Mechanical processing production process

Mechanical processing refers to the process of changing the external dimensions or performance of a workpiece through a mechanical device. According to the difference in processing methods, it can be divided into cutting processing and pressure processing.

The production process of the machine refers to the entire process of making products from raw materials (or semi-finished products). For machine production, it includes the transportation and preservation of raw materials, preparation for production, manufacturing of blanks, processing and heat treatment of parts, assembly and commissioning of products, painting and packaging. The content of the production process is very extensive. Modern enterprises use the principles and methods of system engineering to organize production and guide production, and regard the production process as a production system with input and output.

In the production process, the process of changing the shape, size, position and nature of the production object to make it into a finished product or semi-finished product is called a process. It is the main part of the production process. The process can be divided into casting, forging, stamping, welding, machining, assembly and other processes. The mechanical manufacturing process generally refers to the sum of the machining process of the parts and the assembly process of the machine, and the other processes are called auxiliary Processes, such as transportation, storage, power supply, equipment maintenance, etc. The process is composed of one or several sequentially arranged processes, and a process consists of several steps.

The process is the basic unit of the machining process. The so-called process refers to a part (or a group of) workers, on a machine tool (or a work place), for the same workpiece (or several workpieces at the same time) that part of the process is completed continuously. The main feature of a process is that it does not change the processing objects, equipment and operators, and the content of the process is completed continuously
The working step is under the condition that the processing surface is unchanged, the processing tool is unchanged, and the cutting amount is unchanged.
Walking tool is also called working stroke, and it is the working step completed by the processing tool once processing on the processing surface.

When formulating the machining process, it is necessary to determine the number of procedures and the order in which the workpiece will go through. Only a brief process of listing the name of the main procedure and its processing sequence is called the route.

The preparation of the process route is to formulate the overall layout of the process. The main task is to select the processing method of each surface, determine the processing order of each surface, and the number of steps in the entire process. The preparation of technological routes must follow certain principles.

Heat treatment of spring steel

The strength level of traditional spring steel is difficult to meet the requirements of modern industrial development. As we all know, the mechanical properties of spring steel depend on the heat treatment process under the premise of material quality assurance, and the heat treatment process should also be determined according to the materials used. An important way of high strength spring steel is to give full play to the role of alloy elements to achieve the best alloying effect.
heat treatment

Spring steel requires higher strength and fatigue limit, which is usually used in the state of quenching + tempering at medium temperature to obtain higher elastic limit. Heat treatment technology has an important influence on the internal quality of spring. Therefore, how to further improve the fatigue life of spring needs further research, especially the chemical surface modification heat treatment, shot peening strengthening and so on have an important impact on the spring fatigue life.

 

In order to further strengthen the surface strength, increase the compressive stress and improve the fatigue life of the valve spring, the valve spring should be further nitrided, low temperature liquid carbonitriding or sulfur nitrocarburizing treatment after forming, and then shot peened. For example, in Japan, the fatigue limit of f4mm Si CR oil quenched steel wire can be increased by 240 MPa by comparison with that of low temperature bulk carbonitriding at 450 ℃× 4.5H and tempering at 400 ℃× 15min. At the same time, the high temperature strength of the valve spring after nitriding and low temperature liquid carbonitriding is increased.

 

The deformation amount at 150 ℃ is 0.2% (the specified value is 0.5%), and the deformation amount at 250 ℃ is 0.56%. The thermal stability and anti relaxation stability of valve spring are improved, but the time of nitriding and liquid carbonitriding should be Otherwise, the network sulfide and network nitride will be formed, and the fatigue strength will be reduced.

Two kinds of shot peening can be used to improve the strength of valve spring. One is 0.8 mm in diameter and its microhardness is 720hv0.2, and the other is 0.25 mm in diameter and its microhardness is 800hv0.2. The third shot peening can achieve better strengthening effect and improve the surface quality.

Production technology of spring steel

Spring steel has hot rolled steel, cold drawn steel, cold rolled steel strip and so on. The smelting of spring steel is very important. It is necessary to ensure excellent metallurgical quality, not only to ensure accurate chemical composition, but also to have high purity. The content of sulfur, phosphorus, oxygen and nitrogen should be low, and the uniformity and stability of steel should be good. The metallurgical quality of spring steel should reach the level of high quality steel and high quality steel.
The surface quality and dimensional accuracy of spring steel have great influence on the performance and life of spring.

 

Because the surface of the spring steel in the delivery state is no longer processed except the surface shot peening, so it is the working surface of the finished spring, and various defects on the original surface are also left to the spring. Therefore, the surface requirements of spring steel are very high, and the restrictions on decarburization, crack, folding, scab, inclusion and delamination are very strict. All kinds of surface defects will make the spring early damaged and shorten its life. Take the steel wire with circular cross-section as an example, the strength and stiffness of the finished spring are directly proportional to the third and fourth power of the steel wire diameter respectively.

 

The small change of the steel wire diameter will lead to the huge fluctuation of the spring performance, so the shape and dimension tolerance of the steel should be strictly controlled. In this way, the processing deformation of spring steel (hot rolling, cold rolling, cold drawing, etc.) is very important and must be taken seriously.

