Table 2: The best-reported surface roughness (R_a) and related cutting parameters for the hard turning of AISI 4140 steel.

Reference	Type of Insert	Feed Rate (mm/rev)	Depth of Cut (mm)	Cutting Speed (m/min)	Nose Radius (mm)	Hardness (HRC)	Best Ra (µm)
Current study	CBN	0.08	0.5	200	1.2	60	0.202
[Akkuş et al.]	Coated carbide	0.18	0.2	120	0.8	62	0.940
[Asiltürk et al.]	Coated carbide	0.18	0.6	150	0.8	57	1.170
[Elbah et al.]	Wiper ceramic	0.08	0.2	115	0.8	60	0.240
[Elbah et al.]	Ceramic	0.08	0.2	150	0.8	60	0.640
[Chavoshi et al.]	CBN	0.1	0.3	190	1.2	60	0.222
[Das et al.]	Al2O3 + TiCN mixed ceramic	0.05	0.2	180	0.8	52	0.714
[Aslan et al.]	Al2O3 + TiCN mixed ceramic	0.05	0.25	100	0.8	63	0.688