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Home - News - Mechanical Analysis of Deep Hole Machining and Simulation of Drilling Process

Mechanical Analysis of Deep Hole Machining and Simulation of Drilling Process

October 17, 2022

Abstract: Deep hole machining is overlapped under the closed sill state, and the cutting condition of the tool cannot be directly observed. The metal plastic forming simulation software DEFORM-3D is used to simulate the deep hole drilling process dynamically with the finite element method, predict the temperature and stress changes in the processing process, compare the changes of temperature and equivalent stress under different drilling parameters, and obtain the change curves of cutting temperature and equivalent left force under different cutting speeds. The results show that the cutting temperature increases with the increase of the cutting depth, and tends to be stable gradually; The cutting temperature is proportional to the cutting speed, while the effect force does not change much with the change of cutting parameters.

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Key words: deep hole Rugong; D eform -3D; Drilling
Deep hole machining is one of the most difficult processes in hole machining, and deep hole solid drilling technology is recognized as the key technology of deep hole machining technology. The traditional processing method is time-consuming and labor-intensive, and the precision of deep hole processing is not high, there is also the problem of frequent tool change and the risk of tool breakage [1]. Gun drilling is an ideal processing method at present. In the process of deep hole processing, the drill pipe is thin and long, easy to deflect, generate vibration, and the generated heat and cutting shoulder are not easy to discharge. It is not possible to directly observe the cutting condition of the tool. At present, there is no ideal way to monitor the temperature change and distribution in the cutting area in real time [w]. Only experience can be used to judge whether the cutting process is normal by listening to the cutting sound, watching the chips, touching the vibration and other appearance phenomena.

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In recent years, with the rapid development of computer hardware technology and numerical simulation, simulation technology provides an efficient scientific and technological way to solve this problem [4]. Simulation drilling is of great significance for improving the machining accuracy, stability and efficiency of deep holes. At present, some scholars can indirectly judge or predict the processing process in advance through some advanced measurement methods and software analysis. For example, Ding Zhenglong of Xi'an Jiaotong University and other scholars set up an online measurement platform to measure the inner diameter of deep holes [5], but the processing process could not be monitored online; some engineers improved the processing technology of deep holes by changing the traditional structure of the machine tool. For example, in order to prevent the cutting shoulder from scratching the hole wall after processing, the machine tool spindle was used in an inverted structure, and the self weight of the cutting fluid and the cutting shoulder was used to make the chips more smoothly discharged from the V-shaped groove of the drill pipe [6] and other measures, Effectively improve drilling quality.

In this paper, Def 〇 rm-3D metal plastic forming simulation software is used to dynamically simulate the drilling process; The temperature and stress changes under different cutting speeds are obtained, and the processing effect of deep hole is predicted in advance, which provides a basis for the design and implementation of deep hole processing coolant.

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1. Working principle and drilling technology of gun drill
1.1 Working principle of gun drill
Gun drill is the main tool for machining deep holes. It has the characteristics of good accuracy and low surface roughness after one drilling [7]. The basic structure of gun drill is shown in Figure 1.
Figure 1 Basic Structure of Gun Drill
Gun drill consists of head, drill pipe and handle. The head is the key part of the whole gun drill, which is generally made of cemented carbide. There are two types: integral type and welded type, which are usually welded with the drill pipe. The drill pipe of gun drill is generally made of special alloy steel and heat treated to make it have good strength and rigidity, and must have sufficient strength and toughness; The handle of the gun drill is used to connect the tool with the machine tool spindle, and is designed and manufactured according to certain standards.

1.2 Gun drilling process
During operation, the handle of the gun drill is clamped on the spindle of the machine tool, and the drill bit enters the workpiece through the guide hole or guide sleeve for drilling. The unique structure of the drill blade plays the role of self guidance, ensuring the cutting accuracy. First process the pilot hole, and then reach 2~5 m m on the pilot hole at a certain feed speed, that is, the point in Figure 2. At the same time, open the coolant by intercooling; Start machining at normal speed after the pilot hole is reached. During the machining process, adopt intermittent feeding, and feed every time! 2 depth, realizing deep hole and short shoulder; When the machining is finished and leaves the entity, first withdraw the tool at a fast speed to a certain distance from the hole bottom, then exit the pilot hole at a low speed, and finally quickly leave the machining workpiece and turn off the coolant. The whole process is shown in Figure 2. The dotted line in the figure represents rapid feed, and the solid line represents slow feed.

