About Mechanical design and processing’s several issues

In the market economy, enterprises must be invincible in the competition, relying on high-quality, low-cost products. Mechanical design work is important because it is the first step in preparing production technology and will seriously affect the quality and cost of the product. Although many factors affect the cost of the product, they are mainly related to design, manufacturing, and raw materials.

Selection of materials in mechanical design

Material selection is appropriate in the design and manufacture of mechanical parts. It directly affects the parts’ performance, service life, and manufacturing costs.

When encountering parts of the material selection problem, generally refer to the same or similar parts of the material program and select traditionally used materials (this method is known as the experience of material selection method).

When there is no precedent to follow, at the same time on the material’s performance (such as corrosion resistance, etc.) and no special requirements, they are only based on simple calculations and manuals to provide data, hand-selected a more versatile material.

In practice, mechanical engineers often see parts of the material selection as a simple and less important task.

1. mechanical parts material selection should meet the basic requirements

1.1. performance requirements

Material performance in the use process, that is, the use of performance, is the selection of materials that should be considered to meet the fundamental requirements. Different parts required by the use of performance are very different. Some parts mainly require high strength, some require high wear resistance, and some have no strict performance requirements, only a beautiful appearance.

Therefore, the first task in selecting materials is to determine the main performance required by the parts accurately.

1.2 Requirements for Process Performance

Material technology reflects the material itself’s ability to adapt to various processing requirements, that is, the requirements of the selected materials in the processing and manufacturing of the first to create a finished product. It can facilitate manufacturing while guaranteeing quality.

1) Thermal process performance Thermal process performance mainly refers to casting, forging, welding, and heat treatment performance.

2) cutting performance of metal cutting performance is generally used for tool durability of 60min cutting speed V60 to indicate that the higher the V60, the better the metal cutting performance.

1.3 Economic Performance Requirements

Selection of parts and materials to minimize the cost of obtaining the maximum economic benefits. At the same time, to meet the premise of performance, the choice of materials should also pay attention to minimizing the total cost of parts.

1) Material prices accounted for a large proportion of the total cost of the product, up to 30% ~ 70%.

2) Improved utilization of precision casting, die forging, and cold-drawn blanks can reduce the waste of cutting surface materials.

3) Parts of the processing and maintenance costs should be as low as possible.

4) A combination of structures, such as a worm gear ring with good wear reduction of precious metals and other parts using inexpensive materials.

5) Reasonable substitution of materials for the production of large quantities of parts, taking into account the resource situation in China, the source of materials to be rich, try to avoid the use of scarce materials; as far as possible, use high-strength cast iron instead of steel, heat treatment methods, and other methods to strengthen the carbon steel instead of alloy.

2. Mechanical parts material selection methods

2.1. the impact of material selection on product life cycle cost: the selection of materials greatly affects the various components of the product life cycle cost. Engineering practice, to ensure that the reasonable function of the product (or performance) under the premise, although generally the selection of inexpensive materials, can reduce the life cycle cost of the product;

But at the same time, we should pay more attention to is that sometimes if the choice of high-cost but better-performance material due to the product weight reduction, service life extension, maintenance cost reduction, energy cost reduction, and other favorable factors from the point of view of the product life cycle cost considerations, on the contrary, it is economical.

2.2. The choice of manufacturing method is an inseparable factor in material selection; that is, the structural design, material selection, and available processing methods should be viewed as an organic whole.

The selection of materials should consider the cost of a single part of the processing process and the total cost of the entire processing route involved in all processing procedures.

Mechanical design standardization is the main way to improve product quality and reduce costs.

1. Mechanical parts are the basic components of the machine; for mechanical parts design work, the role of standardization is very important. The so-called standardization of parts, through the size of the parts, structural elements, material properties, inspection methods, design methods, drawing requirements, etc., to develop various common standards.

The superiority of standardization is shown in the following.

1.1 The most advanced method, centralized manufacturing, can be used in specialized factories to produce large quantities of the most widely used parts to improve quality and reduce costs.

1.2 Harmonization of material and component performance specifications to enable comparisons and increase component performance reliability.

1.3 Adopting a standard structure and parts and components can simplify the design work, shorten the design cycle, and improve the design quality.

2. Improving the standardization of the design stage is an important way to reduce product costs in the market economy. Manufacturers should be based on market demand changes, constantly update product variety, improve product quality, reduce material consumption, and improve economic efficiency.

To achieve these purposes, which are inseparable from standardization, standardized means must be used, starting with the strict good product design of this pass, to enable enterprises to survive and develop in market competition and accelerate the development of new products.

