The main function of the thread is to achieve mechanical coupling, and to transfer motion by converting rotary/linear motion into linear/rotary motion to achieve mechanical benefits.
The thread profile defines the geometry of the thread, which includes the part diameter (major diameter, middle diameter and minor diameter), thread profile angle, pitch, radius and helix angle. Each thread processing method and tool has its own advantages under specific circumstances. Today we will discuss in detail the knowledge and key points of metal turning thread processing.
1. Important basic knowledge of thread processing
Marking of thread processing inserts and shim:
Thread shape and related applications:
2. Thread insert type and clamping scheme
1). Multi-tooth blade
-Reduce the number of feeds
-Very high productivity
-Need for stable clamping
-After the thread is machined, enough space to retract the tool is required
2). Full tooth blade
-Better control of thread shape
-One blade can only cut one pitch
3). V-shaped blade
-Flexibility, the same blade can be used to process several pitches.
-It will cause burrs and need to be removed.
3, three different types of feed methods
The feed method can have a significant impact on the threading process.
Such as, cutting control, blade abrasion, thread quality, tool life.
1). Improved lateral feed
Most CNC machine tools can use this feed method through cycle programs:
-Cutting and traditional turning types-easier to shape and guide
-Axial cutting force reduces the risk of vibration
-The cutting is thicker, but only touches one side of the blade
-Reduced heat transfer to the blade
-First choice for most threading operations
2). Radial feed
The most commonly used and the only method that can be used on older non-CNC lathes:
-Produce hard "V"-shaped cutting
-Uniform blade abrasion
-The insert is exposed to high temperatures, which limits the depth of penetration
-Suitable for processing fine thread
-Vibration may occur abrasion machining coarse threads and poor cutting control
-First choice for work hardening materials
3). Alternate feed
-Recommended for large teeth
-Achieve uniform blade wear and longest tool life when machining threads with very large pitches
-Cutting are guided in two directions, so it is difficult to control
4, Methods to improve processing results
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 cutting area remains unchanged)
Able to achieve a constant cutting area, which is the most commonly used method in CNC programs.
-The deepest move for the first time
-Follow the recommended values on the feed table in the catalog
-More "balanced" cutting 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.
-Higher requirements for blades
-Ensure optimal cutting control
-Not used when the pitch is greater than TP1.5mm or 16TP
Use the extra margin to fine-tune the thread crest:
Before processing the thread, it is not necessary to turn the blank to the exact diameter, and use the extra margin/material to finish the thread crest. For finishing tooth crest inserts, the material of 0.03-0.07mm should be reserved in the previous turning process to make the tooth crest correctly shaped.
Recommended external thread feed value (ISO metric system):
Ensure that the workpiece and the tool are aligned:
Use the maximum centerline deviation ±0.1mm. If the cutting edge position is too high, the clearance angle will decrease and the cutting edge will be scratched (cracked); if the cutting edge position is too low, the thread profile may be incorrect.
5. 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 tooth crest allowance.
2) Precise positioning of 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 blade geometry (A, F or C).
5) Ensure a sufficiently large and uniform gap (blade-inclined shim) by selecting the appropriate shim to obtain the correct flank gap.
6) If the thread is unqualified, check the entire clamping including the machine tool.
7) Check the available CNC programs for thread turning.
8) Optimize the feed method, number of feeds and size.
9) Ensure the correct cutting speed to meet the application requirements.
10) If the thread 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 with a minimum distance of 3 times the pitch.
12) High-precision coolant can extend tool life and improve cutting control.
13) Quick change system can ensure simple and fast clamping.
When selecting tools for thread turning operations, you should consider:
-Check the overhang and any gaps required (e.g. shoulder, counter spindle, etc.)
-Minimize tool overhang for quick clamping
-For rigid clamping, choose inserts with lower cutting force
-High-precision coolant can extend tool life and improve cutting control
-Use plug-and-play coolant connector to easily connect coolant
-In order to improve productivity and tool life, it is recommended that the multi-tooth insert is the first choice, and the second choice is the single-edged full-tooth insert. The lowest productivity and the shortest tool life is the V-tooth insert.
Blade abrasion and tool life:
Feed method, optimize feed method, number of passes and depth
Blade inclination angle to ensure a sufficiently large and uniform gap (blade-tilted shim)
Blade geometry, to ensure that the correct blade geometry (A, F or C geometry) is used
Blade material, choose the right material according to the material and toughness requirements
Cutting parameters, if necessary, change the cutting speed and the number of passes.