We deal with machining every day. Do you know what kind of precision tolerance grades can be achieved by turning, milling, planing, grinding, drilling and boring?
The tolerance grade refers to the grade that determines the degree of dimensional accuracy. The national standard stipulates that it is divided into 20 grades, from IT01, IT0, IT1, IT2 to IT18. The larger the number, the lower the tolerance grade (processing accuracy), the larger the allowable range of variation (tolerance value) of the size, and the lower the processing difficulty.
According to the different functions of product parts, different processing accuracy needs to be achieved, and the selected processing forms and processing processes are also different. This article introduces the processing accuracy that can be achieved by several common processing forms such as turning, milling, planing, grinding, drilling and boring.
IT standard tolerance grade table (click on the picture to enlarge and view) Note: When the basic size is less than 1mm, there is no IT14 to IT18
01 Turning
The workpiece rotates, and the turning tool moves in a straight line or curve in the plane for cutting. Turning is generally performed on a lathe to process the inner and outer cylindrical surfaces, end faces, conical surfaces, forming surfaces and threads of the workpiece.
The turning accuracy is generally IT8~IT7, and the surface roughness is 1.6~0.8μm.
1) Rough turning strives to use large cutting depth and large feed rate to improve turning efficiency without reducing cutting speed, but the processing accuracy can only reach IT11, and the surface roughness is Ra20~10μm.
2) Semi-finishing and finishing turning try to use high speed and small feed rate and cutting depth, the processing accuracy can reach IT10~IT7, and the surface roughness is Ra10~0.16μm.
3) High-speed finishing of non-ferrous metal parts on high-precision lathes with finely polished diamond turning tools can achieve processing accuracy of IT7~IT5, and the surface roughness is Ra0.04~0.01μm. This kind of turning is called "mirror turning".
02 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 is the same or opposite to the workpiece feed direction during milling, 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 during rough milling is IT11~IT13, and the surface roughness is 5~20μm. 2) The machining accuracy during semi-finishing milling is IT8~IT11, and the surface roughness is 2.5~10μm. 3) The machining accuracy during fine milling is IT16~IT8, and the surface roughness is 0.63~5μm.
03 Planing
Planing is a cutting method in which a planer is used to perform 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 precision 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.
04 Grinding
Grinding refers to the processing method of using abrasives and grinding tools to remove excess materials on the workpiece. It is a finishing process that is widely used in the machinery manufacturing industry.
Grinding is usually used for semi-finishing and finishing, with a precision of IT8~IT5 or even higher, and 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.
05 Drilling
Drilling is a basic method of hole processing. Drilling is often performed on drilling machines and lathes, and can also be performed 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.
06 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.
