Machining: Milling Parameters and Processes PDF

Milling Before delving into the topic of accuracy, which represents the first phase of evolution in milling, it is essential to recap some key concepts. For example, when dealing with rotational part, the most efficient process is turning. In contrast, when de...

Milling Before delving into the topic of accuracy, which represents the first phase of evolution in milling, it is essential to recap some key concepts. For example, when dealing with rotational part, the most efficient process is turning. In contrast, when dealing with prismatic parts, milling is often the best solution. 💡 A fundamental principle in milling is the use of a rotational tool, where the cutting speed is derived from the tool’s tangential velocity. One advantage of milling is that it is a non-continuous cutting process, which aids in chip breaking. In turning, the tool is continuously in contact with the material, whereas in milling, the cuter intermittently enters and exits the material, making it easier to break chips. ⚠️ Chip formation can present challenges, especially in materials that flow well, as the chips may not break easily. This can lead to issues such as surface damage, tool breakage, or excessive heat build-up. The heat carried away by the chips is essential for maintaining optimal working conditions, and when chips are not properly discharged, temperatures can rise, causing additional problems. Milling parameters Milling 1  Mill diameter Dc : one of the key factors in milling is the diameter of the tool. This is crucial ​ because it directly determines the size of the tool being used. Since the tool rotates during milling, the diameter allows us to calculate the tangential cutting speed, which is essential for understanding the dynamics of the process. Cutting speed vc : this parameter is typically calculated using a standard formula derived from ​ the theory of circular motion: πDc ⋅ n m vc = [ ] ​ 1000 ​ ​ ​ min The cutting speed is connected to the spindle speed, expressed in rpm. 💡 When selecting a tool for a particular operation, it’s not up to the machinist to choose the material of the tool. Instead, manufacturers have already determined which tool material is best suited for the job. Manufacturers will recommend tools based on factors like the workpiece material and the type of operation, ensuring optimal results. Cost should not be the sole determining factor when selecting tools, as focusing solely on the price can lead to poor decisions. Many companies make the mistake of choosing tools based only on their cost, ignoring the potential time and efficiency savings that come with higher-quality, more expensive tools. To fully benefit from an advanced milling machine, you need to use tools that match the machine’s capability. 💡 Once a suitable tool is chosen, the manufacturer will provide guidance on the optimal cutting speed. Milling 2  Cutting speed is closely related to tool life. The faster the cutting speed, the higher the heat generated, which can accelerate wear and reduce the lifespan of the tool. Therefore, balancing cutting speed with tool longevity is critical. The heat generated during milling can significantly impact the tool and workpiece. Spindle speed n: since vc is generally a suggested value, the spindle speed can be calculated ​ based on the tool diameter and the cutting speed: vc ⋅ 1000 n= [rpm] ​ ​ πDc ​ ⚠️ A large-diameter tool will require a slower spindle speed, while a smaller tool will need a faster speed to achieve the suggested vc  ​ It’s the machinist’s responsibility to determine the correct spindle speed, as the machine itself doesn’t automatically account for these parameters. Feed rate vf : another important factor in milling is the concept of feed, which refers to the ​ relative velocity between the cutting tool and the workpiece. Essentially, it describes how fast the tool and the workpiece move in relation to each other. For instance, if the tool remains fixed while the workpiece moves, the feed is the speed of the workpiece’s motion. In some machines, the tool rotates and stay in place, while the part moves beneath it. Regardless of which component is moving, the key concern is always their relative motion, not the motion of each individually. The overall feed rate can be computed as: ] mm vf = fn ⋅ n[ ​ ​ ​ min Feed per revolution fn : to explain the concept of feed further, let’s consider what happens in ​ one revolution of the tool. After one full revolution, the tool returns to its original angular position. During that time, the space covered by the spindle axis is known as feed per revolution. The feed per revolution fv [ ] mm ​ ​ rev is the space accomplished by the tool during one revolution. Feed per tooth fz : if the tool has multiple teeth, each tooth removes a layer of material, and the ​ thickness of this layer is determined by: Milling 3  [ ] fn mm fz = ​ Z rev ⋅ tooth ​ ​ ​ The feed per tooth represents the amount of material each tooth removes in one pass. Feed per tooth is crucial because it directly relates to chip thickness. ⚠️ Each tooth operates independently, meaning the forces applied to each one are unique. To properly dimension the tool, especially the cutting inserts, it’s necessary to calculate the forces acting on each individual tooth. Another key point to consider is the trajectory of the cutting tool as it engages with the workpiece. This motion combines both the linear feed motion and the rotational motion of the tool. The result is a cycloidal path, which can vary depending on the relative speeds of both movements. In practice, this cycloidal motion can often be approximated as an arc of circle, simplifying the analysis while remaining for accurate for purposes. This assumption is valid since generally vc ≫ vf ​ ​ therefore the feed motion can be neglected with respect to the rotational motion. The distance between the successive passes of the cutting tool in the feed direction is crucial, and this distance is determined by the feed per tooth fz . However, when looking at the chip thickness, ​ things become more complex. As each tooth engages with the workpiece, the chip thickness varies throughout the arc of engagement. Initially, when the tool first makes contact, the chip thickness is small. As the tool moves through the material, the chip thickness increases before reducing again toward the end of the cut. ⚠️ A critical issue arises when the chip thickness becomes too small, particularly near the edge of the tool. When this happens, cutting becomes difficult because the material tends to resist being cut. Milling 4  This phenomenon occurs because the cutting pressure increases sharply when the material is too thin. To avoid these situations, it’s important to ensure that the chip thickness is neither too large nor too small. Proper control of feed rate and cutting parameters helps maintaining an optimal balance, ensuring efficient cutting without unnecessary wear on the tool or damage to the workpiece. Chip section area AD : when calculating the force generated during cutting, it is essential to ​ multiply the cutting pressure by the are of the chip being removed. This involves understanding the cross-sectional area of the chip. If the chip thickness is very small, the pressure may be high, but the actual cutting force is negligible because the chip area is so minimal. In these situations, no material is being removed; instead, the tool is merely pushing the material without cutting. Chip thickness hD : the chip thickness represents the radial distance between the cutting ​ passes of the tool. It is somehow liked with the feed per tooth, with the only difference that fz  ​ represents the axial distance between two subsequent passages whereas hD represent the ​ radial distance between two subsequent