Transfer from China Machinery Community
We deal with CNC processing every day, I believe you must have a deep impression on the machining accuracy, but you used to express the machining accuracy of the “tolerance level” know how much?
Tolerance grade refers to the grade of determining the accuracy of dimensions. The National Standard stipulates that there are 20 grades. From IT01, IT0, IT1, IT2 to IT18, the larger the number, the lower the tolerance grade (machining accuracy) , and the larger the allowable range (tolerance value) of dimensions, the less difficult it is to process.
According to the different function of product parts, the machining precision is different, and the machining form and technology are also different. This paper introduces the machining precision that can be achieved by turning, milling, planing, grinding, drilling and boring.
A cutting process in which the workpiece is rotated and the turning tool moves in straight or curved lines in a plane. Turning is usually carried out on a lathe, which is used to process the inner and outer cylindrical surface, end surface, conical surface, forming surface and thread of the workpiece.
Turning machining accuracy is generally IT8~IT7, surface roughness is 1.6~0.8μm。
（1）Under the condition that the cutting speed is not reduced, the high cutting depth and feed rate are adopted to improve the turning efficiency, but the machining precision can only reach the IT11, surface roughness of Rα20~10μm.
（2）Semi-finished and finished cars adopt high speed and small feed and cutting depth, the machining accuracy can reach IT10 ~ IT7, the Surface roughness is Rα10~0.16μm。
（3）The high-speed fine non-ferrous metal parts of diamond turning tool with fine finishing on high-precision lathe can make the machining precision reach Rα0.04IT7~IT5,0.01μm, the surface roughness of the lathe is called “mirror turning “.
Milling refers to the use of rotating multi-edge cutting tools to cut workpieces, is a high efficiency machining method. Suitable for machining planes, grooves, various forming surfaces (such as splines, gears and threads) and special shape surfaces of molds. According to the same or opposite direction of the main moving speed and the feed direction of the workpiece during milling, it is divided into forward milling and reverse milling.
Milling accuracy can generally reach IT8~IT7, surface roughness of 6.3~1.6μm.
（1）Machining precision IT11~IT13, surface roughness 5~20μm. during milling.
（2）Machining precision IT8~IT11, surface roughness 2.5~10μm. in semi-finishing milling.
（3） Machining precision IT16~IT8, surface roughness 0.63~5μm. during finishing milling.
Planing is a method of using planer to make horizontal relative linear reciprocating motion to the workpiece, which is mainly used for the contour machining of parts.
Machining accuracy is generally up to IT9~IT7, surface roughness is Ra6.3~1.6μm.
（1）Rough machining accuracy IT12~IT11, surface roughness 25~12.5μm.
（2） The machining precision of semi-finish planer is up to IT10~IT9, surface roughness is 6.2~3.2μm.
（3）The precision of finish planing can reach IT8 ~ IT7 and the Surface roughness is3.2~1.6μm.
Grinding refers to the use of abrasives, abrasives to remove excess materials on the workpiece processing methods, belong to finishing in the machinery manufacturing industry is more widely used.
Grinding is usually used for semi-finishing and finishing with an accuracy of IT8-IT5 or higher, and generally Surface roughness1.25~0.16μm.
(1)Surface roughness of precision grinding is 0.16~0.04μm.
(2) Surface roughness of ultra-precision grinding 0.04~0.01μm.
(3)surface roughness of mirror grinding can reach below 0.01μm.
Drilling is a basic method of hole processing. Drilling is often done on drilling machines and lathes, or on boring machines or milling machines.
The machining accuracy of drilling is low, generally can only reach IT10, surface roughness is generally 12.5~6.3μm, After drilling, reaming and reaming are often used for semi-finishing and finishing.
Boring is a kind of inner diameter cutting process which uses cutting tools to expand holes or other circular contours. Its application range is generally from semi- Rough machining to finishing, and the tool used is usually a single-edge boring tool (called boring bar).
(1)The boring accuracy of steel material is generally up to IT9~IT7, surface roughness is 2.5~0.16μm.
(2) Precision boring can achieve IT7~IT6, surface roughness of 0.63~0.08μm.