Mr. Bevan experimented on iron wood-screws 2 inches long, 22/100diameter at exterior of threads, threads 35/1000 deep, 12 to the inch. These were driven into boards 1/2 inch thick. The force required to extract them was - from hard woods about 790 lbs., from soft woods about half that amount.1
Making Screws is a subject which is beyond the scope of these Notes.
Wood screws ordinarily used by the carpenter and joiner are made by machinery, the thread being turned in a sort of lathe. Very large screws are also turned in lathes in ordinary workshops.
They hardly come within the range of notes on materials, and it i& impossible, for want of space, to describe them at all.