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What Is CNC Tolerance?
2021.08.27
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Tolerance is a controlled margin of error. It's often expressed as a value of "+/-". The process of design and production is complicated. The engineer should have a permissible degree between design and object.
 
Choose the right tolerance is important to any project. The tolerance determines the future application possibilities, delivery, and price. The closer or tighter precision controlled, the higher cost it gets.
 
Engineers may add tolerance to drawing specifications to ensure the machined parts have controlled size and geometry. It is inefficient to adding tolerances to every single part. Unless otherwise specified by the customers, 2fprototypes controls the tolerance according to ISO 2768. If the design’s dimension is correct, the machinist would be clear about the designer’s intent and assembly requirements. 
 
Why need ISO 2768 standard?
ISO 2768 is an international manufacturing standard for linear dimensions, angular dimensions, and geometrical tolerances. It is intended to simplify drawing specifications for mechanical tolerances and preventing misunderstandings. The standard makes design and manufacturing more convenient. Cooperation between different companies gets closer and easier. 2fprototype’s CNC machining service adheres to ISO 2768 requirements.
 
ISO 2768 can be divided into two parts. The ISO 2768-1 is for linear and angular dimensions and the ISO 2768-2 is for geometrical tolerances.
 
The linear and angular dimension includes external sizes, internal sizes, step sizes, diameters, distances, chamfer heights for broken edges, etc. The precision level is defined as f-fine, m-medium, c-coarse, v-very coarse.
 
The geometrical tolerance includes flatness, straightness, cylindericity, and circularity. The precision level is defined as H-fine, K-medium, L-coarse.


Engineers should note the precision grade on the drawings if ISO 2768 is applied, such as ISO 2768-mK. Please choose the correct precision level according to the requirements of machining and design.

 

Linear Dimensions
(For nominal sizes below 0.5 mm, the deviations shall be indicated adjacent to the relevant nominal size(s).)

 

External Radii and Chamfer Heights
(For nominal sizes below 0.5 mm, the deviations shall be indicated adjacent to the relevant nominal size(s).)

Permissible deviations in mm for ranges in nominal lengths

Tolerance Class Designation (Description)

 

Permissible deviations in mm for ranges in nominal lengths

Tolerance Class Designation (Description)

f (fine)

m (medium)

c (coarse)

v (very coarse)

 

f (fine)

m (medium)

c (coarse)

v (very coarse)

0.5-3

±0.05

±0.1

±0.2

 

0.5-3

±02

±0.2

±0.4

±0.4

3-6

±0.05

±0.1

±0.3

±0.5

 

3-6

±0.5

±0.5

±1.0

±1.0

6-30

±0.1

±0.2

±0.5

±1.0

 

Over 6

±1.0

±1.0

±2.0

±2.0

30-120

±0.15

±0.3

±0.8

±1.5

 

 

 

 

 

 

120-400

±0.2

±0.5

±1.2

±2.5

 

 

 

 

 

 

400-1000

±0.3

±0.8

±2.0

±4.0

 

 

 

 

 

 

1000-2000

±0.5

±1.2

±3.0

±6.0

 

 

 

 

 

 

2000-4000

±2.0

±4.0

±8.0

 

 

 

 

 

 

 

Angular Dimensions

Permissible deviations in mm for ranges in nominal lengths
单位:mm

Tolerance Class Designation (Description)

f (fine)

m (medium)

c (coarse)

v (very coarse)

Up to 10

±1º

±1º

±1º30′

±3º

10-50

±0º30′

±0º30′

±1º

±2º

50-120

±0º20′

±0º20′

±0º30′

±1º

120-400

±0º10′

±0º10′

±0º15′

±0º30′

Over 400

±0º5′

±0º5′

±0º10′

±0º20′

 

 

General Tolerances on Straightness and Flatness

 

General Tolerances on Perpendicularity

Ranges of nominal lengths in mm

Tolerance Class

 

Ranges of nominal lengths in mm

Tolerance Class

H

K

L

 

H

K

L

Up to 10

0.02

0.05

0.1

 

Up to 100

0.2

0.4

0.6

10-30

0.05

0.1

0.2

 

100-300

0.3

0.6

1

30-100

0.1

0.2

0.4

 

300-1000

0.4

0.8

1.5

100-300

0.2

0.4

0.8

 

1000-3000

0.5

1

2

300-1000

0.3

0.6

1.2

 

 

 

 

 

1000-3000

0.4

0.8

1.6

 

 

 

 

 

 

General Tolerances on Symmetry

 

General Tolerances on Circular Run-Out

Ranges of nominal lengths in mm

Tolerance Class

 

Tolerance Class
Ranges of nominal lengths in mm

H

K

L

 

H

K

L

Up to 100

0.5

0.6

0.6

 

0.1

0.2

0.5

100-300

0.5

0.6

1

 

 

300-1000

0.5

0.8

1.5

 

 

 

 

1000-3000

0.5

1

2

 

 

 

 

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