Hot rolled spring steel has round steel, square steel, flat steel and so on. Steel surface quality requirements are high, can be delivered as hot rolling or after heat treatment, to ensure that the hardness does not exceed the specified value, easy to spring. The coil spring can be made of hot rolled material in cold state, and the spring with larger section size needs to be heated. After forming, the spring is quenched and tempered at medium temperature. The tempering temperature is about 350-550 ℃ to obtain high yield ratio and good comprehensive mechanical properties.

Due to the large cross-section size of hot-rolled products, the grade with enough hardenability should be adopted to ensure the core hardenability. If there are non martensitic transformation products in the center, such as ferrite and bainite, the properties after tempering will be reduced, especially the fatigue properties.

Classification of spring steel

1、 Classification by chemical composition
According to GB / T 13304 standard, spring steel is divided into non alloy spring steel (carbon spring steel) and alloy spring steel according to its chemical composition.

Carbon spring steel
The carbon content (mass fraction) of carbon spring steel is generally 0.62% ~ 0.90%. According to its manganese content, it can be divided into general manganese content (mass fraction) (0.50% ~ 0.80%), such as 65, 70, 85 and higher manganese content (mass fraction) (0.90 ~ 1.20%), such as 65Mn.

Alloy spring steel
Alloy spring steel is a kind of steel which can improve the mechanical properties, hardenability and other properties of the steel by adding one or several alloy elements on the basis of carbon steel, so as to meet the requirements of manufacturing various kinds of spring.

The basic composition series of alloy spring steel include silicon manganese spring steel, silicon chromium spring steel, chromium manganese spring steel, chromium vanadium spring steel, tungsten chromium vanadium spring steel, etc. On the basis of these series, some brands have added molybdenum, vanadium or boron to improve their properties in some aspects.
In addition, some brands are selected from other steel categories, such as high-quality carbon structural steel, carbon tool steel, high-speed tool steel and stainless steel.

2、 Classification according to production and processing methods
1. Hot rolled (forged) steel includes hot rolled round steel, square steel, flat steel and steel plate, and forged round steel and square steel.
2. Cold drawn (rolled) steel includes steel wire, steel strip and cold drawn material (cold drawn round steel).
3、 Classification according to steel delivery status
1. Hot rolled (forged) steel
A. The steel delivered in hot rolled (forged) state is hot formed into spring, and then quenched and tempered.
B. The steel delivered in annealed condition is cold formed into spring, then quenched and tempered.
All of the above shall meet the requirements of the corresponding delivery state hardness.

2. Cold drawn (rolled) steel
A. Steel wire
① After the steel wire is made into spring, it only needs to be tempered at low temperature to eliminate the stress.
② After the steel wire is cold drawn to the required size, it is subjected to continuous heating, continuous oil quenching and lead tempering. After the steel wire is made into spring, only the bottom temperature fire is needed to eliminate the stress.
③ Cold drawn steel wire (i.e. steel wire without quenching and tempering treatment)
a. Delivery in cold drawn condition.
b. Delivery in annealed, normalized or tempered condition.
The steel wire delivered in the above two states shall be quenched and tempered after being made into spring.
B. Steel strip
① After being delivered in cold rolling condition, the spring should be tempered at low temperature to relieve stress.
② After being delivered in quenched and tempered condition, the spring shall be tempered at low temperature to remove the stress.
③ After being delivered in annealed condition, the spring shall be quenched and tempered
C. Cold drawn steel
The steel delivered in annealed condition is cold formed into spring, then quenched and tempered.

4、 Other classification methods
In addition to the above, there are other classification methods, such as:
According to the different delivery conditions, it can be divided into delivery by chemical composition (mechanical properties) and delivery by hardenability.
According to the working conditions of spring, it can be divided into static load bearing spring steel, impact load bearing spring steel, high (low) temperature resistant spring steel and corrosion-resistant spring steel.

Characteristics and application of tension spring

Characteristics of tension springMany different terminal devices or “hooks” are used to ensure the tension source of the tension spring. The working principle of tension spring is opposite to that of compression spring. The compression spring reverses when it is compressed, while the stretching spring acts reversely when it is stretched or pulled apart. When the ends of the stretch springs are pulled apart, the springs try to pull them back together.

Like compression springs, tension springs also absorb and store energy. But unlike compression springs, most tension springs are usually under a certain degree of tension, even without any load. This initial tension determines the tightness of the tension spring coil without any load.

Application of tension spring

Tension spring is also known as spiral tension spring. It is usually of equal pitch, and its cross section is mostly circular. It can be used in many occasions, such as production assembly, experiment, research and development, maintenance, etc.

Tension spring plays an important role in the global market and is widely used in the fields of national defense, ocean, computer, electronics, automobile, mold, medicine, biochemistry, aerospace, railway, nuclear power, wind power, thermal power, engineering machinery, mining machinery, construction machinery, elevator, etc.