passages, therefore: hD = hD (θ) ​ ​ The chip thickness hD matches fz along the axial direction. ​ ​ Another important concept is the depth of cut, which can be defined in two ways: axial depth of cut ap : is measured along the axis of the tool. This remains true whether the tool ​ is positioned perpendicular to the workpiece or parallel to it. radial depth of cut ae : in cases where the mill is fully engaged with the workpiece (such as ​ when the tool is cutting a slot) the radial depth of cut must be carefully measured. When the mill is fully engaged with the material, the situation becomes more complex and delicate, particularly in full-slot cutting operations Milling 5  where the tool engages the workpiece along its entire width. Angles in milling In advanced milling applications, such as precision steel machining, the cutter’s geometry becomes crucial. The chip flow and how well the material is removed depend heavily on these geometric factors. First, the chip slides along what we call the rake face, the surface of the tool where the chip flows. The rake angle γ0 influences how easily the material flows over the rake face. ​ ✅ A positive rake angle facilitates smoother chip flow because the rake face offers less resistance. This can be compared to slicing butter with a knife: if the knife is inclined in the right direction, the cutter flows smoothly off the blade. The more the tool is inclined in this direction, the higher the wake angle, and the easier it becomes for the material to be cut. Conversely, if you hold the knife more perpendicularly to the butter and push against it, the resistance increases, and it takes more force to create the chip. In most of milling operations, especially with delicate materials, the rake angle is kept positive to ensure smooth material flow and to reduce the cutting force required. Up milling and Down milling In milling operations, two fundamental techniques are down milling and up milling. 1. Down milling, also known as climb milling, gets its name from the motion of the tool relative to the workpiece. Milling 6  Imagine the tool is pushing down as it engages the material, creating a downward force against the workpiece. This method is favourable for fixture stability since the downward force helps keep the part securely in place, reducing vibrations. The cutting tool engages the surface, and the chip thickness is already significant from the start. This can be a drawback with certain materials, such as ceramics, which are prone to breaking on initial contact due to the high forces involved. In peripheral milling, where the periphery of the tool is engaged, down milling can be advantageous. 2. Up milling, on the other hand, is also known as conventional milling, and works in the opposite direction. The tool engages the workpiece from zero thickness, meaning it starts cutting from the very edge of the material. ⚠️ While this technique may reduce initial forces, it can be more dangerous in terms of wear. The material tends to spring back, causing friction and increased wear on the cutting tool Additionally, up milling generates more heat, which can affect tool life and the quality of the machined surface. The choice between down and up milling should still consider the type of material, the rigidity of the setup, and the conditions of the cutting tool. Milling 7  In up milling, especially during roughing, if the radial engagement is too large, the forces acting on the cutter can be problematic. As the cutter rotate, it is attracter further into the material, potentially removing more material than intended. This could lead to errors in the subsequent passes, where there may be no material left to cut. ⚠️ Up milling in roughing operations is generally discouraged because of this force imbalance However, up milling can be beneficial in finishing operations, particularly in high-precision applications. In finishing, the material removal is minimal, so the forces acting on the cutter are less intense. While the tool may still be pulled slightly into the material, this can result in a smoother surface finish. Despite this, up milling still results in higher tool wear due to the nature of the forces involved. ✅ Down milling is particularly advantageous in roughing operations, as the forces generated help stabilize the process. Even though the forces acting on the cutter are strong, they push the cutter out of the material rather than pulling in it. Manufacturers generally recommend using down milling for most operations because it provides a balance between stability and material removal Milling 8  efficiency. This distinction also extends to the forces involved in milling. The primary cutting force, denoted by Fc acts ​ perpendicularly to the material surface and is responsible for the majority of material removal. This force is tangential, mush like the cutting speed vc . ​ When calculating the power required for the operation, we can approximate it by P = Fc ⋅ vc ​ ​ However, there are other forces at play, such as Fp , which is the force acting perpendicularly to the ​ cutting motion, and Ff , the feed force, which acts in the direction of the feed. ​ While Fp does not contribute much to the power consumption, Ff can play ​ ​ an important role, though it is usually less significant compared to Fc since ​ vc ≫ vf  ​ ​ The composition of forces also influences the choice between milling methods. For example, in down milling, the force composition FD is perpendicular to the cutting edge, which prevents the ​ tool from pushing down into the material. This is beneficial in situations where you’re machining a thin wall or flow (first case in the picture) Rounded cutting edge Let’s explore the situation with round inserts, which are somewhat different from the typical inserts discussed earlier. Since the insert is round, the straightforward decomposition of forces is not directly applicable. Instead, we use a different approach by drawing a line from the tangent point of the insert to its intersection with the surface being machined. This helps us understand how the force vectors, such as the vertical force Fd , ​ behave in this context. The inclination of Fd is influenced by the axial ​ depth of cut. As the depth changes, so does the inclination of Fd . ​ Milling 9  This means that the behaviour of the round insert can vary depending on the shape and size of the insert, offering some flexibility in how the forces are applied. However, it’s essential to exercise caution. When making a pass with a very small ap , the vertical ​ force can become quite significant, which may lead to potential issues. 💡 Rounded inserts are generally robust due to their shape, which provides a larger cross- sectional area of resistance. Lastly, in high-precision and micro-milling applications, material characteristics play a crucial role. When dealing with very small cuts or high-precision work, the material’s homogeneity becomes significant. Researchers have shown that materials with a different grain structure can pose challenges, as they may cause vibrations during the machining process. This is because the cutting tool may experience sudden changes as it moves through different grains or phases of the material, leading to instability and poor surface finish. 