**4. The analysis of technological machinability indicators**

Machined surface texture and tool wear are the essential factors determining cutting ability in practical applications. One of the most popular geometrical tool wear indicators is tool wear on the flank face designated by the *VB*. Its value can be measured using stereoscopic microscopes. The method of tool wear measurement is depicted in Figure 12. Machined surface texture can be examined using three and two dimensional (3D, 2D) measurements. 3D measurements can be achieved using stationary profilometer Hommelwerke T8000 (Figure 13). Two dimensional measurements can be made by T500 profilometer (Hommelwerke), equipped with T5E head and Turbo DATAWIN software. The sampling length *lr* = 0.8 mm, the evaluation length *ln* = 5·*lr* = 4.8 mm, the length of wave cut – off *λc*  (cut – off) = 0.8 mm and ISO 11562(M1) filter are usually applied in the measurements. As a result of 2D measurements the surface profile charts are received. On the basis of surface profile charts the *Ra* and *Rz* parameters can be calculated using appropriate software.

Machining Characteristics of Direct Laser Deposited Tungsten Carbide 113

**Figure 14.** a), b) Tool wear in function of cutting time *ts* for two investigated cutting speeds *vc*; c) tool wear comparison for exemplary dullness criterion *VBc* = 0.2 mm (*z1*, *z2* – number of tooth, *T1*, *T2* – tool life)

Figure 15 compares the surface texture of tungsten carbide sample manufactured by DLD

**Figure 15.** Surface texture of tungsten carbide manufactured by DLD technology, before and after

technology before and after milling.

machining: a) 2D surface profile, b) image of surface

**Figure 12.** Tool's flank wear measurement

**Figure 13.** The view of stationary profilometer Hommelwerke T8000

Figure 14 depicts the tool wear progress in function of cutting time during face milling of tungsten carbide (manufactured by DLD technology) with CBN cutters. As it can be seen, tool wear process for each tooth is similar, i.e. there are no significant deviations of *VBc* values for respective teeth. Introducing arbitrary dullness criterion *VBc* = 0.2 mm, it can be seen that twofold cutting speed *vc* increase, caused almost eightfold tool life *T* decrease. On the basis of acquired data the *s* exponent used in Taylor`s equation (*T* = *CT*/*vc* s , where *CT* is constant dependent of workpiece properties) can be estimated, but it is necessary to emphasize that determining the *s* exponent from two experimental values is not very accurate. After consideration of the *vc1*, *vc2*, *T1* and *T2* the *s* = 2.65 was obtained. This value is located in the range of the *s* exponents characteristic for high speed milling of hardened steel, thus the intensity of cutting speed *vc* influence on tool life *T* in tungsten carbide milling is similar to those for hardened steel. Moreover the tool wear concentrates on the flank face of the tool (see Figure 14c). Because of this, the relations between the tool wear and both forces and vibrations in thrust direction were observed (see Figure 7).

**Figure 12.** Tool's flank wear measurement

**Figure 13.** The view of stationary profilometer Hommelwerke T8000

Figure 14 depicts the tool wear progress in function of cutting time during face milling of tungsten carbide (manufactured by DLD technology) with CBN cutters. As it can be seen, tool wear process for each tooth is similar, i.e. there are no significant deviations of *VBc* values for respective teeth. Introducing arbitrary dullness criterion *VBc* = 0.2 mm, it can be seen that twofold cutting speed *vc* increase, caused almost eightfold tool life *T* decrease. On

constant dependent of workpiece properties) can be estimated, but it is necessary to emphasize that determining the *s* exponent from two experimental values is not very accurate. After consideration of the *vc1*, *vc2*, *T1* and *T2* the *s* = 2.65 was obtained. This value is located in the range of the *s* exponents characteristic for high speed milling of hardened steel, thus the intensity of cutting speed *vc* influence on tool life *T* in tungsten carbide milling is similar to those for hardened steel. Moreover the tool wear concentrates on the flank face of the tool (see Figure 14c). Because of this, the relations between the tool wear and both

s

, where *CT* is

the basis of acquired data the *s* exponent used in Taylor`s equation (*T* = *CT*/*vc*

forces and vibrations in thrust direction were observed (see Figure 7).

**Figure 14.** a), b) Tool wear in function of cutting time *ts* for two investigated cutting speeds *vc*; c) tool wear comparison for exemplary dullness criterion *VBc* = 0.2 mm (*z1*, *z2* – number of tooth, *T1*, *T2* – tool life)

Figure 15 compares the surface texture of tungsten carbide sample manufactured by DLD technology before and after milling.

