Internal turning

Internal turning operations machines the inner diameter of the workpiece. Long overhangs and poor chip evacuation are two of the challenges with internal turning. Long overhangs can cause problems with both deflection and vibrations. Vibrations and poor chip evacuation can cause insert breakages. Difficulties with chip evacuation can also lead to poor surface finish.

The basic application areas for internal turning are longitudinal turning (1), and profile turning (2).

Longitudinal turning

The choice of tool is very much restricted by the component hole diameter and length (the depth of the hole with overhang). A general rule is to select a tool with the shortest overhang and the largest possible tool size.

Selecting the right tool for the operation, applying it correctly and clamping it properly all have an effect on minimizing tool deflection and vibration.

Insert shape

In internal turning, it is advantageous to use positive basic-shape inserts as they have lower cutting forces compared to negative inserts. A small nose angle, as well as a small nose radius, also contribute to reduced cutting forces.

Entering angle

The entering angle affects the direction and magnitude of axial and radial forces. A large entering angle (small lead angle) produces a large axial cutting force, while a small entering angle (large lead angle) results in a large radial cutting force. An entering angle close to 90° (0° lead angle) and never less than 75° (15° lead angle) is recommended.

Tool holder

If there is a shoulder to machine in the component, choose a tool with 91–95° entering angle (-1 to -5° lead angle). It is recommended to use a triangular or a D-style (55°) insert.

If a stronger edge is needed, choose a C-style (80°) insert.

If there is no shoulder to be machined and the conditions are stable, increase productivity by using a square insert at 75° entering angle (15° lead angle).

Turning tools for longitudinal turning

Profile turning

In internal profiling, the tool is exposed to both radial and tangential cutting forces. The radial cutting forces will deflect the tool away from the workpiece and the tangential forces will force the tool downwards and away from the centre line.

When boring small-diameter holes, it is particularly important that the clearance angle of the insert is high enough to avoid contact between tool and wall of hole.

Insert shape

In profile turning the cut can vary as cutting depth, feed and speed. The largest suitable nose angle on the insert should be selected for strength, cost efficiency and accessibility. The most frequently used nose angles are 55° and 35°.

Entering angle

Entering angle and insert nose radius are both important factors for accessibility. The workpiece profile has to be analyzed in order to select the most suitable copying angle.

A free cutting angle of at least 2° between workpiece and insert has to be maintained. However, for reasons of surface finish and tool life, at least 7° is recommended.

Tool holder

First choice is a tool with 93° entering angle (-3° lead angle) and a D-style (55°) insert. If a larger ramping angle is needed, use a V-style (35°) insert.

For the possibility to profile the face or to make a corner relief, choose a holder with a 107–117° entering angle (-17 to -27° lead angle).

There are specific tools for back boring, especially designed to machine a shoulder in the opposite direction