Grinding is a machining method in which a grinding wheel is used as a cutting tool to machine the workpiece on a grinding machine. The characteristics of this method are:
(1) Since the abrasive grains of the grinding wheel themselves possess high hardness and heat resistance, grinding can be used to machine materials with extremely high hardness, such as hardened steel, cemented carbide, etc.
(2) The characteristics of the grinding wheel and the grinding machine determine that the grinding process system can perform uniform micro-cutting. Generally, the depth of cut ap = 0.001 - 0.005 mm; the grinding speed is quite high, usually reaching v = 30 - 50 m/s; the grinding machine has good rigidity; and it adopts hydraulic transmission. Therefore, grinding can economically achieve high machining accuracy (IT6 - IT5) and a small surface roughness (Ra = 0.8 - 0.2 μm). Grinding is one of the main methods for finishing parts.
(3) Due to intense friction, the temperature in the grinding area is very high. This will cause stress and deformation in the workpiece and even result in surface burns on the workpiece. Therefore, a large amount of cooling liquid must be injected during grinding to reduce the grinding temperature. The cooling liquid can also play the roles of chip removal and lubrication.
(4) The radial force during grinding is quite large. This will cause elastic deflection in the machine tool - grinding wheel - workpiece system, making the actual depth of cut smaller than the nominal depth of cut. Therefore, when the grinding is about to be completed, it is necessary to perform a finishing grind without feeding the tool to eliminate errors.
(5) After the abrasive grains become dull, the grinding force also increases, causing the abrasive grains to break or fall off, and sharp cutting edges are exposed again. This characteristic is called "self-sharpening". The self-sharpening property enables grinding to proceed normally within a certain period of time. However, after a certain working time, manual dressing should be carried out to avoid vibration, noise, and damage to the surface quality of the workpiece caused by the increase in grinding force.
grinding wheel
A grinding wheel is a cutting tool for grinding. It is a porous object made by bonding many tiny and hard abrasive grains with a bonding agent. The abrasive grains are directly responsible for the cutting work and must be sharp and possess high hardness, heat resistance, and a certain degree of toughness. The commonly used abrasives are two types: aluminum oxide (also known as corundum) and silicon carbide. The aluminum oxide abrasives have high hardness and good toughness, and are suitable for grinding steel materials. The silicon carbide abrasives have even higher hardness, are sharper, and have good thermal conductivity, but are relatively brittle and are suitable for grinding cast iron and cemented carbide.
For grinding wheels made of the same abrasive, due to the difference in their coarseness, the surface roughness of the machined workpiece and the machining efficiency will vary. Those with coarse abrasive grains are used for rough grinding, while those with fine abrasive grains are suitable for finish grinding. The coarser the abrasive, the smaller the grain size number.
The bonding agent serves to bond the abrasives. The commonly used one is the ceramic bonding agent, followed by the resin bonding agent. Different selections of bonding agents will affect the corrosion resistance, strength, heat resistance, and toughness of the grinding wheel.
The more firmly the abrasive grains are bonded, the less likely they are to fall off the grinding wheel. This is referred to as the hardness of the grinding wheel, that is, the hardness of the grinding wheel indicates the ease with which the abrasive grains on the surface of the grinding wheel fall off under the action of external forces. Those that fall off easily are called soft, and vice versa are called hard. The hardness of the grinding wheel and the hardness of the abrasive are two different concepts. If the surface of the workpiece to be ground is relatively soft, the cutting edges (corners) of the abrasive grains will not be easily worn, so the abrasive grains can be used for a longer time. In other words, a grinding wheel with a stronger bond (a grinding wheel with higher hardness) can be selected. Conversely, a grinding wheel with lower hardness is suitable for grinding workpieces with higher hardness.
Grinding wheels work under high-speed conditions. To ensure safety, they should be inspected before installation and should not have defects such as cracks. To make the grinding wheel work smoothly, a dynamic balance test should be carried out before use.
After the grinding wheel has been working for a certain period of time, its surface voids will be clogged with grinding chips, the sharp angles of the abrasives will become dull, and the original geometric shape will be distorted. Therefore, it must be dressed to restore its cutting ability and correct geometric shape. The grinding wheel needs to be dressed with a diamond pen.
The Structure and Grinding Motions of a Surface Grinding Machine
There are many types of grinding machines, mainly including surface grinding machines, cylindrical grinding machines, internal grinding machines, universal cylindrical grinding machines (which can also grind internal holes), gear grinding machines, thread grinding machines, rail grinding machines, centerless grinding machines (for grinding external circles), and tool grinding machines (for grinding cutting tools), etc. Here, the surface grinding machine and its motions will be introduced.
1.The structure of the surface grinding machine (Taking M7120A as an example, where: M - represents grinding machine type; 71 - indicates horizontal spindle rectangular table surface grinding machine; 20 - means the width of the worktable is 200mm; A - represents the first major improvement.)
(1) Grinding Wheel Head - It is used to install the grinding wheel and drive the grinding wheel to rotate at a high speed. The grinding wheel head can make a manual or hydraulic transverse intermittent movement along the dovetail guide rail of the slide carriage.
(2) Slide Carriage - It is used to install the grinding wheel head and drive the grinding wheel head to move up and down along the column guide rail.
(3) Column - It supports the slide carriage and the grinding wheel head.
(4) Worktable - It is used to install the workpiece and is driven by the hydraulic system to make a reciprocating linear movement.
(5) Bed - It supports the worktable and installs other components.
(6) Cooling Liquid System - It provides cooling liquid (saponified oil) to the grinding area.
(7) Hydraulic Transmission System - Its components are:
1.Power Element - The oil pump, which supplies pressure oil to the hydraulic transmission system;
2.Executive Element - The oil cylinder, which drives the movement of components such as the worktable;
3.Control Element - Various valves, which control pressure, speed, direction, etc.;
4.Auxiliary Element - Such as oil tank, pressure gauge, etc.
Compared with mechanical transmission, hydraulic transmission has the advantages of smooth transmission, overload protection, and the ability to achieve stepless speed regulation within a relatively wide range
2.Surface Grinding Motions
(1) Main Motion - The high-speed rotating motion of the grinding wheel.
(2) Feed Motions
1)Longitudinal Feed - The reciprocating linear motion of the workpiece driven by the worktable;
2)Vertical Feed - The movement of the grinding wheel in the depth direction of the workpiece;
3)Transverse Feed - The intermittent motion of the grinding wheel along its axis.
