The method of cylindrical grinding and the selection techniques of grinding wheel and grinding allowance

一. The type of cylindrical grinding

1. Central cylindrical grinding

2. Centerless cylindrical grinding

3. Cylindrical grinding of end face

2. Grinding method of outer circle and stepped surface

1. Cylindrical grinding method

(1) Longitudinal grinding method

The longitudinal grinding method is the most commonly used grinding method. During grinding, the worktable is reciprocated longitudinally, and the grinding wheel is periodically laterally fed. The grinding allowance of the workpiece must be ground in multiple reciprocating strokes.

The characteristics of the longitudinal grinding method (referred to as the longitudinal method):

1) In the whole width of the grinding wheel, the working conditions of the abrasive grains are different. The sharp corner of the left end surface (or right end surface) of the grinding wheel bears the main cutting action, and the grinding allowance of the workpiece part is cut off by the abrasive grains at the sharp corner of the grinding wheel, while Most of the abrasive grains on the width of the grinding wheel are responsible for reducing the surface roughness value of the workpiece. The longitudinal grinding method has small grinding force and good heat dissipation conditions, and can obtain higher machining accuracy and smaller surface roughness values.

2) Low labor productivity.

3) The grinding force is small, suitable for grinding slender, precise or thin-walled workpieces.

(2) Plunge grinding method

The plunge grinding method is also called the transverse grinding method. The length of the outer circle of the workpiece to be ground should be less than the width of the grinding wheel. During grinding, the grinding wheel makes continuous or intermittent lateral feed movement until all the allowance is ground. There is no longitudinal feed movement when the grinding wheel is grinding. Higher cut-in speed can be used for coarse grinding; lower cut-in speed can be used for fine grinding to prevent burns and heat deformation of the workpiece.

The characteristics of the plunge grinding method (referred to as the plunge method):

1) The working conditions of the abrasive grains on the entire width of the grinding wheel are the same, and the grinding effect of all abrasive grains is fully exerted. At the same time, the basic time of grinding is shortened due to the continuous lateral feed, so it has high production efficiency.

2) The radial grinding force is large, and the workpiece is prone to bending deformation, which is generally not suitable for grinding thinner workpieces.

3) Large grinding heat is generated during grinding, and the workpiece is easily burned and deformed due to heat.

4) The shape of the grinding wheel surface (dressing marks) will be copied to the surface of the workpiece, affecting the surface roughness of the workpiece. In order to eliminate the above defects, a small longitudinal movement can be made at the end of the cutting method.

5) Due to the limitation of the width of the grinding wheel, the cut-in method is only suitable for the outer circular surface with a short grinding length.

(3) Sectional grinding method

The segmented grinding method is also called the comprehensive grinding method. It is a comprehensive application of the cut-in method and the longitudinal method, that is, the workpiece is roughly ground in sections by the cut-in method, leaving a margin of 0.03~0.04mm, and finally finely ground to size by the longitudinal method. This grinding method not only utilizes the advantages of high production efficiency of the plunge method, but also has the advantages of high machining accuracy of the longitudinal method. When grinding in sections, there should be an overlap of 5~10mm between two adjacent sections. This grinding method is suitable for workpieces with good grinding allowance and rigidity, and the length of the workpiece should also be appropriate. Considering the grinding efficiency, a wider grinding wheel should be used to reduce the number of segments. The best state is when the length of the machined surface is about 2 to 3 times the width of the grinding wheel.

(4) Deep grinding method

This is a more commonly used grinding method, which uses a larger amount of back engagement to grind all the grinding allowance of the workpiece in one longitudinal feed. Since the basic grinding time is shortened, the labor productivity is high.

Features of deep grinding method:

1) Suitable for grinding workpieces with good rigidity

2) The grinder should have greater power and rigidity.

3) When grinding, use a small single-direction longitudinal feed. The longitudinal feed direction of the grinding wheel should face the headstock and lock the tailstock sleeve to prevent the workpiece from falling off. The hardness of the grinding wheel should be moderate and have good grinding performance.

