QuickBolt is a tool written by Peter Inkpen to quickly create accurately-dimensioned screws, nuts and bolts. The tool will refuse to work in any layer that already has geometry to avoid the possibility of corrupting existing geometry. This doesn't mean you need to start a completely fresh object to use QuickBolt, just a fresh layer is needed.
The QuickBolt panel is divided into two main regions: The top region which allows you to specify the general characteristics of the bolt or screw object and also allows you to save and load parameter value files, and the lower region which comprises five tabs labeled Presets, Thread, Thread Extras, Head and Nut where you define the detailed characteristics of the bolt or screw or nut.
The QuickBolt plugin will remember the last settings that you used to generate a screw/bolt/nut so that you can use them again, or use them as a baseline from which to produce a variant. If you want to quickly clear the previous settings, just click on the Reset To Defaults button at the top of the QuickBolt panel.