Two types of angular clearance between the insert and thread are necessary for precise, accurate threading. These are:
- Flank clearance
- Radial clearance
Cutting edge clearance between the sides of the insert and thread flank is essential to ensure that tool wear develops evenly, to give consistent and high quality threads. The insert should therefore be tilted to gain maximum symmetrical clearance from the flanks (flank clearance angle).
The tilt angle of the insert should be the same as the helix of the thread, to ensure success.
Selecting shims to tilt the insert for flank clearance
Insert shims are used to give different tilts to the insert, so that the angle of insert inclination is the same as the helix of the thread.
- The standard shim in the holder is 1°, the most common angle of inclination
- Shims are available in 1° steps, in the range -2° to 4°
- Negative-inclination shims are required when turning left-hand threads with right-hand tools, and vice versa
λ= the angle of insert inclination
The flank clearance angle of the insert is adjusted by changing the shim under the insert in the tool holder. Standard tool holders have a 1° insert inclination angle.
Two alternative ways to select the correct shim
A. Use the diagram
B. Use the formula to calculate the helix angle to choose the corresponding shim
Workpiece diameter and pitch influence inclination angles
||Lead (Pitch) mm
For a pitch of 6 mm and a workpiece diameter of 40 mm, a 3° shim is required. For a pitch of 5 threads per inch and a diameter of 4 inches, a 1° shim is required.
P = Pitch
d2 = Effective diameter of thread
λ = Angle of inclination
To give adequate radial clearance, inserts are tilted in the tool holder, 10° or 15°.
It is important to use internal inserts with internal tool holders, and vice-versa, to ensure that the correct thread form is achieved.
|Insert sizes 11, 16 and 22 mm
(1/4, 3/8 and 1/2 inch)
|Insert size 27 mm (5/8 inch)|