**2.3 Cutting tools and their geometry effect on micro-cutting**

Cemented carbide tools are an excellent choice for micro milling because of their high hardness, acceptable wear resistance, and strength, as well as their affordability. The performance of solid carbide tools is chiefly dependent upon the composition (i.e., cobalt content) and grain size. A lower grain size enables lower values of cuttingedge radius on the tool, which is important for achieving high surface roughness and accuracy in micro milling. The most popular choice for micro milling tools is fine grain size (<0.6 μm) cobalt bonded tungsten carbide [18–20]. Fine grain size tungsten carbide is an excellent choice for micro milling tools due to its high hardness, good wear resistance, high strength, and affordability. However, it is important to be aware of its brittleness and sensitivity to heat when using it.

The generic geometry of the micro cutting tool is shown in **Figure 5**, and raw material properties are given in **Table 2**.

The material properties of cemented carbides can be adjusted according to the requirement of micro milling cutter applications by altering cobalt content, average grain size, and concentration of composite carbides, as shown in **Figure 6a** and **b** [21].

### **2.4 Tool geometry and design considerations**

Micro milling tools are small and delicate, and therefore they are more susceptible to breakage than larger tools. High stiffness is essential to prevent the tool from breaking, especially when machining hard materials. The optimal tool geometry for micro milling depends on the tool diameter. For tools with a diameter smaller than 200 μm, an asymmetrical tool geometry is preferred. This is because asymmetrical tools are more tolerant of fabrication errors. For tools with a diameter larger than 200 μm, symmetrical tool geometry is preferred. This is because symmetrical tools are more efficient. The relief angles, tool peripherals, and tool bottom surfaces should be

**Figure 5.** *SEM image of cutter having diameter of 70 μm [20].*


#### **Table 2.**

*Raw material types for micro milling cutters and their properties [20].*

#### **Figure 6.**

*(a) Scanning electron microscopy (SEM) images, (b) schematic of chemical components for micro-milling cutter materials [21].*

carefully designed to avoid unnecessary contact with the workpiece. This is important to reduce friction and heat generation, which can lead to tool wear and poor surface finish. The critical small geometry features of the tool, such as the size of the smallest grinding wheels and thinnest electrode wire, must be considered during design. This is because these features can limit the minimum feature size that can be machined. Chip disposal spaces must be considered, especially when machining under severe cutting conditions such as no cutting fluid and high-pressure air. This is to ensure that the chips are removed from the cutting zone efficiently. Small cutting-edge radii are essential for achieving a high-quality machined surface. However, it is important to balance the need for a small cutting-edge radius with the need for wear resistance. The rake angle is an important parameter that affects the cutting forces, chip formation, and surface finish. A small positive rake angle is preferred for machining ductile materials. This is because a small positive rake angle produces lower cutting forces and a better surface finish. A highly negative rake angle is preferred for machining hard and brittle materials. This is because a highly negative rake angle produces higher cutting forces but a better surface finish. In addition to the criteria listed above, there are

a number of other factors that need to be considered when designing micro milling tools, such as the material of the tool, the coating of the tool, and the cooling system. By carefully considering all of these factors, it is possible to design micro milling tools that can produce high-quality machined surfaces with high precision and accuracy [18, 22].

#### **2.5 Minimum quantity lubrication (MQL)**

Cutting fluid plays an important role because of its capacity to reduce friction and its ability to dissipate the heat generated between the micro tools and the workpiece. As cutting zones are very tiny, one of the most challenging aspects of the micromilling scale is effective cooling and/or lubricating fluid delivery. More than 90% of the mechanical energy in micro-milling processes is converted into thermal energy, resulting in a temperature increase in the cutting zone and cutting edges. When machining oxygen-free copper and aluminum alloys, the cutting zone temperature is frequently below 100°C, whereas over 200°C is frequently what is observed when machining hard and brittle materials [23]. The conventional flood cooling method is not effective in micro-milling due to the small size of the cutting zone and the significant surface tension force [24, 25]. In order to improve cutting fluid penetration into the small cutting zone, a minimum quantity lubrication (MQL) method is considered as the suitable alternative for precisely delivering the lubricant and/or coolant and meeting the requirements of being environmentally friendly. In the MQL system, lubricant is pulverized, allowing to better reach the cutting edge. The turbulence created by tool rotation is a major challenge in micro-milling. It can lead to poor surface finish, tool wear, and chip formation. MQL systems help to overcome this challenge by delivering a small amount of lubricant to the cutting edge in the form of a mist. The mist is able to penetrate the turbulence and reach the cutting edge, where it can reduce friction, heat generation, and chip formation [26]. According to Li et al. [47], minimum quantity lubrication, also known as near dry machining (NDM), refers to the use of cutting fluids in tiny quantities, which is only about ten-thousandths of the amount of cutting fluid used in flood-cooled machining. MQL has been shown to be effective in improving tool life, surface finish, and burr formation in micro-milling.

