Screw

Step 1: Create a New layer.

Step 2 : Create an elliptical marquee. Hold down shift to make it a perfect circle

Step 3: Reset you colours (by pressing d) then swap foreground and background colours (press x)

Now select the gradient tool. use a radial gradient. Click in the middle of you selection and then drag out of your selection and keep going for a bit past the border of you selection. So You have something that looks like this.

Step 4: Create a new layer and select you pencil tool, colour black. and hold down shift and draw a line right down the middle of your ball.

Step 5: Select white as you colour, create a new layer and on the left of the black line, draw a white line down (remember to hold shift to keep it straight) set the layer opacity to about 45% and then merge with the black layer.

Step 6: Now you need on you layer with you black and white line, press ctrl+t to enter free transform mode. and drag the handles to make the lines a bit wider. and then set the opacity down to about 50%.

And that is you complete Screw head. If you are using in a design you may need to add a drop shadow to make it fit in correctly.