Selection of Threading Inserts for CNC Lathes
When performing threading on a CNC lathe, it is crucial to select the most suitable threading inserts for your application based on the insert type, flank/radial clearance, and insert profile. These factors significantly impact chip control, insert wear, tool life, and thread quality.
1. Common Types of Threading Inserts
There are three common types of inserts for threading on CNC lathes: full-profile, V-profile, and multi-tooth inserts. Each type has its advantages and disadvantages.
2. Full-Profile Inserts
Full-profile inserts are the most common type. They are used to cut the complete thread profile, including the crest.
2.1 Advantages of Full-Profile Inserts
Ensure the required thread depth, root, and crest.
No deburring of the thread profile is necessary.
Larger nose radius, requiring fewer passes compared to V-profile inserts.
Ensure efficient threading performance.
2.2 Disadvantages of Full-Profile Inserts
Each pitch and profile require a specific insert.
Note: Leave an extra allowance/material of 0.05-0.07 mm (0.002-0.003 inches) on the workpiece to ensure the final threading process can finish to the crest.
3. V-Profile Inserts
V-profile inserts do not cut the thread crest. Therefore, the screw's outer diameter and the nut's inner diameter must be turned to the correct diameter before threading.
3.1 Advantages of V-Profile Inserts
High flexibility; the same insert can be used for different pitches, provided the thread profile angle (60° or 55°) and radius are the same.
Requires minimal tool inventory.
3.2 Disadvantages of V-Profile Inserts
Smaller nose radius, leading to shorter tool life.
Possible burr formation issues.
4. Multi-Tooth Inserts
Multi-tooth inserts are similar to full-profile inserts but have multiple cutting edges (NT > 1). A double-edged insert can double productivity, a triple-edged insert can triple it, and so on.
4.1 Advantages of Multi-Tooth Inserts
Reduced number of passes required, extending tool life, increasing productivity, and lowering tool costs.
4.2 Disadvantages of Multi-Tooth Inserts
Longer edge cutting contact length increases cutting force, requiring stable working conditions.
Adequate space must be left behind the last thread to allow the final tooth of the insert to pass through, achieving full thread depth.
5. Selection of Insert Shim
After selecting the appropriate insert for a CNC lathe, choosing the right insert shim is essential. Insert shims provide different inclinations, making the insert rake angle (λ) equal to the thread's helix angle. Refer to the table below for selecting the correct insert shim.
Many standard shims in tool holders have a 1° rake angle, the most common rake angle.
Negative rake angle shims are required when using right-hand tools to cut left-hand threads and left-hand tools to cut right-hand threads.
Examples:
Pitch = 6 mm, workpiece diameter = 40 mm: requires a 3° shim.
Pitch = 5 t.p.i., workpiece diameter = 4 inches: requires a 1° shim.
