## Selecting a Shim for the MMT Series

### Flank Angle and Lead Angle

Lead angle (α) depends on a combination of thread diameter and pitch. Select a shim so that the lead angle of the thread can coincide with the flank angles of the thread and insert (β1、β2). No need to change a shim for general threading with an MMT holder.
When threading with a small diameter or large pitch, change the shim depending on the lead angle, referring to the table and graph below. When threading left hand threads, change to a shim with a negative inclination angle.

(Note) Back turning in the case of left hand threads.

### Shim Reference Graph (Thread Angle 60˚ and 55˚)

(Note) When a thread lead angle < the tool flank angle, change the shim to prevent side interference with the insert.
(Refer to the table below for the calculation of thread lead angle and tool flank angle.)

(Note) Back turning in the case of left hand threads.

### Shim Reference Graph (Thread Angle 30˚ and 29˚)

(Note) When a thread lead angle < the tool flank angle, change the shim to prevent side interference with the insert.
(Refer to the table below for the calculation of thread lead angle and tool flank angle.)

### Selection Table

(Note) Back turning in the case of left hand threads.

 When replacing a shim,check if the difference between the thread lead angle and shim inclination angle is within: 2.5°─0.5° where thread helix angle is 60° (55°) 2°─1° where thread helix angle is 30° (29°) *Inclination angle of a standard shim is 0°.*The holder has a 1.5° lead angle.

### Example of Selecting a Shim

1. In the case when the thread helix angle is 60°
(2.2° lead angle) ─ (2.5─0.5°) = -0.3°─1.7° shim inclination angle is appropriate.
Threading with a standard shim (0° inclination angle) is possible. But, replacing with a shim with a 1° inclination angle is recommended.

2. In the case when the thread helix angle is 30°
(2.2° lead angle) ─ (2─1°) = -0.2°─1.2° shim inclination angle is appropriate.
Replacing with a shim with a 1° inclination angle is recommended.

### Relief Angle of an Insert Set on a Holder

Thread Helix Angle Internal Relief Angle External Relief Angle
60° 8.8° 5.8°
55° 7.9° 5.2°
30° 4.1° 2.7°
29° 2.6°

• Relief angles (β2, β1) of an insert become small when the thread helix angle of a trapezoidal, round, or other thread is small.Take care when selecting a shim.