Short Answer:

In engineering drawings, holes and slots must be dimensioned clearly to ensure they are manufactured accurately. The main rules include specifying the diameter, position, depth, and type of hole or slot using standard symbols and dimensions. All dimensions should be clear, precise, and not repeated elsewhere on the drawing.

For holes, symbols like ⌀ (diameter) and depth (⏐ or depth note) are used, while for slots, length, width, and position must be shown. Proper dimensioning helps machinists understand the exact location, size, and shape of the hole or slot for correct machining and assembly.

Detailed Explanation:

Rules for dimensioning holes and slots

In mechanical engineering drawings, holes and slots are common features used for fasteners, clearance, fitting components, and motion guidance. If not dimensioned properly, they can lead to wrong part fabrication, misalignment, or assembly failures.

That’s why specific dimensioning rules are followed in standard drafting practices (ISO, ASME, BIS), which help engineers, machinists, and inspectors to understand and manufacture parts correctly.

Dimensioning holes

Holes can be through holes, blind holes, counterbore, countersink, or tapped (threaded) holes. Each type has different dimensioning needs.

Key rules:

1. Use ⌀ symbol before the diameter of the hole
   Example: ⌀20
2. Indicate depth if it is a blind hole
   Example: ⌀12 depth 25 mm
   Or using the depth symbol → ⌀12 ⏐25
3. For multiple holes, mention quantity and pattern
   Example: 4 × ⌀10 equally spaced on ⌀50 PCD
4. If threaded, use thread notation
   Example: M10 × 1.5 – 6H depth 20 mm
5. Avoid repeated dimensions
   A hole’s size and location should be shown only once.
6. Use center marks and dimension from a datum
   All holes must be dimensioned from a fixed reference edge (datum line or point).
7. Use leader lines and feature control frames (if needed for GD&T)

Dimensioning slots

Slots are elongated holes, usually rectangular or semicircular at ends. They require length, width, and position to be completely defined.

Key rules:

1. Show width and length clearly
   Example: 10 × 40 (Width × Length)
2. Dimension center point of the slot
   It helps to position the slot relative to other features or edges.
3. Indicate radii if the slot ends are semicircular
   Example: R5 for both ends
4. Use a centerline to represent symmetry if slot is symmetrical
5. Show slot orientation using angular or directional reference (if not aligned with main axes)
6. Use notes or feature control symbols for tolerances or precision slots

Best practices for dimensioning holes and slots

• Use consistent units and avoid mixing mm and inch in the same drawing
• Keep all dimensions outside the view whenever possible
• Avoid crowding by spreading dimensions clearly and neatly
• Use views or sections to show hidden holes or slot depths
• Label hole types (e.g., threaded, countersunk, counterbored)

Common mistakes to avoid

• Missing ⌀ symbol before hole size
• Giving incomplete slot data (like width only)
• Not defining depth for blind holes
• Overdimensioning or duplicating measurements
• Not aligning dimension lines or reference datums

Conclusion:

Dimensioning holes and slots in engineering drawings must be done carefully using standard rules and symbols. Holes require clear indications of diameter, depth, and location, while slots need length, width, position, and end details. Following these rules ensures the parts are machined accurately, reducing errors during manufacturing and ensuring correct fit and function in assembly. Properly dimensioned drawings also help in inspection and quality control.