Precision machining refers to a machining technology with a machining prsicion of 1~01μm and a surface roughness of Ra01~001μm. However, this precision limit enjoys constant change as the developing of machining technology. The precision machining we are talking today could be the general machining tomorrow.
1. Precision machining accuracy including geometric tolerance, dimensional accuracy and surface condition;
2. Precision machining efficiency. Some machinings can achieve better machining accuracy, but it is difficult to achieve high machining efficiency.
Precision machining includes machining technologies such as micro machining, ultra-micro machining, and finishing. Traditional precision machining methods include abbrasive belt grinding, precision cutting, honing, precision grinding and polishing.
(1) Abrasive belt grinding is to process the workpiece by using the abrasive blended fabric as the abrasive . It is categorized into grinding by using coated abrasives and enjoys the properties of high productivity, good surface quality and wide application.
(2) Precision cutting, also known as single point diamond tuming (SPDT), uses high-precision machining tools and single crystal diamond tools for cutting. It is mainly used for precision machining of soft metals such as copper and aluminum that do not go for grinding. For instance, the computer drums, magnetic disks and metal mirrors for high-power lasers etc. It is one to two grades higher than the general cutting in machining precision.
(3) Honing refers to a kind of manching method of moving back and forth along the surface of workpiece under a certain pressure by adopting a honing head made from oilstone abrasive bars. The surface roughness after machining can reach Ra0.4~0.1µm, and up to Ra0.025µm at the best. Honing is mainly used for processing cast iron and steel rather than nonferrous metals with poor hardness and good toughness.
(4) Precision grinding and polishing is a machining method which makes the workpiece reach the required dimension and precision by means of mechanical friction between the workpiece and the tool through abrasives and machining fluid between the workpiece and the tool. Precision grinding and polishing can achieve precision and surface roughness that can not be realized by other machining methods for both metal and non-metal workpieces. The roughness of the grounded surface is Ra≤0.025µm,which means minor processing deterioration layer and high surface quality. The equipment for precision grinding is simple and this method is mainly used to process flat surfaces, cylindrical surfaces, gear surfaces and mating parts subject to sealing requirements. It can also be employed for the smoothing of gauges, gauge blocks, fuel injectors, valve bodies and valve cores.
(5) Polishing is a type of micro-machining on the workpiece surface through mechanical, chemical, and electrochemical methods. It is mainly aimed to reduce the surface roughness of the workpiece. The commonly used methods include hand or mechanical polishing, ultrasonic polishing, chemical polishing, electrochemical polishing and electrochemical mechanical combined machining, etc. After hand or mechanical polishing, the surface roughness of the workpiece is Ra≤0.05 µm, which can be used for the polishing of flat, cylindrical, curved surfaces and mold cavities. The machining precision of ultrasonic polishing can reach 0.01~0.02µm, and the surface roughness is Ra0.1µm. The surface roughness of chemical polishing is generally Ra≤0.2µm while the electrochemical polishing can be increased to Ra0.1~0.08µm.
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