Factors affecting the physical and mechanical properties of machined parts surface layer

Due to the role of cutting force and cutting heat in machining, the physical and mechanical properties of the surface layer will change greatly, resulting in differences in the performance of the base material. These changes are mainly manifested in the surface layer of the metallurgical organization and hardness of the changes in the surface layer and the surface layer of the residual stress.

1. Changes in surface layer metallographic organization

The machining process in the processing area is due to the processing of the heat consumed by the vast majority of the conversion into thermal energy to make the processing of the surface temperature rise. When the temperature rises to more than the critical point of metallurgical organization changes, the surface layer metallurgical organization will change.

In a general cutting, the chip takes away most of the cutting heat, so the impact is also small. However, for grinding processing, because the cutting heat generated per unit area is dozens of times larger than in the general cutting method, the high temperature of the cutting zone will cause the phase change of the surface layer metal.

Factors affecting grinding burns are:

1.1. Grinding wheel material For a grinding wheel with too high a hardness, the passivated abrasive particles are not easy to remove, and the wheel is easily blocked by cutting. Therefore, it is generally good to use a soft grinding wheel.

1.2. Grinding dosage When the grinding depth increases, the temperature of the workpiece’s surface and the different depths under the surface will increase, which can easily cause burns; when the workpiece’s longitudinal feed increases, the temperature of the grinding zone increases, but the time of the heat source’s action decreases, which can reduce the burns.

However, increasing the workpiece’s speed will increase its surface roughness value. Increasing the grinding wheel speed can compensate for this deficiency. The practice has proved that, at the same time, improving the workpiece speed and grinding wheel speed can reduce the workpiece’s surface burns.

1.3 A cooling method using cutting fluid to remove the heat of the grinding zone can avoid burns. However, due to the rotation of the grinding wheel surface to produce a strong airflow layer, cutting fluid is not easy to adhere to, so not much cutting fluid can enter the grinding zone.

Therefore, high-pressure, high-flow cooling can increase the cooling effect. On the other hand, the grinding wheel surface can be flushed so that the chips do not block the grinding wheel.

2. Cold work hardening of machined surfaces

Processing the surface layer of metal plastic deformation between the crystals produces shear slip, serious distortions in the lattice, and broken and fibrous grain elongation. Strengthening the material improves its strength and hardness. This change is called cold hardening.

Processing of the surface layer cold hardening indicators to harden the depth of the layer, its microhardness, and its degree of hardening. Generally, the greater the degree of hardening, the greater the depth of the hardened layer.

The main factors affecting cold work hardening:

2.1. As cutting speed increases, the tool and workpiece contact extrusion time is short, and plastic deformation is small. The temperature also increases when the speed is high, which helps the recovery of cold hardening, but the cold hardening is weaker. The cutting force increases when the feed increases, the plastic deformation increases, and the hardening strengthens.

However, when the feed is small, the degree of hardening increases due to the increase in the number of times the rounded corner of the tool’s cutting edge is squeezed per unit length of the machined surface.

2.2. As the tool edge radius increases, the surface layer extrusion effect becomes large, increasing cold hardening; tool vice rear face wear increases, increasing machined surface friction, increasing cold hardening; and tool front angle increases can reduce plastic deformation, reducing cold hardening.

2.3. Workpiece material: The lower the hardness of the workpiece material, the greater the plastic deformation and the more serious the phenomenon of cold hardening after cutting.

3. Residual stress of the surface layer

Cutting process of metal materials in the surface layer of the organization of the shape and organization changes, in the surface layer of metal and the junction of the base material will produce a mutually balanced elastic stress, the stress is the surface residual stress. There are three main reasons for the generation of residual stresses in the surface layer:

3.1. Residual stresses caused by cold plastic deformation

Under the action of the cutting force, the processed surface undergoes strong plastic deformation and the volume of the surface layer of metal changes when the base metal is affected by the elastic deformation state.

After the cutting force is removed, the base metal tends to recover. Still, because the surface layer has produced plastic deformation of the limitations, recovery can not return to its original state and thus produces residual stress in the surface layer.

3.2. Residual stresses caused by thermal plastic deformation

When the base metal temperature is low, the workpiece’s surface is processed in the cutting heat under thermal expansion, producing thermal compressive stress. When the cutting process is over, the surface temperature decreases because the surface layer has produced thermoplastic deformation and is limited by the substrate, thus producing residual tensile stresses.

3.3. Residual stresses caused by changes in metallurgical organization

The high temperature generated during cutting will cause changes in the metallurgical organization of the surface layer. Due to the different metallurgical organizations’ different densities, the surface layer metallurgical organization changes are caused by volume changes when the surface layer volume expands due to the limitations of the matrix, resulting in compressive stress. When the volume of the surface layer shrinks, tensile stress is generated.