💡 For micro-milling, materials with fine, homogeneous grain structures are preferable. If the material is heterogeneous it is often beneficial to perform a thermal treatment to refine and homogenize the grains. This process ensures consistent material properties, which helps maintain stability and achieve better results during milling. Minimum chip thickness effect Let’s start examining a crucial aspect of milling operations: understanding the impact of tool geometry and chip thickness on the machining process. Milling 10  To illustrate this, consider a simplified model of the cutting process. In this model, we have the cutting speed, which is the main relative motion and is typically much higher than the feed speed. This picture shows also the clearance angle, which is the angle between the flank of the tool and the machined surface. This angle is significant because it influences how the material springs back after being cut. As the tool moves away, his energy causes the material to spring back, which can wear down the tool’s flank. In micro-milling, where chip thicknesses are extremely small, traditional models don’t always apply. For instance, in micro-milling, achieving a sharp cutting edge is nearly impossible. The roundness of the edge, even if minimal, affects how the tool interacts with the material. When the chip thickness is on the order or micrometres, the tool often uses only the rounded portion of its cutting edge. This can result in less effective material removal and increased tool wear. Additionally, in high-precision applications, the tool must maintain a chip thickness that is sufficiently large relative to the tool’s radius. Empirical guidelines suffect that the chip thickness should be at least: hD,min = 0.3 ÷ 0.4 ⋅ re ​ ​ to avoid issues such as excessive deformation without actual material removal. If the chip thickness is too small, the material might just be deformed rather than removed, leading to poor machining results. Milling 11  Another important effect in micro-machining operations is that the rake angle does not depend only on the geometry of the tool. The effective rake angle defines the fact that the chip is formed and slides on the tool rounding instead of sliding on the rake face. The nominal rake angle γ does not play a role anymore and its role is taken by γeff . ​ ⚠️ In micro-milling application, the effective rake angle γeff gets highly negative, negatively ​ effect the chip sliding. In this case ploughing forces become more relevant than shearing forces. Milling strategies One key difference in micromilling compared to conventional milling is the size effect. At the microscale, materials often display non-homogeneous behaviour due to the presence of defects and the scale of the material’s grain structure. For this reason, in micro-milling it’s crucial to recognize that the material may exhibit different properties than expected because defects, or their absence, significantly influence material strength. Another challenge in micromilling is that achieving a perfectly sharp tool edge is virtually impossible. Tools at this scale always have some degree of rounding at the cutting edge. ⚠️ In micromilling, the cutting action transitions from shearing, which is typical in larger-scale milling, to more of a compressive action. This shift is significant because it explains why micromilling is particularly effective for machining brittle materials like glass or ceramics. When machining brittle materials the use of compression rather than shearing helps in creating a continuous chip, which is crucial for effective material removal. Let’s delve into the complex interactions that occur during micromilling, especially when it involves curved paths and the effect of rotation on feed rates. Milling 12  Consider a milling operation where the tool is moving along a curved path. As the milling tool approaches a turn or curve, it must navigate around the center of the curve to create a precise feature. During this process, the feed rate changes along different points of the curve, which affects the chip thickness and the cutting forces. In a curved path, the feed rate varies depending on the radius of the curve. The machine tool typically only knows the center of the mill and does not account for the diameter of the tool itself. ⚠️ If the machine is set based on standard parameters without adjusting for the curve, the actual feed rate could exceed the tool’s capacity, leading to excessive cutting forces and potential tool breakage. To solve this, you need to compute the feed rate adjustment based on the radius of the trajectory and the tool’s diameter. An important parameter is the difference between the workpiece radius and the tool radius: Dvf = Dm − Dcap ​ ​ ​ If the tool diameter Dcap is too close to the diameter or the curve path Dm  ​ ​ this parameter tends to zero, and the tool center has to follow a curve with a too small radius. ⚠️ If the mill diameter is similar to the diameter of the feature to machine, the peripheral feed rate is too high respect to the suggested one, and this has serious consequences on the tool, on the mill or on the workpiece. In order to solve this issue, we have to set the suggested fz at the tool periphery. ​ The feed rate of the mill axis will be therefore: Dvf vf = vf ,m ​ ​ ​ ​ Dm ​ where vf ,m is the feed rate at the periphery which satisfies the fz constraints. ​ ​ Milling 13  When analysing the effects of milling, it’s crucial to consider the engaged arc, especially during turns. In fact, when ae is kept constant along the tool path, as it happened with traditional ​ machining strategies, the engagement angle varies at the curves, as shown in the figure: In principles, as the mill diameter approaches the diameter of the curve to manufacture, things get more and more complicated. Initially, one may think that if only one cutter is in contact, the engaged arc wouldn’t significantly impact the force or torque because it doesn’t directly affect the cutter’s contact force. However, it does affect the cutting time. A larger engaged arc means the cutter engages with the material for a longer period, which can lead to more significant forces and torque due to the increased length of engagement. Modern CAM systems address these issues by compensating for the engaged arc. For instance, during a turn, the system can adjust the axial depth to keep the engaged arc constant, ensuring the cutter performs as if it were moving in a straight line. This is done to maintain consistent forces and avoid excessive strain on the cutter. Another important point is to avoid selecting a milling tool whose radius is very close to the radius of the turn or pocket. ⚠️ If the tool’s radius matches the radius of the pocket, the cutter may have to handle extreme variations in chip thickness, leading to increased wear or even damage. Therefore: Milling 14  It is better to choose a milling tool with a smaller radius relative to the pocket radius. Let’s now discuss an important equation related to ball end mills. This shape allows for a wide range of functionalities. The main characteristic is that the cutting speed vc depends on the ​ axial depth of cut ap . ​ We define Dc as the diameter of the cylindrical ​ tool, and Dcap as the diameter of the cutting ​ tool engaged inside the workpiece These two diameters can be related by: Dcap = 2 ​ ap (Dc − ap ) ​ ​ ​ ​ Even though it’s common to assume that the standard configuration is the most effective, it’s not always the case. One significant issue arises when we consider the behaviour of the cutting tool at its center. Since the center has no radius, the tangential cutting speed is zero at that point. This creates a problem because the tool struggles to form chips, as there’s no tangential speed to drive the cutting process. One option is to tilt the axis of the tool. Milling 15  By doing so, the center becomes less involved in the cutting, preserving it from wear. However, even with tilting, you must carefully compensate for changes in velocity or decide whether compensation is necessary. In this case: Dcap = Dc sin(θ + δ) ​ ​ where θ is the inclination angle and 2ap cos(δ) = 1 − Dc​ ⚠️ As θincreases, the effective cutting radius grows. At this point, the cutting speed vc  ​ becomes critical. If the tool’s effective radius increases, then the maximum tangential speed increases as well. This