What is the process of grinding?
2021-12-06

What is the process of grinding?

Grinding is cutting processing in which the high-speed rotation of the grinding wheel is the main movement, and the low-speed rotation and linear movement of the workpiece is used as the feed movement. It is one of the most commonly used mechanical parts processing and mechanical manufacturing methods.


In the grinding process of a single abrasive grain, the function of cutting into the workpiece is divided into three stages:


(1) Slippage stage. Abrasive particles rub and squeeze on the surface of the workpiece, which makes the workpiece elastically deform. At this time, the abrasive particles have no cutting effect, which is called the sliding stage.


(2) Scribing stage. The abrasive grains scribe grooves on the surface of the workpiece. This stage is called the scribing stage.


(3) Cutting stage. The front edge of the abrasive particles slides along the shear surface to form chips. This stage is called the cutting stage.


It can be seen that the grinding process of an abrasive particle makes the grinding surface go through three stages: sliding, scribing (uplifting), and cutting. The common forms of the formed wear debris are band-shaped, nodular, tadpole-shaped, and ashes.


Grinding movement and dosage

When grinding, there are generally four movements.

1. The main movement.

The rotational movement of the grinding wheel is called the main movement. The main movement speed υc (m/s) is the linear velocity of the outer circle of the grinding wheel, that is, υc=πd0n0/1000. In the formula, d0 is the diameter of the grinding wheel (mm); n0 is the speed of the grinding wheel (r/s).

In ordinary grinding, the main motion speed υc is 3035m/s; when υc>45m/s, it is called high-speed grinding.


2. Feed movement. 

There are three types of feed motions:

1) Radial feed movement. 

The radial feed movement is the movement of the grinding wheel into the workpiece. The radial feed fr refers to the distance the workpiece moves radially relative to the grinding wheel in each double (single) stroke of the worktable, and the unit is mm/double stroke. When the grinding wheel is continuously fed, the unit is mm/s. Under normal circumstances, fr (or ap)=0.050.02mm/double stroke.

2) Axial feed movement. 

The axial feed movement is the axial movement of the workpiece relative to the grinding wheel. Axial feed refers to the axial movement distance of the workpiece relative to the grinding wheel within each revolution of the workpiece or each double stroke of the worktable. The unit is mm/r or mm/double stroke. Under normal circumstances, fa (or f) = (0.20.8) B, B is the width of the grinding wheel, and the unit is mm.

3) Circular (or linear) feed motion of the workpiece. 

Workpiece speed υw refers to the linear velocity of the circumferential feed movement of the workpiece or the linear feed movement velocity of the worktable (together with the workpiece), and the unit is m/s.


Grinding stage

During grinding, the back force Fp is very large, which causes elastic deformation of the processing system, making the actual grinding depth differ from the value displayed on the dial of the grinder. Therefore, the actual grinding process of ordinary grinding is divided into three stages.


1. The initial grinding stage. 

When the grinding wheel first contacts the workpiece, due to the elastic deformation of the processing system, the actual grinding depth is smaller than the radial feed indicated by the grinder dial. The worse the rigidity of the processing system, the longer the initial grinding stage.


2. Stable stage.

In the sound stage, when the elastic deformation of the processing system reaches a certain level, when the radial feed is continued, the actual grinding depth is equal to the radial feed.


3. The cleaning stage. 

After grinding away the main machining allowance, the radial feed can be reduced, or no feed at all can be ground for a while. At this time, due to the gradual recovery of the elastic deformation of the processing system, the actual grinding depth is greater than the radial feed. As the workpiece is ground layer by layer, the actual grinding depth approaches zero, and the grinding sparks gradually disappear. The cleaning stage is mainly to improve the grinding accuracy and surface quality.


About grinding fluid

Grinding can obtain high dimensional accuracy and low surface roughness. When grinding, the grinding speed is high, the heat is large, and the grinding temperature can be as high as 800~1000℃ or even higher. It is easy to cause burns on the surface of the workpiece, surface cracks and deformation of the workpiece due to thermal stress, wear and passivation of the grinding wheel, and abrasive particles falling off. In addition, abrasive debris and grinding wheel powder are easy to splash and fall on the surface, affecting processing accuracy and surface roughness. When processing tough and plastic materials, the grinding debris is embedded in the gap on the working surface of the grinding wheel, or the grinding debris and the processed metal are fused on the surface of the grinding wheel, which will make the grinding wheel lose its grinding ability. Therefore, to reduce the grinding temperature, wash away the grinding debris and grinding wheel powder, and improve the surface quality of the workpiece, it is necessary to use a cutting fluid with good cooling and cleaning performance and a certain degree of lubrication and rust resistance.


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