Guidelines for Threaded Holes in CNC Machining？

When creating threaded holes in CNC machining, there are several key guidelines and best practices to follow to ensure proper functionality, manufacturability, and durability of the threads. Here’s a comprehensive overview:

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Design Guidelines for Threaded Holes

1. Thread Depth

• Rule of Thumb: Thread engagement should typically be 1 to 1.5 times the major diameter of the screw/bolt.
  • For example, an M6 screw should have a thread depth of around 6mm to 9mm.
• Avoid over-threading. Excess depth doesn’t significantly increase strength and just adds machining time.

2. Thread Start Location

• Try to start threads at least 0.5–1.0 mm below the hole surface (countersinking) to help with thread lead-in and avoid burrs.

3. Minimum Thread Size

• Avoid threading holes smaller than M2 or #2-56 in metals; they are difficult to machine and prone to damage.
• In plastics, use inserts instead of small threads due to wear and strength issues.

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Thread Types and Creation Methods

1. Tapping

• Used for internal threads.
• Can be done with hand taps (manual) or using a thread tapping cycle (rigid tapping) on a CNC.
• Ensure correct tap drill size is used (refer to standard tap drill charts).

2. Thread Milling

• More flexible than tapping.
• Allows custom thread sizes, multi-start threads, and partial threads.
• Ideal for large threads or difficult materials.
• Allows for easy thread repair or adjustment.

3. Helical Interpolation

• A technique used with thread mills to cut threads using a helical tool path.
• Good for internal threads in hard-to-machine materials.

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Hole Preparation

1. Hole Diameter

• Use the correct tap drill size (depends on thread type and material).
  • Example: For M6 x 1, the recommended drill size is 5.0 mm.
• Undersized holes = hard tapping.
• Oversized holes = weak threads.

2. Chamfer the Hole

• Always chamfer or countersink before threading to reduce burrs and help tap/fastener engagement.
• A 45° chamfer is common.

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General Best Practices

• Use thread inserts (e.g., Heli-Coils) for threads in soft materials (like aluminum) or high-stress applications.
• Avoid threading blind holes too close to the bottom – leave 1.5x pitch clearance.
• When possible, use through-holes instead of blind holes for better chip removal and easier machining.
• Clearly specify thread standards (e.g., ISO Metric, UNC/UNF, etc.) in your drawing.
• Use thread callouts like M6x1.0 - 6H or 1/4-20 UNC - 2B to avoid confusion.

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Material Considerations

• Aluminum: Easy to thread; use standard taps or thread mills.
• Steel: Use coated taps and thread mills for tool life.
• Plastics: Use coarse threads or thread inserts.
• Titanium/Stainless: Use slow speeds, high lubricity tools, and pecking cycles.