Coolant has special uses in the lathe machining process. In addition to extending tool life, coolant in some cases can reduce force and smooth the surface of the manufactured product. This study aims to determine the effect of differences in the use of coolant on the surface roughness of the hinge cylindrical turning (hinge) of ST 41 iron material. The choice of this material is because it is often used in machining tools and the price is economical. This type of research is experimental research conducted in a laboratory. This study uses quantitative methods. In the multilevel straight turning process as many as 3 hinge workpieces using 3 types of coolant (coolant). Hinge workpiece A uses water cooling without a mixture, hinge workpiece B uses water cooler with a mixture of coolant (coolant) and hinge workpiece C uses lubricant coolant. The turning of the three hinge workpieces is done by cylindrical turning. The results showed that the use of the lowest mean surface roughness cooling type was water cooler with a mixture of coolant (1,254 mm), the highest type of surface roughness average value was oil cooler (1,850 mm). While the average value of moderate surface roughness is the type of coolant (1,489 mm).