Axial laminated cutting methods have been proposed for turning threads with large screw pitches. Key process control variables are revealed through a study of tool contact relationships and cutting layer parameters. It also describes the effect of the torsional spiral angle on the front and rear working angles of the left and right cutting edges, and the effect of the cutting order on the cutting efficiency.
The design goal is to reduce machining efficiency and surface consistency on the left and right thread surfaces. Tool shapes, cutting parameters, and cutting order are used as design variables. We proposed a method for designing an axially layered cutting process for large pitch threads. We design and grind two tools, propose matching process designs, and perform lathe comparison experiments with high-pitch threading techniques.\
Experimental results show that the process scheme obtained by this design method is as follows. Large pitch threads have significantly improved pitch error, machined surface morphology and distribution to meet the requirements of large pitch thread processing quality.
Threads with a pitch greater than 4 mm are defined as large pitch threads. It belongs to non-standard parts and has a wide and deep tooth profile. These large pitch screws are commonly used for screws and nuts in large press adjustment assemblies and for self-propelled gun port screws and play an important role in each device. Therefore, in order to study how to cut large pitch threads, it has been proposed that the process design method for large pitch threads is essential to ensure and improve adjustment and assembly accuracy. Existing research mainly deals with machining and precision control methods for small and medium pitch threads and does not consider the case of large pitch threads with non-standard pitches. It is not possible to specifically disclose the machining and precision control methods for large pitch screws. Existing research has mainly focused on medium and small pitch thread machining and precision control methods, without considering the situation of non-standard large pitch threads, and large pitch thread machining and precision control methods. Cannot be clarified concretely. Threads with a large pitch have a large removal margin, so it is not possible to finish a thread in a single cut. The finishing step for large pitch screws must be completed with multiple feeds. In axial layered cutting of large pitch threads, too few cuts will result in a threaded surface that meets the machining quality requirements. If the number of cuttings is large, the tool life will be shortened if cutting heat and cutting force accumulate during multiple cuttings. This affects the surface quality of thread processing and reduces the processing effect. Therefore, when using axial laminated turning for high pitch thread finishing, there is a need for a corresponding design method to ensure that the number of cuts is used for the best thread quality. In this paper, the cutting efficiency and surface consistency of the left and right thread surfaces are designed on the assumption that they meet the technical requirements. The tool shape angle, cutting parameters, and cutting time are used as design variables to clarify the constraint relationships between the variables. We proposed a method for designing an axially layered cutting process for large pitch threads. According to this method, two different process methods were designed and a comparative experiment of large pitch thread finishing was conducted to verify the accuracy of the design method.
1. Axial layered cutting method and its cutting layer parameters
It is impossible to achieve the machining accuracy and surface quality of large pitch screws by the conventional radial layered cut method. In this paper, an axial layered cutting method is proposed due to the machining properties of large pitch threads.
- n is the rotation speed of the workpiece,
- vf is the axial feed rate of the tool,
- Vc is the main movement speed.
- Κr1 is the main deflection angle of the cutting tool for cutting the left end.
- Κr2 right end, tool lead angle during cutting.
- d is the outer diameter of the test piece,
- D2 is the diameter of the center of the test piece,
- D1 is the small diameter of the test piece.
- Ap is the total radial cutting depth.
- Zlj is on the far left and has a single machining allowance.
- Zrk is on the far right and has a single machining allowance.
- The left end of the hDl tool, the cutting thickness during turning.
- Cutting thickness at the right end of the hDr tool, turning.
- P is the screw pitch of the test part, and R1 and R2 are the radii of the left and right teeth of the test piece, respectively. R1 and r2 are the radii of the left and right cusps of the tool, and α is the angle of the thread.
Due to the completion of roughing and semi-finishing, the radial and shape dimensions of the male threads meet the finishing requirements. Therefore, when a large pitch thread is completed, only the left and right cutting edges are used, and the machining allowance is eliminated by cutting the layers alternately in the axial direction.\
Then, when finishing a thread with a large pitch, only the left and right cutting edges are used to eliminate the process margin by alternating multiple feeds along the axis. Until the error of the machined surface roughness and screw diameter of the left and right threads is controlled below the specified machining quality index. As can be seen from Figure 1, the axial layered cutting method is full edge cutting. During each cut, the cutting depth ap is constant, equal to the thread height H, and the area of the cutting layer is only related to a single axial machining allowance.