2. Analysis of deep hole drilling force
Compared with other metal cutting methods, the most significant difference between deep hole drilling and other metal cutting methods is that the deep hole drilling uses the positioning and support of the guide block to drill in the closed cavity. The contact between the tool and the workpiece is not the single contact of the blade+91, but also the contact between the additional guide block on the tool and the workpiece.
As shown in Figure 3. The deep hole drill is composed of three parts: cutting tool body, cutter tooth and guide block. The cutter body is hollow. The cutting shoulder enters from the front end and discharges through the drill pipe cavity. The rear thread is used to connect with the drill pipe. The main cutting edge on the cutter teeth is divided into two, namely, the outer edge and the inner edge.
Taking the cobalt in the deep hole of the multi blade inner shoulder as an example, the auxiliary blade and two guide blocks are on the same circumference, and the three-point fixed circle is self guided. The force on it is analyzed. The simplified mechanical model is shown in Figure


4. (1) Cutting force F. The cutting force on deep hole tools can be decomposed into mutually perpendicular tangential forces F,,, and radial forces F, And axial force radial force will directly lead to tool bending deformation, axial force increases tool wear, while tangential force on cutting edge mainly produces torque. In the process of processing, it is always hoped to reduce the axial force and torque as much as possible on the premise of ensuring the processing quality and efficiency. Generally, the service life of the tool is directly linked to the axial force and torque. Excessive axial force makes the drill bit easier to break, and excessive torque will also accelerate the wear and break of the tool until it is scrapped [1 °].
(2) Friction F/. Friction/and/2 are generated when the guide block rotates relative to the hole wall; The axial friction between the guide block and the hole wall when it moves along the axis is/lu and 7L;
(3) Extrusion force The extrusion force is caused by the elastic deformation of the hole wall. The extrusion force between the guide block and the hole wall is M and ^ 2. According to the principle of force system balance, it can be known that:
Where: is the resultant force of vertical cutting force; F ,. Is the resultant of the radial cutting force; F is the resultant of circumferential cutting force. Assuming that only Coulomb friction coefficient is considered, the axial friction and circumferential friction on the guide block are equal. It can be straight through experiment
Connect the torque M and F a measured during deep hole processing.
For a given drill bit, its nominal diameter is and the position angle of the guide block is determined. In addition, the empirical axial force of the cutting force is half of the main cutting force. By synthesizing the above formula, the cutting force components and the force on the guide block can be calculated.

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3. Drilling simulation of gun drill
The deep hole drilling of inner shoulder is carried out in a closed or semi closed condition. The cutting heat is not easy to disperse, the shoulder is difficult to arrange, and the rigidity of the process system is poor. When the coolant produced in drilling cannot enter the cutting area, resulting in poor cooling and lubrication, the tool temperature will rise sharply, accelerating tool wear; With the increase of the drilling depth, the tool overhang increases, and the rigidity of the drilling process system decreases. All these put forward some special requirements for the deep hole drilling process with internal chip removal. This paper predicts the heat and cutting force generated in the cutting process through the reproduction simulation of the actual processing conditions, which provides a basis for optimizing the deep hole drilling process. 3.1 Definition of drilling parameters and material properties DEFORM is a set of finite element based process simulation system for analyzing metal forming process. By simulating the whole processing process on the computer, engineers and designers can predict the adverse factors under various working conditions in advance and effectively improve the processing process nM2]. In this paper, the 3D modeling software Pm/E is used to draw the simulation tool model, and the model is saved as The STL format is imported into Defo rm - 3 D. The set cutting parameters and conditions are shown in Table 1.
(1) Setting of working conditions: select drilling as the machining type, the unit standard is SI, input the cutting speed and feed rate, the ambient temperature is 20t:, the friction factor of the workpiece contact surface is 0.6, the heat transfer coefficient is 45 W/m2. 0C, and the thermal melting is 15 N/mm2/X.
(2) Setting of tool and workpiece: the tool is rigid, the material is 45 steel, the workpiece is plastic, and the material is WC carbide.
(3) Set the relationship between objects: The master slave relationship of D e fo rm is that rigid body is the main part and plastic body is the slave, so the tool is active and the workpiece is driven.
Table 1 Main Parameters of Workpiece and Tool
In order to compare the influence of different process parameters on the changes of temperature, stress and strain in the cutting process, the simulation is carried out under different drilling parameters as shown in Table 2, and the results are observed.
Table 2 Gun drilling parameters

