Posted by David Colwill on 27/02/2020 09:42:12:
I have noticed that the threads can look quite rough and then suddenly as you get to correct depth magically get much better. Has anyone else noticed this?
Sometimes, on steel. Haven't seen it on cast iron or brass. I start with large depths of cut,up to 20 thou depending upon the thread pitch, and reduce DOC once I'm close to full depth. For stringy materials like steel it's possible the swarf on the first cuts rubs against the flanks as it tries to escape. With smaller DOC the swarf is smaller and thinner, so more pliable, and will have less of an effect on the flanks. For brass and cast iron where the swarf is fine particles the issue doesn't seem to occur. I do all my screwcutting by plunging straight in.
Insert profiles are rather more involved than DC31k propounds. Here are three inserts:

Which are full form and which partial?
The one on the left is marked 16ERAG60, so a partial profile for a 60 degree angle thread. The middle insert is marked 16ER-8W, a full profile insert for 8tpi Whitworth, where the partial tooth does indeed form the rounded crest, as specified in the Whitworth form. The righthand insert is marked ER16-2.0ISO, a full profile insert for 2mm pitch ISO threads. These threads have a flat crest, so the second tooth is not needed to fully form the crest, a flat is sufficient.
Andrew