Threading Process Demystified: Feeds, Inserts and Critical Terminology

The primary methods for metal thread cutting include turning, milling, and tapping. Today, we’ll discuss the essential technical knowledge of the most common method: thread turning in production.

 

1. Important basic knowledge of thread processing

1. Definition of terms

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Helix angle:
The helix angle is influenced by both the diameter and the pitch of the thread. The blade clearance angle can be adjusted by replacing the shim. The cutting edge inclination angle is denoted as γ. The most commonly used cutting edge inclination angle is 1°, which is the standard shim used in the tool holder.

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Cutting forces when cutting into and out of the thread:
The highest axial cutting forces in threading operations occur when the cutting tool engages with the workpiece during entry and exit. Excessive cutting parameters may cause the insert to move if it is not securely clamped.

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Tilt the insert to get clearance:
The rake angle can be adjusted by placing shims under the insert in the holder. Please refer to the chart in the tool catalog to select the appropriate shim. All holders are provided with a standard shim set to a 1° rake angle.

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Choose the shim according to the rake angle. The rake angle is influenced by the workpiece diameter and thread pitch. In the figure below, the workpiece diameter is 40 mm, and the thread pitch is 6 mm. Therefore, the required shim must have a rake angle of 3°. (standard shims cannot be used).

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Marking of threading inserts and shims:

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Thread shapes and their applications:

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2. Thread insert types and clamping solutions

1. Multi-tooth inserts

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Advantages:
Reduce the number of feeds.
Very high productivity.

 

Disadvantages:
Requires stable clamping.
Requires sufficient space for retraction after thread processing.

 

2. Full-tooth blade

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Advantages:
Better control of thread shape;
Fewer burrs.

 

Disadvantages:
One blade can only cut one pitch.

 

3. V-tooth blade

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Advantages:
Flexibility, the same insert can be used for several thread pitches.

 

Disadvantages:
Causes burr formation, which requires deburring.

 

Clamping concept i-LOCK:
The threading is extremely rigid, utilizing fixed inserts. Each insert is positioned accurately along the guide rail. A screw presses the insert against a radial stop located at a contact surface (marked in red) within the insert seat. This reliable insert interface helps ensure a longer tool life and improved thread quality.

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Various knife handles:

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3. 3 Different types of feed methods

The feed rate significantly influences the threading process by affecting cutting control, tool wear, thread quality, and tool life.The feed method can have a significant impact on the threading process. It affects: cutting control, insert wear, thread quality, and tool life.

 

1. Improved side feed
Most CNC machines can utilize this feed method through cycle programs. The chips produced are similar to those from traditional turning, making them easier to shape and guide. Axial cutting forces help minimize the risk of vibration. Although the chips are thicker, they only contact one side of the cutting insert, which reduces the heat transferred to the insert. This method is typically the preferred choice for most threading operations.

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2. Radial feed
The most common method available on older non-CNC lathes has the following characteristics:

- It produces hard, “V”-shaped chips.
- It results in uniform wear of the insert.
- The insert seat is exposed to high temperatures, which limits the feed depth.
- It is suitable for machining fine threads.
- It may cause vibration and poor chip control when machining coarse threads.
- It is preferred for machining hardened materials.

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3. Alternating feed

Recommended for large tooth profiles, this tool maximizes insert wear uniformity and extends tool life when machining threads with very large pitches. However, the chips are guided in two directions, making them difficult to control.

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4. Methods to improve processing results

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1. Cutting depth decreases layer by layer (chip area remains unchanged)

The process can achieve a consistent chip area, which is the most commonly used method in CNC programming. The first pass should be the deepest. Be sure to follow the recommended values in the feed table provided in the catalog for optimal results. This approach promotes a more balanced chip area. Finally, the last pass should be set to approximately 0.07 mm.

 

2. Constant cutting depth

Regardless of the number of passes, the depth for each pass remains constant. There are stricter requirements for the blade to ensure optimal chip control. It should not be used when the pitch exceeds TP1.5mm or 16 TP.

 

Using extra stock to finish the thread crest:
Before CNC custom machining the thread, it is not necessary to turn the stock to the exact diameter. You can use the extra material to finish the thread crest. During the initial turning process, leave approximately 0.03 to 0.07 mm of material to ensure that the crest can form correctly during the finishing process.

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Recommended external thread feed values ​​(ISO metric):

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Ensure that the workpiece and tool are aligned:
Use a maximum centerline deviation of ±0.1 mm. If the cutting edge is positioned too high, the back angle will be reduced, which could result in the cutting edge becoming scratched or broken. Conversely, if the cutting edge is positioned too low, the thread profile may become incorrect.

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5. Tips for Successful Thread Turning Applications

- Before thread turning, check the workpiece diameter to ensure it has the correct machining allowance and add a crest allowance of 0.14 mm.
- Position the tool accurately in the machine.
- Verify the setting of the cutting edge in relation to the pitch diameter.
- Ensure that the correct insert geometry is used (A, F, or C).
- Select the appropriate shim (insert-tilted shim) to achieve sufficient and uniform clearance, ensuring the correct flank clearance.
- If the thread produced is not acceptable, review the entire clamping process, including the machine setup.
- Check the CNC programs that are available for thread turning.
- Optimize the feeding method, the number of passes, and the size.
- Ensure the cutting speed is appropriate to meet application requirements.
- If the workpiece thread has the incorrect pitch, check the machine tool settings for the correct pitch.
- It is recommended that the tool starts at a minimum distance of three times the pitch before engaging with the workpiece.
- Using high-precision coolant can extend tool life and enhance chip control.
- A quick-change system enables simple and rapid clamping.

 

When selecting a tool for a thread turning operation, consider:
- Check the overhang and any necessary clearance (e.g., shoulders, sub-spindles, etc.).
- Minimize tool overhang to enable quick clamping.
- For less rigid clamping situations, choose inserts that produce lower cutting forces.
- Using high-precision coolant can prolong tool life and enhance CNC cutting service control.
- Easily connect coolant systems using plug-and-play coolant adapters.
- For optimal productivity and tool longevity, prefer multi-thread inserts; single-edge full-thread inserts are the next best option, while V-thread inserts are the least productive and have the shortest tool life.

 

Insert wear and tool life:
- Feed Method: Use an optimized feed method, considering the number of passes and cutting depth.
- Insert Angle: Ensure that there is sufficient and uniform clearance by using a tilted shim for the insert.
- Insert Geometry: Confirm that the correct insert geometry is employed, choosing from A, F, or C geometries.
- Insert Material: Select the appropriate insert material based on the specific material and toughness requirements.
- Cutting Parameters: Adjust the cutting speed and the number of passes as needed.

 

 

 

If you want to know more or inquiry, please feel free to contact info@anebon.com.

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