Introduction
If you have ever seen a fastener with both internal and external threads — such as an M6 external thread with an M4 internal thread — you may have wondered: how is it possible to machine both threads on such a thin-walled part without distorting either?
This article explains the manufacturing process for internal-external thread fasteners (double-threaded screws and nuts), why conventional thread rolling cannot be used, and how thread milling on automatic cam lathes solves the problem efficiently and economically.
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What Are Internal-External Thread Fasteners?
Internal-external thread fasteners are custom precision components that carry both an external thread and an internal thread on the same part. Common examples include:
- Internal-external thread screws: e.g., M6 external thread + M4 internal thread
- Internal-external thread nuts: external thread on the OD + internal thread on the ID, including self-tapping external thread variants used as press-in or thread-forming inserts in soft materials
These are typically custom, non-standard parts produced to customer drawings. They are widely used in electronics, automotive, and precision mechanical assemblies where space is limited and a single component must serve dual connection functions.
Why Thread Rolling Cannot Be Used
For standard external threads, thread rolling (using a thread rolling machine or roll-threading attachment) is the most efficient production method. However, for internal-external thread fasteners, thread rolling is not applicable for the following reasons:
- Thin wall section: When the external thread (e.g., M6) and internal thread (e.g., M4) are close in size, the wall thickness between them is very small. Thread rolling applies radial compressive force that would deform the thin wall and distort the internal thread.
- Non-standard external thread profiles: Some internal-external thread nuts use self-tapping or cutting-type external threads with sharp, non-standard profiles that have no overlap with standard thread geometry. These profiles cannot be produced by rolling or thread-grinding.
- Internal thread integrity: The internal thread must be formed before or independently of the external thread. Rolling the external thread after tapping would collapse the internal thread.
The Solution: Thread Milling Attachment on Automatic Cam Lathes
The external thread on these parts is produced using a thread milling head (铣牙座) — a specialized attachment mounted directly on the automatic cam lathe.
How the Thread Milling Head Works
- The thread milling head is installed on the cam automatic lathe as an accessory unit
- Its drive belt connects to the main spindle, creating a fixed transmission ratio (the belt is toothed/synchronous)
- The transmission ratio is adjusted according to the required thread pitch
- Different insert blades are selected for different thread profiles (standard metric, self-tapping, cutting-type, custom profiles)
- As the workpiece rotates in the spindle and advances axially, the milling head cuts the helical thread form
Advantages of Thread Milling
- No radial force on the workpiece: Unlike rolling, milling removes material rather than displacing it, eliminating the risk of wall deformation or internal thread distortion
- Any thread profile: Standard metric, self-tapping, cutting-type, and fully custom profiles can all be produced by selecting the appropriate insert
- Integrated into the cam lathe cycle: Thread milling occurs as part of the automatic lathe machining cycle — no secondary operation required
- High efficiency: Far more economical than CNC turning for high-volume production of these parts
The Internal Thread: Standard Tapping
The internal thread is produced by conventional tapping (using a tap / 丝攻) — the same method used for any standard internal thread. The tap is driven by the tail spindle unit of the automatic lathe as part of the same machining cycle. This is standard practice and requires no special explanation for anyone familiar with machining.
Why Not Just Use CNC?
CNC turning can produce any thread profile. However, for high-volume production of small precision parts, CNC is not economically competitive with cam automatic lathes:
| Factor | CNC Lathe | Cam Auto Lathe + Thread Milling Head |
|---|---|---|
| Cycle time (simple turned part) | Slower | Up to ~30 parts/min |
| Setup time | Shorter (program change) | Longer (cam adjustment) |
| Unit cost (high volume) | Higher | Significantly lower |
| Thread profile flexibility | Any profile | Any profile (with correct insert) |
| Minimum viable batch size | Small batches OK | Best for large batches |
For parts like these internal-external thread fasteners, even with zero material cost, CNC machining cost alone would exceed ¥1 per piece. The cam lathe with thread milling head brings this cost down to a fraction of that at production volumes.
Discussing CNC machining without considering cost is not a useful engineering conversation.
Related Guides
- Automatic Screw Machine Types and Specifications
- Beginner’s Guide to Operating an Automatic Screw Machine
- Common Bar Stock Materials for Automatic Lathes
- C3604 Free-Cutting Brass — The Most Common Automatic Lathe Material
Content authorized for publication by FULLERKREG.