#### **2.6 Accuracy and thermal effects**

Micromilling is significantly affected by factors that are often overlooked in conventional milling, such as tool wear, tool runout, and chatter [28]. Tool wear is high in micro-milling due to the lower chip loads and large effective rake angles. Tool runout is also higher in micro-milling due to the low rigidity of the tool and higher cutting speeds. Chatter is a common problem in micro-milling due to the low stiffness of the tool and the interrupted cutting process. Another challenge of micro-milling is that the stiffness of the tool decreases rapidly as the diameter of the tool decreases. This is because the tool can be considered as a cantilever, and the stiffness of a cantilever decreases with the fourth power of the diameter [29, 30]. Also, micro-milling is an interrupted cutting process where cutting forces vary with the rotation angle of the cutter. To overcome these challenges cutting paths and machining strategies should be selected and optimized in micro-scale milling to maintain tighter tolerances, reduce the cutting force, maintain constant chip load, and avoid premature tool failure [31, 32]. Gracia et al. [48] have discussed the sources of geometrical and surface errors affecting the machining accuracy in Micro milling (**Figure 7**). The key characteristics

of micro-milling are shown in **Figure 8** in three broad categories i.e. Tooling, Work material and machine tool [11].

Micromachining processes for THz waveguide are particularly sensitive to the surface roughness and the precision of achieved machining geometrical accuracies. Apart from the inaccuracies of the setup and positional errors, another problem in CNC precision is that of achieved control versus desired control, where actuator backlash, wearing of gears can all add to the problems of deviations. Tool deflections occur in micro-milling, and vibrations occur based on the large tool length to small diameter ratios; such problems only add to increased geometrical inaccuracies. Finally, another problem to be considered is in terms of temperature gradients that can impact material machining accuracy.

**Figure 8.** *Key characteristics in micro milling [11].*

#### *2.6.1 Higher spindle speeds (RPM) in micro-milling*

High spindle speeds also have the effect of reducing the chip size by requiring a smaller feed rate per tooth, fT (since fT∝ 1/ N). Smaller chip loads result in improved surface finish. Good surface finishes are crucial for cutting down on waveguide losses [33, 34].

#### *2.6.2 Temperature control and thermal effects*

The most important issue in precision machining is maintaining a constant temperature of the positional stages and the tool holders. Due to the stringent tolerances required for fabricating high-frequency waveguide components, temperature changes of a few degrees result in significant errors. To overcome the thermal effects, liquid cooled spindles and slideways are part of the precision machine design. As well as before commencing actual machining, the machine is usually run for 10–20 min to reach temperature equilibrium (warmup).

#### **2.7 Chatter in machining**

Chatter has been and still is a very important topic in micro-milling. It is a highly complex phenomenon due to the diversity of elements that can compose the dynamic system and its behavior. Negative effects of chatter are poor surface quality, unacceptable accuracies, excess noise, disproportionate tool wear, low material removal rate (MRR), material wastage, cost of rework or repair, wastage of energy, and tool damage. Chatter generates through the self-excitation mechanism. It can be primary chatter or secondary chatter.

*Primary chatter* is by friction between tool and work, it produces a thermomechanical effect on chip formation, and it is due to mode coupling. At the same time, *Secondary chatter* regenerates waviness on the workpiece surface. This is the main cause of chatter. In most publications, chatter is referred to as regenerative chatter. However, it is possible to distinguish between frictional chatter, thermomechanical chatter, mode coupling chatter, and regenerative chatter on the self-excitation mechanisms that cause the vibration [36].

#### *2.7.1 Strategies for ensuring stable machining process*

The control strategies are based on in-process control and out-of-process control. The first group is composed of all those methods that ensure a stable machining process by selecting cutting parameter combinations in the stable zone of the side lobe diagram (SDL) and making most of it. It is possible to distinguish between out of the process and in-process methods. The Methods aim to predict the location of the stability boundary of the cutting process to select stable cutting parameter combinations. The SLD identification is made out of the process before machining. On the other hand, In-process includes those methods that detect chatter during the metal cutting process, allowing the parameters to be corrected, so the cut migrates to the stable zone. It is possible to distinguish between passive and active mechanisms as follows:

### *2.7.2 Passive modes of chatter suppression*

Strategies based on modifying certain machine tool elements to passively change the behavior of system composed of machine tool, for example, cutting tool and tool holder.

#### *2.7.3 Active modes of chatter suppression*

Based on certain elements capable of modulating the quantity of work provided, absorbing or supplying the energy with the aim to actively raise or at least change the stability frontier.

#### **2.8 Burr formation**

One of the main micro-milling-induced geometrical defects is the burr occurrence on the machined edges of the material. Burrs can be defined as the undesirable projections on the surface beyond the edge of the workpiece which are formed due to the bending of chips at the end of the cut. It has been reported and accepted that burr formation at the microscale is also affected with size effects analogous to the surface roughness.

