Hole callouts look simple, but an unclear hole note can create a real manufacturing problem. A production drawing needs to communicate the hole type, size, quantity, depth and any required thread or counterbore information without forcing the machinist to guess.
This guide explains a practical way to create clear hole callouts in CAD drawings and shows the checks a mechanical designer should make before releasing a drawing.
1. Start With the Hole Function
Before choosing a callout, identify what the hole actually does. Is it for a clearance bolt, a tapped fastener, a locating feature, a bearing seat or simply weight reduction? The function determines the diameter, tolerance, depth and manufacturing process.
2. Common Hole Types
- Through hole: The hole passes completely through the part. A diameter and quantity are normally enough when no special tolerance is required.
- Blind hole: The drawing should communicate the required depth or end condition.
- Threaded hole: Specify the thread standard, nominal size, pitch where applicable, and thread depth or full-depth requirement.
- Counterbore: Used when a socket-head screw or similar fastener needs its head below the surface.
- Countersink: Used when a flat-head fastener needs to sit flush with the surface.
3. Do Not Confuse Hole Size With Fastener Size
A bolt size and its clearance-hole diameter are not automatically the same. A clearance hole normally needs additional diameter to allow assembly. The exact value depends on the fastener, fit requirement, standard and manufacturing process.
4. Add Tolerance When the Function Requires It
A general diameter may be acceptable for a non-critical hole, but locating holes, bearing fits and precision interfaces may require a specific tolerance. Do not add tight tolerances simply because the CAD software allows them. Tight tolerances can increase machining and inspection cost.
5. Check Hole Location and Datums
A perfectly specified diameter can still produce a bad part if the hole location is unclear. Use functional dimensions and appropriate datums for features that control assembly. Where positional accuracy matters, consider GD&T rather than relying on long chains of basic dimensions.
6. CAD Drawing Release Checklist
- Hole diameter and type are clear.
- Through or blind condition is defined.
- Thread standard and depth are stated where needed.
- Counterbore or countersink dimensions are complete.
- Quantity and pattern are unambiguous.
- Critical location tolerances are identified.
- Units and drawing standard are consistent.
- The 3D model and 2D drawing agree.
Practical CAD Tip
After creating the drawing, inspect the model and drawing together. A drawing should describe design intent, not merely repeat dimensions that already exist in the model. For more drawing guidance, explore the GetCADFile Design Guide and our Engineering Unit Converter.
Image: Mechanical engineering drawing and CAD blueprint. Source: Unsplash.