creates excessive wear on the tool unless properly compensated for. There’s also a modern milling strategy known as the barrel method. This approach uses a tool with a small radius at its tip and a large radius along its side, allowing for a smoother cutting with fewer passes. Productivity When purchasing a machine, it’s crucial to examine its specifications, particularly the characteristic curve of the spindle. In the case of machine tools, particularly in micromachining, torque and power are critical parameters. Milling 16  Let’s look at a common example. In this diagram, the red curve represent the relationship between speed and power, while the blue curve shows the relationship between speed and torque. These curves are typical for machine tools. Notably, the torque curve remains almost flat, meaning that the machine can deliver consistent torque across a wide range of speeds. This characteristic is advantageous because it allows the machine to maintain performance even as the spindle speed varies. You may have noticed multiple curves in this diagram, labelled as S1 and S3. There represent different service modes: S1 is continuous service, meaning the machine runs at constant speed without stopping. In this mode, the system must deal with the formation of heat in the spindle, which requires cooling systems to manage. S3 is intermittent service. According to standards, this might mean running the machine for 10 minutes and then pausing for 25% of the time. This allows the machine to handle temporary peaks in torque or power without overheating or damaging the motor. 💡 In intermittent service the machine provides a higher torque for short periods, but in continuous service, it must operate at a lower level to avoid overheating. The same logic applies to power: the higher curve corresponds to intermittent service, while the lower curve represents continuous operation. This is important because, during machining, there are moments when you need extra torque or power, and your machine should be able to handle these peaks. Milling 17  This diagram also shows the relationship between torque and power as a function of spindle speed. As the spindle speed increases, the power output also increases because power is the product of torque and speed. This is a key factor to keep in mind when interpreting the machine’s capability from a catalogue. Often, catalogues list the maximum power output, but this is not the power available at all operating speed. This lead to a critical point regarding tool selection. When choosing a tool, such as a mill from a catalogue, the software will often recommend a range of suitable mills based on the material and task at hand. However, it’s essential to verify these recommendations against the machine’s actual power and torque curves to ensure that the tool can perform as expected. To calculate the power requirement we can use the following formula: MRR kc,m ⋅vf ⋅ ap ⋅ ae Pc = ​ ​ ​ ​ ​ 600 ⋅ 1000 ⋅ η ​ ​ In some cases, you may select a large mill for cutting a big pocket, but upon checking the power available at the required spindle speed, you realize that it’s too low. 💡 Generally, the smaller the mill, the higher the power available since the spindle speed is generally higher. Given a suggested cutting velocity vc , we can recall the following formula: ​ 1000 ⋅ vc n= ​ ​ πDc ​ Therefore, the smaller the mill, the higher the spindle speed, the higher the power available. For high-precision machining, the feed rate vf is a critical factor in determining productivity. ​ A higher feed velocity allows you to complete a machining path more quickly, which increases the number of parts you can produce. ✅ In micromachining, the feed rate is tied to the rotational speed of the spindle and the design of the tool. By running the machine faster, you can improve productivity without compromising accuracy (balance of chord error) Milling 18  Force modelling in milling Generally, chip removal processes require higher pressure compared to forming processes, which are more energy-efficient. Despite this, we still use chip removal when precision is essential, particularly for parts with tight tolerances. In modelling the force, there is a key parameter: kc1 [ ] N mm2 ​ ​ which comes from material catalogues. This parameter represent the reference cutting pressure. Once you have: kc1 : the reference cutting pressure; ​ hc : the chip thickness; ​ x: a tool material coefficient obtained from catalogues; you can calculate the cutting pressure: kc = kc1 h−x ​ c ​ ​ Multiplying the cutting pressure by the chip section area: 1−x Fc = kc ⋅ Ad = kc ⋅ (hc ap ) = kc ap ⋅ (fz sin θ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ hc ​ gives a full representation of the forces at play. To decompose the force along the machine’s axis, you must consider the machine’s capacity to handle loads. Load cells are often used to measure the forces acting along different axes. By decomposing the force into components, we can verify the model and ensure it align with the actual forces measured during process. Milling 19  To calculate the chip section area, we need to fine the relationship between the chip thickness and other variables, like θ Despite the oscillating nature of the force, we typically calculate the average power in our equations, not considering there fluctuations. For practical purposes, however, we need to dimension machines to handle maximum force, particularly during up-milling operations. ex (1 − ) γ0 hex = fz sin(φ) → kc,ex = kc1 h−x ​ 100 ​ ​ ​ ​ ​ ​ Fc,max = kc,ex Ad = kc,ex hex ap ​ ​ ​ ​ ​ ​ When calculating the force, we also account for rake angle, which can affect the cutting pressure. ✅ As γ0 increases, the material becomes softer and the cutting pressure decreases ​ accordingly. Note that a negative rake angle increases the required force. If the tool bends during machining, it introduces an error on the workpiece. When working within tight tolerances, such as those required for precision components, any deformation or bending becomes problematic. The following equation will help us address this issue by allowing us to calculate the amount of tool deflection: 1 Fc L3 d= ​ 3 EJ ​ ​ where: πDc4 J= ​ 32 ​ is the moment of inertia of the tool. Milling 20  In practice, we should also consider that the milling tool isn’t a solid bar but has an empty core. This makes it less rigid than a solid bar, meaning we should adjust our calculations to account for the reduced rigidity. The length of the milling tool plays a crucial role. Keeping the tool as short as possible will minimize bending and ensure more accurate machining. ⚠️ The length’s effect is significant because it acts at the cubit power, making even slight increases in length greatly magnify the bending issue. Milling 21  Micromilling Size effect ⚠️ One key issue is that the material cannot always be considered homogeneous at smaller scales. This inconsistency arises unless the material has been specifically treated, such as through grain refinement or other techniques. In micromachining, the parameters involved are so small that it becomes challenging to form a chip. This is due to the limited forces that can be applied, which leads to very thin chip formation. However, when the chip thickness becomes comparable to the radius of the cutting edge, the material behaves as though the rake angle is very negative. As a result, when the chip thickness is extremely small, the material primarily undergoes elastic deformation. After the toll passes, the material experiences full elastic recovery, leaving no permanent deformation. If the chip thickness increases slightly but still remains below the minimum chip thickness, a different effect occurs. There is combination of elastic and plastic deformation, meaning the tool begins to stress the material, introducing residual stresses. Part of the energy is stored elastically, resulting in partial recovery. However, not all the material returns to its original shape, leaving behind a slight deformation. Once the chip thickness surpasses the minimum value, true chip formation occurs. Ideally, we want to operate in this range. However, a major difficulty arises because most micro- milling tools or precision turning inserts have a rounded cutting edge. ⚠️ This rounding can be on the scale of several microns, typically around 5 microns, which becomes problematic if you are working with a chip thickness of 1 micron. In such cases, achieving efficient material removal is extremely challenging. Micromilling 1  It is also important to note that the minimum chip thickness is typically around 30% of the cutting edge radius. Therefore, the first step when selecting a tool is to verify the value of the cutting edge radius. 