I agree with you in most respects.
COMMON carbide inserts are NOT very suitable for small lathes. It's true that negative rake carbide (or even HSS) tools do NOT work well except in rigid machines. It's also true that small low powered machines cannot make the BEST use of carbide.
There are, however, families of small uncoated inserts, mostly triangular, that are VERY sharp, have considerable relief, NO rake, cut with minimal pressure, and work well at most any speed. I use these regularly, on a 6" Atlas, and even on my old round-way Unimat SL. While the rake is zero (flat on top), these cut with less pressure than most positive rake tools (which are less sharp). I can take as heavy a cut, with as good a finish, with these carbides as I can with HSS ... even on the Unimat!
I also use these, and a variety of more common carbide inserts on a 10" Logan. This heavier machine is more forgiving of the commonly available carbides inserts. It still doesn't like negative rake tools much, however.
The advantages of the carbides are that they stay sharp longer than a HSS bit in the same service. I find this especially advantageous in turning soft but abrasive plastics. It is also useful when one has to turn hard materials. The inserts work reasonably well with almost all materials, but there is no advantage when turning softer steels, brass, etc.
Disadvantages include problems with interrupted cuts, the inability (easily) to custom grind odd shaped tools, and higher cost (considerably).
That said, I use HSS for perhaps 90% of my turning needs. The carbide is less versatile, and more expensive. On a small lathe, carbide is NOT the best choice for general work. But, to say it's NOT suitable at all for use in a small lathe, even a Unimat, is just plain WRONG!
Dan Mitchell ==========
Ted Edwards wrote: