During machining, as the tool plunges into the workpiece, the metal in front of it is compressed. When it exceeds the strength limit, the metal separates from the workpiece and separates as a chip. The shear plane rises upward from the attachment in front of the tool. The value of the cutting angle depends on the type of material and the cutting conditions (tool inclination, speed of rotation, etc.). The smaller the angle, the longer the cutting path, resulting in a thicker chip and higher cutting force, and vice versa. Secondary shearing occurs due to friction as the chip moves along the face. Resistance increases the temperature of the cutting process, causing overheating of the derived materials of the CNC process. In this article, we will try to give you some general knowledge useful to start our adventure, but it is worth mentioning that this is only the tip of the iceberg, and the field of CNC itself is quite extensive and undoubtedly very interesting.
Types of chips used in CNC milling machines
Below are some common categories of chips used in CNC tools:
Discontinuous chips
Discontinuous chips have a non-uniform shape and are usually deformed by multiple cracks. Workpieces made of hard and brittle metals such as cast iron, brass, and bronze have been known to produce discrete, small chips. They can also be generated from very tough workpieces where the friction between the workpiece and the tool is high. The production of discontinuous chips is caused by low feed rate, high cutting speed, low rake angle, deep cutting of the material, and so on. Their discontinuous formation on brittle materials improves surface finish and reduces energy consumption. However, at the same time, when machining ductile workpieces, it causes poor machining of the field we are developing and increases the duration of the entire process.
Continuous chips
Also called ribbon chips, they are homogeneous and have no cracks or segments, which makes them by all means unique. Moreover, they are interconnected to form a long coil and are obtained when cutting plastic materials such as aluminum, mild steel, mild steel, etc. They are characterized by having small notches on the top side, while the underside is smooth and shiny. Continuous chips are formed when machining parts made of hard materials. High cutting speed, high rake angle, low depth of cut, low material share, and low friction are other factors that contribute to the formation of continuous chips. Generally, this type of waste material also occurs with the use of lubricants or coolants and the use of a sharp cutting edge. The ribbon filings discussed in this paragraph provide a clean surface finish, longer tool life, and less power consumption. On the other hand, removing these types of chips presents a challenge to the operator. To improve the disposal conditions, chip breakers are necessary.
Heterogeneous chips
Heterogeneous chips, also known as serrated chips, are semi-continuous. They appear saw-like due to low shear deformation and high notch deformation areas. They are typically made of materials with low thermal conductivity or thermally softened mechanical strength. Titanium alloys, nickel, and austenitic stainless steels are examples of workpiece raw materials that can form uneven chips during machining. One reason for this type of filing is the large deformation of the tool chip surface when cutting hard materials at moderate cutting speeds.
Continuous chips with built-up edges
These chips are very similar to the continuous chips discussed above but are not as smooth as them. They are mainly formed when cutting hard materials that are processed at high temperatures and speeds. In this case, the friction created between the tool and the workpiece is too great. Therefore, the chips stick to the edges of the machine tool, and when these chips overlap in successive layers, a build-up is created on the edge of the machine being used. During the cutting process, the dimensions of these build-up edges are constantly changing. High temperature and pressure between the tool and the workpiece, the use of improper coolant, high friction on the surface of the machine, and the machining of hard plastics are some of the most common reasons for the formation of this type of swarf.
Insert properties
The machining of ductile metals, such as steel, with high cutting speeds and large rake angles, results in long chips. The swarf generated at elevated temperatures and continuous high speeds can compromise the safety of machine tool operators, damage products through entanglement in tools, and make swarf removal difficult. Chips can fall off spontaneously or as a result of a forced fracture. When machining plastics, waste materials tend to curl up due to differences in temperature and flow velocity.
Although there are various methods of getting rid of the interfering plastic, such as mechanical; using coolant; or „by force,” the most common method of forced detachment is the use of a chip breaker.
Chip breakers
They are used for inserts with a variety of coatings and substrates, and their most basic function is to force chips to curl more than they would under natural conditions. Forced wrapping can cause chips to break when they strike the workpiece or tool. Chip breakers increase machining productivity by better controlling waste and reducing cutting forces. Most modern tools of this type take the form of grooves or obstacles in the form of notches in the cutting tools. The goal of chip breaker design is to find the best geometry for a given process scenario, which then puts pressure on the filing and makes it susceptible to fracture because they have a small groove behind the front cutting edge. The geometry of this groove determines the radius of curvature of the chip. Interlocking type chip breakers have a characteristic geometry similar to a step. Obstacle-type chip breakers, on the other hand, have a characteristic geometry similar to a step. The barrier can be integrated into the cutting tool or attached to it. In the case of the „installed” type, they can be adapted to different machining conditions.
Summary – chip control
The machining process is a clever combination of physics, materials science, and mechatronics. During machining, material removal is the result of the force between the workpiece and the tool. The nature of these interactions determines the color and size of the chips. Chips are valuable test and diagnostic data for CNC process engineers; however, if not handled properly, a chip can even reduce machine performance.
There are three different types of chips during machining:
- segmented chips,
- continuous chips
- chips with built-up edges.
The formation of material waste depends on the choice of material and the parameters of the cutting process. Chip removal is an important factor to consider when improving overall productivity and planning autonomous machine operations. Although segmented and continuous chips are self-destructive under certain cutting conditions, the use of chip breakers in systems is a practical principle. Unwanted waste is broken to the proper length by such a tool, preventing chips from becoming entangled in the machine, reducing vibration, and preventing damage. Chip breakers also reduce cutting resistance by preventing chipping and breaking of the cutting edge. Chip control is an extremely important component when we are doing metal cutting. Chip formation and edge overbuilding can prove to be a problem for our CNC machine. For this reason, it is a good idea to constantly analyze this phenomenon. When using a chip breaker, choose the one that is right for the job. For turning operations such as finishing, medium, and roughing, we need to choose a matching accessory for each, as using the correct chip breaker is critical depending on the desired depth of cut, feed rate, spindle speed, and desired poor surface finish.