Threaded holes are a common feature in CNC machining and are essential for assemblies that require fasteners. Here are key guidelines to consider for efficient and high-quality threaded holes:
Choose the Right Thread Type and Size
Standard Thread Types: Use standard thread types like UNC (Unified National Coarse) or UNF (Unified National Fine) for American systems and M (metric) threads for international applications. Choosing common sizes reduces tooling costs.
Limit Fine Threads in Soft Materials: Fine threads work well in hard materials but can strip more easily in softer materials, so adjust accordingly.
Size Range: For best results, avoid very small (M20) threaded holes unless required, as these are difficult and costly to machine.
- Thread Depth Guidelines
Practical Thread Depth: The standard thread depth is typically around 1.5 times the diameter of the hole. Deeper threads are possible but lead to higher machining costs.
Avoid Overly Deep Threads: Deep threading adds tool wear and cycle time, especially in harder materials. Where feasible, use shallower threads and rely on fasteners to provide strength.
Blind Hole Consideration: For blind holes, avoid threading all the way to the bottom. Leave a “thread relief” gap at the base, typically 1–2 thread pitches. - Hole Preparation
Pilot Hole Size: Ensure the pilot hole is the correct diameter for the specific thread being cut. Using the right pilot size avoids weak threads or tool damage.
Chamfering: Chamfering the hole entry before tapping helps align the tap and prevent thread deformation. Chamfer size is typically around 0.2–0.5 mm larger than the thread’s major diameter. - Tool Selection and Tapping Strategy
Use the Right Tap: Select taps appropriate for the material (e.g., spiral-point taps for through holes, spiral-flute taps for blind holes).
Rigid Tapping: For high precision and consistency, use rigid tapping rather than a floating tap holder in CNC machines. This allows better control over depth and avoids misalignment.
Thread Milling: For harder materials or custom threads, thread milling may offer more precision and tool life. - Tolerance and Quality Control
Thread Tolerances: Standard tolerances for threads are typically 6H (metric) or 2B (imperial) class. Adjust tolerance requirements based on the fit needed.
Inspection: For critical applications, inspect threaded holes using gauges or threaded plugs to verify fit and depth. - Consider Alternatives for High Volume or High Wear Applications
Thread Inserts: In softer materials like aluminum, consider using threaded inserts, such as helicoils, to increase strength.
Avoid Internal Threads Where Possible: For high-volume or cost-sensitive applications, consider through-holes with nuts rather than threaded holes, reducing machining time.
These guidelines can help balance strength, precision, and machining efficiency when creating threaded holes in CNC machining.