**Figure 15.** Surface texture of tungsten carbide manufactured by DLD technology, before and after machining: a) 2D surface profile, b) image of surface

It can be seen, that tungsten carbide sample manufactured by DLD technology has an unsatisfactory geometric accuracy and unreasonable surface roughness. Furthermore, from the surface profile and the FFT analysis (Figure 16) it is resulting, that surface texture after DLD process has a random character. The FFT analysis of surface profile consists also of constituent related to the half of the evaluation length (2.4 mm), which means that DLD surface profile is affected by the waviness. Therefore, it needs further finishing process. After milling, machined surface is much smoother and characterized by significantly lower values of surface roughness parameters.

Machining Characteristics of Direct Laser Deposited Tungsten Carbide 115

**Figure 17.** 3D surface roughness chart and corresponding Power Density Function during: a) milling

From these charts no influence of feed per tooth *fz* on surface roughness is seen, despite for *fz* = 0.1 mm/tooth. In this case the theoretic value of *Rzt* is comparable to real *Rz* value. It is commonly known that the increase of feed per tooth *fz* is accompanied by the increase of surface roughness. Theoretically, the lower feed is fixed, the lower surface roughness is generated. Nevertheless in practice, differences between theoretical and real surface roughness values are increasing with feed decrease. Similar conclusions can be proposed

Twofold *ap* and fourfold *fz* growth caused insignificant *Ra* and *Rz* change. Therefore in the range of conducted research non monotonic increase of surface roughness in

with cutting speed *vc* = 68 m/min, b) milling with cutting speed *vc* = 150 m/min

from cumulative *Ra* and *Rz* charts for all *ap* and *fz* combinations (see Figure 20).

function of investigated factors was stated.

**Figure 16.** FFT analysis of surface profile after DLD process of tungsten carbide

Figure 17 depicts 3D surface roughness charts and power density spectra (PDS) obtained after milling of tungsten carbide.

It can be seen, that 3D surface topographies after milling (Figure 17) are affected by the cutter's projection into the workpiece. This observation is also confirmed by the power density spectra which represent wavelengths of surface irregularities generated during machining. Surface profiles consist of wavelengths related to the feed per tooth value (*fz* = 0.05 mm) which is related to the kinematic-geometric projection of cutter into the workpiece, and feed per revolution value (*f* = 0.1 mm) which can be induced due to radial run out phenomenon.

Figure 18 depicts examples of profile charts and corresponding to them *Ra* and *Rz* parameters for various feed per tooth *fz* values. As it can be seen the fourfold feed per tooth *fz* increase did not make any significant qualitative and quantitative surface texture changes. It denotes that feed insignificantly influences surface roughness, what is not in full agreement with the results shown in Figures 17a and 17b. For some instances, characteristic kinematic-geometric projection of cutting edge into the workpiece can be seen, however in a wider surface roughness range, there is no typical relation. Figure 19 depicts surface roughness parameters *Ra* and *Rz* (for *vc* = 68 m/min) in function of feed per tooth *fz*.

values of surface roughness parameters.

after milling of tungsten carbide.

phenomenon.

tooth *fz*.

It can be seen, that tungsten carbide sample manufactured by DLD technology has an unsatisfactory geometric accuracy and unreasonable surface roughness. Furthermore, from the surface profile and the FFT analysis (Figure 16) it is resulting, that surface texture after DLD process has a random character. The FFT analysis of surface profile consists also of constituent related to the half of the evaluation length (2.4 mm), which means that DLD surface profile is affected by the waviness. Therefore, it needs further finishing process. After milling, machined surface is much smoother and characterized by significantly lower

**Figure 16.** FFT analysis of surface profile after DLD process of tungsten carbide

Figure 17 depicts 3D surface roughness charts and power density spectra (PDS) obtained

It can be seen, that 3D surface topographies after milling (Figure 17) are affected by the cutter's projection into the workpiece. This observation is also confirmed by the power density spectra which represent wavelengths of surface irregularities generated during machining. Surface profiles consist of wavelengths related to the feed per tooth value (*fz* = 0.05 mm) which is related to the kinematic-geometric projection of cutter into the workpiece, and feed per revolution value (*f* = 0.1 mm) which can be induced due to radial run out

Figure 18 depicts examples of profile charts and corresponding to them *Ra* and *Rz* parameters for various feed per tooth *fz* values. As it can be seen the fourfold feed per tooth *fz* increase did not make any significant qualitative and quantitative surface texture changes. It denotes that feed insignificantly influences surface roughness, what is not in full agreement with the results shown in Figures 17a and 17b. For some instances, characteristic kinematic-geometric projection of cutting edge into the workpiece can be seen, however in a wider surface roughness range, there is no typical relation. Figure 19 depicts surface roughness parameters *Ra* and *Rz* (for *vc* = 68 m/min) in function of feed per **Figure 17.** 3D surface roughness chart and corresponding Power Density Function during: a) milling with cutting speed *vc* = 68 m/min, b) milling with cutting speed *vc* = 150 m/min

From these charts no influence of feed per tooth *fz* on surface roughness is seen, despite for *fz* = 0.1 mm/tooth. In this case the theoretic value of *Rzt* is comparable to real *Rz* value. It is commonly known that the increase of feed per tooth *fz* is accompanied by the increase of surface roughness. Theoretically, the lower feed is fixed, the lower surface roughness is generated. Nevertheless in practice, differences between theoretical and real surface roughness values are increasing with feed decrease. Similar conclusions can be proposed from cumulative *Ra* and *Rz* charts for all *ap* and *fz* combinations (see Figure 20).

