Process-Specific Tolerance Capabilities in Metal Cutting Operations

Do you know the precision tolerance grades that can be achieved through turning, milling, planing, grinding, drilling, and boring?

 

Tolerance grade refers to the level of dimensional accuracy in manufacturing. National standards classify these grades into 20 categories, ranging from IT01, IT0, IT1, and IT2 to IT18. Higher numbers indicate lower tolerance grades (which means less precision), a larger allowable range of dimensional variation (the tolerance value), and reduced manufacturing difficulty.

 

The required processing accuracy varies depending on the function of the product parts, which also influences the choice of processing method and technology. This article discusses the processing accuracy that various common manufacturing methods, such as turning, milling, planing, grinding, drilling, and boring, can achieve.

The processing accuracy that can be achieved by common processing methods1

 

 

01 Turning

The workpiece rotates while the turning tool moves in a straight line or along a curve within the plane. Turning is typically performed on a lathe to process the inner and outer cylindrical surfaces, end faces, conical surfaces, forming surfaces, and threads of the workpiece.

The standard turning accuracy ranges from IT8 to IT7, with a surface roughness of 1.6 to 0.8 μm.

1) Rough Turning: This process aims to achieve high efficiency by utilizing a large cutting depth and feed rate without decreasing the cutting speed. However, the resulting processing accuracy only reaches IT11, and the surface roughness is between Ra20 and Ra10 μm.

2) Semi-Finishing and Finishing Turning: These techniques focus on high speed, along with a small feed rate and cutting depth. They can achieve a processing accuracy of IT10 to IT7, and the surface roughness can range from Ra10 to Ra0.16 μm.

3) Mirror Turning: Using a finely polished diamond turning tool on a high-precision lathe for high-speed finish turning of non-ferrous metal parts can attain a machining accuracy of IT7 to IT5 and a surface roughness between Ra0.04 and Ra0.01 μm.

The processing accuracy that can be achieved by common processing methods2

02 Milling

Milling is a process that uses rotating multi-edged tools to cut workpieces, making it a highly efficient method for various machining tasks. It is suitable for processing flat surfaces, grooves, and different types of formed surfaces, such as splines, gears, threads, and specialized mold surfaces. Milling is categorized into two types based on the direction of the main motion speed relative to the workpiece feed direction: forward milling and reverse milling.

The processing accuracy of milling typically falls between IT8 and IT7, with surface roughness ranging from 6.3 to 1.6 μm. The details for different milling stages are as follows:

- Rough Milling: Processing accuracy is between IT11 and IT13, with surface roughness ranging from 5 to 20 μm.
- Semi-Finishing Milling: Processing accuracy is between IT8 and IT11, with surface roughness ranging from 2.5 to 10 μm.
- Fine Milling: Processing accuracy is between IT16 and IT8, with surface roughness ranging from 0.63 to 5 μm.

The processing accuracy that can be achieved by common processing methods3

03 Planing

Planing is a cutting method that utilizes a planer to create horizontal linear motion on the workpiece, primarily for shaping parts. The accuracy of planing typically ranges from IT9 to IT7, with a surface roughness of Ra from 6.3 to 1.6 µm.

1. Rough planing can achieve an accuracy of IT12 to IT11, with a surface roughness of 25 to 12.5 µm.
2. Semi-fine planing can reach an accuracy of IT10 to IT9, resulting in a surface roughness of 6.2 to 3.2 µm.
3. Fine planing can attain an accuracy of IT8 to IT7, with a surface roughness of 3.2 to 1.6 µm.

 

The processing accuracy that can be achieved by common processing methods4

 

04 Grinding

Grinding is a processing method that utilizes abrasives and grinding tools to remove excess material from a custom CNC parts. As a finishing process, it is commonly employed in the machinery manufacturing industry.

This technique is typically used for semi-finishing and finishing, achieving accuracy levels ranging from IT8 to IT5 or even higher. The resulting surface roughness generally falls within the range of 1.25 to 0.16 micrometers (μm).

1) The surface roughness achievable with precision grinding is between 0.16 and 0.04 μm.
2) Ultra-precision grinding can achieve surface roughness values of 0.04 to 0.01 μm.
3) Mirror grinding can produce surface roughness of less than 0.01 μm.

The processing accuracy that can be achieved by common processing methods5

 

05 Drilling

Drilling is a fundamental method for creating holes. It is commonly carried out on drilling machines and lathes, but can also be performed on boring machines or milling machines.

The processing accuracy of drilling is generally low, typically around IT10, and the surface roughness usually ranges from 12.5 to 6.3 μm. To improve the finish and accuracy, expansion and reaming are often used after drilling for semi-finishing and finishing processes.

 

The processing accuracy that can be achieved by common processing methods6

 

06 Boring

Boring is a machining process that involves using a tool to enlarge the inner diameter of a hole or other circular contour. This process can be applied in various stages, ranging from semi-roughing to finishing. Typically, a single-edged boring tool, known as a boring bar, is used for this purpose.

1) The boring accuracy for steel materials generally achieves an international tolerance grade (IT) between IT9 and IT7, with a surface roughness ranging from 2.5 to 0.16 micrometers (μm).
2) In precision boring, the processing accuracy can reach IT7 to IT6, with a surface roughness ranging from 0.63 to 0.08 μm.

The processing accuracy that can be achieved by common processing methods7

 

The processing accuracy that can be achieved by common processing methods8

 

 

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