In the process of gray cast iron cutting, the cutting force of gray cast iron directly determines the wear, damage, service life, machining accuracy and machined surface quality of tools, which is the main evaluation index of material cutting performance. Under the same cutting conditions of gray cast iron, the material with small cutting force of gray cast iron has better machinability of gray cast iron; On the contrary, it is poor.
The cutting force of gray cast iron refers to the interaction force between the workpiece and the tool caused by the tool cutting the workpiece (test piece), as shown in Figure 1. When cutting workpieces, the cutting force of gray cast iron is caused by the deformation and friction in the cutting process of gray cast iron. For the cutting tool, the cutting force of gray cast iron comes from two aspects: one is the resistance caused by the elastic and plastic deformation of the workpiece material in the three deformation zones; The second is the resistance caused by the friction between workpiece, chip and tool. The total resultant force F of the cutting force of gray cast iron acting on the tool (or workpiece) is called the cutting force. Because the size and direction of the cutting force of gray cast iron change with the cutting conditions, it is not easy to calculate and measure, and it is difficult to be directly applied in the research and analysis of practical problems. In order to meet the needs of solving problems, it is also convenient to measure and calculate, F is often decomposed into components in some directions, which is called cutting component of gray cast iron. F is often decomposed into the following three components in turning. They are the main cutting force FZ (component force in the cutting speed direction of gray cast iron, also known as tangential force), feed resistance FX (component force in the feed movement direction, also known as axial force in cylindrical turning), and cutting depth resistance FY (component force in the cutting depth direction, also known as radial force in cylindrical turning).
In order to obtain the value of cutting force under a specific cutting condition of gray cast iron, a special device for measuring the cutting force of gray cast iron – dynamometer can be used for measurement. There are many kinds of force measuring instruments. According to different working principles, they can be divided into mechanical type, resistance type, inductive type, piezoelectric type, etc. at present, the resistance strain force measuring instrument is widely used. Piezoelectric force measurement has high precision, but it is expensive. The resistance strain dynamometer is used in the comparative test of cutting performance of multicomponent alloy strengthened and pure copper strengthened gray cast iron under the sameconditions.
The resistance strain dynamometer consists of qb-07 double parallel octagonal ring dynamometer (as shown in Figure 2), bridge circuit and dh3817 dynamic and static strain gauge (as shown in Figure 3). Qb-07 double parallel octagonal ring dynamometer is an elastic element that can convert the change of cutting force of gray cast iron into the change of electric quantity. The octagonal ring part in the middle is an elastic element, which is divided into upper ring and lower ring. The front end is provided with a square hole for installing turning tool, and the rear round hole is used for installing and fastening on lathe tool holder.
Qb-07 double parallel octagonal ring dynamometer can measure FZ, FX and FY at the same time, or a single component force. During measurement, several pieces of resistance strain gauges whose resistance value can change with the deformation of the elastic element shall be pasted at the appropriate part of the elastic element, and they shall be connected to the bridge circuit, so as to convert the change of resistance value into readable electrical signal (current or voltage) and output.
The cutting performance of gray cast iron was compared by measuring the cutting force of different materials under the same processing parameters. The gray cast iron cutting machine tool is a CA6140 horizontal lathe. In order to ensure the consistency of tool angle, the machine clamp turning tool is selected, and the blade is YG8 cemented carbide blade of the same batch of products produced by the same manufacturer. The tool bar is connected with the slide box of the lathe through QB − 07 double parallel octagonal ring dynamometer. The resistance change of the strain gauge caused by the change of cutting force of gray cast iron during the test is measured by dh3817 dynamic and static strain gauge. Since the cutting length of the cutting edge of each sample for measuring the cutting force of gray cast iron is very small, the impact on the wear of the tool can be ignored. In the later tests, the same tool is installed to complete the cutting force test of gray cast iron with a set of processing parameters. During the process of changing the sample, the tool is not changed and the tool is not reinstalled, This also avoids the influence of the change of tool reinstallation angle on the cutting force of gray cast iron in the same group of machining tests. During the test, only one clamp blade is used to cut four samples in the same group of machining parameters. During the cutting test, the spindle speed (160 R · min-1) and feed rate (0.294 mm · R-1) of the machine tool remain unchanged, and only the cutting depth of gray cast iron is changed in each group of test parameters, The cutting depth is 1 mm, 1.25 mm, 1.5 mm and 1.75 mm respectively.
During force measurement, when the turning tool fastened in the tool hole of QB − 07 double parallel octagonal ring dynamometer is subjected to the cutting force of gray cast iron, the diameter and length of the resistance wire in the strain gauge will change with the deformation of the elastic element, so its resistance value will change slightly, the resistance value will increase under tension, decrease under compression, and its change will change with the deformation. In order to facilitate measurement, the bridge circuit is usually used to convert it into voltage (or current) signal, and then measured and output by dh3817 dynamic and static strain measuring instrument. Within the allowable range of elastic elements, the output electrical signal is directly proportional to the cutting force of gray cast iron. Through calibration, the relationship curve (calibration curve) between gray cast iron cutting force and electrical signal can be obtained. During actual gray cast iron cutting, the corresponding gray cast iron cutting force can be obtained on the curve through the measured electrical signal.