2. Grinding method of step surface

The step surface of the workpiece can be ground by the end surface of the grinding wheel by moving the worktable by hand after grinding the outer circle. When grinding, the grinding wheel needs to be withdrawn laterally a little, and the workbench is shaken by hand. After the grinding wheel contacts the end surface of the workpiece, feed intermittently, and pay attention to pouring sufficient cutting fluid to avoid burning the workpiece. Usually, the end face of the grinding wheel can be trimmed into a concave shape to reduce the contact area between the grinding wheel and the workpiece and improve the grinding quality.

When grinding the step surface, the grinding wheel is under great side pressure. Therefore, the workbench must be moved carefully during operation. When the end surface of the workpiece is in contact with the grinding wheel, the longitudinal feed handwheel can be lightly tapped with the hand to ensure a small and uniform feed.

3. Selection of external grinding wheel

1. The principle of reasonable selection of grinding wheel

The choice of grinding wheel not only affects the machining accuracy and surface quality of the workpiece, but also affects the loss, service life, production efficiency and production cost of the grinding wheel. To achieve the purpose of choosing a grinding wheel reasonably, the following basic principles should be observed:

(1) Abrasive grains should have good grinding performance.

(2) The grinding wheel should have suitable "self-sharpening" during grinding.

(3) The grinding wheel should not be blunted and has a long service life.

(4) A small grinding force is generated during grinding.

(5) Small grinding heat is generated during grinding.

(6) It can achieve high machining accuracy (dimensional accuracy, shape accuracy, position accuracy).

(7) A smaller surface roughness value can be achieved.

(8) There are no burns and cracks on the surface of the workpiece.

2. Selection of main characteristics of cylindrical grinding wheel

The outer round grinding wheel is generally a flat grinding wheel with a medium structure, and the size of the grinding wheel is selected according to the machine tool specifications. The selection of the main characteristics of the external grinding wheel includes the selection of abrasive grain, hardness and grain size.

(1) Selection of abrasives: The selection of abrasives mainly corresponds to the material and heat treatment method of the workpiece to be processed. Among various artificial abrasives, brown corundum and white corundum are the most commonly used.

(2) Selection of hardness: In addition to following the general principles of hardness selection, the impact on the "self-sharpening" of the grinding wheel and the contour of the micro-blade should also be considered.

(3) Selection of grain size: The thickness of the abrasive grains of the grinding wheel directly affects the roughness of the workpiece surface and the grinding performance of the grinding wheel. For fine grinding, a finer grain size should be selected; for coarse grinding, the opposite is true. When grinding workpieces that are easily deformed, the particle size should also be selected coarser.

4. Selection of grinding allowance and grinding amount

1. Allocation of grinding allowance

Reasonable determination of grinding allowance plays an important role in improving production efficiency and ensuring workpiece quality. When determining the grinding allowance, a series of factors should be considered, such as the shape, size, technical requirements, process sequence, heat treatment method, processing method adopted, equipment conditions, etc. of the part. The general principles are as follows:

(1) When the shape of the workpiece is complex, the technical requirements are high, and the process sequence is complex, the grinding allowance should be large, such as high-precision machine tool spindles and sleeves and other parts.

(2) The workpiece is slender or thin-walled, and the grinding allowance should be larger.

(3) For workpieces that require heat treatment, the grinding allowance should be larger in consideration of heat treatment deformation.

(4) The larger the workpiece size, the more processing error factors, and the possibility of deformation caused by grinding force and internal stress increases, so the margin should be increased accordingly.

(5) The grinding allowance decreases in the order of coarse grinding, semi-finishing, fine grinding, and precision grinding.

2. Selection of cylindrical grinding dosage

The choice of grinding amount has an impact on the surface roughness of the workpiece, machining accuracy, productivity and process cost.

