**2. Machining of tungsten carbide**

104 Tungsten Carbide – Processing and Applications

in production of functional prototypes, modify or repair components which have excellent hardness, toughness, corrosion and abrasion wear-resistance, e.g. machine parts for the automotive industry – Figure 2. In the near future DLD technology will be used in

**Figure 1.** Direct laser deposition technology (DLD): a) the scheme of process, b) the view of process [5]

**Figure 2.** The application of DLD technology for the crankshafts (a, b) and parts for the automotive

industry (c) [5]

manufacturing of spare parts in long term space missions [7] or submarines [8].

Tungsten carbide has excellent physicochemical properties such as, superior strength, high hardness, high fracture toughness, and high abrasion wear-resistance. These properties impinges wide application of tungsten carbide in industry for cutting tools, molds and dies. On the other hand, these unique properties can cause substantial difficulties during machining process, which can result in low machinability. Therefore, machining of tungsten carbide 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. The primary objective of post-process machining of tungsten carbide is to achieve satisfactory geometric and physical properties of its surface texture.

The most popular finishing method of tungsten carbides applied in the tooling industry is grinding with the diamond and CBN (cubic boron nitride) wheels. However, in order to produce optical components made of cemented carbide (e.g. spherical mirrors) the profile quality requires a low surface roughness, a stringent form accuracy on the submicron scale, as well as a low amount of surface damage [10]. Traditional grinding with the diamond wheels can cause machining-induced cracks and damages to the material. To remove these cracks and damage and to obtain a mirror finish, lapping and polishing with fine diamond abrasives are usually employed. Nevertheless, these processes can cause the deterioration of form accuracy and increase the machining cost.

Recently, ultraprecision grinding has been developed that substantially decreases subsurface damage and can precisely control the geometry of the finished surface [11, 12]. This kind of process is conducted on the ultraprecision CNC grinding machines, with threeaxes movements, and micro-system to deterministically generate, fine, and pre-polish a plano or spherical surface. Very often these machines have motors with power exceeding 1kW and maximal rotational speeds above 80 000 rpm. The example of ultraprecision set-up is shown in Figure 3a.

Tools applied in the ultraprecision grinding processes are usually selected as metal-bond diamond cup wheels (Figure 3b) with grit sizes between 15÷25 µm. The selected CNC grinding program includes two parts, i.e. stock removal and spark out. During the stock removal step, the grinding speed is selected in the range of 10÷15 m/s (for a small tool diameters it corresponds to rotational speeds up to 40 000 rpm). The vertical feed rates of the tool spindle are usually selected in the range of 0.05÷0.2 mm/min, and the workpiece spindle rotated at 1000 rpm. During the spark out phase, the workpiece is rotated with a 1000 rpm for about 180 rotations.

Machining Characteristics of Direct Laser Deposited Tungsten Carbide 107

Cutters applied in the ductile cutting experiments, are made of diamond (MCD, PCD) or CBN (cubic boron nitride) materials. The example of turning and milling tool applied in carbide's machining process is presented in Figure 5. These tools have usually negative geometry (rake *γ* angles lower than 0), and a small values of tool cutting edge inclination angle *rn* < 6 µm, which is needed to initiation of the ductile cutting. To obtain a crack free surface the tool feed rate *f* and the cutting depth *ap* must be very low. Their values are usually selected as: *f* ≈ 1÷75 µm/rev and *ap* ≈ 2÷10 µm. Cutting speeds can be selected in the

**Figure 5.** Tools applied in machining of carbides: a) diamond turning tool [15], b) CBN torus end mill

In order to finish plane surface, made of tungsten carbide, obtained using DLD technology, one can apply face milling process (Figure 4b). Surfaces obtained using DLD technology have significantly higher roughness than ones manufactured by powder metallurgy technology. Therefore, cutting parameters during machining of these surfaces can be higher than those applied in machining of powder metallurgy surfaces, and selected as follows: feed per tooth *fz* ≈ 25÷100 µm/tooth, axial depth of cut *ap* = 20 µm, radial depth of cut *ae* = *D*/2

In this chapter the analysis of main physical machinability indicators, such as: cutting forces and vibrations will be presented. The set-up of cutting forces and vibrations measurements

The hook up into bed of a machine piezoelectric force dynamometer was used to measure total cutting forces components [16]. Instantaneous force values were measured in feed force *Ff*, normal feed force *FfN* and thrust force *Fp* directions. Force dynamometer's natural frequency is equal to 1672 Hz. In order to avoid disturbances induced by proximity of forcing frequency to gauge natural frequency, the band – elimination filter was applied. The acceleration of vibrations of tungsten carbide workpiece during milling was measured using piezoelectric accelerometer. These vibrations were measured in the same directions as

**3. The analysis of physical machinability indicators** 

during face milling process is presented in Figure 6.

following range: *vc* = 50÷600 m/min.

