Screw-Screw processing

Screw-Screw processing:

Screw is a high precision parts, it can accurately determine the coordinates of the location table, the rotary motion into linear motion, surface and also pass a certain power, so in accuracy, strength and wear resistance and other aspects have high demands. So, screw machining from rough to finished product of each process must be carefully considered in order to improve its precision.

1, screw classification

Machine screw their friction characteristics can be divided into three categories: the sliding screw, screw and scroll static screw.

As the sliding screw simple structure, convenient, so the machines used widely.


Sliding screw-type, mostly trapezoidal teeth. This tooth type triangular teeth than with the effect of high pay, driving performance, high precision, and easy processing.

Rolling ball screw and roller screw into screw-defined two categories. Compared to ball screw and roller screw, the friction is small, high transmission efficiency, accuracy is also high, and thus more commonly used, but the manufacturing process more complicated.


Static pressure screw has many advantages, is often used in precision machine tools and CNC machine tool feed institutions. The trapezoidal thread form thread form with the same standard. However, tooth-shaped than the same size 1.5 to 2 times the standard thread, the purpose is to get a good seal and improve the carrying capacity. But the adjustment is too much trouble, and need a hydraulic system, process complexity, high cost.

2, screw the structural characteristics and technical requirements

(1) screw structure of the process characteristics

Is a slender flexible screw axis, its length L and diameter D, the ratio of large, generally 20 to 50, less rigid. Structure of complex shape, with a high thread surface requirements, as well as steps, grooves, etc. So, in the process prone to deformation.

(2) screw accuracy grade

In the national standard GB785-65, the precision of ordinary trapezoidal thread is divided by the diameter tolerance. There are five basic parameters: the diameter D, inner diameter D1, the diameter D2, and the tooth pitch T-shaped half-width α / 2. As the screw to pass accurate movement, so, according to JB2886-81 requirements, screw and pitch accuracy, according to the requirements are divided into six levels: 4,5,6,7,8,9 (in order of decreasing accuracy).

Levels of precision lead screw applications are as follows: four for the most advanced, generally rarely used; 5 secrets for precision instruments and machine tools, such as the jig borer, thread grinder, etc.; 6 for precision instruments, precision machine tools and CNC machine tools; 7 threads for precision lathes, gear cutting machines and CNC machine tools; 8 for general machine tools, such as lathe, milling machine; 9 for planer, drill press and general machine tool feed mechanism.

Generally referred to 5,6,7-level precision screw is screw. Precision screw has hardened and not hardened screw screw two. The former wear better, longer time to maintain machining accuracy, but the processing complex, there must be a special high-precision thread grinding and heat treatment equipment, which only requires precision screw lathe.

Ball screw and ball screw accuracy classes are divided into six levels.

3) screw technology

The technical requirements for the screw can be divided into the following items:

① accuracy class;

② surface roughness;

③ single pitch tolerance and cumulative tolerance on length;

④ in diameter roundness tolerance;

⑤ outside diameter tolerance for equality;

⑥ diameter runout tolerance;

⑦ tooth-shaped half-width tolerance;


⑧, the outer, inner diameter tolerance of other items.


3, the choice of materials


Screw the choice of materials is critical to ensure the quality of the screw, the general requirements are:


(1) has excellent processing properties, grinding is not easy to crack, to get a good surface finish and less residual stress, tool wear is less important.


(2) ultimate tensile strength of no less than 588MPA.


(3) have a good process of heat treatment, hardenability, and not quench crack, homogeneous, heat deformation small, can obtain a higher hardness, wear resistance and screw in order to ensure dimensional stability.


(4) uniform hardness, microstructure meet the standards. Commonly used materials are: not hardened screw used T10A, T12A and 45, etc.; hardened screws are often used in 9MN2V, CRWMN and so on. Which 9MN2V better technology and stability, but poor hardenability, commonly used in precision screw diameter ≤ 50MM; CRWMN steel has the advantage of small deformation after heat treatment, for the production of precision parts, but its easy to crack, wear cutting process is poor.


The higher the hardness of the more wear-resistant screw, but not easy to manufacture grinding.


4, the typical screw machining process


In the screw in order to obtain high precision machining, plus. The following process should consider the following points:


(1) External round and can be divided into multiple processing threads, gradually reduce the amount of cutting, so as to gradually reduce the cutting forces and internal stress, reduce processing errors and improve accuracy.


(2) cylindrical roughing and roughing each thread should be carried out after the aging treatment for the elimination of internal stress. The higher the precision lead screw, the number of aging the more.


(3) after each aging treatment should be re-grinding center punch the center hole or to repair aging treatment only when the deformation; and remove the oxide, so that the processing of a reliable and accurate positioning of the base surface.


(4) for each processing thread before plus two cylindrical screw L (cutting very small), then screw cylindrical and positioned at both ends of the center hole as a base surface gading: thread, gradually increase the thread machining accuracy.


Lieutenant Colonel straight screw machining and heat treatment process procedures, to ensure precision screw, to prevent bending of the key processes. However, straightening itself will produce internal stress, which is a high precision lead screw is unfavorable. Because stress has a tendency to disappear, due to the disappearance of stress within the screw will cause the deformation, which affects screw accuracy remain. Therefore, high precision, large diameter precision screw, in the process is not straight, but by increasing the radial margin and total margin between the method of successive processes cut bending, time after repeated processing and manufacturing processes into a more detailed approach to solve the distortion problem.


