Selecting the correct indexable inserts – what do I need to know?

In order to prevent wrong decisions from being made, indexable inserts are standardised internationally. This ISO standardisation specifies the size, shape, mounting, material properties and coating. The cutting machine operator uses the ISO indexable insert designation to select the appropriate indexable insert needed for the relevant application.

The ISO code can include up to twelve characters. The first to seventh characters indicate mandatory information. The eighth and ninth provide optional information which is used as required. The tenth to twelfth are optional manufacturer details and are added separately to the ISO code using a hyphen.

The seven mandatory fields indicate the insert shape or the clearance angle and other core features of the cutting insert. The characters have individual identification letters and numbers which clearly identify an indexable insert. In order to be able to assign the appropriate dimension to these identification letters, tables conforming to DIN4983 are available which can be looked up in a table book.

Manufacturer-specific details are indicated after a hyphen and, depending on the company, specify the chamfer width, chamfer angle, cutting material or shape of the chip breaker. The relevant legend is taken from the manufacture catalogues.

The ISO code comprises 9 symbols, of which symbols 8 and/or 9 are only used as required. The manufacturer can add other symbols, which are added to the ISO code by a hyphen (e.g. for the shape of the chip breaker).

To achieve good chip control and the best machining results, the geometry, grade, shape (point angle), size, corner radius and setting angle of the indexable insert should be selected carefully.

For roughing, we recommend a combination of large cutting depth and high feed rate. Roughing applications place high demands on the cutting edge dependability. Finishing is an application which requires low cutting forces as shallow cutting depths and low feeds are generally needed.

A large point angle is stable, requires increased machine output, but can also tolerate higher feed rates. High cutting forces are therefore possible, but the tendency for vibrations also increases. With a small point angle, the cutting edge is less stable and it has a small cutting edge engagement; thereby increasing the thermal sensitivity. The insert has a smaller cutting force.

Select the indexable insert size depending on the machining requirements and the space which is available for the tool in the application. A larger indexable insert offers greater stability. For difficult machining, the indexable insert size is usually above IC 25 mm (1 inch). During finishing, the size can often be reduced in most cases. Proceed as follows: First, determine the largest cutting depth and then set the required cutting length, whilst taking into account the setting angle of the tool holder. You can then select the correct cutting edge length for the insert.

The corner radius is selected depending on the cutting depth and feed and influences the surface quality, chip breakage and stability of the indexable insert – the corner radius is therefore an extremely important factor when it comes to turning operations. A small corner radius is ideal for shallow cutting depths, reduces vibrations and produces a good chip breakage. However, the cutting edge is less stable than with a large corner radius. This allows a high feed with greater cutting depths and high cutting edge dependability. However, with the large corner radius, greater radial forces occur. This can impair the cutting effect and lead to a poorer surface quality. You should therefore select a smaller corner radius if there is a tendency for vibrations to be generated with your configuration.  Always select a corner radius which is not larger than the cutting depth.

The setting angle KAPR is the angle between the cutting edge and feed direction. It influences the chip formation, direction of the cutting forces and the cutting edge length being engaged. With a large setting angle, the forces are directed towards the chuck, thereby producing a reduced tendency to generate vibrations. It allows shoulders to be turned and produces higher cutting forces during entry and exit. However, with the large setting angle there is a tendency towards notch wear in HRSA and case-hardened materials. The smaller setting angle increases the tendency for vibrations to be generated as higher radial forces are directed into the component. But on the other hand, the cutting edge is under less strain and a thinner chip is created, which allows for increased feed rates, and the notch wear is lower. However, it is not possible to turn against a 90-degree shoulder.