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What kind of precision tolerance can be achieved by turning, milling, drilling and boring respectively?

In the field of mechanical manufacturing, processing accuracy directly affects product performance and quality. As common processing methods, turning, milling, grinding, drilling and boring can achieve different precision tolerance levels. The tolerance level is a key indicator to determine the degree of dimensional accuracy. The national standard divides it into 20 levels, from IT01 to IT18. The larger the number, the lower the processing accuracy, the larger the allowable range of dimensional variation, and the lower the processing difficulty. According to the different functions of product parts, it is very important to reasonably choose the processing form and process. The following is a detailed analysis of the accuracy of these processing methods.

Turning

Turning is a cutting method in which the workpiece is rotated and the turning tool moves in a straight line or curve in the plane. It is mainly completed on a lathe and can be used to process the inner and outer cylindrical surfaces, end faces, conical surfaces, forming surfaces and threads of the workpiece. Its processing accuracy is closely related to the processing stage.

The turning processing accuracy is generally IT8~IT7, and the surface roughness is 1.6~0.8μm.

• Rough turning: To improve turning efficiency, large cutting depth and large feed rate are used without reducing cutting speed. The machining accuracy at this stage can only reach IT11, and the surface roughness is Ra20 - 10μm.
• Semi-finishing and finishing turning: By adopting high-speed cutting, with smaller feed rate and cutting depth, the machining accuracy can be improved to IT10 - IT7, and the surface roughness can reach Ra10 - 0.16μm.
• Mirror turning: On a high-precision lathe, high-speed finishing turning of non-ferrous metal parts using a finely polished diamond turning tool can improve the machining accuracy to IT7 - IT5, and the surface roughness can be as low as Ra0.04 - 0.01μm, achieving a "mirror" effect.

Milling is a high-efficiency machining method that uses a rotating multi-edge tool to cut workpieces. It is suitable for machining planes, grooves, various forming surfaces (such as splines, gears and threads) and special surfaces of molds. According to the relationship between the main motion speed direction and the workpiece feed direction during milling, it is divided into forward milling and reverse milling. The milling processing accuracy is as follows:

The processing accuracy of milling can generally reach IT8~IT7, and the surface roughness is 6.3~1.6μm.

• Rough milling: IT11~IT13, and the surface roughness is 5~20μm.
• Semi-finishing milling : IT8~IT11, and the surface roughness is 2.5~10μm. :
• Fine milling: IT16~IT8, and the surface roughness is 0.63~5μm.

Drilling

Drilling, as a basic method for hole processing, can be performed on a drilling machine, lathe, boring machine or milling machine. However, its processing accuracy is relatively low, generally only IT10, and the surface roughness is usually 12.5 - 6.3μm. Therefore, after drilling, semi-finishing and finishing by reaming and boring are often required to improve the processing quality of the hole.

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).

• Steel materials: generally reach IT9~IT7, and the surface roughness is 2.5~0.16μm.
• Precision boring: reach IT7~IT6, and the surface roughness is 0.63~0.08μm.

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