Machining Methods

Basic Knowledge Of Grinding Technology

Basic Knowledge Of Grinding Technology

Grinding is a method of cutting a workpiece using a grinding wheel as a cutting tool on a grinding machine. The characteristics of this method are:

(1) Due to the high hardness and heat resistance of the grinding wheel abrasive grains, grinding can process materials with high hardness, such as hardened steel and cemented carbide.

(2) The characteristics of the grinding wheel and the grinding machine determine that the grinding process system can make uniform micro-cutting, generally ap=0.001~0.005mm; the grinding speed is very high, generally up to v=30-50m/s; the grinding machine has good rigidity; adopt Hydraulic transmission, so grinding can economically obtain high machining accuracy (IT6~IT5) and small surface roughness (Ra=0.8~0.2μm). Grinding is one of the main methods for finishing parts.

(3) Due to severe friction, the temperature in the grinding zone is very high. This will cause stress and deformation of the workpiece, and even cause burns on the surface of the workpiece. Therefore, a large amount of coolant must be injected during grinding to reduce the grinding temperature. The coolant can also play the role of chip removal and lubrication.

(4) The radial force during grinding is large. This will cause the elastic retreat of 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, the tool should not be fed for smooth grinding to eliminate errors.

(5) After the abrasive particles are blunt, the grinding force also increases, causing the abrasive particles to break or fall off, and the sharp edge is exposed again. This feature is called “self-sharpness”. The self-sharpening makes grinding normal in a certain period of time, but after a certain working time, it should be manually trimmed to avoid vibration, noise and damage to the surface quality of the workpiece caused by increased grinding force.

Grinding wheel

The grinding wheel is a cutting tool for grinding. It is a porous object composed of many small and hard abrasive particles and a bonding agent. The abrasive grains are directly responsible for the cutting work, and must be sharp and have high hardness, heat resistance and certain toughness. Commonly used abrasives are alumina (also known as corundum) and silicon carbide. Alumina abrasives have high hardness and good toughness, suitable for grinding steel. Silicon carbide abrasives have higher hardness, sharper, good thermal conductivity, but are more brittle, suitable for grinding cast iron and cemented carbide.

The grinding wheels of the same abrasive have different surface roughness and processing efficiency after the workpiece is processed. The coarser abrasive particles are used for coarse grinding. The finer abrasive particles are suitable for fine grinding. The coarser the abrasive, the smaller the particle size.

The binder acts as a bonding abrasive. The ceramic bond is commonly used, followed by the resin bond. The choice of binder materials is different, which affects the corrosion resistance, strength, heat resistance and toughness of the grinding wheel.

The stronger the abrasive particles are bonded, the harder it is to fall off the grinding wheel, which is called the hardness of the grinding wheel, that is, the hardness of the grinding wheel refers to the difficulty of the abrasive particles on the surface of the grinding wheel falling off under the action of external force. Easy to fall is called soft, otherwise it is called hard. The hardness of the grinding wheel and the hardness of the abrasive are two different concepts. The surface of the workpiece to be ground is soft, and the edges (corners) of the abrasive particles are not easy to wear, so that the abrasive particles can be used for a longer time, that is to say, the grinding wheel with stronger adhesion (higher hardness grinding wheel) can be selected. Conversely, low-hardness grinding wheels are suitable for grinding high-hardness workpieces.

The grinding wheel works under high-speed conditions. In order to ensure safety, it should be checked before installation, and there should be no defects such as cracks; in order to make the grinding wheel work smoothly, a dynamic balance test should be carried out before use.

After working for a certain period of time, the surface gap will be blocked by abrasive debris, the sharp angle of the abrasive will be blunt, and the original geometry will be distorted. Therefore, it must be trimmed to restore the cutting ability and the correct geometry. The grinding wheel needs to be trimmed with a diamond pen.

Structure and grinding movement of surface grinder

There are many types of grinders, mainly surface grinders, cylindrical grinders, internal grinders, universal cylindrical grinders (which can also grind inner holes), gear grinders, thread grinders, guide rail grinders, centerless grinders (grinding outer circles) and tool grinders ( Grinding tools) etc. Here we introduce the surface grinder and its movement.

1. Structure of surface grinder (Take M7120A as an example, among them: M-grinder type machine tool; 71-horizontal axis rectangular table surface grinder; 20-working table width 200mm; A-first major improvement.)

(1) Grinding wheel frame-install the grinding wheel and drive the grinding wheel to rotate at high speed. The grinding wheel frame can be moved along the dovetail guide rail of the carriage for manual or hydraulic lateral clearance movement.

(2) Sliding seat-install the grinding wheel frame and drive the grinding wheel frame to move up and down along the column guide rail.

(3) Uprights-support the slide and grinding wheel frame.

(4) Worktable-install the workpiece and drive it by the hydraulic system for reciprocating linear motion.

(5) Bed—support worktable and install other components.

(6) Coolant system-to provide coolant (saponified oil) to the grinding area.

(7) Hydraulic transmission system-its components are:

1) Power component-is an oil pump, supplying pressure oil of hydraulic transmission system;

2) Actuating element-it is an oil cylinder, which drives the worktable and other components to move;

3) Control elements-for various valves, control pressure, speed, direction, etc.;

4) Auxiliary components-such as fuel tank, pressure gauge, etc.

Compared with mechanical transmission, hydraulic transmission has the advantages of stable transmission, overload protection, and stepless speed regulation in a large range.

2. Surface grinding motion

(1) Main motion-the high-speed rotary motion of the grinding wheel.

(2) Feed movement

1) Longitudinal feed-the worktable drives the reciprocating linear motion of the workpiece;

2) Vertical feed-the movement of the grinding wheel in the depth direction of the workpiece;

3) Lateral feed-the grinding wheel moves along the gap of its axis.

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