Twofold *ap* and fourfold *fz* growth caused insignificant *Ra* and *Rz* change. Therefore in the range of conducted research non monotonic increase of surface roughness in function of investigated factors was stated.

Machining Characteristics of Direct Laser Deposited Tungsten Carbide 117

**Figure 19.** Surface roughness *Ra* and *Rz* in function of feed per tooth *fz*

**Figure 20.** Surface roughness *Ra* and *Rz* in function of feed per tooth *fz* and depth of cut *ap*

Figure 21 depicts schemes of tungsten carbide products manufacturing processes.

The development of modern tool materials such as diamonds (PCD, MCD) and cubic boron nitrides (CBN), as well as ultraprecision and rigid machine tools enables machining of tungsten carbides. These materials have excellent physicochemical properties such as, superior strength, high hardness, high fracture toughness, and high abrasion wearresistance. On the other hand, these unique properties can cause substantial difficulties during machining process, which can result in low machinability. From the carried out experiments it can be seen, that during machining of tungsten carbides, excessive values of

**5. Summary and conclusions** 

vibrations and intense tool wear growth can occur.

**Figure 18.** Examples of profile charts for various feed per tooth *fz* values: a) *fz* = 0.025 mm/tooth, b) *fz* = 0.05 mm/tooth, c) *fz* = 0.075 mm/tooth, d) *fz* = 0.1 mm/tooth

**Figure 19.** Surface roughness *Ra* and *Rz* in function of feed per tooth *fz*

**Figure 20.** Surface roughness *Ra* and *Rz* in function of feed per tooth *fz* and depth of cut *ap*

#### **5. Summary and conclusions**

116 Tungsten Carbide – Processing and Applications

**Figure 18.** Examples of profile charts for various feed per tooth *fz* values: a) *fz* = 0.025 mm/tooth, b) *fz* =

0.05 mm/tooth, c) *fz* = 0.075 mm/tooth, d) *fz* = 0.1 mm/tooth

The development of modern tool materials such as diamonds (PCD, MCD) and cubic boron nitrides (CBN), as well as ultraprecision and rigid machine tools enables machining of tungsten carbides. These materials have excellent physicochemical properties such as, superior strength, high hardness, high fracture toughness, and high abrasion wearresistance. On the other hand, these unique properties can cause substantial difficulties during machining process, which can result in low machinability. From the carried out experiments it can be seen, that during machining of tungsten carbides, excessive values of vibrations and intense tool wear growth can occur.

Figure 21 depicts schemes of tungsten carbide products manufacturing processes.

Machining Characteristics of Direct Laser Deposited Tungsten Carbide 119

**Author details** 

**6. References** 

Paweł Twardowski and Szymon Wojciechowski

Publication No 597, Vol. 97. [5] http://www.lasercladding.com/

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**Figure 21.** The schemes of manufacturing processes of various products made of tungsten carbide: a) cutting insert, b) spherical surface, c) end product made by DLD technology

The application of ductile cutting to production of cutting inserts (Figure 21a) shortens manufacturing process by the elimination of one partial process (e.g. polishing). However ductile cutting occurs only in the range of extremely low values of depths of cut and feeds. Therefore, this kind of process can be achieved only on very rigid and ultraprecision machine tools, what is substantial limitation of this method. Ultraprecision machine tools can be also applied to grinding of very accurate spherical surfaces. This process also shortens manufacturing process by the elimination of polishing or lapping (Figure 21b). In case of tungsten carbide products obtained by DLD (direct laser deposition) technology (Figure 21c), grinding or cutting (e.g. milling, turning) can be applied as the finishing process. However cutting enables also the shaping of manufactured part, by the possibility of higher cutting conditions application in comparison to grinding. Nevertheless, during cutting of tungsten carbide, intense tool wear growth can occur, and thus this process requires the selection of appropriate cutting conditions.

Deliberations presented in this chapter reveal, that efficient machining process of tungsten carbide parts is feasible, however it requires the knowledge about the physical effects of the process, as well as appropriate selection of machining method and cutting conditions, enabling desired technological effects.
