The main methods of metal cutting thread processing include turning, milling, tapping, etc. Today, the editor has brought the most common thread turning technical knowledge in production, hoping to be helpful to everyone.
1. Important basic knowledge of thread processing
1. Definition of terms
①Tooth base ②Tooth side ③Tooth top
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Spiral angle:
- The helix angle depends on the diameter and pitch of the thread.
- Adjust the flank relief angle of the blade by changing the blade shim.
- The edge inclination angle is γ. The most common edge angle is 1°, which corresponds to a standard shim in the tool holder.
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Cutting forces during thread entry and exit:
- The highest axial cutting forces in threading operations occur during the cutting tool's entry into and exit from the workpiece;
- Cutting parameters that are too high may cause movements of unreliably clamped inserts.
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Tilt the blade for clearance:
The edge angle can be set using a shim under the blade in the holder. Refer to the chart in the tool catalog to choose which shim to use. All toolholders come with standard tool shims with an edge angle set to 1°.
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Select the tool shim according to the edge inclination angle. Workpiece diameter and pitch will affect the edge angle. As can be seen from the picture below, the workpiece diameter is 40mm, the pitch is 6mm, and the required tool shim must have a 3° edge inclination angle (standard tool shims cannot be used).
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Markings of threading inserts and shims:
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Thread shapes and their applications:
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2. Threaded insert types and clamping solutions
1. Multi-tooth blade
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advantage:
- Reduce the number of feeds;
- Very high productivity.
shortcoming:
- Requires stable clamping;
- Sufficient retraction space is required after machining the thread.
2. Full profile blade
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advantage:
- Better control of thread shape;
- Less burrs.
shortcoming:
- One type of insert can only cut one type of pitch.
3. V-shaped blade
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advantage:
- Flexibility, the same insert can be used to process several pitches.
shortcoming:
- Can cause burrs to form and require removal.
3. 3 different types of cutting methods
The cutting method plays an important role in the threading process. It affects cut control, insert wear, thread quality and tool life.
1. Improved lateral feed. Most CNC machines use this feed method through cycle programs.
- Chips are easier to form and guide than traditional turning types;
- Axial cutting forces reduce the risk of vibration;
- The chips are thick but only in contact with one side of the blade;
- Reduced heat transfer to the blade;
- First choice for most threading operations.
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2. Radial feed. The most commonly used method is also the method that can be used by earlier non-CNC lathes.
- Produce hard "V" shaped chips;
- Uniform blade wear;
- The insert seat is exposed to high temperatures, which limits the depth of penetration;
- Suitable for processing fine threads;
- Possible vibration and poor chip control when machining coarse threads;
- First choice for work hardened materials.
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3. Alternate feed.
- Recommended for large teeth;
- Uniform insert wear and maximum tool life when machining threads with very large pitches;
- 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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The depth of cut decreases layer by layer (left), and the depth of cut is constant (right).
1. The depth of cut decreases layer by layer (the chip area remains unchanged)
Able to achieve a constant chip area, this is the most common method used in CNC programs.
- The first pass is the deepest;
- Follow the recommended values on the feed table in the catalog;
- More balanced chip area;
- The last pass is actually about 0.07mm.
2. Constant depth of cut
Regardless of the number of passes, the depth of each pass is the same.
- Have higher requirements for blades;
- Ensure optimal chip control;
- It should not be used when the pitch is greater than TP1.5mm or 16TP.
Use extra allowance to finish the thread crest:
Instead of turning the blank to the exact diameter before machining the thread, use the extra allowance/material to refine the thread crest. For finishing crest inserts, 0.03~0.07mm of material should be left in the previous turning process to allow the crest to be correctly formed.
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Recommended feed value for external threads (ISO metric):
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Make sure the workpiece and tool are aligned:
Use maximum centerline deviation of ±0.1mm. If the cutting edge position is too high, the relief angle will be reduced and the cutting edge will be scratched (broken); if the cutting edge position is too low, the thread profile may be incorrect.
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5. Thread turning application skills
1) Before thread turning, check whether the workpiece diameter has the correct machining allowance, and add 0.14mm as the crest allowance.
2) Precisely position the tool in the machine tool;
3) Check the setting of the cutting edge relative to the pitch diameter;
4) Make sure to use the correct insert geometry (A, F or C);
5) Ensure a large enough and uniform gap by selecting the appropriate shim (insert-tilt shim) to obtain the correct flank clearance;
6) If the thread is unqualified, check the entire clamping including the machine tool;
7) Check the CNC programs available for thread turning;
8) Optimize the feed method, number of passes and size;
9) Ensure correct cutting speed to meet application requirements;
10) If the pitch of the workpiece thread is wrong, check whether the machine tool pitch is correct;
11) Before cutting into the workpiece, it is recommended that the tool should start at a minimum distance of 3 times the pitch;
12) High-precision coolant can extend tool life and improve chip control;
13) The quick change system ensures simple and fast clamping.
When selecting tools for thread turning operations, you should consider:
- Check overhangs and any clearances required (e.g. shoulders, sub-spindles, etc.);
- Minimize tool overhang for fast clamping;
- For clamping with poor rigidity, choose an insert with smaller cutting force;
- High-precision coolant extends tool life and improves cutting control;
- Connect coolant easily with plug-and-play coolant connections;
- In order to ensure productivity and tool life, multi-tooth type inserts are the first choice, followed by single-edged full-tooth type inserts. The choice with the lowest productivity and shortest tool life is V-tooth type inserts.
Blade wear and tool life:
- Feeding method, optimize the feeding method, number of passes and depth;
- Blade inclination to ensure a sufficiently large and uniform gap (blade-tilt pad);
- Insert geometry, make sure to use the correct insert geometry (A, F or C geometry);
- Blade material, choose the correct material according to material and toughness requirements;
- Cutting parameters, changing cutting speed and number of passes if necessary.
