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Wear and Friction for machining and sheet metal operation

Abstract:

In this paper the importance of tool wear due to friction, when oblique cutting using milling cutter tool in different machining operations is explored. In particular, friction on the flank face is incorporate in the tribological balance. The force components are recorded during tool wear tests. Different mode of tool wear is covered. It was seen that that both friction coefficients change substantially during tool wear progression. Importance of feed rate (fn), depth of cut (dp), flank angle (?), rake angle (?), cutting speed (Vc), are covered.

Introduction:

During the 100 years effort by scientific community, wear and friction are still important areas in machining research. Knowledge of friction at present is insufficient to accurately predict metal cutting operations using, 1) thermal, 2) tribological phenomena. The friction coefficient determination is based on simplified methods, orthogonal cutting tool model and Coulomb'slaw [W.Grzesik, 2008. M.C. Shaw, 1989]. Models based on adhesive properties between the workpiece and the tool [F. Zemzeni, J. Rech, W. Ben Salem, A. Dogui, P. Kapsa, 2009, W.Grzesik, 1999]. Using Finite Element Method (FEM) prediction method for cutting operation characteristics are given by Özel [T. Özel 2006] and Arrazo la et al. [P.J. Arrazola, D. Ugarte, X. Dominguez 2008]. Coulomb's friction coefficient is able to generate about 50% differences for cutting forces predictions in comparison to the measured values. Three dimensional (3D) models and simulations methods using oblique cutting and cutting tools of inclination angle (?s) (W.Grzesik, 2008.). Commercial turning tools has inclination angle(?s = -6o to -7o ) which quite small while for helical drills, milling cutters, shaving tools and broaching tools it is of the order of tens of degrees.

With CBN and ceramic cutting inserts with negative rake angles for the chamfer which is in the range of -20o to -30o, this helps in protecting cutting edges against premature failure [H. Ren, Y. Altintas 2000, J.M. Zhou, H. Walter, M. Andersson, J.E. Stahl, 2003]. The rake angles and inclination angle causes resultant force components of cutting force to change significantly. Hence the friction on cutting tool contacting point should be adjusted as required [W.Grzesik, 1990]. This is because magnitude of friction depends on normal force and friction force acting on the flank faces and rake faces, which defines the apparent / mean friction coefficient (?) as per Coulomb's law [W.Grzesik, 2008, P.J. Arrazola, D. Ugarte, X. Dominguez 2008].

Machining:

Machining from cemented carbide cutting tools is well-known and versatile machining operation [Usui E, Shirakashi T 1982]. The cutting tool usually consists of shaft made of steel and cemented carbide cutting tool insert which gets replaced on wearing out. The cutting tools or their tips are coated with thin film to enhance tool life and to reduce friction. Cutting tools vary in size, alloying materials and geometry different material grades. Geometries of cutting tool include different surfaces and their angles which are defined in different planes. The cutting parameters presented in fig. 1 and 2, defined by ISO 3002/1 [J.A. Arsecularatne, L.C. Zhang, C. Montross], have been found to be of great importance during machining and have impact on the cutting tool life time as well as the machined surface finish and structure [V.P. Astakhov 2010].

The velocity of workpiece relative to the cutting tool is defined as cutting speed (Vc). Feed rate, fn, is the motion which leads to continuous removal of workpiece material to create the machined surface. Depth of cut (dp), is defined as width to depth ratio of the cut. Cutting tool surface closest to the chip is the Rake face, which is the front edge of the cutting tool. The workpiece material is cut by the cutting edge, which is the line of intersection between rake face and flank face. The flank face is the surface of the cutting tool that the newly created surface flows against.

Cutting forces

The main component of the total force is the cutting force (Fc) and is a perpendicular projection of total force acting in the direction of primary motion.

Fc is the cutting force which causes the power consumption. Feed force or in-feed force (Ff) which is axial force component. It is perpendicular projection of the total force along the feed direction. Back force (Fp) is the force perpendicular to both the primary motion and the feed motion [S.K. Choudhury, K.K. Kishone 2000].

Tool wear is detected by measuring the cutting- force signals [S.K. Choudhury, K.K. Kishone 2000] and is indirect detection techniques. Force measurements correlates with cutting tool wear and is used as a tool wear indicator. Force measurements can be performed during machining using a piezoelectric quartz crystal dynamometer. The cutting forces applying pressure on the piezoelectric crystal cause an electromagnetic force proportional to the force or pressure exerted on the cutting tool.

Wear

Material wear processes are found at all places where materials are in mechanical contact with

each other [S.S. Ingle 1993]. Wear is combinations of several physical wear mechanisms and most likely to be present are: abrasive, adhesive, diffusive, chemical, and wear due to plastic deformation [F. Luigino, M. Fabrizio, L. Settineri, D. Umbrello 2007]. The dominating wear mechanism depends on the surfaces, the contact area between them, materials, topography, hardness, etc.

Abrasive wear

Abrasion arises due to hard particles that abrade on a softer material. The mechanism works on almost all contact surfaces that have a relative velocity against each other and is dependent on the relative hardness of the abrading particles and abraded material. Most abrasion starts with two-body abrasion but switches to three-body abrasion at a certain point, after formation of wear-off particles. Two-body abrasion has higher wear rates than three-body abrasion. The abrasive wear increases with increasing temperature, as hardness decreases with temperatures [S.K. Choudhury, K.K. Kishone 2000, P.K. Wright, A. Bagchi 1981].

Adhesive wear

Adhesion occurs at high temperatures and pressure. Small particles gets welded together when metals are forced together [S.K. Choudhury, K.K. Kishone 2000, F. Luigino, M. Fabrizio, L. Settineri, D. Umbrello 2007]. When metals have relative velocity against each other, as in metal cutting, the small welds are formed by adhesion and they cause micropieces of tool to break loose. The adhesion of workpiece material onto the cutting tool forms a built up edge (BUE) [19] and in that case the wear rate due to adhesive wear is very high. In this case when high speeds and temperatures cause adhesion junctions to form between workpiece material flowing past the flank face and the cutting tool material, carbide particles can be plucked from the cutting tool into the chip. This wear mechanism is referred to as attrition wear.

Conclusion

To reduce the tool wear feed need to be kept at optimum level, the depth of cut will depend on material hardness and must be reduced if temperature goes up, and must be below the optimal cutting temperature. Change in friction coefficient during tool wear due to cutting forces is explained in terms of increments and resolving to normal and tangential frictional forces acting at the interfaces of tool- work piece and tool-chip. The friction coefficient values for flank faces and the rake are same for fresh tools but changes with as tool wear progresses.