Actually there is an easy way to do it, first start the knob body as a cylinder, then make a rectangular sketch on any of the planes (i.e. Front plane), extrude that sketch untill it comes out from one side of your cylinder with the measurement you need, then create an axis (insert-reference geometry axis) using two planes, choose front and left planes and create an axis in the middle of your cylinder, then go to (insert-pattern/mirror-circular pattern, use your axis as middle, on the angle use 360/number of grooves you want to create including the first one you created, write the quantity of grooves you want, choose the extrude you made as features to pattern and voil