Basic information of tension spring

The tension spring is a spiral spring which bears the axial tension. The tension spring is generally made of circular cross-section material. In the case of no load, the tension spring between the coils is generally tight and there is no gap.
Each end of the loop can be pulled tightly around the loop.

The following is the necessary information for an extension spring:
(1) Free length: (a) total length, (b) length of all loops, (c) length within the hook.
(2) Control diameter: (a) outer diameter, (b) inner diameter, (c) inner diameter of casing.
(3) Wire size “wire diameter”.
(4) Material (type, grade).
(5) Number of turns: (a) total number of turns and (b) right-handed or left-handed.
(6) The form of the end.
(7) Load in the hook.
(8) Load rate, deflection, pounds per inch.
(9) Maximum tensile length.

A compression spring is a spiral spring that bears axial pressure

Compression spring is a spiral spring that bears axial pressure. Because it can bear pressure, both ends can be open or closed, or wound flat or ground flat.

 

The cross section of the material used is mostly circular It is made of rectangular and multi strand steel coils. The spring has equal pitch and variable pitch. The shape of compression spring is cylindrical, conical, convex and concave, and a small number of non-circular or even various tail end variants.

 

The products can be shaped according to the design. There is a certain gap between the coils of the compression spring. When subjected to external load, the spring shrinks and deforms and stores the deformation energy.

Notes on heat treatment of compression spring

1. The heat treatment of spring generally requires quenching, fine grain and less retained austenite. The depth of decarburized layer on each side shall be in accordance with: < Φ 6mm steel wire or steel plate, Φ 6mm steel wire and steel plate, < 1.0% diameter or thickness

2. The large spring is processed in hot state and then quenched and tempered; the medium spring is processed in cold state (the raw material requires spheroidized structure or most spheroidization), and then quenched and tempered. The small spring is processed with cold rolled steel strip and cold drawn steel wire, and then tempered at low temperature

3. Shot peening after treatment
Using 40-50n / cm2 compressed air or 70m / s centrifugal machine, cast iron shot or hardened steel shot of Φ 0.3-0.5mm (for small parts, valve spring, gear, etc.) and Φ 0.6-0.8mm (for leaf spring, crankshaft, axle shaft, etc.) are sprayed onto the spring surface to strengthen the surface layer. The fatigue cycle times can be increased by 8-13 times and the service life can be increased by 2-2.5 times.

Design and production of compression spring

Compression spring is a kind of hardware fittings with spiral geometry formed by line. It is more difficult to harden and strengthen the spring than that of plane hardware.

 

Related standard GB / T_ 1239[1].2–2009_ Technical specification for cold rolled cylindrical coil spring, GB / t2089 — 94, etc

Design and production
1. Select spring material according to the standard according to the actual product demand;
2. According to the product force value, compression stroke, fatigue degree and other requirements, determine the internal and external diameter, effective number of coils, and the range of wire diameter;
3. Put the relevant parameters into the design parameter formula;
4. It is reasonable and feasible for spring manufacturer to design and produce spring.

In the process of processing multi strand spring, attention should be paid to

Due to the limitation of product structure, multi strand spring has the characteristics of high strength and good performance. The material is required to guarantee the final performance in spring strength and toughness. In the process of processing multi strand spring, the following points should be paid attention to:

 

1. Cold and hot methods can be selected for supporting ring according to product requirements. It is not allowed to heat the spring to spark or whiten by hot bonding method, and the temperature of silicon manganese steel shall not be higher than 850 ℃. The bearing ring and the effective ring shall be in effective contact, and the clearance shall not exceed 10% of the nominal clearance between rings.

 

2. The characteristics of multi strand spring can be determined by adjusting lead, and the cable distance can be adjusted when winding. The screw pitch can be 3 ~ 14 times of wire diameter, but generally 8 ~ 13 times is better. The spring force is also closely related to the free height, the end coil, the outer diameter and the properties of the steel wire, which can be changed by adjusting one or more of them.

 

3. The spring head shall not be welded for the spring without supporting ring and the spring with too small steel wire diameter, but the end wire rope shall not be obviously loose and burr shall be removed. The length of the welding part of the multi strand spring to be welded shall be less than 3 times of the cable diameter (the longest length shall not be greater than 10 mm). The heating length should be less than one circle, and it should be polished smoothly after welding. The welding parts should be annealed at low temperature during gas welding.

 

4. Generally, the surface treatment of spring can be done by phosphating or other treatments. When zinc and cadmium are to be plated, dehydrogenation treatment shall be carried out after electroplating, and 3% (no less than 3 pieces) shall be taken for retest after dehydrogenation, and there shall be no fracture during retest. The surface dirt, salt mark and oxide scale should be removed by sand blowing or gasoline cleaning, but acid washing is not allowed.

 

5. The compression time of the important spring is 24 hours, and that of the ordinary spring is 6 hours or 3-5 times continuously, holding for 3-5 seconds each time. The clearance between the spring and the mandrel should be 10% of the mandrel diameter. If the clearance is too small, it is difficult to operate. If the clearance is too large, it is easy to cause the spring to bend and deform. If one of the springs is broken during pressing, the rest should be treated again.