Cutting motion and cutting amount
Cutting motion and cutting parameters are fundamental concepts in metal cutting, directly impacting CNC machining quality, production efficiency, and tool life. Cutting motion refers to the relative movement between the tool and the workpiece, which removes excess material from the workpiece to create a desired surface finish. Cutting parameters, including cutting speed, feed rate, and depth of cut, represent the magnitude of the cutting motion. A thorough understanding of the components of cutting motion and the principles for selecting cutting parameters is crucial for optimizing the cutting process and improving CNC machining efficiency.
Cutting motion can be divided into two types based on its function: the primary motion and the feed motion. Together, they constitute the composite motion of the cutting process. The primary motion is the most basic motion required to remove excess material from the workpiece. It is characterized by the highest speed and the greatest power consumption. In turning, the primary motion is the rotation of the workpiece; in milling, it is the rotation of the milling cutter; and in planing, it is the reciprocating linear motion of the planer. The direction and speed of the primary motion directly determine the type and efficiency of the cutting process. For example, in turning, the higher the workpiece rotational speed, the more material is removed per unit time, and the higher the CNC machining efficiency. The feed motion is the motion that continuously adds new cutting layers to the cutting process. It has a lower speed and consumes less power. In turning, the feed motion is the axial or radial movement of the tool; in milling, it is the movement or rotation of the workpiece; and in drilling, it is the axial movement of the drill bit. The magnitude of the feed motion determines the thickness and width of the cutting layer, affecting the surface quality and tool wear rate. The composite motion trajectory of the main motion and feed motion determines the shape of the workpiece’s machined surface. For example, when turning an outer circle, the workpiece’s rotational motion and the tool’s axial feed motion are combined to form a cylindrical surface.
Cutting speed is the linear velocity of the primary motion, represented by the symbol “v” and measured in meters per minute (m/min). The formula for calculating cutting speed varies depending on the type of primary motion: for rotational motion (such as turning and milling), cutting speed v = πdn/1000, where d is the diameter of the workpiece or tool (mm) and n is the rotational speed (r/min); for reciprocating motion (such as planing), cutting speed v = 2Ln/1000, where L is the stroke length (mm) and n is the number of reciprocating strokes per minute (str/min). Cutting speed is the most significant factor affecting tool wear. Excessively high cutting speeds can lead to a sharp increase in cutting temperature, softening the tool material, accelerating wear, and even causing tool burning. Excessively low cutting speeds reduce production efficiency and can easily cause built-up edge (when CNC machining plastic materials), affecting surface quality. Therefore, it is necessary to select the appropriate cutting speed according to the tool material and workpiece material. The cutting speed of high-speed steel tools is lower (generally 10-60m/min), the cutting speed of carbide tools is higher (generally 50-300m/min), and the cutting speed of ceramic tools is even higher (up to 500m/min or more).
Feed rate is a parameter of feed motion, referring to the amount the tool moves relative to the workpiece in the feed direction, represented by the symbol “f.” In turning, feed rate refers to the distance the tool moves in the feed direction per workpiece revolution, measured in mm/r. In milling, feed rate can be expressed in three ways: feed per tooth (fz, mm/z), feed per revolution (f, mm/r), and feed speed (vf, mm/min). The relationship between the three is vf = f×n = fz×z×n (z is the number of tool teeth). In drilling, feed rate refers to the axial distance the drill moves per revolution, measured in mm/r. The feed rate directly affects cutting forces and surface roughness. Increasing feed rate increases cutting forces and surface roughness, but improves CNC machining efficiency. Reducing feed rate reduces cutting forces and surface roughness, but reduces CNC machining efficiency. Therefore, when selecting the feed rate, a balance must be struck between processing quality and efficiency. A larger feed rate (0.1-0.5 mm/r) can be selected for roughing to quickly remove the excess; a smaller feed rate (0.05-0.1 mm/r) should be selected for fine CNC machining to ensure surface quality.
The depth of cut, denoted by the symbol “ap,” is the depth of penetration of the tool into the workpiece, expressed in mm. In external turning, the depth of cut is the perpendicular distance between the workpiece surface and the machined surface, defined as ap = (dw – dm)/2, where dw is the diameter of the workpiece surface (mm) and dm is the diameter of the machined surface (mm). In milling, the depth of cut is the perpendicular distance the tool penetrates the workpiece. In drilling, the depth of cut is half the drill diameter. The depth of cut is the primary factor influencing cutting forces. Increasing the depth of cut increases cutting forces proportionally, increasing the load on the machine tool and tool and potentially causing vibration. Decreasing the depth of cut reduces cutting forces, but requires more feeds, reducing CNC machining efficiency. Therefore, a larger depth of cut (2-10 mm) should be used for roughing, removing most of the stock in one go and reducing feeds. A smaller depth of cut (0.5-2 mm) should be used for semi-finishing and finishing, gradually improving CNC machining accuracy. For workpieces with less rigidity or slender shafts, a smaller depth of cut should be used to avoid workpiece deformation.
The three key elements of cutting parameters (cutting speed, feed rate, and depth of cut) are interrelated and mutually constrained. Properly matching cutting parameters is key to optimizing the cutting process. When selecting cutting parameters, the principle of “deep cut first, feed rate second, and cutting speed last” should be followed: First, determine the depth of cut based on the workpiece’s stock and rigidity, aiming to remove all roughing stock in one operation whenever possible. Then, select an appropriate feed rate based on the workpiece material, tool material, and surface finish requirements. Finally, determine the cutting speed based on tool durability. For example, when rough turning a 45 steel workpiece with a carbide turning tool, select a depth of cut ap = 3-5mm, a feed f = 0.2-0.4mm/r, and a cutting speed v = 80-120m/min. For finish turning, select ap = 0.5-1mm, f = 0.1-0.15mm/r, and v = 120-180m/min. The selection of cutting parameters also needs to consider the power and rigidity of the machine tool to avoid overloading the machine tool or generating strong vibration due to excessive cutting force.
