Regression equations of specific cutting pressure components
Machinability can be defined as the relative susceptibility of the work material to the decohesion phenomenon and chip formation, during cutting and grinding. This feature depends on work and tool’s material physic-chemical properties and condition, method of machining, as well as cutting conditions . Therefore, there is no unique and unambiguous meaning to the term machinability. This feature, can be described by many various indicators. Each one of them carries out a wide variety of operations, each with a different criteria of machinability. A material may have good machinability by one criterion, but poor machinability by another .
To deal with this complex situation, the approach adopted in this chapter is to divide machinability indicators into two groups, namely: physical and technological indicators. Physical machinability indicators include i.a. temperatures, cutting forces, vibrations and residual stresses generated during machining process, because their value have the direct influence on the ensemble of the remaining machining effects. Technological indicators include mainly machined surface texture and tool’s life (relatively tool wear).
The most popular method for producing tungsten carbide components is by powder metallurgy technology. Nonetheless, for individual, small quantity production or product prototyping this method is too costly and time consuming. The alternative to powder metallurgy is Direct Laser Deposition (DLD) technology, which can be used to quickly produce metallic powder prototypes by a layer manufacturing method [3, 4] – Figure 1. The primary objective of DLD technology is the regeneration of machine parts or machine parts manufacturing with the improved surface layer properties, e.g. higher corrosion, erosion and abrasion resistance. Direct Laser Deposition is an extension of the laser cladding process, which enables three dimensional fully-dense prototype building by cladding consecutive layers on top of one another . The DLD technology is increasingly being used 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 manufacturing of spare parts in long term space missions  or submarines .
Unfortunately, DLD technology has also significant disadvantage. Presently most components produced by DLD technology has an unsatisfactory geometric accuracy as well as surface roughness and requires some post-process machining to finish them to required tolerances . Therefore, the machinability of DLD manufactured materials (e.g. tungsten carbide), require further and extensive studies.
2. Machining of tungsten carbide
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 . 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 three-axes 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.
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 (
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 B-axis 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.
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
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
3. The analysis of physical machinability indicators
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 during face milling process is presented in Figure 6.
The hook up into bed of a machine piezoelectric force dynamometer was used to measure total cutting forces components . Instantaneous force values were measured in feed force
Figure 7 depicts the tool wear (
On the base of conducted investigations, clear relation between progressing tool wear and
In order to analyze forcing frequencies affecting cutting force components during milling of tungsten carbide, the FFT (Fast Fourier Transform) spectra were determined (Figure 8). From the Figure 8 it is resulting, that primary forcing frequency is tooth passing frequency
Figure 9 compares cutting forces in function of feed per tooth obtained during milling of tungsten carbide and hardened X153CrMoV12 steel (with 60 HRC hardness). It was observed that both in milling of tungsten carbide and hardened steel, cutting forces (
Figure 10 depicts
From the Figure 10 it can be seen, that feed per tooth
In order to estimation of cutting forces in the broad range of cutting conditions, cutting force models can be applied. Majority of models assume that cutting force is proportional to sectional area of cut and the specific cutting pressures. Figure 11 depicts, empirically determined course of the specific cutting pressure in function of mean uncut chip thickness
From the Figure 11 it can be seen, that mean uncut chip thickness
|Specific cutting pressure component||Specific cutting pressure |
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
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
Figure 15 compares the surface texture of tungsten carbide sample manufactured by DLD technology before and after milling.
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.
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 (
Figure 18 depicts examples of profile charts and corresponding to them
From these charts no influence of feed per tooth
5. Summary and conclusions
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 wear-resistance. 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.
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.
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