3 Measured Factors Of Machined Surface Quality After Cutting

The machined surface quality after cutting shall meet the machining requirements, including surface roughness, surface hardening degree, residual stress of machined surface, etc.

（1） Surface roughness

In cutting, due to knife marks, plastic deformation, vibration and friction, the machined surface will produce small peaks and valleys.

The height and spacing of these small peaks and valleys are called surface roughness.

Surface roughness has a great influence on the wear resistance, corrosion resistance and matching properties of parts.

It directly affects the service performance and service life of the machined surface of parts.

The national standard GB / T1031-2009 specifies the evaluation parameters and values of surface roughness.

The commonly used parameter for evaluating surface roughness is the arithmetic mean deviation Ra value of contour.

The surface roughness value generally achieved by common processing methods is shown in table 1-7.

Table 1-7 tolerance grade and surface roughness achieved by common processing methods

Surface microscopic characteristics		Ra/um	Machining accuracy	Processing method	Application
No processing	Remove burrs		IT16-IT14		Castings, forgings, weldments, stamping parts
Rough machining	Visible knife marks	≤80	T13-IT10	Rough turning, rough planing, rough milling, drilling, filing, sawing	Used for non fit dimensions or unimportant fits
	Visible knife marks	≤40	IT10		It is used for one strand requirements, mainly for the matching of length and size
	Slight knife marks	≤20	IT10-IT8
Semi finishing	Visible machining marks	<10	IT10~ Π eight	Semi fine turning, fine turning, fine milling and rough grinding	For important fit
	Slight machining marks	≤5	IT8-IT7
	No machining marks	≤2.5	IT8-IT7
Finish machining	Discernible machining trace direction	≤1.25	IT8-IT6	Fine turning, fine planing, fine grinding and comparison
	Micro identification of machining trace direction	≤0.63	IT7-IT6		For precision fit
	Do not distinguish the direction of machining marks	≤0.32	IT7-IT6
Super-finishing	Dark glossy surface	≤0.16	IT6-IT5	Fine grinding, grinding, mirror grinding, superfinishing	Finishing of gauge blocks, gauges, precision instruments and precision parts
	Glossy surface	≤0.08	IT6~IT5
	Mirror like glossy surface	≤0.04
	Foggy luster	≤0.02
	Mirror	≤0.01

Generally speaking, the smaller the surface roughness value of the part, the better the service performance of the part and the longer the service life, but the manufacturing cost of the part will increase accordingly.

（2） Surface tissue changes and white layer

After machining, the mechanical, physical and chemical properties of the surface layer of the part will be different from the base material, such as work hardening, generation of residual stress, change of fatigue strength and decline of corrosion resistance, which directly affect the service performance of the part.

There is a close relationship between parts processing quality and processing cost.

The high requirement of machining accuracy will complicate the machining process and increase the cost.

Therefore, when determining the machining accuracy and surface roughness of parts, the general principle is to select lower accuracy grade and larger surface roughness value as far as possible on the premise of meeting the service performance requirements of parts and the requirements of subsequent processes.

The so-called white layer is a special tissue form formed along with the dry and hard cutting process and exists on the surface and sub surface of the workpiece.

It is white and bright under the optical microscope, so it is called “white layer” (see Fig. 1-42).

It can be seen from fig. 1-42 that there is often an over tempered layer under the white layer, which is usually called black layer.

The experimental test shows that the white layer has special physical properties: on the one hand, the hardness is higher than that of the substrate material and the corrosion resistance is good. Fig. 1-43 shows the hardness comparison among the white layer, black layer and matrix;

On the other hand, it is relatively brittle.

If the processing parameters are not well controlled, micro cracks will occur in the organization. During the use process, the surface material of the part will fall off with the expansion of the crack.

Fig. 1-42 white layer on machined surface observed by SEM

Fig. 1-43 hardness comparison of white layer, black layer and matrix

（3） Work hardening and hardened layer depth

1. Work hardening

After cutting, the hardness of the surface is often 1 ~ 2 times higher than that of the matrix, that is, the surface is hardened, and the hardening depth ranges from tens of microns to hundreds of microns.

This hardening phenomenon caused by cutting without heat treatment is called work hardening.

The work hardening of the surface layer can improve the wear resistance of the parts, but the brittleness increases and the impact resistance decreases.

The hardened surface layer increases the difficulty of subsequent processing and tool wear.

2. Depth of hardened layer

Work hardening is usually based on the degree of hardening N and the depth of hardened layer Δ hd.

(1) The degree of hardening N is

Where

• H0 — microhardness value of matrix, unit: HV;
• H — microhardness value of hardened layer, unit: HV.

(2) Hardened layer depth Δ hd, that is, the distance between the hardened layer and the matrix, in μm meter.

General N and Δ hd is related to workpiece materials and processing methods (see table 1-8).

Table 1-8 hardened layer depth △ hd and hardening degree N of steel surface

Processing method	Average hardened layer depth △ hd/ μ m	Average degree of hardening N (%)
High speed turning	30-50	120~150
Fine car	20-60	140~180
Drill hole	180~200	160~170
Broaching	20~75	150~200
Gear hobbing	120~150	160~200
Cylindrical grinding (not quenched)	30-60	140~160
Grind	3~7	110-117

3. Measures to reduce work hardening

Choose a larger γO (see Fig. 1-44) , αO and smaller rn;

Select vc as high as possible (see Fig. 1-45) and f as small as possible (see Fig. 1-46).

ap has little effect on work hardening (see fig. 1-47);

The use of effective cutting fluid can reduce work hardening.

Fig. 1-44 △ hd- γ O relationship curve

Fig. 1-45 △ hd- vc relation curve

Fig. 1-46 △ hd-f relation curve

Fig. 47 △hd-ap relationship curve

（4） Residual stress state

In the process of machining, the workpiece will be affected by various processes and other factors.

When these factors disappear, if the above functions and effects on the workpiece cannot disappear completely, and some functions and effects remain in the workpiece, the residual functions and effects are called residual stress.

Residual stress is extremely unfavorable to the normal operation of precision parts.

Compressive stress can sometimes improve the fatigue strength of parts, but tensile stress will produce cracks and reduce the fatigue strength;

In addition, uneven stress distribution will deform the parts, which will affect the accuracy of the parts and even affect the normal operation, as shown in Fig. 1-48.

The causes of residual stress can be summarized as follows:

① Stress caused by plastic deformation;

② Thermal stress caused by cutting temperature;

③ Volume stress caused by phase transformation.

Fig. 1-48 typical residual stress distribution characteristics

（5） Effect of surface quality on product performance

Although the deterioration of processing layer only occurs in the very thin surface layer, practice has proved that it will affect the service performance of mechanical parts, and then affect the performance and service life of the whole machine.

Its impact mainly includes the following aspects:

① Affect wear resistance;

② Affect fatigue strength;

③ Affect corrosion resistance;

④ Affect the coordination nature.