Another negative feature is the use of special coolants for non-ferrous materials, containing special additives and other substances to facilitate the machining process and to prevent the formation of build-up on the blade. Considering the composition of these coolants, they are not suitable for machining steel and cast iron, and it is therefore necessary to switch the coolant when the machining of aluminium alloys is finished; this results in additional costs and delays due to replacement, disposal of the coolants, cleaning and overall maintenance of the CNC machine. The approach to the health of employees is also essential, as they are constantly exposed to the action of relatively aggressive semi-synthetic fluids, used in machining.
Document DE 197 13 817 A1 discloses a method of grinding aluminium alloys which forms the basis for the preamble of claim 1.

Essence of invention

The deficiencies mentioned above are, to a large extent, eliminated by the method of grinding workpieces of aluminium alloys to achieve high surface quality, as specified in the invention according to claim 1. Its essence is that the workpiece is machined in an environment comprising a semi-synthetic liquid with a concentration of 3 to 7%, by a grinding wheel with the grain of green silicon carbide of a size smaller than 200µm, in a bonding agent consisting of synthetic resin, at a circumferential speed of the workpiece in the range of 20 to 30 m.min^-1, where the cutting speed of the grinding wheel is in the range of 35 to 40 m.s^-1.
The feed rate during plunge grinding, i.e. the radial feed, is advantageously in the range of 0.07 to 0.13 mm.min^-1.
The feed rate during longitudinal grinding is advantageously in the range of 0.01 to 0.04 m.min^-1.
The essence of the the use of a combination of cutting conditions involving a specific grinding tool, the ratio of the cutting speed of the workpiece and of the tools, the infeed or feed rate and the specific cutting environment.
The grain of the grinding tool comprises silicon carbide - SiC - marked as C49, i.e., green silicon carbide. The grain of the material influences the surface so that the smaller the grain, the better the values of surface roughness achieved, starting from grain size 80, which corresponds with a grain size of approximately 200µm and smaller. Synthetic resin is used as the bonding agent. The structure of the grinding wheel must be open to very open pursuant to CSN ISO 525. The hardness of the wheel based on the aforementioned standard is in the range of "soft".
The cutting speed ratio is based on the maximum circumferential speed of the wheel, its diameter and the diameter of the workpiece. The cutting speed of the grinding wheel must be between 35 to 40 m.s^-1 and the circumferential speed of the workpiece between 20 to 30 m.min^-1.
The feed rate during plunge grinding is in the range of 0.07 to 0.13 mm.min^-1. If longitudinal grinding is used, the feed rate is between 0.01 to 0.04 m.min^-1.
To achieve a high quality of the surface finish in case of grinding aluminium alloys, it is necessary to use a cutting environment comprising a semi-synthetic liquid.
The advantage of this solution is a very high precision of the machined surface in terms of the required parameters of geometric accuracy and surface roughness requirements, its profile and material share. This technology makes it possible to machine aluminium alloys without additional costs associated with the purchase of expensive CNC machines because manufacturers are not able to find a solution for the grinding of aluminium alloys, and speeds up the process of manufacturing machine parts, especially in single-part production. The cutting environment comprises a semi-synthetic liquid which makes it possible to achieve high-quality machined surfaces during grinding without their replacement and disposal after each grinding of of aluminium alloys, for example during turning and milling; when a charge of the workpiece material causes a change of the cutting environment. This feature is surely positive also in relation to the environmental aspects of manufacture and the health of workers.
The high material share, i.e. minimum pits and grooves, also supports subsequent application - surface anodizing, when a homogeneous layer, able to prevent abrasive wear and pitting during the use of the parts, is formed on the surface thanks to the minimum differences in the height of the profile and microscopic unevenness - parameters Rz and Rt.

Clarification of drawings

The method of grinding workpieces of aluminium alloys to achieve high surface quality, as specified in this invention, will be described in greater detail on specific embodiments with accompanying drawings, where Figure 1 shows a chart with surface roughness values based on parameter Ra for three aluminium alloys. Figure 2 is an illustration of the material share of the surface profile in three aluminium alloys and Figure 3 shows the surface profile for three aluminium alloys. Figure 4 shows examples of the surface of aluminium alloys EN AW 2007 during grinding using a grinding wheel based on artificial corundum using random cutting conditions. Figure 5 shows examples of the surface of aluminium alloys EN AW 2007 during grinding with silicon carbide grains and using optimum cutting conditions and cutting environment.

Embodiments of the invention

In practical application, the grinding wheel C49 150K 8V was used and three aluminium alloys with a different chemical composition were ground, namely the following alloys: EN AW 7075 - AlZn5,5Cu1,5Mg2,5, EN AW 2007 - AlCu4MgMn and EN AW 6082 - AlMgSiSnBi. The circumferential speed of the grinding wheel was 35 m.s^-1, the used circumferential speed of the workpiece was 20 m.min^-1, the feed rate was 0.13 mm.min^-1 during plunge grinding and the environment comprised the semi-synthetic liquid Semix Al 4000 CZ with 5% concentration. The outcome of the process of grinding these aluminium alloys was the achieved mean arithmetic roughness in the range of 0.22 to 0.34µm, see Figure 1, a high material share of the surface profile, see Figure 2, and its own profile, see Figure 3.
The differences between the use of conventional grinding materials based on artificial corundum - Al₂O₃ and the grains of green silicon carbide - SiC with the defined parameters of cutting conditions and the cutting environment are shown in the photographs of the machined surface of the alloy EN AW 2007, see Figure 4 and Figure 5. The achieved surface parameters do not require energy-intensive, high-performance machines; ordinary workshop equipment is enough. The main essence consists in the parameters of the cutting conditions, the grinding wheel and the cutting environment, as specified in this patent application.

Industrial use

A method of grinding workpieces of aluminium alloys to achieve high surface quality as specified in this invention can be used if high surface quality of aluminium alloys is required. The method can be used for grinding rotary surfaces of aluminium alloys in all grinding machines for round shapes and in central grinders. Using the described method; it is possible to achieve a high support share of the material on the surface. The method is advantageously used if high surface accuracy and high roughness quality are required, when the mean arithmetic roughness Ra is required to be in the range of 0.2 to 0.3µm with a high support share of materials, as completion using the super-finishing or honing method.

Claims

A method of grinding workpieces of aluminium alloys to achieve high surface quality, wherein the workpiece is machined by a grinding wheel with the grain of green silicon carbide of a size smaller than 200µm, in a bonding agent consisting of synthetic resin, the method being characterised in that the workpiece is machined in an environment comprising a semi-synthetic liquid with a concentration of 3 to 7%, at a circumferential speed of the workpiece in the range of 20 to 30 m.min^-1, where the cutting speed of the grinding wheel is in the range of 35 to 40 m.s^-1.
A method of grinding workpieces as in Claim 1, wherein the feed rate during plunge grinding is in the range of 0.07 to 0.13 mm.min^-1.
A method of grinding workpieces as in Claim 1, wherein the feed rate during longitudinal grinding is in the range of 0.01 to 0.04 m.min^-1.