# Comparative Study on Thermal Behaviour, Tool Wear and Surface Roughness in Milling EN8 Steel for Sustainable Machining

## TL;DR

This study compares up-milling and down-milling techniques for machining EN8 steel, finding that down-milling reduces tool wear, improves surface finish, and lowers cutting temperatures, supporting sustainable manufacturing.

## Contribution

The study introduces a kinematic–tribological coupling concept to optimize dry machining of harder materials through down-milling.

## Key findings

- Down-milling reduced tool wear by 12.4% and surface roughness by 45.9% compared to up-milling.
- Down-milling lowered peak cutting temperatures by 47 °C due to better heat dissipation and lower friction.
- A mid-range cutting speed and low feed rate in down-milling provided optimal multi-objective performance.

## Abstract

Dry machining of medium-carbon steels plays an important role in sustainable manufacturing; however, high tool wear and thermal instability pose challenges. The study aims to evaluate the kinematic–tribological performance of EN8 steel during dry milling and compare up-milling and down-milling to trade-off tool life and surface finish. The experiments were conducted using a central composite design (CCD) as part of response surface methodology (RSM), with 36 runs to evaluate interactions among spindle speed, feed rate, and depth of cut. Down-milling outperformed up-milling, achieving 12.4% less tool wear, 45.9% better surface finish, and a 47 °C lower peak temperature from cutting. The above benefits are attributed to the unique kinematics of chip formation during down-milling, which offers lower friction at entry and better heat dissipation, contrasting with the high-friction ploughing phase of up-milling. Grey relational analysis (GRA) found that down-milling with a mid-range cutting speed (22.31 m/min) and a low feed rate (25 mm/min) provided a multi-objective optimum. The findings support the existence of a kinematic–tribological coupling, providing a solid single approach to optimising the dry machining of harder materials.