Selection of shim
Insert shims are used to give different tilts to the insert, so that the angle of insert inclination (λ) is the same as the helix of the thread. See table below for methods on how to select the correct insert shim.
- The standard shim in many holders is 1° which is the most common angle of inclination
- Negative shims are used when turning left-hand threads with right-hand tools and vice versa
Lead (Pitch) mm||
- Pitch=6 mm and workpiece= Ø40 m: a 3° shim is required
- Pitch=5 threads per inch and workpiece= Ø4 inches: a 1° shim is required
Threads with small profile angles
ALP = Radial clearance
ALF = Flank clearance
For ACME, trapezoidal and rounded threads it is especially important to choose the correct shim to tilt the insert, because the pressure on the cutting edge is higher and the flank clearance is smaller.
Flank clearance (ALF) depending on profile
|Thread profile||Profile angle|
To set the correct radial clearance, the inserts are tilted 10° or 15° in the tool holder. It is important to use internal inserts with internal tool holders, and vice-versa, to ensure that the correct thread form is achieved.
11, 16 and 22 mm
(1/4, 3/8 and 1/2 inch)
27 mm (5/8 inch)