When the ratio of cut to the cutting-edge radius is small, plowing dominates the cutting process rather than shearing, resulting in high biaxial compressive stresses that push material toward the free surface and generate large-top burrs. Also, the path the tool follows when entering or exiting the work surface also significantly affects burr formation [38, 39]**.** The cutting parameters, workpiece material properties, tool geometry, coatings, and coolant lubricants also affect the burr formation significantly micro-milling [41]. **Figure 9** shows different types of burr and their location.

Higher feed per tooth and smaller cutting width values have a positive effect on the burr formation mechanisms [42]. **Figure 10** gives the Ishikawa diagram is showing various possible factors which may affect the surface quality and burr formation of the micro-milled surface.

**Figure 9.** *Different burr types and their locations [38].*

*Micromachining and Its Applications for Electronics DOI: http://dx.doi.org/10.5772/intechopen.113892*

**Figure 10.** *Fish bone diagram of factors affecting surface quality.*

#### **2.9 Machine tools for micro-scale processing**

Micro-scale processing is used to produce small and precise features in a wide range of industries, including automotive, aerospace, astronomy, medical, optics, and metrology. To meet the demands of these industries, it is necessary to develop machine tools that can provide the required accuracy and precision.

**Figure 11** shows that a thermally and dynamically stable machine tool structure is important for micromachining because it helps to ensure that the workpiece is machined accurately and precisely. Thermal distortion and vibration can cause errors in the machining process, which can lead to defective products. New structural materials, such as advanced ceramics, advanced composite materials, engineering plastics, fiber-reinforced plastics (FRPs), and fiber-reinforced metals, can be used to improve the thermal and mechanical stability of machine tools for micromachining. **Figure 12** shows conceptual design of a thermally and dynamically stable machine tool. Design to minimize thermal distortion and vibration can be done by using materials with a low coefficient of thermal expansion (CTE), designing the structure to be stiff and rigid, and isolating the error sources from the machining area. By using vibration dampers and designing the structure to be symmetric and balanced, isolation of the error sources can be achieved. High-precision components, designing the structure to be easy to assemble and adjust, and using materials with a low CTE will lead to minimization of the error sources and feedback sensors will contribute to controlling of the error sources [49].



**Figure 11.** *Micro scale processing and machine tool structure.*

**Figure 12.**

*Concept of stable machine tool structure for micro-and nanometer-scale processing [49].*

#### *2.9.1 Abbe's principle*

The Abbe principle states that the length to be measured and the measuring scale must lie on the same axis. This is important because if the measurement is taken at an angle, it will introduce an error. Conventional machine tools often do not satisfy the Abbe principle because the motion axis and the machine element (its reference point or its center of gravity) do not lie on a single line. This can lead to errors in the machining process. **Figure 13** shows two different machine tool designs. **Figure 13b** shows a C-frame with a single ball screw column. In this design, the tool axis line and measuring line are separated. This means that the Abbe principle is not satisfied, and angular deflection will be translated into a displacement error at the tool. **Figure 13a** shows a double-column and twin ball screw

design. In this design, the distance between the tool axis and each ball screw axis compensates for the angular deviation effect on the linear axis. The Abbe principle is an important consideration for the design of machine tools and other precision instruments. By following the Abbe principle, engineers can design machines that are more accurate and precise.

#### *2.9.2 Machine bed material*

Thermal stability is important for machine tool beds in micro-cutting because it helps to ensure that the machine bed maintains its shape and dimensions even when it is subjected to heat from the machining process. This can be achieved by using materials with a low thermal expansion coefficient, low specific heat capacity, and good damping properties. Granite and mineral casting are two materials that meet these requirements, and they are commonly used for machine tool beds in microcutting applications. Granite is used mostly in the field of ultra-precision machine beds because of its low thermal expansion coefficient of 6.5 μm/mK which is nearly 2.5 times less than the one of mineral casting. Additionally, granite with a density of 2.8 kg/dm3 is beneficial in terms of Eigen frequency and dynamic properties (refer **Table 3**).

Mineral casting materials are a good alternative to granite for machine tool beds because they have similar material properties, but they are less expensive and easier to manufacture. The material properties of mineral casting are comparable to those of granite, but the heat expansion coefficient is about two to three times higher. However, the thermal distortion of mineral casting machine tool beds can be minimized by using appropriate design techniques and by carefully controlling the temperature of the machine tool environment [45].

#### **2.10 Touch-trigger probing and laser measuring systems**

Probing systems are an indispensable part of micromachining. Probing systems can help to improve the accuracy, efficiency, and quality of manufactured products. On-machine probing is used to measure and inspect workpieces while they are still on the CNC machine tool. This can help to reduce setup and inspection times and improve the overall efficiency of the machining process. In-process probing is used to monitor the machining process and to make adjustments as needed. This can help to improve the accuracy and quality of the finished product.


#### **Table 3.**

*Material properties for structural elements in precision machine tools.*
