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What tolerance grades can be achieved by kinds of machining process

We deal with machining every day. Do you know what precision tolerance levels can be achieved by turning, milling, planing, grinding, drilling, and boring?

Tolerance level refers to the level that determines the accuracy of dimensions. The national standard specifies 20 levels, ranging from IT01, IT0, IT1, IT2 to IT18. The larger the number, the lower the tolerance level (machining accuracy), the larger the allowable range of dimensional variation (tolerance value), and the smaller the machining difficulty.

The machining precision required for product components varies depending on their functions, and the chosen machining form and process also differ. This article introduces the machining accuracy that can be achieved by several common machining forms such as turning, milling, planing, grinding, drilling, and boring.

IT Standard Tolerance Grade Table

Note: When the basic size is less than 1mm, there is no IT14 to IT18

01 Turning

The workpiece rotates and the cutting tool moves in a straight or curved line in a plane during cutting. Turning is generally performed on a lathe to machine the inner and outer cylindrical surfaces, end faces, conical surfaces, forming surfaces, and threads of workpieces.

The machining accuracy of turning is generally IT8~IT7, and the surface roughness is 1.6~0.8 μ m.

1) Coarse turning strives to improve turning efficiency by using large cutting depths and feed rates without reducing cutting speed, but the machining accuracy can only reach IT11, with a surface roughness of Ra20~10 μ m.

2) Semi precision and precision machining should use high-speed and smaller feed rates and cutting depths as much as possible, with machining accuracy of IT10~IT7 and surface roughness of Ra10~0.16 μ m.

3) High speed precision machining of non-ferrous metal parts using finely polished diamond turning tools on high-precision lathes can achieve machining accuracy of IT7~IT5 and surface roughness of Ra0.04~0.01 μ m. This type of turning is called "mirror turning".

02 Milling

Milling refers to the use of rotating multi blade cutting tools to cut workpieces, which is an efficient machining method. Suitable for processing flat surfaces, grooves, various shaped surfaces (such as flower keys, gears, and threads), and special shaped surfaces of molds. According to whether the main motion speed direction during milling is the same or opposite to the workpiece feed direction, it can be divided into forward milling and reverse milling.

The machining accuracy of milling can generally reach IT8~IT7, with a surface roughness of 6.3~1.6 μ m.

1) The machining accuracy during rough milling is IT11~IT13, and the surface roughness is 5-20 μ m. 2) The machining accuracy during semi precision milling is IT8~IT11, and the surface roughness is 2.5~10 μ m. 3) The machining accuracy during precision milling is IT16~IT8, and the surface roughness is 0.63~5 μ m.

03 Planing

Planing machining is a cutting method that uses a planer to perform horizontal relative linear reciprocating motion on a workpiece, mainly used for the shape machining of parts.

The machining accuracy of planing can generally reach IT9~IT7, and the surface roughness is Ra6.3~1.6 μ m.

1) The precision of rough planing can reach IT12~IT11, and the surface roughness is 25~12.5 μ m. 2) The semi precision machining accuracy can reach IT10~IT9, and the surface roughness is 6.2~3.2 μ m. 3) The precision machining accuracy can reach IT8~IT7, and the surface roughness is 3.2~1.6 μ m.

04 Grinding

Grinding refers to the machining method of using abrasives and grinding tools to remove excess material from a workpiece. It belongs to precision machining and is widely used in the mechanical manufacturing industry.

Grinding is usually used for semi precision machining and precision machining, with an accuracy of IT8~IT5 or even higher, and a surface roughness of 1.25~0.16 μ m is generally ground.

1) The surface roughness of precision grinding is 0.16~0.04 μ m. 2) The surface roughness of ultra precision grinding is 0.04~0.01 μ m. 3) The surface roughness of mirror grinding can reach below 0.01 μ m.

05 Drilling

Drilling is a basic method of hole machining, and drilling is often carried out on drilling machines and lathes, as well as on boring or milling machines.

The machining accuracy of drilling is relatively low, generally only reaching IT10, and the surface roughness is generally 12.5-6.3 μ m. After drilling, drilling and reaming are often used for semi precision and precision machining.

06 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 rough machining to precision machining, and the tool used is usually a single edge boring tool (called a boring bar).

1) The boring accuracy for steel materials can generally reach IT9~IT7, with a surface roughness of 2.5~0.16 μ m. 2) The machining accuracy of precision boring can reach IT7~IT6, with a surface roughness of 0.63~0.08 μ m.

The following figure provides a more intuitive display of the precision levels of various processes