Why are machining tolerances so important in manufacturing?

The processing of machining tolerances is a rigorous and complex task that mechanical designers and manufacturers must pay close attention to. At present, as the market’s requirements for product quality continue to rise, machining tolerances have become a key parameter that must be carefully considered in the product manufacturing process. It is an important guarantee to ensure that the product remains consistent at the micro level.

1、Definition of machining tolerance

Machining tolerance refers to the range of geometric parameters allowed to vary during machining. It is an intuitive reflection of the precision requirements for part manufacturing and also reflects the difficulty of machining. Generally speaking, the smaller the tolerance value, the higher the precision of the part. However, in actual machining operations, there is no perfect process without errors. Even if modern manufacturing technology continues to develop and the tolerance value has been reduced as much as possible, it will never reach zero.

2、Explanation of terms related to tolerance calculation

1.Linear dimensions
Linear dimensions are mainly divided into two categories: linear dimensions and angular dimensions. Among them, linear dimensions refer to the distance between two points, such as diameter, width, height, depth, thickness and center distance.

2.Nominal dimensions
Nominal dimensions are dimensions determined at the product design stage, and they are the basis for subsequent dimension calculations and processing.

3.Limit size (limit size)
The limit size is the two extreme values ​​of the allowable size of a hole or shaft. The larger limit value is called the upper limit size (Dmax), and the smaller one is the minimum size (Dmin).

4.Actual size (actual size)
The actual size is the size measured after the part is processed. Because there will be errors during measurement, the actual size measured is not the real size. The nominal size and limit size are given during design, and the actual size is between the upper limit size and the lower limit size.

5.Dimension deviation
Dimension deviation is the algebraic difference between a certain size (such as limit size, actual size) and the nominal size. This algebraic difference can be positive, negative or zero, and the deviation value must be preceded by a positive or negative sign. Deviation is divided into limit deviation and actual deviation.
Upper limit deviation = upper limit dimension – nominal dimension
Lower limit deviation = lower limit dimension – nominal dimension.

6.Dimension tolerance
Dimension tolerance t (that is, the variation range of qualified actual dimension) = upper limit tolerance dimension – lower limit tolerance dimension. Because the upper limit dimension (deviation) is always larger than the lower limit dimension (deviation), the tolerance is an unsigned number and cannot be negative or zero.

3、Types of Tolerance

1.Dimensional tolerance
It refers to the amount of change allowed in the size, that is, the absolute value of the algebraic difference between the maximum limit size and the minimum limit size. This type of tolerance is directly related to the accuracy and range of variation of the part size.

2.Shape tolerance
This is the sum of the allowed changes in the shape of a single actual element, covering six aspects: straightness, flatness, roundness, cylindricity, line profile and surface profile.

3.Position tolerance
The sum of the allowable changes in the position of the relevant actual elements relative to the reference. It can limit the relative position relationship between two or more points, lines and surfaces of the part, including parallelism, verticality, inclination, coaxiality, symmetry, position, circular runout and total runout. Accurate position tolerance can ensure the accuracy and overall performance of parts during assembly.

4.ISO 286 standard tolerance grade (IT grade) comparison table (IT01 ~ IT18)

公差等级	特征说明	精度等级	典型应用示例/说明

IT01

超高精度

超高精密级

高级实验室计量仪器、量块制造

IT0

极高精度

超精密级

精密测量仪器、航空仪表零件

IT1

非常高精度

精密级

光学零件、超精密轴承、精密主轴装配

IT2

精密

精密级

高精度模具、定位销、特种机械零件

IT3

精密

精密级

高精密配合，如滑动轴承/孔轴配合

IT4

精细

精密~中等之间

精密联轴器、齿轮副配合

IT5

较高精度

中高精度

机床主轴、滚动轴承配合

IT6

通用精密机械加工

中精度

常规轴承孔、过渡配合、公差带小的传动件

IT7

通用机械加工

中精度

机床部件、滑块、导轨

IT8

中等精度

中等精度

普通联接件、轴承座壳体、一般机械零件

IT9

中等~粗略精度

中粗精度

通用结构件、公差要求不严的轴孔配合件

IT10

粗加工

粗略精度

铸造件初加工、一般冲压件

IT11

粗加工

粗略精度

粗车零件、非配合面

IT12

初级加工

较粗略

焊接结构件、结构支撑件

IT13

粗略加工

粗略

粗车零件、机械支撑座

IT14

非配合零件

粗略

建筑机械结构件、零配件的外壳

IT15

非配合零件

很粗

模糊尺寸要求、无精密要求的构件

IT16

粗糙结构用途

非精密

焊接粗糙结构、框架件

IT17

极粗加工

非精密

大型铸件、初级构造件

IT18

极粗加工

极低精度

大型焊接结构、土木工程组件

5、Points to note when using tolerance

When dealing with machining tolerances, proper planning in advance can effectively utilize resources, reduce the error rate of finished products, and improve work efficiency. However, the smaller the tolerance, the better. The smaller the tolerance, the higher the precision requirement and the higher the cost. The tolerance should be determined according to the actual needs of the parts.

When controlling strict tolerances, special attention should be paid to the following points:

1.Cost
Based on the prototype and usage of the designed part, map the relationship between cost increase and tolerance zone decrease, find the critical value that meets the acceptable range of the specific project, and then adopt the minimum machining tolerance that fits the budget. This can not only meet the basic requirements of the product, but also achieve optimal cost control.

2.Inspection
After the product is completed, it enters the quality inspection stage, which is to determine whether the final product is qualified. If it is not qualified, it will be rejected. Depending on the strictness of the machining tolerances, the time for the quality inspection phase is different. The stricter the tolerances, the higher the time cost and the more complex the testing equipment required.

3.Processing technology and methods
Different processing techniques require different mechanical equipment, and these equipment have a significant impact on the machining tolerances of parts. Since some parts need to go through multiple different machines during processing, it is necessary to understand the tolerance levels of different machines in detail and adjust the processing sequence in a planned way, so that the tolerances of the finished product can be effectively controlled.

4.Surface roughness
The surface of all products has roughness distortion, and different materials have different roughness. So when choosing tight tolerances for CNC machining, existing surface roughness conditions must be considered. If tight machining tolerances are the goal, rough surfaces will cause trouble and may affect the final quality and accuracy of the product.

6、Conclusion