Are you tired of struggling with dimensioning in your engineering drawings? Look no further! Our comprehensive guide on dimensioning of part drawings has got you covered. From the rationality of part dimensions to the methods and steps for dimensioning parts, we provide all the necessary information to ensure your dimensions are complete, clear, and reasonable.

We understand that meeting design and process requirements can be challenging, which is why we provide practical production knowledge and relevant professional knowledge to help you master the art of dimensioning. Our guide covers everything from distinguishing primary and secondary dimensions to selecting appropriate dimensional tolerance and dimension datum.

We also provide helpful tips on closed dimension chains, marking dimensions according to the processing sequence, and precautions for dimensioning. Plus, our dimension annotation method of assembly section provides examples and guidance on dimension classification, dimension datum, and overall dimension annotation.

Say goodbye to dimensioning headaches and hello to accurate and efficient engineering drawings with our dimensioning guide.

1. Dimensioning of part drawings

1. Rationality of part dimensions

2. Methods and steps for dimensioning parts

3. Precautions for dimensioning

Basic requirements for dimensioning in part drawings

The dimensions in the part drawing shall be marked in accordance with the standard, complete, clear and reasonable.

Requirements for reasonable dimensioning:

(1) Meet the design requirements to ensure the quality of the machine,

(2) Meet the process requirements to facilitate structural manufacturing and inspection.

To meet these requirements, we must master certain practical production knowledge and relevant professional knowledge.

1. Rationality of part dimensions

The dimensions in the part drawing shall not only meet the requirements of correctness, integrity and clarity, but also make the dimensions reasonable.

Definition of rationality of dimensions on part drawing:

Meet the design requirements and ensure the service performance of components;
Meet the requirements of processing technology and make processing and measurement convenient;
Low cost.

Whether the dimensions are reasonable or not, the key points are as follows:

Distinguish the primary and secondary dimensions of parts;
Select appropriate dimensional tolerance;
Select the dimension datum correctly.

1. Main and non main dimensions

Main dimensions:

Dimensions that affect the specification and performance of components or machines, such as mating dimensions, dimensions to determine the exact position of parts in components, connection dimensions, installation dimensions, and dimensions that affect the interchangeability and working accuracy of parts.

Non main dimensions:

Such as external outline dimension, non fit dimension, dimension used to meet the mechanical properties, structural shape and process requirements of parts, etc.

Main dimensions shall be directly indicated:

2. Dimension datum of parts

Datum: select a group of geometric elements on the part as the basis for determining the mutual position relationship of other geometric elements.

According to different purposes, the benchmark is divided into design benchmark and process benchmark.

Design datum: the datum used to determine the exact position of the part in the assembly. Usually, one of them is selected as the main dimension datum.

Process benchmark: the benchmark for processing and measurement, which is often used as the auxiliary dimension benchmark.

Elements often used as datum: important bottom surface, end face, symmetrical plane, assembly joint surface, axis of main hole or shaft, etc.

Dimension datum selection:

There shall be a dimensional connection between the auxiliary datum and the main datum.

3. Closed dimension chain

The dimensions of parts in the same direction can be connected end to end and listed in the form of dimension chain.

However, it should be avoided to form a closed dimension chain.

4. Only one non machined surface can be connected with the machined surface in the same direction.

The blank size and processing size shall be marked separately as far as possible to facilitate drawing reading.

5. Mark the dimensions according to the processing sequence

Dimensions for stepped shafts

Processing sequence

1. Blanking;
2. Turning φ 10 long 38 shaft section;
3. Turning φ 8 long 22 shaft section, turning and retracting the cutter slot;
4. Thread turning and chamfering.

2. Methods and steps for dimensioning parts

General methods and steps:

(1) Analyze the function of part structure shape and understand the combination relationship with adjacent parts;

(2) Distinguish the primary and secondary dimensions, determine the design basis and mark the main dimensions;

(3) Select the process benchmark and note all non main dimensions with shape analysis method.

Example: vane pump shaft

(2) Mark the main dimensions and their deviations from the design basis.

Radial dimension;
Axial dimension;
Keyway size

(3) Considering the processing and manufacturing requirements, select the appropriate process benchmark, note all other dimensions, select the left and right end faces of the shaft as the process benchmark, and mark other dimensions.

Mark the surface structure requirements, geometric tolerance and other technical requirements.

(4) Check

Whether the main dimensions and design basis are appropriate, whether there are omissions, whether the dimension value and its deviation meet the design requirements, and whether they are coordinated with the relevant dimensions on the part drawing of relevant parts.

Check whether the dimensions are complete.

Check whether the setting size and positioning size are complete according to the structural shape of the parts.

Check whether it meets the national standard.

3. Precautions for dimensioning

1. Earnestly implement national standards.

There are standard values for length, diameter, angle, taper and its deviation, which should be selected generally;

Standard structural elements (gear teeth, threads, etc.) shall be marked with dimensions and deviations according to regulations.

2. The dimensions shall be complete, but there shall be no redundant dimensions.

3. In order to facilitate the reading of drawings, the dimensions required for the same type of work shall be marked centrally, such as the dimensions of keyway on shaft.

4. The size of the contact shall be consistent.

Each part in the component has the relationship of fit, connection, transmission, position, etc. When marking the size of their connection, the dimension datum, marking content and marking form shall be coordinated as far as possible.

5. Consider the requirements of convenient measurement and inspection.

The marked dimensions are convenient for measurement and inspection

The dimensions marked are not easy to measure