💎 Tools designed for high-precision operations often have extremely sharp cutting edges, sometimes as small as 3 or 4 microns. For even finer work, diamond-coated tools may be required to achieve very smooth surfaces. Additionally verification steps are needed to be taken in machining processes. In milling, the chip thickness can be divided into slices. Each of these slices contributes to the total force exerted during the cutting process. For instance, if we look at the last slide (3), we can observe that the chip thickness material flows along the rake face of the tool, utilizing the designed rake angle. This is the nominal rake angle intended for the tool. However, for slices closer to the cutting edge (1 and 2), the material does not flow according to the nominal rake angle. Instead, it follows the “effective rake angle”, which can be significantly smaller or even negative. This is a crucial consideration when analysing the forces involved in milling. 📖 Several models have been developed to calculate the forces exerted during milling operations. These models work by dividing the chip thickness into slices, calculating the force for each slice with a proper rake angle, and then integrating the results to predict the total force acting on the tool. ❓ Why is it important to put so much effort into modelling the forces in machining? The reason is quite straightforward. The forces involved in chip removal processes provide critical insights into what is happening during machining. These forces represent the interaction between the tool and the workpiece, and from them, we can derive the energy required for the process. By understanding the forces, we can calculate how much energy the machining operation demands from the electrical network. Essentially, the forces govern every aspect of the process. Micromilling 2  Additionally, the forces can be used for monitoring purposes. If something goes wrong, such as the tool becoming dull or breaking, the forces provide an immediate indication. Even with very small tools, such as those with diameters as small as 50 microns, the force measurement can detect whether the tool is machining effectively. A load cell can register changes in force, allowing the system to stop the operation if a tool breaks or make decisions based on real-time data. Built-up edge In manufacturing and chip removal processes, particularly in relation to the nominal rake angle and tool behaviour, it’s essential to understand how the material and tool interact. Imagine the material remains fixed while the tool moves to the right. Alternatively, you can picture the tool as stationary, with the material flowing towards it at the same cutting speed. What matters here is the relative speed between the two. As the tool moves, material slides against it, forming a built-up-edge. 💡 BUE occurs when layers of material deposit on the rake face of the tool due to the high pressures in the cutting zone. The material sticks to the tool, rather than sliding off, because the conditions prevent easy movement. Over time, the BUE grows as layers accumulate. This increases the cutting resistance until, at some point, the force causes the BUE to break off. When BUE breaks, portions of materials may remains on the finished part, which becomes an issue because these fragments are brittle and harder than the base material. This is a result of high plastic deformation in the BUE. If you were to make a second pass with the tool, the increased hardness of these fragments could damage it. In standard machining, the first step to remove the BUE is often to address the strain hardening effect that causes it. One counterintuitive solution is to increase the cutting speed. Micromilling 3  💡 Increasing the cutting speed is used in macromachining for increasing the working temperature and reducing the strain hardening effect, which is at the base of the BUE formation. Interestingly, in micromachining the BUE tends to stabilize. In this context, it’s almost as if there is a cap sitting on the top of the cutting edge, and it stays consistent throughout the process. Extensive research, including simulations, has shown that unlike in macromachining, where the BUE is always something to avoid, in micromachining it is nearly impossible to prevent it. The cutting edge is usually round, meaning it interacts with the material differently, leading to a small but stable BUE. The stable BUE brings to a new model for the chip formation in microcutting called slip-line field model. This model assumes the presence of a dead metal cap at the edge rounding that can be thought as the dummy zone in hot metal extrusion, i.e. a stagnation region naturally created by the target material itself to find a easier and stable way to deform. This model substitutes the classical Merchant model that is based on the hypothesis of perfectly sharp cutting edge. The material flow is divided in two parts, one is dominated by shearing and flows on the side of the dead metal cap that is in line with the nominal rake face of the tool and the other flows beneath the dead metal cap and is dominated by ploughing. Micromilling 4  Molecular dynamic simulation, though not widely used, offers a fascinating way to study the behaviour of materials on a very small scale. This simulation focuses on tiny portions of material and models the interactions between atoms, accounting for forces such as electrostatic interactions. It provides highly detailed insights into how materials behave at the atomic level, allowing research to simulate actions and predict outcomes with great precision. In these simulations, you’ll often see denser nodes along certain lines, which are known as slip lines. These lines are crucial because they represent where portion of the material, particularly the chip, flow and shear. By tracking these slip lines, we can understand the behaviour of the material during cutting or machining processes. When the chip thickness is reduced to 20% of the cutting edge radius or less, the simulation shows a stable BUE that divides the material flow into two parts. This phenomenon, which leads to a negative effective rake angle, explains the difficulty in forming a proper chip when the chip thickness is tool small. As chip thickness increases relative to the radius of the cutting edge, the simulation reveals a significant change. The BUE remains, but now the effective rake angle becomes positive, making it easier to form the chip. When the thickness reaches 0.9 the process becomes even more efficient, though the BUE still exists. Variation of the cutting force In micromachining, the common models able to predict the cutting force are not enough in microcutting as Fc is not the main component anymore as, when the chip thickness is low, the feed ​ force Ff dominates the chip formation. ​ Micromilling 5  ⚠️ In micromachining, the forces involved change, and ploughing becomes dominant, especially in milling processes. In these cases, the chip thickness at the edges is often below the minimum threshold, making ploughing unavoidable. If we look deeper into the mechanics of cutting, it becomes evident that the cutting force, which acts along the cutting speed direction, behaves differently than the feed force. The cutting force increases steadily as the chip thickness increases. However, the feed force, which results from ploughing, behaves differently. 