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#1221
Oh don't get me wrong I'm a big fan of learning how to do things I make loads of tutorials when I can to assist in this area, but sometimes small things like the screw head which are insignificant in the overall picture are just quicker and easier to get from the net.
If the screw head was a major focal point then yes try and redraw it but otherwise use the resources that are available. :)
Your skills are way beyond simply drawing a circle with a cross in the middle and a bit of shading, so maybe take the time on learning how the shadow would fall if the screw wasn't tightly screwed in or maybe making it look old and rusty or shiny and new etc.