[16]

(half of tool's diameter).

cutting force components.

**Figure 3.** The ultraprecision grinding process of a spherical mirrors: a) set-up [10], b) schematic presentation of the diamond tool [12]

Apart of grinding, recently are seen tendencies to cutting (mainly turning and milling – Figure 4) brittle materials such as, tungsten carbide and reaction-bonded silicon carbide (RB-SiC) by a superhard CBN (cubic boron nitride) and PCD (polycrystalline diamond) cutters in cutting conditions assuring ductile cutting [13, 14]. This technique of cutting can be achieved when depths of cut and feeds (expressed as uncut chip thickness) are extremely low and a quotient of the tool cutting edge inclination angle to uncut chip thickness is greater than unity (*rn*/*h*>1). In milling process of tungsten carbide by CBN tools, the transition from ductile to brittle cutting occurs at critical depth of cut *apcr.* equal to approximately 4.78 µm. Machining with very low cutting conditions is feasible only on ultraprecision machine tools with high rigidity, which is substantial limitation of this technique.

Figure 4a depicts the schematic diagram of the numerically controlled three-axis ultraprecision lathe used in ductile turning experiments. The lathe has two perpendicular hydrostatic tables along the X- and Z-axis direction, in addition to a B-axis rotary table built into the X-axis table. Both X-axis and Z-axis tables have linear resolutions of 1nm, and the Baxis rotary table has an angular resolution of one ten millionths of a degree. The sample can be rotated with the spindle and moved along the Z-axis direction, while the cutting tool can be moved along the X-axis direction and also rotated around the B-axis.

**Figure 4.** The set-up of cutting process: a) ductile turning of carbides [15], b) face milling of DLD tungsten carbide [16]

Cutters applied in the ductile cutting experiments, are made of diamond (MCD, PCD) or CBN (cubic boron nitride) materials. The example of turning and milling tool applied in carbide's machining process is presented in Figure 5. These tools have usually negative geometry (rake *γ* angles lower than 0), and a small values of tool cutting edge inclination angle *rn* < 6 µm, which is needed to initiation of the ductile cutting. To obtain a crack free surface the tool feed rate *f* and the cutting depth *ap* must be very low. Their values are usually selected as: *f* ≈ 1÷75 µm/rev and *ap* ≈ 2÷10 µm. Cutting speeds can be selected in the following range: *vc* = 50÷600 m/min.

106 Tungsten Carbide – Processing and Applications

presentation of the diamond tool [12]

tungsten carbide [16]

which is substantial limitation of this technique.

**Figure 3.** The ultraprecision grinding process of a spherical mirrors: a) set-up [10], b) schematic

Apart of grinding, recently are seen tendencies to cutting (mainly turning and milling – Figure 4) brittle materials such as, tungsten carbide and reaction-bonded silicon carbide (RB-SiC) by a superhard CBN (cubic boron nitride) and PCD (polycrystalline diamond) cutters in cutting conditions assuring ductile cutting [13, 14]. This technique of cutting can be achieved when depths of cut and feeds (expressed as uncut chip thickness) are extremely low and a quotient of the tool cutting edge inclination angle to uncut chip thickness is greater than unity (*rn*/*h*>1). In milling process of tungsten carbide by CBN tools, the transition from ductile to brittle cutting occurs at critical depth of cut *apcr.* equal to approximately 4.78 µm. Machining with very low cutting conditions is feasible only on ultraprecision machine tools with high rigidity,

Figure 4a depicts the schematic diagram of the numerically controlled three-axis ultraprecision lathe used in ductile turning experiments. The lathe has two perpendicular hydrostatic tables along the X- and Z-axis direction, in addition to a B-axis rotary table built into the X-axis table. Both X-axis and Z-axis tables have linear resolutions of 1nm, and the Baxis rotary table has an angular resolution of one ten millionths of a degree. The sample can be rotated with the spindle and moved along the Z-axis direction, while the cutting tool can

be moved along the X-axis direction and also rotated around the B-axis.

**Figure 4.** The set-up of cutting process: a) ductile turning of carbides [15], b) face milling of DLD

**Figure 5.** Tools applied in machining of carbides: a) diamond turning tool [15], b) CBN torus end mill [16]

In order to finish plane surface, made of tungsten carbide, obtained using DLD technology, one can apply face milling process (Figure 4b). Surfaces obtained using DLD technology have significantly higher roughness than ones manufactured by powder metallurgy technology. Therefore, cutting parameters during machining of these surfaces can be higher than those applied in machining of powder metallurgy surfaces, and selected as follows: feed per tooth *fz* ≈ 25÷100 µm/tooth, axial depth of cut *ap* = 20 µm, radial depth of cut *ae* = *D*/2 (half of tool's diameter).