💡 Below the minimum chip thickness, the feed force spikes because the material is being ploughed without forming a chip. Once the chip thickness exceeds the minimum threshold, the feed force begins to decrease and eventually follows the same trend as the cutting force. As the chip thickness increases, the cutting force returns to be the main component of the force, reflecting the macromachining behaviour. Micromilling 6  Finally, when looking at the cutting and feed forces over multiple revolutions of the tool, the simulation confirms that, once a stable value is reached, the forces become consistent across each revolution. The feed force remains stable when the correct chip thickness is chosen. If the chip thickness is too small, the mill struggles to remove material, resulting in an accumulation of elastic deformation. As the mill continues to pass over the material without removing it, the elastic deformation builds up, and eventually, after several revolutions, the force becomes sufficient to create a chip. However, if the chip thickness is too small, the mill behaves like a spring, bending as it moves forward until it finally recovers its elastic deformation and removes the material. This results in a fluctuating force, causing the mill to vibrate along the feed direction. These fluctuations can cause fatigue in the tool, leading to premature failure. ⚠️ As the tool moves, the material build up, and once the chip is finally formed, it is much thicker than expected. Choosing a too small chip thickness lead to chips that are thicker than expected. This not only results in poor surface finish due to vibrations but also causes tool wear and eventual breakage. Micromilling 7  To avoid these issues, it’s essential to select a feed rate that is not only above the minimum chip thickness but significantly higher. Other typical issues in microcutting Mechanical deformation: everything involved in the process deforms to some degree, including the tool and the fixture walls. For those, mechanical deformation plays significant role, as the precision is critical and tolerances are often within a few microns. Thermal deformation: another factor to consider is thermal deformation. While thermal effects from the tool are typically localized, the environment in which the machining takes place can have a much greater impact. For instance, if you are working on a large part in an environment without temperature control, thermal expansion or contraction can lead to significant deviations in the final product. Surface integrity: although we can scale down the cutting process, we can’t scale down the radius of the cutting edge. This creates challenges in achieving the desired surface finish. For example, with diamond tools, the surface may appear needle-like due to the extremely small rounding of around 1 micron. This allows for exceptional surface roughness, as the cutting edge creates very fine, closely spaced striations that result in a smooth, mirror-like finish. Reference: another consideration is the reference system. In larger machines, positioning the part against a precise structure is relatively straightforward. However, when working with very small parts, positioning becomes much more difficult, and achieving micron-level accuracy is nearly impossible. In such cases, the best strategy is to avoid removing the part from the machine during the process. 💡 Using a five-axis machine allows you to position the part once and machine all necessary surfaces without changing its orientation, minimizing errors. Another common challenge in micromachining is the appearance of defects along the edges of the part, particularly burrs. Burrs form when material is pushed or clogged around the edge during cutting, resulting in edges that are not clean or sharp. These burrs can be difficult to remove, and in some cases, innovative solutions have been developed. For example, some people use CO2 as a lubricant, which can modify the properties of the material, making it more brittle and easier to remove burrs. Chip clogging: another effect on the process comes from the tool itself, especially when it is small and slender. Micromilling 8  Thin tools are more prone to problems such as chip clogging, which occurs when the chips produced during cutting are not evacuated properly. When chips accumulate inside the tool, the machine continues to advance, but the tool eventually breaks because of the excess material trapped in the flutes. Even if the tool doesn’t break, chip clogging can cause the hole diameter to deviate, leading to dimensional inaccuracies. Runout: runout is another issue that affects the precision of the machining process. Runout occurs when the tool doesn’t rotate precisely around its intended axis, resulting in inconsistent cutting. Runout can stem from three primary sources: the tool itself may have intrinsic errors due to poor manufacturing the tool holder may not hold the tool concentrically, causing it to rotate off-center the spindle, which rotates the tool, may already have runout. Managing runout is crucial, especially when working with very small tolerances. It is also important to invest in high-quality tool holders. Often, people are willing to spend money on expensive machines but try to cut costs on tool holders or tools, which can significantly impact the overall machining quality. This is counterproductive, as the tool holder and tool are just as crucial for precision as the machine itself. If runout goes undetected, the tool may start cutting in a misaligned configuration. For example, one cutting edge may be more exposed to the material than the other. In extreme cases, one cutting edge may not even be engaged with the material, which doubles the feed on the other edge. This can lead to tool breakage, or at the very least, reduced cutting efficiency 🔍 We can detect runout by monitoring the cutting forces. If the force peaks associated with one cutter are significantly higher than those for the other, it’s a clean indication of runout. Chord error Let’s now discuss the final point, which deals with the accuracy of our machine. Machining aims at achieving two main objectives: speed and precision. These two goals, however, often conflict with each other. The best parameter used to quantity productivity is the material removal rate Micromilling 9  MRR = vf ⋅ ap ⋅ ae ​ ​ ​ However, maximizing RR has its limitations. While we might want to push for higher productivity by increasing feed rates, spindle speed, or depth of cut, we need to be careful. There are inherent constraints in micromachining. For instance, we know that the feed per tooth fz cannot be too small because it must be at least 30% of the tool’s cutting edge radius. This means that there is a lower limit we cannot cross. On the other hand, it we exaggerate and push too much, the forces involved increases. This can cause bending or even breakage of the tool or workpiece, leading to damage or defects. Now, let’s focus on the machining process itself. Machining starts with a CAD model. Once the design is ready, it’s passed to the CAM (Computed- Aided Manufacturing) system to develop the machining strategy, including tool selection, toolpath, and cutting parameters. The CAM system generates a file with a list of instructions, which is post- processed to convert it into a format that the CNC (Computer Numerical