Threading: infeed methods, passes and cutting data

An internal thread with a fine pitch has a small nose radius on the insert. Because of that, form errors in the pre-machined hole — out of roundness, taper, drill marks — affect insert life far more than you would expect: the edge cuts deeper into the metal at one point and barely works at another.

Before the first working pass, run a zero pass — a pass at zero depth of cut. It follows the contour without removing stock and evens out the form error of the drilled hole. The first working pass then runs on a prepared surface.

If a long continuous chip forms during cutting, it tangles around the toolholder or the machine components and damages the insert. So the character of the chip has to be watched from the very first part.

1. First part — step through the program stage by stage. Make sure the coolant washes the chip away after every pass. Enter the thread start coordinate 50 mm from the face of the surface being machined.
2. Second part — make sure the chip is not tangling, and only then run the full threading cycle.

On every pass the cutting edge moves towards the centre of the thread. The most common method.

For relatively fine thread pitches.

A V-shaped chip forms, and evacuating it can be difficult — depending on the part material. Both flanks of the profile work at once, so cutting pressure is the highest of the three methods.

The infeed follows the direction in which the thread profile is formed rather than straight towards the centre.

For coarse thread pitches.

The chip is directed to one side — that is the main advantage. The price is accelerated wear on the right-hand cutting edge of the insert, which works at zero depth of cut.

Infeed at an angle of 3…5°, redistributing the work between both flanks of the profile.

Recommended for coarse pitches and materials that produce long chips.

It gives even wear on both sides of the insert and the lowest cutting pressure of the three methods. For production work this is the most predictable option.