The relationships between the variables are as follows:
It can be known from the above formula. This embodiment uses a perfect cutting edge, after which each feed does not change regardless of the cutting length of the cutting edge. The area of the cutting layer during cutting is related to the single axial tolerance and the total radial depth of the cut. Therefore, when a large pitch thread is machined by axial layered cutting, the formation of the thread surface is closely related to the condition of the tool edge.
Since the cutting force is closely related to the parameters of the cutting layer, the cutting force should be less than the maximum bearing capacity of the workpiece material. Therefore, the area of the cut layer should be smaller than a certain fixed value. That is, you need to control the disconnect parameters during processing. Therefore, you can get the following expression:
- Zi is a single machine cost for axial layer cutting.
- γ0 is the rake angle of the cutting edge.
- α01 is the rear angle of the left cutting edge.
- α02 is the angle of the right cutting edge.
- εr1 is the cutting edge angle of the left cutting edge.
- εr2 is the right cutting edge angle.
- j is the left end, the number of continuous cuts,
- k is the right end, the number of continuous cuts,
- T is the number of left and right edges and cutting cycles.
2. Axial laminated cutting process design method
Due to the presence of the helix angle, a sharp axially inclined threading process is performed and the cut surface changes. During the cutting process, the working angles of the left and right edges change, which is no longer equal to the actual marked angle and the difference between the left and right edges is large. This affects the consistency of the left and right surfaces of the thread. Therefore, when designing the tool, it is necessary to consider the effect of the helix angle on the working angle of the left and right blades and perform a rational structural design. The main impact on the cutting process is the rake and back angles of the tool. The working front angle, working back angle, and spiral angle of the left cutting edge of the tool have the following relationships.
- In the formula,
- γ0e1 is the working angle of the left cutting edge.
- α0e1 is the receding angle of the left cutting edge.
- γ0e2, this is the rake angle of the right cutting edge.
- α0e2, effective cutting angle of right cutting edge,
φ is the helix angle. Therefore, in order to make the screw a consistent design of the left and right thread surface treatment, process design, and rational design of the left and right cutting edge angles. The design must be followed, the rake angle of the left cutting edge is smaller than that of the right cutting edge, and the difference is approximately equal to twice the spiral angle. The back angle of the left cutting edge is larger than that of the right cutting edge, and the difference is almost equal to twice the spiral angle.
At the same time, when high-pitch thread finishing is performed by the axial layer cutting method, if the number of cuttings is too small, a thread surface that satisfies the machining quality cannot be obtained. When the number of cuttings is large, cutting heat and cutting force are accumulated during multiple cuttings, which shortens the tool life. This will affect the thread surface and reduce the treatment effect. Therefore, the values of j, k, and t should be reasonably designed to be minimized on the assumption that thread surface treatment requirements are met. The relationship is as follows:
Cutting efficiency has a certain relationship not only with the cutting order but also with three factors of cutting amount. Reasonable design of cutting parameters is very important for cutting production efficiency, processing cost and product quality. Using the right cutting parameters can significantly reduce processing costs and improve processing efficiency. Therefore, when designing your process plan, you need to choose the appropriate disconnect parameters. The order of selection is as follows: First, try selecting the maximum radial depth of the cutting ap. Next, select the appropriate machining allowance zi according to the machining conditions. And finally, for the life of the tool or the output the machine allows, select the appropriate cutting speed vc. From the above analysis, high efficiency and high surface consistency are important for thread cutting during threading. Consistency is the key to ensuring the highest quality and productivity of threading.
Therefore, as shown in Table 2, process design goals for the axial laminated cutting method for large pitch threads have been proposed.
η is the processing efficiency and is closely related to the tool life and processing order. ΔP is the pitch error of the thread surface and is divided into the left-hand thread surface pitch error and the right-hand thread surface pitch error. Both must be less than the technical requirement ΔP0 for the screw. γij is used to clarify the consistency of the pitch error of the left and right thread planes, and the higher the value, the higher the consistency. Ra is the arithmetic mean deviation of the surface profile and is used to reveal the surface roughness of the left and right threaded surfaces. Both must be less than the threaded surface of the technical requirement for a surface roughness value of Ra0.
From the above analysis, the design goals are based on the assumption that they meet the technical requirements, cutting efficiency, and surface consistency of the left and right thread surfaces. As shown in Fig. 2, we propose a design method for a layered cutting process in the axial direction of a large pitch screw, using the tool shape angle, cutting parameters, and cutting time as design variables.