The flaring process is achieved by cold forming which requires a suitable material with sufficient plasticity.

The tool to perform the deformation is a cylindrical press bolt that rotates in an angle to the respective work piece while it is firmly held down on this.

Since test coupler bodies are manufactured rationally and in large numbers on multi spindle lathes, the material used is necessarily free cutting steel ( i.e. : 9 S Mn Pb 28 ).

The use of this material is necessary because its content of sulphur and lead make it  relatively brittle. This in turn is beneficial during the machining process as it causes short and easy to remove cutting swarf. It would therefor be unreasonable or impossible to expect this material to have sufficient plasticity and strength to withstand high loads where it is flared by cold forming.