The main methods of metal cutting thread processing include turning, milling, tapping, etc. Today, the editor brings you the important technical knowledge of the most common thread turning in production.
1. Important basic knowledge of thread processing 1. Definition of terms ① Root of thread ② Flank of thread ③ Top of thread
Helix angle:
The helix angle depends on the diameter and pitch of the thread. The flank clearance angle of the insert is adjusted by replacing the shim. The rake angle is γ. The most common rake angle is 1°, corresponding to the standard shim in the tool holder.
Cutting force when cutting into and out of the thread: The highest axial cutting force in the thread processing process occurs during the cutting of the cutting tool into and out of the workpiece. Excessive cutting parameters may cause movement of the unreliably clamped insert.
Tilt the insert to obtain clearance:
The rake angle can be set with a shim under the insert in the tool holder. You can refer to the chart in the tool catalog to choose which shim to use. All tool holders are equipped with a standard shim with a rake angle set to 1°.
Select the shim according to the rake angle. The workpiece diameter and pitch affect the rake angle. As can be seen from the figure below, the workpiece diameter is 40mm and the pitch is 6mm. The required shim must have a 3° blade inclination angle (standard shims cannot be used).
Marking of thread processing inserts and shims:
Thread shape and its application:
II. Thread insert types and clamping solutions
1. Multi-tooth insert
Advantages: Reduce the number of feeds Very high productivity Disadvantages: Stable clamping is required Sufficient retraction space is required after thread processing
2. Full-tooth insert
Advantages: Better control of thread shape Less burrs
Disadvantages: One insert can only cut one pitch
3. V-tooth insert
Advantages: Flexibility, the same insert can be used to process several pitches.
Disadvantages It will cause burrs to form and need to be removed.
Clamping solution i-LOCK: Extremely rigid threading with fixed inserts Guided by the guide rail, the insert is positioned in the correct position The screw presses the insert on the guide rail back to the radial stop at one contact surface (red contact surface) in the insert seat Reliable insert interface ensures longer tool life and higher thread quality
Various tool holders:
3. 3 different types of feed methods
The feed method can have a significant impact on the threading process. It affects: cutting control, insert wear, thread quality, tool life
1. Improved side infeed
This infeed method can be used by most CNC machines through a cycle program:
Chips are easier to shape and guide than conventional turning typesAxial cutting forces reduce the risk of vibrationThicker chips but only in contact with one side of the insertLess heat transferred to the insertPreferred for most threading operations
2. Radial infeed
Most common method - the only method available to older non-CNC lathes:
Produces hard "V" shaped chipsUniform insert wearThe insert seat is exposed to high temperatures, which limits the infeed depthSuitable for fine threadsPoor vibration and chip control when machining coarse threadsPreferred for hardened materials
3. Alternating infeedRecommended for large profiles
Uniform insert wear and maximum tool life when machining very large pitch threadsChips are guided in two directions, making them difficult to control
IV. Methods for improving machining results
Decreasing depth of cut (left), constant depth of cut (right)
1. Cutting depth decreases layer by layer (chip area remains unchanged)
It is possible to achieve a constant chip area, which is the most commonly used method in CNC programs. The deepest first pass follows the recommended value on the feed table in the sample. The chip area is more "balanced" and the actual last pass is about 0.07mm
2. Constant cutting depth
Regardless of the number of passes, the depth of each pass is equal. It has higher requirements on the blade to ensure the best chip control. It should not be used for pitches greater than TP1.5mm or 16TP
Use extra stock to finish the thread crest:
Before machining the thread, it is not necessary to turn the blank to the exact diameter, and use extra stock/material to finish the thread crest. For the finishing crest insert, the previous turning process should leave 0.03-0.07mm of material to form the crest correctly.
Recommended external thread feed value (ISO metric):
Ensure the workpiece and tool are centered:
Use the maximum centerline deviation of ±0.1mm. If the cutting edge position is too high, the back angle will be reduced and the cutting edge will be scratched (broken); if the cutting edge position is too low, the thread tooth profile may be incorrect.
V. Successful thread turning application skills
1) Check whether the workpiece diameter has the correct machining allowance before thread turning, and add 0.14mm as the crest allowance. 2) Accurately position the tool in the machine tool. 3) Check the setting of the cutting edge relative to the center diameter. 4) Ensure the correct insert geometry is used (A, F or C ). 5) Ensure adequate and uniform clearance by selecting the appropriate shim (insert-tilted shim) to obtain the correct flank clearance. 6) If the thread is not acceptable, 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 the correct cutting speed to meet the application requirements. 10) If the workpiece thread has the wrong pitch, check the machine tool for the correct pitch. 11) It is recommended that the tool should start at a minimum distance of 3 times the pitch before cutting into the workpiece. 12) High-precision coolant can extend tool life and improve chip control. 13) Quick-change systems ensure simple and fast clamping.
When selecting tools for thread turning operations, consider:
Check the overhang and any clearance required (such as shoulders, Sub-spindle, etc.) Minimize tool overhang for fast clamping For poor rigidity clamping, choose inserts with lower cutting forces High-precision coolant can extend tool life and improve cutting control Use plug-and-play coolant adapters to easily connect coolant To ensure productivity and tool life, multi-tooth inserts are the first choice, and single-edge full-tooth inserts are the second choice. The lowest productivity and shortest tool life are V-tooth inserts.
Insert wear and tool life:
Infeed method, optimize the feed method, number of passes and depth Insert inclination, ensure sufficient and uniform clearance (insert-tilted shim) Insert geometry, ensure the correct insert geometry (A, F or C geometry) is used Insert material, choose the correct material according to material and toughness requirements Cutting parameters, change cutting speed and number of passes if necessary
