Precautions for Part Eng'g Drawings

The following is a comprehensive summary of precautions for producing part engineering drawings, covering key points in the entire process from basic settings to detailed annotations:

I. Core Principles of View Layout

1. Selection of Drawing Format and Scale

• Drawing format: Prioritize using the horizontal A3/A4 drawing format (for easy binding).
• Scale priority: 1:1 > Enlarged scale (e.g., 2:1) > Reduced scale (e.g., 1:2).
• Complex parts: Multiple scales can be combined (local enlarged views should be marked with their own scales separately).

2. View Planning

• Main view: Reflect the main outline of the part (preferably select the machining position or working position).
• Auxiliary views: Sectional views (full section/half section/stepped section), directional views, and local views.
• Layout examples:
  • The main view occupies 1/3 - 1/2 of the drawing area.
  • Reserve 1/4 of the drawing area for dimension annotation.
  • Local enlarged views are arranged along the edge of the drawing frame (marked with Roman numerals such as Ⅰ, Ⅱ).

II. Dimension Annotation Specifications

1. Annotation Hierarchy

• Key dimensions: Dimensions that directly affect assembly/function (tolerances should be marked).
• Important dimensions: Dimensions that affect the manufacturing process (e.g., mating surfaces).
• General dimensions: Non - key outline dimensions (un - specified tolerances can be referenced).

2. Annotation Techniques

• Avoid chain dimensioning (to prevent cumulative errors) and use datum dimensioning.
• Group dimensions of the same features (e.g., mark uniformly distributed holes as "6×Ø8 EQS").
• Use leader lines for counterbored holes/countersunk holes (e.g., "Ø10×90°").

III. Tolerances and Technical Requirements

1. Tolerance Annotation Steps

• Mark geometric tolerances first (e.g., flatness, coaxiality).
• Then mark dimensional tolerances (preferably select common fits such as h7/g6).
• Un - specified tolerances are marked according to the m - level of GB/T1804.

2. Content of Technical Requirements

1. Material: 45 steel, quenched and tempered to HRC28 - 32.
2. Un - specified fillets are R2, and un - specified chamfers are C1.
3. Surface galvanized to 8 - 12μm (standard: GB/T 9799).
4. The surface roughness of key mating surfaces is Ra≤1.6 (Ra3.2 for the rest).

IV. Marking of Quality Characteristics

1. Importance Classification

• Critical characteristics (⚫): Those that affect safety/regulations (e.g., the wall thickness of pressure vessels).
• Important characteristics (▲): Those that affect the main function (e.g., bearing mating surfaces).
• Marking position: On the right side of the dimensional tolerance or stated in the technical requirements.

V. Avoidance of Common Errors

• Closed dimension chain: Avoid forming a closed - loop dimensioning (e.g., marking the total length and the lengths of each section simultaneously).
• Redundant annotation: Repeating the same dimension in different views.
• Process conflict: Dimensions marked contradicting the machining sequence (e.g., the selection of the datum for turning first and then milling).

VI. Verification after Drawing Production

1. Cross - Check List

2. DFM Verification

• Confirm the machining feasibility with the process engineer.
• For sheet metal parts, verify the bending radius using the developed view.

By systematically implementing the above steps, the engineering drawing can ensure three core values: manufacturing guidance (clear dimensions/tolerances), inspection traceability (clear key characteristics), and standardization (compliance with GB/enterprise specifications).