I do as Jason, even on an Emco 5 lathe which is very much lighter than your Harrison L5, I normally use full form inserts for external metric threads from 0.50mm to 1.50mm pitch but for imperial sizes just a 55 deg tool with a minimum nose radius.

Emgee

Why 27 degrees. Tad under 30 I would have thought.

regards Martin

For coarse threads it s best to flank feed the tool. On medium pitches ether flank feed or plunge feed is acceptable. On fine threads plunge feed is preferred.

Definitions:

Plunge feed is feeding with the cross slide.

Flank feed is done with the compound. The compound is set 1/2* or so steeper than the trailing flank of the thread. The idea is to do the major amount of metal with the leading flank of the tool. Th trailing flank takes a very fine cut to prevent steps in the thread form.

Is it possible you are cutting a 1.25mm pitch? 1.25 tpi is an extremely coarse thread, not one I would expect in model engineering?

A 1.25 tpi would have a lead of 0.8 inches. My 16" lathe will not cut that coarse a thread. I figure it was a typo

Recently I had to cut numerous various sized threads in steel & brass, Metric and imperial using both HSS and Inserts. My lathe is only a moderate sized bench job. I ended up using a variety of methods both straight plunge in and angled compound slide ( the angle isn't that critical – it's only there to ensure the cut only takes place on one side of the tool so reducing the loading )

Overall, I couldn't really see much difference between any of them! Maybe my standards aren't as high as some guys on here, but essentially as long as the thread is the right size and fits the job as I want, I'm happy.

My only advice, echoing old mart's comment, is to carefully reduce the in feed the deeper you get – may seem tedious but try to keep the feed to quite small amounts ( as long at it still cuts as opposed to rubbing ) that way you should get a good finish and accurate thread form. Like most things, the more you practice the better your results will be.

p.s because I can't get a slow enough speed on my machine, all threads were hand-cranked.

regards