Machining Surface Quality

Machining Surface Quality

The destruction of mechanical parts usually starts from the surface layer. The performance of the product, especially its reliability and durability, largely depends on the quality of the surface layer of the part. The purpose of researching the machined surface quality is to grasp the laws of the influence of various technological factors on the machined surface quality, so as to control the processing process by using these laws, and ultimately achieve the goal of improving the surface quality and the service performance of products.

1. The influence of machined surface quality on machine performance

(1) Effect of Surface Quality on Wear Resistance

Effect of Surface Roughness on Wear Resistance

The contact area between the two contact surfaces of a newly machined friction pair is much smaller than the theoretical contact area. There is a very large unit stress at the contact peak, which results in plastic deformation, elastic deformation and shear fracture between the two contact surfaces at the actual contact area. Bad, cause serious wear and tear.

The wear of parts can generally be divided into three stages: initial wear stage, normal wear stage and severe wear stage.

Surface roughness has a great influence on surface wear of parts. Generally speaking, the smaller the surface roughness, the better the wear resistance. But the surface roughness value is too small, the lubricating oil is not easy to store, and the molecular bonding between the contact surfaces is easy to occur, and the wear is increased. Therefore, the roughness of the contact surface has an optimum value, which is related to the working condition of the parts. When the working load increases, the initial wear amount increases, and the optimum value of the surface roughness also increases.

Effect of Surface Cold Hardening on Wear Resistance

The cold work hardening of the machined surface improves the microhardness of the metal on the surface of the friction pair, so the wear resistance is generally improved. But it is not that the higher the degree of cold work hardening, the higher the wear resistance. This is because excessive cold work hardening will cause excessive loosening of the metal structure, and even cracks and exfoliation of the surface metal, which will reduce the wear resistance.

(2) Effect of Surface Quality on Fatigue Strength

The fatigue failure of metals under alternating loads often occurs on the surface of parts and under the surface hard layer, so the surface quality of parts has a great influence on the fatigue strength.

Effect of Surface Roughness on Fatigue Strength

Under the action of alternating load, the stress concentration and fatigue crack are easy to occur in the concave part of surface roughness. The bigger the surface roughness is, the deeper the marks on the surface and the smaller the radius of the bottom, the worse the anti-fatigue ability of the bottom.

EFFECT OF RESIDUAL STRESS AND COLD WORKING HARDENING ON FATIGUE STRENGTH

The residual stress has a great influence on the fatigue strength of parts. The residual tensile stress of the surface layer will expand the fatigue crack and accelerate the fatigue failure, while the residual stress of the surface layer can prevent the fatigue crack growth and delay the fatigue failure.

Surface hardening is usually accompanied by residual stress, which can prevent cracks from occurring and prevent the propagation of existing cracks. It is beneficial to improve fatigue strength.

(3) Effect of Surface Quality on Corrosion Resistance

The corrosion resistance of parts depends largely on the surface roughness. The greater the surface roughness, the more corrosive substances accumulate in the valley. The worse the corrosion resistance is.

The residual tensile stress of the surface layer will cause stress corrosion cracking, which will reduce the wear resistance of the parts, while the residual compressive stress can prevent stress corrosion cracking.

(4) Effect of Surface Quality on Coordination Quality

The value of surface roughness will affect the quality of matching surface. For clearance fit, roughness value will increase wear and clearance, and destroy the required fit property. For interference fit, part of the surface bumps are extruded during assembly, and the actual interference decreases, which reduces the connection strength between fittings.

2. Factors Affecting Surface Roughness

(1) Factors affecting surface roughness in cutting

Representation of Tool Geometry

When the tool moves relative to the workpiece, the residual area of the cutting layer is left on the machined surface, and the geometric shape of the tool is reconstructed when its shape is changed. The height of the residual area can be reduced by reducing the feed rate, the main deflection angle, the secondary deflection angle and increasing the radius of the tool tip arc.

In addition, it is also an effective measure to reduce the surface roughness by properly increasing the rake angle of the cutting tool to reduce the degree of plastic deformation during cutting, reasonably selecting lubricant and improving the grinding quality of the cutting tool to reduce the plastic deformation during cutting and restrain the formation of knife tumors and scales.

Properties of Workpiece Materials

When processing plastic materials, the extrusion of the metal by the cutter produces plastic deformation, and the tearing effect of the cutter forces the chips to separate from the workpiece increases the value of surface roughness. The better the toughness of the workpiece material, the greater the plastic deformation of the metal, the rougher the machined surface.

When processing brittle materials, the chips are granular, and many pits are left on the machined surface due to the chips crushing, which makes the surface rough.

Cutting parameters

(2) Factors Affecting Surface Roughness in Grinding

Just like the formation of surface roughness in cutting, the formation of surface roughness in grinding is determined by geometric factors and plastic deformation of surface metals.

The main factors affecting grinding surface roughness are as follows:

GRAIN OF GRINDING WHEEL

Hardness of grinding wheel

Dressing of grinding wheel

grinding speed

Cooling Lubricant

Grinding Radial Feed and Grinding Number

Workpiece Circumferential Feed Velocity and Axial Feed

3. Factors Affecting Physical and Mechanical Properties of Machining Surface Layer

In the cutting process, the physical and mechanical properties of the surface metal change due to the effect of cutting force and heat. The most important changes are the change of microhardness, metallographic structure of the surface metal and the change of the surface metallographic structure.