Control) machine can interpret. When the program is loaded onto the CNC machine, the machine interprets the positions it needs to move through to cut the part. These positions are represented as discrete points, known as “control points” (Desired Tool Cutter Locations). The CNC machine doesn’t just move from one point to the next in a straight line. The CNC machine interpolates between these points to create smooth curves, which represent to toolpath. This interpolation introduces a small error called the interpolation error, as the machine cannot perfectly follow the discrete control points. Micromilling 10  This process is similar to navigating a race track. If the track is narrow, it forces the car to slow down, especially around sharp turns. The same principle applies to CNC machining: if the tolerance is tight, the machine needs to slow down to stay within the tolerance, especially in complex toolpaths. ✅ The CNC interpolator does its best to satisfy the machining constraints (accuracy, acceleration and jerk) while maintaining the desired productivity. This is why the toolpath does not always pass precisely through each control point, but instead follows a smooth, optimized path that balances speed and precision. However, there is another problem associated to CNC, which is related to its sampling time. Sampling time refers to the period between two consecutive measurements of the machine’s position. Every CNC machine continuously monitors the position of each axis by acquiring data from sensors, which help determine the exact location of the tool or workpiece. These acquisitions happen at intervals, and the time between them is called the sampling time Ts . ​ To illustrate this, imagine the machine starts at a point (marked as a black dot). The CNC system knows the tool’s position at that moment. After one sampling time, the machine may have moved to a new position. However, the machine might not be precisely where it should be on the ideal yellow path due to external factors such as cutting forces, which push the tool away from the planned trajectory. To correct for this, the CNC machine frequently checks its position and adjust the tool’s path. At each new position, the CNC calculates where the tool should be next. The distance between points along the path depends on the feed rate and the sampling time. Micromilling 11  ⚠️ If the feed rate is higher, the machine covers more distance between consecutive points within the same sampling time. This brings us to the concept of chord error, which is the deviation between the actual path and the intended one. If the machine moves too fast, the chord error increases, meaning the tool deviates further from the ideal path. To reduce the chord error, the solution is simple: reduce the feed rate. By slowing down, the machine follows a more precise path and minimizes deviations. This is particularly important when dealing with tight curves or intricate features, where a high feed rate would otherwise cause excessive errors. This situation is especially challenging in micromachining. In traditional macromachining, the size of the tools can be relatively small compared to the features being machined, and the tool’s diameter can often be neglected in calculations. In micromachining, however, the tools are much smaller, but so are the features being produced. There’s a limit to how small tools can be made. In these cases, the ratio between the tool size and the feature size becomes critical. When the tool is tool large relative to the feature, it’s impossible to machine certain details. To calculate the appropriate feed rate, we consider not only the radius of the curvature of the desired path, denoted as ρ, but also the tools radius (or diameter) denoted as Dc . ​ Micromilling 12  In micromachining, the feed rates are generally slower than in macromachining. This is due to the smaller tool sizes and tighter feature tolerances, which impose stricter constraints on the maximum allowable feed rate. As we can notice, sampling time is crucial in CNC machining. ✅ Machines with a faster sampling time can more frequently update the tool’s position, effectively reducing chord error We have seen that there are two types of errors to consider: interpolation and chord errors. While interpolation errors may seem more significant, in machining chord errors typically have a more direct impact on the final product. Using complex interpolation methods can slow down the machine, potentially leading to longer sampling times and reduced accuracy. Therefore: ✅ It is often better to use simpler interpolation methods, such as linear and circular interpolation, which are faster and allow the machine to operate at a lower sampling time The key takeaway is that maintaining a fast sampling time is more important than achieving extremely high levels of interpolation accuracy. In cases where machining errors occur, such as polygonal surface instead of a smooth curve, the cause is often related to the sampling time and feed rate. To verify this, one can calculate the distance the machine travels in one sampling period by multiplying the feed rate by the sampling time. If the calculated distance matches the error observed on the part, it is likely that the issue lies with the machine’s inability to handle the given feed rate at the current sampling time. Micromilling applications When we talk about manufacturing parts, it’s crucial to remember that our role as technologists is to meet the functional requirements of the designs. This often involves dealing with challenging aspects, such as ensuring the accuracy of parts and handling complex geometries. One typical example is the manufacturing of thin walls. Micromilling 13  Thin walls are prevalent in different context. For instance, in computer heat exchangers, thin walls are used to transfer heat through convection. Similarly, in turbine blades are designed with thin walls to manage their complex shapes and thermal loads. Generally, the thinness of these components is emphasized by their high aspect ratios, which is critical for both machining and mechanical performances. The aspect ratio influences the part’s susceptibility to bending under force. ✅ If the aspect ratio is high, techniques like EDM are often preferred, as they do not rely on physical forces, which can be problematic with conventional machining methods. Thin-walled components are complex due to their intricate geometries. These parts often require precise roughing operations and careful consideration on manufacturing processes. To illustrate, our study involves creating a specimen with varying nominal thicknesses and dimensions to examine these challenges in detail. In manufacturing such specimens, it’s crucial to ensure accuracy through onboard machining. For instance, by machining both sides of a part on the same machine, you reduce potential discrepancies. Even then, some errors may arise when the part is repositioned, therefore: it’s ideal to machine as much as possible in a single setup to maintain precision. Moreover, achieving high accuracy requires not only precise machining but also meticulous measurements. Micromilling 14  In machining thin walls, different strategies are used depending on the complexity and thickness of the material. For standard, thicker walls, you can machine one side, flip the material, and machine the other side without significant issues. However, as the material becomes thinner, the approach needs adjustment. One effective strategy is the “waterline” method, where you alternate passes on both sides of the wall. This approach minimized the risk of deformation by distributing the machining forces evenly. For more even challenging scenarios, you might use a method where the first step involves machining with one part of the setup, and then the second step