Turning, Milling, Planing, Grinding, Drilling and Boring- What Precision Tolerance Levels Can They Achieve?

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 levels refer to the degree of dimensional accuracy, classified into 20 grades according to national standards, ranging from IT01, IT0, IT1, IT2 to IT18.

The higher the number, the lower the tolerance level (machining accuracy), the larger the permissible variation in size (tolerance value), and the easier the machining process.

Different product components require different machining precisions based on their functions, and thus different machining methods and processes are chosen.

This article introduces the machining precision achievable by common methods such as turning, milling, planing, grinding, drilling, and boring.

IT Standard Tolerance Level Table

Note: No IT14 to IT18 for basic sizes less than 1mm

01 Turning

Turning involves rotating the workpiece while the cutting tool moves in straight or curved paths within a plane. Turning is generally performed on a lathe and is used to machine internal and external cylindrical surfaces, end faces, conical surfaces, shaped surfaces, and threads.

• The typical machining precision for turning is IT8 to IT7, with a surface roughness of 1.6 to 0.8 μm.

1. Rough Turning: Aims to improve efficiency by using large cutting depths and feed rates while maintaining cutting speed, but the machining precision can only reach IT11, with a surface roughness of Ra20 to 10 μm.
2. Semi-Finish Turning and Finish Turning: Uses high speed with smaller feed rates and cutting depths, achieving machining precision of IT10 to IT7 and a surface roughness of Ra10 to 0.16 μm.
3. High-Precision Turning: Achieved with finely honed diamond cutting tools on high-precision lathes for non-ferrous metals, reaching machining precision of IT7 to IT5 and a surface roughness of Ra0.04 to 0.01 μm, known as "mirror turning."

02 Milling

Milling involves cutting the workpiece with a rotating multi-edged tool and is a high-efficiency machining method suitable for processing planes, grooves, various shaped surfaces (such as splines, gears, and threads), and special mold contours. Milling can be classified as climb milling or conventional milling, depending on the direction of the main cutting motion relative to the workpiece feed direction.

• Milling typically achieves machining precision of IT8 to IT7, with a surface roughness of 6.3 to 1.6 μm.

1. Rough Milling: Achieves machining precision of IT11 to IT13, with a surface roughness of 5 to 20 μm.
2. Semi-Finish Milling: Achieves machining precision of IT8 to IT11, with a surface roughness of 2.5 to 10 μm.
3. Finish Milling: Achieves machining precision of IT16 to IT8, with a surface roughness of 0.63 to 5 μm.

03 Planing

Planing involves using a planing tool for horizontal, straight, reciprocating cutting motion, mainly for shaping parts.

• Planing typically achieves machining precision of IT9 to IT7, with a surface roughness of Ra6.3 to 1.6 μm.

1. Rough Planing: Achieves machining precision of IT12 to IT11, with a surface roughness of 25 to 12.5 μm.
2. Semi-Finish Planing: Achieves machining precision of IT10 to IT9, with a surface roughness of 6.2 to 3.2 μm.
3. Finish Planing: Achieves machining precision of IT8 to IT7, with a surface roughness of 3.2 to 1.6 μm.

04 Grinding

Grinding involves removing excess material from the workpiece using abrasives and grinding tools, and is widely used for fine finishing in mechanical manufacturing.

• Grinding is typically used for semi-finishing and finishing, with precision levels reaching IT8 to IT5 or higher, and surface roughness generally ranging from 1.25 to 0.16 μm.

1. Precision Grinding: Achieves a surface roughness of 0.16 to 0.04 μm.
2. Ultra-Precision Grinding: Achieves a surface roughness of 0.04 to 0.01 μm.
3. Mirror Grinding: Achieves a surface roughness of below 0.01 μm.

05 Drilling

Drilling is a basic method for hole machining, often performed on drilling machines and lathes, and can also be done on boring or milling machines.

• Drilling generally achieves lower machining precision, typically IT10, with a surface roughness of 12.5 to 6.3 μm. Often, reaming or boring is used for semi-finishing and finishing after drilling.

06 Boring

Boring is a process of enlarging holes or other circular contours using a single-edged boring tool, applicable from semi-rough to finish machining.

1. Boring Steel Materials: Typically achieves precision levels of IT9 to IT7, with a surface roughness of 2.5 to 0.16 μm.
2. Precision Boring: Achieves machining precision of IT7 to IT6, with a surface roughness of 0.63 to 0.08 μm.