Annealed 4140

[PixMan]

Positive rake inserts will always be around because they're most-often used in small diameter boring bars where you'd need the additional side clearance angle in smaller bores. They also come in small sizes, down to at least a tiny CPGT1.51.2-PM2 that I have for a 1/4" boring bar. It's a 3/16" I.C. size. No way would any negative rake insert I know of work in the minimum bore size that bar can do, which is .310".

Here's another reason for them, in larger I.C. sizes. A negative rake profiling tool such as the MVJNR-163D holder carrying a VNMG331 insert shown in the photo below can (technically speaking) only turn in the direction that it's tipped to a negative angle, as the back edge is then just rubbing. By contrast, a similar shape positive rake insert such as a VBGT331 would sit flat on the holder and have 5º side clearance angle all the way around. Now you can easily use that tool to "back turn", where a VNMG couldn't do well.



There *is* only one type of negative rake insert I've ever seen for really small work, and it wors well. That's a solid carbide internal insert-type threading bar. It's smallest recommended bore is also .310".