In the book "Screwcutting in the Lathe" Martin Cleeve presents three methods of advancing the tool when threading and discusses the issues with each method.? This is a great book for anyone just learning how to thread on the lathe, covering most all aspects of threading.
The issue with the methods shown in the Southbend book is that standard threads have a flat at the bottom of the thread which isn't shown in their diagrams (and tools with a sharp point would fail very quickly).? In practice, the width of this flat depends on the pitch of the thread being cut so, using the methods shown, the tool would need to be reground to have the correct width flat for each pitch to be cut.? This is fine for production when making identical parts but not so good in the home shop where we would prefer to use one threading tool for multiple pitches.
Cleeve suggests grinding the tool to have the flat for the finest thread to be cut. Set the compound parallel to the thread axis and each time a cut is added with the cross slide, advance the compound half that amount - this works almost like setting the compound over.? But when the tool is at depth one can advance the compound to produce the required thread flat which will also thin the thread to the standard dimensions as measured with the 3 wire method.? Cleve notes that brass cuts so easily that straight in using only the cross slide works fine.
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