To avoid the screw caused by bending due to weight, storage corresponding to the vertical, when heat treatment carried out in the shaft furnace.


Generally not hardened by turning the threaded screw is made of hardened screw thread and the thread grinder grind thread. However, half-width of the tooth-shaped large and large pitch, screw, screw the rough or hardened before turning to good.


6, the screw of the heat treatment


The first requirement for heat treatment of rough, due to precision and a general class of materials of different types of screw, their heat treatment is different. Rough heat treatment requirements: (1) the elimination of internal stress generated by rough manufacturing; (2) control plus mechanical hardness to suit. Work of cutting, cutting hardness of the general control HBS140 ~ 248 between appropriate.


Usually carbon content of 0.25% to 0.5% of the carbon steel used in normalizing the carbon content of 0.5% to 0.8% of the sub-eutectoid steel or steel with eutectoid annealing. For the carbon content of 0.8% to 1.2% of the eutectoid steel, exist in the organization because of its coarse mesh flake pearlite and cementite, relatively high hardness, thermal annealing to take the ball (the ball is rough annealing heated to 750 ~ 780 ℃, with its 40 ~ 40 ℃ / speed when cooled to 500 ~ 550 ℃, and then natural cooling in air).
Technical requirements
The technical requirements for the screw can be divided into the following items:


① accuracy class;


② surface roughness;


③ single pitch tolerance and cumulative tolerance on length;


④ in diameter roundness tolerance;


⑤ outside diameter tolerance for equality;


⑥ diameter runout tolerance;


⑦ tooth-shaped half-width tolerance;


⑧, the outer, inner diameter tolerance of other items.
3, the choice of materials
Screw the choice of materials is critical to ensure the quality of the screw, the general requirements are:
(1) has excellent processing properties, grinding is not easy to crack, to get a good surface finish and less residual stress, tool wear is less important.
(2) ultimate tensile strength of no less than 588MPA.
(3) have a good process of heat treatment, hardenability, and not quench crack, homogeneous, heat deformation small, can obtain a higher hardness, wear resistance and screw in order to ensure dimensional stability.
(4) uniform hardness, microstructure meet the standards. Commonly used materials are: not hardened screw used T10A, T12A and 45, etc.; hardened screws are often used in 9MN2V, CRWMN and so on. Which 9MN2V better technology and stability, but poor hardenability, commonly used in precision screw diameter ≤ 50MM; CRWMN steel has the advantage of small deformation after heat treatment, for the production of precision parts, but its easy to crack, wear cutting process is poor.
The higher the hardness of the more wear-resistant screw, but not easy to manufacture grinding.
4, the typical screw machining process
In the screw in order to obtain high precision machining, plus. The following process should consider the following points:
(1) External round and can be divided into multiple processing threads, gradually reduce the amount of cutting, so as to gradually reduce the cutting forces and internal stress, reduce processing errors and improve accuracy.
(2) cylindrical roughing and roughing each thread should be carried out after the aging treatment for the elimination of internal stress. The higher the precision lead screw, the number of aging the more.
(3) after each aging treatment should be re-grinding center punch the center hole or to repair aging treatment only when the deformation; and remove the oxide, so that the processing of a reliable and accurate positioning of the base surface.
(4) for each processing thread before plus two cylindrical screw L (cut very small), then screw cylindrical and positioned at both ends of the center hole as a base surface gading: thread, gradually increase the thread machining accuracy.
Lieutenant Colonel straight screw machining and heat treatment process procedures, to ensure precision screw, to prevent bending of the key processes. However, straightening itself will produce internal stress, which is a high precision lead screw is unfavorable. Because stress has a tendency to disappear, due to the disappearance of stress within the screw will cause the deformation, which affects screw accuracy remain. Therefore, high precision, large diameter precision screw, in the process is not straight, but by increasing the radial margin and total margin between the method of successive processes cut bending, time after repeated processing and manufacturing processes into a more detailed approach to solve the deformation problem.
To avoid the screw caused by bending due to weight, storage corresponding to the vertical, when heat treatment carried out in the shaft furnace.
Generally not hardened by turning the threaded screw is made of hardened screw thread and the thread grinder grind thread. However, half-width of the tooth-shaped large and large pitch, screw, screw the rough or hardened before turning to good.
6, the screw of the heat treatment

the choice of screw-based surface

As the heat treatment so that the screw deformation, and justice does not allow direct method of cold straightening, cutting method must be corrected only. If you still use the original center of the hole is too large will make allowances. In addition, the center hole itself, there will be deformed, hardened screw is not used for the original center hole cut out, re-play center hole (center hole of the last amendment process excluded) method. In the center hole before playing again, to find out the amount of screw radial runout maximum amount of half the circle beating two points, then with the center support bracket and press on these two points cylindrical alignment, cut the original m center hole, re-play center hole, so you can make to reduce the number of total allowances. Each screw can only be used for grinding hardened center hole of the method to be amended.

Processing screw, the theory is the center hole of the main base surface, cylindrical surface of the auxiliary base. In fact, in the processing thread, the outer cylinder itself and roundness, with pocket and lead screw with precision, with the top pocket with two coaxial connections are a factor affecting the accuracy of thread cutting. So process should be: in the heat treatment process after the first cylindrical, then processing thread, to the cylindrical positioning processed. Thus, the final precision cylindrical grinding time also increased accordingly.