A sheet metal formed thereon with a number of spaced projections, wherein the projections are formed by the steps of positioning the sheet metal between a female die formed with a number of spaced recesses at one surface thereof and a male die formed with a concavity to be coupled with the female die and a number of spaced projections located at the bottom of the concavity to correspond with the recesses of the female die and pressing the sheet metal under existence of hydraulic fluid between the sheet metal and the concavity of the male die.