02. Tolerance - (2.1) The fit of the hole and axis [Definition, interpretation of IT tolerance]

In the last post, we looked at what tolerance is and its necessity.

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02. Tolerance - (1) What is tolerance? Necessity and standard of tolerance.

 

In this post, we will look at the fitting of holes and axes,

and the order is as follows, and now, we will look at items 1 and 2 (IT tolerance).

 

1. Definition and Notation of IT tolerances.

2. Meaning and example of IT tolerance.

3. Tolerance Class of holes and axes - Definition & Notation.

4. Interpretation of tolerances and Fundamental Deviation.

5. Fitting method.

6. Recommended fitting area

 

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1. Definition and Notation of IT tolerance.

 

A. Definition

 

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- As shown in the image below, Figure A, it is the tolerance

applied to the diameter dimension of the axis and hole.

(The red number is the IT tolerance.)

 

- It is an internationally established tolerance standard. (It's used worldwide)

 

- As shown in the table below, the defined precision level differs

according to the drawing dimension (the size of the object).

(The dimensions of the axes and holes in the image below are 10 mm, so 20 ea IT grades can be applied.)

Applicable IT Grade regarding Basic size

 

Diameter dimensions of holes and axes (Red letters are IT grade, and IT is omitted and only numbers are indicated.)

 

 

B. Notation format.

 

- IT tolerances are indicated along with basic size and tolerance range as shown in the following image.

(The image below is the dimension used for the axis and hole above.)

 

Notation of IT Tolerance - 1

Notation of IT Tolerance - 2

 

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2. Meaning, necessity, and interpretation of IT tolerance.

 

A. Meaning of IT tolerance.

 

- IT tolerance represents the precision of dimensions.

High precision means that the allowable tolerance range is narrow.

Conversely, low precision means that the tolerance range is wide.

- The larger the IT grade (the closer to IT18 grade), the lower the precision. (Tolerance has a wide range.)

- Precision is defined in each IT tolerance grade, and the (KS) standard for this is (KSB) ISO286-1.

 

B. Necessity of IT tolerance.

 

- It is not necessary to manufacture unnecessary measuring instruments by using standardized IT tolerances.

(If tolerances are set according to the supervision of each designer,

they will be very diverse, resulting in additional works. This also affects product cost.)

- The tolerance range can be expressed more simply than the general tolerance notation.

 

C. Examples of interpretation of IT tolerances.

 

<Example ① - ∅16H7 >

- 위 예시 치수는 정치수, 공차역, IT공차 3가지로 이루어져 있으며, 각각에 대해 해석해본다.

- The above example dimensions consist of three types: Basic size, Tolerance class, and IT tolerances,

and each will be interpreted.

 

1) ∅16 (Basic size)

- It means a hole or axis with a reference dimension of 16.

 

2) H (Tolerance class)

- It represents Tolerance class and has two meanings.

- First, since the alphabet is uppercase, it represents the tolerance grade of the hole.

- Second, the Lower Limit Deviation* is presented, and the value is 0.

* Lower Limit Deviation = Minimum dimensions that can be manufactured - Reference dimensions

(The details of the Tolerance range will be covered in the next post.)

 

3) 7 (IT Grade)

 

- It represents the IT Grade.

- IT grade presents tolerance values (tolerance range).

: The definition of IT7 represents a different value for each reference dimension,

and in this example, the reference dimension is 16.

Therefore, the tolerance is 18 μm = 0.018 mm.

(This is the value defined in (KS B) ISO286-1).

 

Interpretation of Example 1

(Interpretation Description: If the diameter of the hole is processed to a dimension

corresponding to 16mm or more and 16.018mm or less, it is a good product.)

 

<Example ② - ∅9H7 >

- This time, we will interpret each of the Basic size, Tolerance class, and IT tolerances

of the above example dimensions.

 

1) ∅9 (Basic size)

- It is a hole or axis with a reference dimension of 9.

 

2) H (Tolerance class)

- 예시 1과 마찬가지로 공차역이 알파벳 대문자이므로 구멍의 공차등급을 나타내며, 아래 치수 허용차*는 0이다.

- As in Example 1, the tolerance class is an alphabetic uppercase character,

indicating the tolerance grade of the hole, and the dimension tolerance below* is 0.

(The details of the tolerance station will be covered in the next post.)

 

3) 7 (IT Grade)

- It represents the IT grade and presents the tolerance value (tolerance range).

 

: The definition of IT7 represents a different value for each basic size,

and in this example, the basic size is 9.

Therefore the tolerance is 15 μm = 0.015 mm. (This is the value defined in KS B ISO286-1).

 

Interpretation of Example 2

(Interpretation Description: If the diameter of the hole is processed to a dimension

corresponding to 9mm or more and 9.015mm or less, it is a good product.)

 

As seen in the example above, the dimensions of the holes and axes can be fully understood

only when the IT grade and the tolerance class are interpreted.

 

In the next post, we will learn about the tolerance class of axes and holes.

 

The above article is for reference to help the company's purchasing, sales, and design personnel.

Unauthorized distribution is prohibited.