3.2 Drilling simulation and result analysis
(1) Temperature
Most of the energy consumed in metal cutting is converted into heat energy. This heat causes the temperature of the cutting zone to rise It directly affects the tool wear, machining accuracy and surface quality of the workpiece. In high-speed metal cutting, severe friction and fracture make local temperature rise to very high temperature in a short time. In gun drilling, the heat mainly comes from the deformation of the metal cutting shoulder, the friction between the drill support pad and the workpiece hole pad, and the friction of the cutting shoulder on the tool rake face [13]. All these heat needs to be cooled by the cutting fluid. By simulating the drilling process, the temperature changes in the contact area of the workpiece at different speeds and feeds are obtained. These data provide a design basis for optimizing the cooling system during deep hole machining. Due to the high performance requirements of the computer for simulating drilling process, it takes a long time to simulate the complete hole processing process. By setting the step size of drilling simulation, the depth of simulation is controlled to achieve stable processing.
Simulation condition setting The number of simulation steps is set as 1000, the number of simulation interval steps is set as 50, and the data is automatically saved every 50 steps; Deform-3D adopts adaptive mesh generation technology. The workpiece is a plastic body. The mesh generation is used to calculate the cutting force. The absolute element type is shown in Figure 5, and the simulation results are shown in


Table 3.
Fig. 5 Finite element model and drilling process of deep hole drill
Table 3 Data Collection of Cutting Speed and Temperature with Steps
By analyzing and processing the data in Table 3, the curves of the temperature change of the workpiece cutting area with the number of steps under three working conditions are obtained as shown in Figure 6.
Fig. 6 shows that the drilling speed has a great influence on the temperature of the workpiece contact area. At the beginning of drilling, the drill bit and the workpiece begin to contact, and the feed rate is large. The sharp impact of the tool on the workpiece causes the initial temperature to change greatly and rise rapidly. As the drilling tends to be stable, the curve generally becomes gentle but still fluctuates, which is normal for deep hole processing. Because the drill bit diameter is small and the feed rate is large, the vibration will persist.
It can also be seen from Fig. 6 that drilling speed has a great influence on temperature. As the speed increases The drilling temperature is getting higher and higher. From the results of the finite element model, the maximum temperature generated at different drilling speeds occurs in the local deformation area near the drill point, because this is where plastic deformation and friction of the tool shoulder are concentrated.
Fig. 6 Variation Curve of Contact Area Temperature with Cutting Speed

(2) Equivalent stress distribution
Von Mises stress is an equivalent stress based on shear strain energy and a yield criterion. After the introduction of equivalent stress, no matter how complex the stress state of the element body is, it can be imagined as the stress when bearing a unidirectional tension on the numerical value. The corresponding relationship between the equivalent stress and the equivalent strain obtained from the analysis reflects the work hardening of the workpiece material caused by plastic deformation through finite element analysis The equivalent stress changes of gun drill at different drilling speeds are obtained. The simulation interval is 50 steps, and the results are automatically saved every 50 steps, as shown in Table 4.

Table 4 Data Collection of Cutting Speed and Equal Force with Steps
The analysis of the relationship between the equivalent stress and the number of steps is shown in Figure 7. It can be seen that different spindle speeds have little influence on the equivalent stress of the workpiece during processing, and fluctuate within a certain range, but the trend of the maximum equivalent stress change under the three processing conditions is very similar.
The curve in Figure 7 of drilling equivalent stress shows that the stress in the initial stage of drilling is large. As the drilling depth becomes stable, the curve generally drops and becomes gentle. At the same time, through the stress and strain analysis, the maximum equivalent stress of the gun drill is 1550 M Pa, and the overall maximum displacement is 0.0823 m m.

4. Conclusion
The deep hole cutting process is effectively simulated by using the software of Defo rm. The temperature change and stress change in the cutting process are analyzed, and the change curve between the cutting temperature and the cutting speed is obtained. This provides a certain basis for the study of the cutting mechanism of deep hole machining, the selection of cutting parameters and the design of the cooling system in actual machining.