Essential Knowledge for Mechanical Engineers

Publish Time: 2025-10-15 Origin: Site

Box-type parts serve as the outer casings or bases of machines or components, encompassing various bodies (bases), pump casings, valve bodies, tailstock bodies, and more. As core components in machinery, they fulfill critical functions such as supporting internal parts, housing mechanisms, positioning components accurately, and providing a sealed environment.

Structurally, box-type parts are relatively complex. Most are manufactured from castings followed by necessary machining processes, while some are welded. Their overall features include a hollow shell or box structure, with intricate internal cavities and external shapes. They typically incorporate flanges for connection and fixation, shaft holes and ribs for support, base plates for mounting, as well as functional holes like installation holes, threaded holes, and pin holes. Additionally, they often feature manufacturing-related structures such as casting fillets, draft angles, and chamfers to facilitate production.

1. View Selection for Box-Type Part Engineering Drawings

The selection of views, particularly the main view, is fundamental to effectively representing box-type parts. It should primarily be determined based on the part’s working position (to align with its actual application scenario) and shape characteristics (to highlight key structural features). To comprehensively display both internal and external structures, a combination of views and sections (or partial sections) is commonly employed. Specific strategies include:

When the external structure is simple but the internal structure is complex, and the part has a symmetry plane, half-section views are ideal. This allows simultaneous visualization of the external contour and internal details on either side of the symmetry plane.
If the external structure is complex while the internal structure is simple, partial sections or dashed lines can be used. Partial sections focus on revealing local internal features without obscuring the external shape, while dashed lines are suitable for simple internal structures that do not require detailed emphasis.
For parts with both complex internal and external structures:

If their projections do not overlap, partial sections can still be applied.
If projections overlap, the internal and external structures must be represented separately to avoid confusion.
Local views, partial sections, and cross-sections are effective for illustrating local internal and external details, ensuring no critical features are overlooked.

Due to the complex projection relationships of box-type parts, intersection lines (formed by the intersection of two surfaces) and interpenetration lines (formed by the intersection of two solids) frequently appear. Since most box-type parts start as castings, transition lines (used to represent rounded transitions between surfaces in castings) are also common and must be accurately depicted.

2. Dimensioning of Box-Type Part Engineering Drawings

Dimensioning is a precise process that directly impacts the manufacturing accuracy of box-type parts. Key principles and methods are as follows:

2.1 Selection of Datums

Datums are the reference points, lines, or planes used for dimensioning. For box-type parts:

The height direction typically uses the mounting base plate as the primary datum, ensuring the part’s stability during installation.
The length and width directions often rely on symmetry planes, key mounting planes, or the axes/centerlines of important holes. These datums guarantee the positional accuracy of functional components relative to each other.

2.2 Key Dimensioning Requirements

Positioning dimensions: These dimensions define the location of features such as holes and flanges. The distances between the centerlines of various holes must be 标注 directly to ensure precise alignment of assembled components.
Form dimensions: These describe the shape and size of individual features (e.g., the diameter of a hole, the thickness of a rib). They should be 标注 using the shape analysis method—breaking the part into basic geometric shapes (e.g., cylinders, prisms) and dimensioning each shape separately.
Critical dimensions: Dimensions that affect the part’s performance, assembly, or functionality (e.g., the diameter of a mating hole, the flatness of a sealing surface) must be 标注 directly. This avoids cumulative errors from indirect dimensioning and ensures compliance with design requirements.

3. Technical Requirements for Box-Type Part Engineering Drawings

Technical requirements specify non-dimensional constraints that guarantee the part’s performance, quality, and manufacturability. They typically include the following aspects:

3.1 Surface Roughness

Surface roughness (Ra value) determines the smoothness of a part’s surface and affects functions like friction, sealing, and wear resistance:

Mounting base plates and contact planes: Generally require a roughness of Ra 6.3–3.2 μm to ensure stable installation and tight contact.
Mating surfaces, rotating surfaces, and critical holes/planes: Require higher precision, with a roughness of Ra 3.2–0.8 μm to minimize friction and ensure reliable mating.
Non-machined surfaces: Retain their original casting or welding surface state, with no additional machining required.

3.2 Tolerances

Dimensional tolerances: Critical holes and surfaces often specify dimensional tolerances ranging from IT6 to IT10 (per GB/T 1804-2000). For example, a hole for a precision bearing might have a tolerance of H7 (a common fit for rotating shafts).
Geometric tolerances: These control deviations in shape, orientation, location, and runout. Common geometric tolerances include flatness (for sealing surfaces), roundness (for cylindrical holes), perpendicularity (for mounting holes relative to the base plate), parallelism (for two mating planes), coaxiality (for concentric holes), and runout (for rotating surfaces). These tolerances typically comply with GB/T 1184-1996 (Grade K) for general applications, with stricter grades (e.g., Grade H) used for high-precision components.

3.3 Material and Heat Treatment

Common materials:

Cast iron: Grades like HT150 and HT200 are widely used for their good castability, wear resistance, and cost-effectiveness (suitable for non-high-load applications).
Cast steel: ZG230-450 offers higher strength and toughness, making it ideal for parts subjected to heavy loads or impact.
Aluminum alloy: ZL102 is lightweight and corrosion-resistant, suitable for applications where weight reduction is critical (e.g., aerospace components).

Heat treatment: Most castings require stress relief annealing to eliminate internal stresses caused by uneven cooling during casting. This prevents deformation or cracking during machining or service. Some high-performance parts may also undergo quenching and tempering to enhance hardness and strength.

3.4 Casting and Machining Requirements

Casting defects: Castings must be free of defects such as blowholes, cracks, and sand inclusions, which could weaken the part or cause leaks (e.g., in pump casings or valve bodies).
Unspecified features:

Unspecified casting fillets: Typically range from R3 to R5 to prevent stress concentrations and facilitate mold release.
Unspecified chamfers: Common chamfers are C1 or C2 (1mm or 2mm 45° angles) to avoid sharp edges and ease assembly.

Machining allowances: Machined surfaces must be clearly indicated on the drawing. During mold manufacturing, these surfaces require machining allowances (extra material removed during machining), while non-machined surfaces require no such allowances.

3.5 Special Tests

For pressure-bearing parts (e.g., pump casings, valve bodies), a hydraulic pressure test is mandatory. The part must withstand the specified pressure for 5 minutes without leakage to ensure its sealing performance in service.

Mastering the view selection, dimensioning, and technical requirements of box-type part engineering drawings is essential for mechanical engineers. These skills directly influence the manufacturability, assembly accuracy, and service life of machinery. By adhering to the principles outlined above, engineers can create clear, accurate, and practical engineering drawings that guide efficient production and ensure product quality.