(1) Selection of the peripheral speed of the grinding wheel: when the peripheral speed of the grinding wheel increases, the grinding productivity is significantly improved; at the same time, due to the reduction of the grinding thickness cut by each abrasive grain, the surface roughness value of the workpiece is reduced, and the load of the abrasive grains is reduced. . General cylindrical grinding Vs=35m/s, high-speed cylindrical grinding Vs=45m/s. High-speed grinding requires high-strength grinding wheels.

(2) Selection of the peripheral speed of the workpiece: When the peripheral speed of the workpiece increases, the amount of metal removed by the grinding wheel per unit time increases, thereby improving the grinding productivity. However, with the increase of the peripheral speed of the workpiece, the grinding thickness of a single abrasive grain increases, and the plastic deformation of the workpiece surface increases accordingly, which increases the surface roughness value. Generally, Vw should maintain an appropriate proportional relationship with Vs. For cylindrical grinding, take Vw=13~20m/min.

(3) Selection of the amount of back cutting: When the amount of back cutting increases, the productivity increases, the surface roughness of the workpiece increases, and the grinding wheel tends to become blunt. Generally ap=0.01~0.03mm, ap<0.01mm during fine grinding.

(4) Choice of longitudinal feed: The effect of longitudinal feed on processing is the same as that of back cut. During rough grinding, f=(0.4~0.8)B, during fine grinding, f=(0.2~0.4)B, where B is the width of the grinding wheel.

5. The use of positioning datum and fixture

1. The concept of positioning benchmark

The datum is the point, line, and surface on the part used to determine the position of other points, lines, and surfaces. The benchmarks used for positioning are called positioning benchmarks. Positioning is the process of determining the correct position of a part. Two center holes are set on the cylindrically ground shaft parts. Usually, the shaft is clamped with two tops, and its positioning reference is the central axis formed by the two central holes, and the workpiece is rotated to form a cylindrical surface.

2. Center hole

The cylindrical grinding process is considered for general shaft parts, and the design center hole is added as the positioning reference on the part drawing. There are two standards for common center holes. Type A center hole is that the 60° cone is the working part of the center hole, which is supported by the top 60° cone, which plays the role of centering and bears the grinding force and the gravity of the workpiece at the same time. The small cylindrical hole on the front face of the 60° cone can store lubricant to reduce the friction between the tip and the center hole during grinding. Type B center hole has a 120° protection cone, which can protect the edge of the 60° cone from scratches, and is often used in workpieces with high precision and long processing procedures.

3. Technical requirements for the center hole

(1) The roundness tolerance of the 60° cone is 0.001mm.

(2) The 60° conical surface is inspected by the gauge coloring method, and the contact surface should be greater than 85%.

(3) The coaxiality tolerance of the central holes at both ends is 0.01mm.

(4) The surface roughness of the conical surface shall be Ra0.4μm or less, and there shall be no defects such as burrs and bruises.

In order to meet the requirements for the center hole, the center hole can be repaired by the following methods:

1) Repair the center hole with oilstone and rubber grinding wheel

2) Grinding the center hole with a cast iron tip

3) Grinding the center hole with a shaped internal grinding wheel

4) Extrude the center hole with a four-sided carbide tip

5) Grinding the center hole with a center hole grinder

4. Top

The shank of the top is a Morse cone, and the size of the top is expressed by the Morse taper, such as Morse No.3 top. The tip is a universal fixture, widely used in cylindrical grinding.

5. Various mandrels

The mandrel is a special fixture used for clamping sleeve parts to meet the accuracy requirements of cylindrical grinding of parts.

6. Commonly used measuring tools and their use

1. The reading of the vernier caliper

The vernier caliper is composed of a measuring claw, a ruler body, a vernier depth gauge and a fastening screw.

2. The reading of the micrometer

The micrometer is composed of a ruler frame, an anvil, a micrometer screw, a locking device, a fixed sleeve, a differential cylinder, and a force measuring device. Before use, the measuring surface of the micrometer should be cleaned and the zero position of the micrometer should be calibrated. Pay attention to the correct measurement posture when measuring.