Abstract In the last decades, micromachining has gained increased relevance as an alternative method to produce miniaturized parts for a wide range of products. Nevertheless, micromachining differs considerably from conventional machining in many aspects. This work is concerned with the investigation of size effect when micromilling three distinct materials (pure nickel, electrolytic copper and AA 6262-T6 aluminium alloy). The friction coefficient between each work material and (Ti,Al)N coated tungsten carbide (cutting tool material) was determined and employed to calculate the critical chip thickness. Micromilling tests were performed and additionally to the specific cutting force, the critical chip thickness was identified using the acoustic emission signal. The findings indicate the suitability of the model based on the friction coefficient and cutting edge radius to determine the critical undeformed chip thickness. For each work material, it was found that feed rate values below the critical undeformed chip thickness led to ploughing (higher acoustic emission signal), whereas feed rate values above the critical undeformed chip thickness cause chip formation. Furthermore, the reduction in the shear plane area resulted in higher specific cutting force.