Cutting edge wear in high-speed stainless steel end milling

Abstract

Wear of cutting tools has a major impact on production cost, quality, and efficiency of the machining processes. Tool wear depends on many parameters including cutting parameters and conditions, tool geometry and materials (coating and base materials), and the workpiece material. This study examines and compares the performance of three state-of-the-art milling tools for high-speed end milling while cutting the same material, stainless steel. The tools have the same base material (tungsten carbide — WC-Co), with different geometrical parameters and coatings (TiAlN and AlCrN). Systematic microscopic analysis and finite element (FE) simulations are used to study mechanisms of damage at the cutting edge. Microscopic analyses show that the flank wear is the most critical damage mechanism at the cutting edge. Having the highest material removal rate, the MT-1 tool experiences cutting edge wear faster among the studied tools with a maximum wear size of 420 μm. This tool ran with a radial depth of cut (a_e) equal to 0.96 mm and feed per tooth (f_z) of 0.15 mm/tooth, which are maximum values among all the tools. The maximum tool stresses from the FE simulations are obtained equal to 1267, 920, and 1145 MPa for MT-1 (a_e of 0.96 mm and f_z of 0.15), MT-2 (a_e of 0.48 mm and f_z of 0.12), and MT-3 (a_e of 0.6 mm and f_z of 0.15) tools, respectively. This indicates that the radial depth of cut and feed per tooth are the key parameters dictating stresses and degree of wear at the cutting edge.

Appendix – Tool/chip and tool/workpiece interactions

Almost all the cutting power is converted to heat and distributed into two areas: primary and secondary deformation/shear zones, as indicated by Shaw and Cookson [32], Kuo et al. [33] and Abukhshim et al. [34]. Additionally, heat generated in the secondary shear zone leads to rise the cutting tool temperature, while a portion of that also is transferred to the chip. Fig. 13 clearly shows how tool interacts with chip/workpiece and corresponding forces and heats generated during the cutting process.

Fig. 13

Schematic of the end milling along with a single flute representation of the cutting force, heat generated during the cutting process, and illustration of the deformation/shear zones

As can be seen in Fig. 13, Heat generated in the secondary shear zone, one heats up the cutting tool (mainly the cutting edge) and the other goes into the chip. The heat flowing into the cutting tool is defined as:

$$ {Q}_c=\alpha \cdotp {Q}_{fr} $$

(4)

in which α is the portion indicator of heat flows into the tool (typically around 50%) and Q_fr is friction power/heat consumed in the rake face and is defined by:

$$ {Q}_{fr}=F\cdotp {V}_{ch} $$

(5)

where F is the friction force at tool – chip interface and V_ch is the chip velocity. From the force diagram shown in Fig. 13, F can be calculated as:

$$ F={F}_c\sin \beta +{F}_t\cos \beta $$

(6)

in which F_c and F_t are main or orthogonal cutting and thrust forces, respectively, and β is the rake angle. Chip velocity can be obtained by adopting the principles of kinematics as follows:

$$ {V}_{ch}=\frac{V_c\sin {\varphi}_s}{\cos \left({\varphi}_s-\beta \right)} $$

(7a)

$$ {\varphi}_s={\tan}^{-1}\left(\frac{r\cos \beta }{1-r\sin \beta}\right)\kern1.75em \mathrm{with}:r=\frac{h}{h_{ch}}<1.0 $$

(7b)

where φ_s is defined as the shear angle, V_c is defined before as the cutting speed, h is the uncut chip thickness (maximum of feed per tooth f_z), and h_ch is the chip thickness (always becomes larger than the uncut chip thickness).