Characteristics and application of lubricant

To achieve the desired geometry, dimensional accuracy, and surface quality of the workpiece, it is necessary to cut, grind, stamp, roll, and draw the workpiece, etc. Metalworking lubricant is a lubricating and cooling material used in the metalworking process.

1. Role and technical requirements of lubricant

The main purpose of the metal chip lubricant we use is:

(1) Extend the service life of cutting tools;

(2) To ensure and improve the machining dimensional accuracy of the workpiece;

(3) Improve the surface finish of the workpiece;

(4) timely removal of metal chips to ensure that the cutting process is carried out smoothly;

(5) Take away the heat of chips in time, quickly and evenly cool the tool and workpiece, etc.;

(6) Prevent machine tools and workpieces from corrosion and rust;

(7) Improve cutting efficiency and reduce costs. To realize the above purposes, the requirements of metal chip lubricants must have the following aspects of the main performance:

1.1. Cooling performance

In the metal cutting process, the energy consumed, the vast majority (more than 90%) is converted into cutting heat, which not only makes the tool easy to wear but also makes the workpiece is easy to produce localized burns or darkening, affecting the machining accuracy of the surface of the workpiece and machining quality.

Therefore, the requirements of metal cutting lubricants can first reduce friction to reduce heat generation. Secondly, heat conduction, convection, and vaporization can dissipate heat through the cutting fluid.

1.2. Lubrication performance

A cutting tool in the cutting workpiece moment usually withstands a great cutting force action. Good metal cutting lubricant lubrication can reduce the tool’s front face and chip, back face and workpiece surface, or grinding wheel and material friction and wear, reduce the coefficient of friction, and prevent the tool and the chip or the workpiece from adhesion.

Reduce the power consumption and prolong the tool’s service life so that the workpiece can get a good surface finish; more importantly, reduce the chances of chip tumors (i.e., cutter tumors) in the cutting process.

1.3. Cleaning performance

Metal in the cutting (or grinding) process, oil, fine chips, metal powder, and grinding wheel grit, etc., bond to each other and adhere to the workpiece, tool, and machine tool, affecting the quality of the workpiece processing, reducing the service life of tools and grinding wheels, and affecting the accuracy of the machine tool.

Therefore, the metal cutting lubricant should have a good cleaning effect, reduce the fine chips and metal powder and other adhesion to facilitate the cleaning, and at the same time, quickly small chips and metal powder and other timely flushing away.

1.4. Rust Prevention Performance

Water-based metal cutting lubricants in use of most of the process is water (about 80% -98%), for the production of corrosion provide a favorable environment in which the extreme pressure agent and some surfactants tend to exacerbate the corrosion of metal;

Therefore, water-based metal cutting lubricants must have certain anti-rust properties so the workpiece does not produce rust after a short period.

2. Selection of metal cutting lubricants

Selection of metal cutting lubricants: according to the cutting process conditions and requirements, the initial judgment is to select pure oil or water-soluble cutting lubricants. Usually, we can choose according to the recommendation of the machine tool supplier;

Secondly, it can also be selected according to conventional experience, such as the use of high-speed steel tools for low-speed cutting, usually pure oil-based, the use of carbide tools for high-speed cutting, usually can be water-soluble;

For liquid supply difficulties or cutting fluid, reaching the cutting zone is difficult when using pure oily metal cutting fluid (such as tapping, bore broaching, etc.); other cases can usually be used in water-soluble metal cutting fluid.

Secondly, after selecting the cutting fluid, according to the material to be processed and the workpiece’s machining accuracy and roughness requirements, such as the initial selection of cutting fluid varieties.

Then, according to the different characteristics of different materials, selecting different cutting fluid products due to mechanical equipment in the operation process, the load, parts processing accuracy, and other aspects of the impact of friction and wear is inevitable.

With the increasingly harsh working conditions of machinery and equipment or the improvement of work intensity, lubricant performance requirements are also increasing. At the same time, a lubricant often requires a variety of functions.

Practice shows that lubricants play a key role in reducing mechanical wear and prolonging the service life of machinery.

Conclusion

In short, in the market economy, mechanical products are more advanced and more varied to better meet the development of enterprises. This requires us to reduce the factors affecting the physical and mechanical properties of the surface layer and the reasonable use of lubricants in the mechanical design, the choice of materials, and standardized application to a new understanding in the processing process—efforts to reduce our processing costs to a reasonable level, thereby improving the economic efficiency of enterprises.