Machining accuracy is mainly used to characterize the fineness of the product, and is a term for evaluating the geometric parameters of the machined surface. Machining accuracy is measured by tolerance grade. The smaller the grade value, the higher the accuracy. There are 20 tolerance grades from IT01, IT0, IT1, IT2, IT3 to IT18. Among them, IT01 indicates that the part has the highest machining accuracy, and IT18 indicates that the part has the lowest machining accuracy. Generally, factory and mining machinery belongs to IT7 grade, and general agricultural machinery belongs to IT8 grade. According to the different functions of product parts, different machining accuracy needs to be achieved, and the selected machining form and machining process are also different. This article introduces the machining accuracy that can be achieved by several common machining forms such as turning, milling, planing, grinding, drilling, and boring. Turning is a cutting process in which the workpiece rotates and the turning tool moves in a straight line or curve in a plane. Turning is generally carried out on a lathe to process the inner and outer cylindrical surfaces, end faces, conical surfaces, forming surfaces and threads of the workpiece. The turning machining accuracy is generally IT8-IT7, and the surface roughness is 1.6-0.8μm. 1) Rough turning strives to improve turning efficiency by using large cutting depth and large feed rate without reducing cutting speed, but the machining accuracy can only reach IT11, and the surface roughness is Rα20-10μm. 2) Semi-finishing and finishing turning try to use high speed and small feed rate and cutting depth, and the machining accuracy can reach IT10-IT7, and the surface roughness is Rα10-0.16μm. 3) High-speed finishing of non-ferrous metal parts on high-precision lathes with finely polished diamond turning tools can achieve machining accuracy of IT7-IT5, and the surface roughness is Rα0.04-0.01μm. This kind of turning is called "mirror turning". Milling Milling refers to the use of rotating multi-edge tools to cut workpieces, which is a highly efficient processing method. It is suitable for processing planes, grooves, various forming surfaces (such as splines, gears and threads) and special surfaces of molds. According to whether the main motion speed direction during milling is the same or opposite to the workpiece feed direction, it is divided into forward milling and reverse milling. The machining accuracy of milling can generally reach IT8~IT7, and the surface roughness is 6.3~1.6μm. 1) The machining accuracy of rough milling is IT11~IT13, and the surface roughness is 5~20μm. 2) The machining accuracy of semi-finishing milling is IT8~IT11, and the surface roughness is 2.5~10μm. 3) The machining accuracy of finishing milling is IT16~IT8, and the surface roughness is 0.63~5μm. Planing
Planing is a cutting method that uses a planer to make horizontal relative linear reciprocating motion on the workpiece, which is mainly used for the shape processing of parts. The machining accuracy of planing can generally reach IT9~IT7, and the surface roughness is Ra6.3~1.6μm. 1) The machining accuracy of rough planing can reach IT12~IT11, and the surface roughness is 25~12.5μm. 2) The machining accuracy of semi-finishing planing can reach IT10~IT9, and the surface roughness is 6.2~3.2μm. 3) The precision of fine planing can reach IT8~IT7, and the surface roughness is 3.2~1.6μm. Grinding Grinding refers to the processing method of using abrasives and grinding tools to remove excess material on the workpiece. It is a kind of fine processing and is widely used in the machinery manufacturing industry. Grinding is usually used for semi-finishing and finishing, and the precision can reach IT8~IT5 or even higher. The surface roughness is generally ground to 1.25~0.16μm. 1) The surface roughness of precision grinding is 0.16~0.04μm. 2) The surface roughness of ultra-precision grinding is 0.04~0.01μm. 3) The surface roughness of mirror grinding can reach less than 0.01μm. Drilling Drilling is a basic method of hole processing. Drilling is often carried out on drilling machines and lathes, and can also be carried out on boring machines or milling machines. The machining accuracy of drilling is low, generally only IT10, and the surface roughness is generally 12.5~6.3μm. After drilling, reaming and boring are often used for semi-finishing and finishing. Boring Boring is a cutting process that uses a tool to enlarge the inner diameter of a hole or other circular contour. Its application range generally ranges from semi-roughing to finishing. The tool used is usually a single-edged boring tool (called a boring bar). 1) The boring accuracy of steel materials can generally reach IT9~IT7, and the surface roughness is 2.5~0.16μm. 2) The machining accuracy of precision boring can reach IT7~IT6, and the surface roughness is 0.63~0.08μm.