involves machining with a different part. This method adds rigidity to the process, as the material is better supported during machining, reducing deformation. Results from experiments shows that thicker walls tend to have less deformation and better flatness. Additionally, up-milling often result in better performance compared to down-milling, especially in finishing operations, in line with theoretical expectations. The second relevant featured reported in this section are thin pins. A typical strategy used in their production involves a “corridor” technique, where the machining process surrounds the pin to maintain precision. Initially, the beam is machined with a larger diameter, allowing for consistent material removal during the finishing operation. Micromilling 15  This ensures that the same amount of material is removed, achieving the desired final dimensions. Three factors were identified as essential for determining the pin’s quality: error in diameter aspect ratio straightness The experiments showed that larger milling tools attracted the pin during machining, causing deformation. ⚠️ Despite the use of a larger tool to increase stiffness and reduce deformation, the forces applied during milling distorted the pin, proving the importance of selecting the right machining parameters. Micromilling 16  Machining advanced materials In this section we’re addressing a crucial topic in the industrial field, the evolution of materials and how milling remains flexible enough to adapt to this progression by incorporating new tools and strategies. This is particularly important because industries are increasingly working with advanced materials like Inconel, which exhibit exceptional mechanical performance. Inconel One of the standout properties of Inconel is its high mechanical strength, even at elevated temperatures, making it ideal for applications such as energy-producing turbines and aircraft engines. This material is highly resistant to deformation caused by heat, a characteristic that is crucial for components like turbine blades, which must maintain their structural integrity under significant thermal and mechanical stresses. This resistance to creep (gradual deformation under prolonged stress at high temperature) is one of the key performance criteria for materials used in these demanding applications. Inconel’s performance extends beyond aerospace. It’s also employed in oil and gas applications, particularly in high-temperature valves and systems handling extreme heat. 📖 The material’s composition, primarily a nickel-based alloy with chromium and small amounts of iron, is what gives it these impressive characteristics. Machining advanced materials 1  However, with high-performance materials like Inconel, manufacturing becomes a challenge. The difficulty lies in its hardening behaviour during machining. As the material undergoes strain, its hardness increases dramatically, sometimes doubling after just one machining pass. This means that the parameters used in one machining pass must be adjusted for subsequent passes, often requiring tool changes to handle the increased hardness. When it comes to machining Inconel, there are two primary approaches: using traditional carbide tools, but due to Inconel’s properties, this requires very slow cutting speeds (around 30m/min). In comparison, when machining materials like steel, cutting speeds can reach 300m/min, and for aluminium up to 3000 m/min. The stark contrast highlights just how challenging it is to machine Inconel with conventional tools. using ceramic inserts. Although ceramics are brittle, they can withstand the high temperatures encountered while machining Inconel, particularly when the material softens under heat. This allows for faster machining, although there are trade-offs between the durability of the tool and the speed of the operation. Although solid ceramic mills are not widespread in the market, they have been used in specific cases. 💎 The key difference lies in the cutting speed: ceramic tools operate at much higher speeds, allowing them to soften the material as they machine it. This ability makes ceramic particularly useful in applications where fast material removal is necessary, despite the potential need for finishing operation afterward. Machining advanced materials 2  At the top of this graph, we can see different types of Inconel and alternative materials. To enhance Inconel’s properties, a treatment known as “aging” is applied, which plays a crucial role in its performance. One of the major challenges when machining these materials is their tendency to adhere to the rake face of the tool, which prevents the chip from breaking cleanly. This result in heat accumulation on the tool, significantly increasing tool wear. When using carbide inserts of solid carbide mills, the feed rate is kept low to maintain stability, particularly in delicate applications like turbine machining, where the materials are already pushed to their limits. With low cutting velocity, the rotational speed must also be reduced. This results in slow machining operations. The feed rate must be adjusted accordingly. This approach contrasts with ceramic tools, which operate at much higher cutting velocities, up to 1000 m/min. Because ceramic tools rotate at high speeds, the feed rate increases significantly. However, this comes with challenges. The torque and power required by the spindle increase substantially, especially during roughing operations with ceramic inserts. Machining advanced materials 3  This table highlights the key differences between ceramics and carbide tools, with the most significant contrast being in cutting speed. This speed difference is the most critical factor in their comparison, although there are other differences that are less important. ⚠️ One crucial aspect when using ceramic insets or mill is that lubricants and coolants cannot be used. The thermal shock would be too severe, causing the ceramic inserts to crack or break. As a result, ceramic machining is typically a dry operation. In fact, the heat generated is so intense that it can turn the material into droplets as it reaches its melting point during machining. Milling has an advantage in this scenario. Since milling is an intermittent cutting process, the cutters are not continuously in contact with the material, as it happens in turning. This allows the tool to cool down during each back pass. Machining advanced materials 4  This graph provides a comparison and helps determine when to use ceramic or carbide tools, serving as a map to guide tools selection based on cutting temperature. When the cutting temperature is low, due to a lower cutting speed, carbide inserts are typically used. This places the operation in a more conservative zone on the left side of the graph, where materials like Inconel are machined in their increased working field, where they perform best. However, the machining parameters are slower as a result. In certain cases, such as the example of a turbine, slower machining with carbine tools is advantageous. 💡 When working with thin-walled parts, preserving the geometry is essential. By avoiding excessive thermal stress, even if the operation is slower, the material’s integrity is maintained. Inconel’s resistance to deformation ensures that the part retains its shape, even under minimal thermal stress. On the other hand, using ceramic tools on the right side of the graph allows for much faster cutting speeds. However, the downside is that higher temperatures can lead to part deformation, requiring additional passes to correct the material. This graph emphasize the properties of ceramic inserts to withstand high temperatures. Ceramics, unlike tougher materials, are fragile, making smooth machining operations essential. To optimize the use of ceramic mills, careful too