Woodturning today !
Here is a hollowing tool :
The blade is made of HSS, it is sold for an edge deburring tool.
It is screwed on a flat section , filed following the diagonal of a square section steel bar.
Those little blades are very interresting, when the cutting edge is no more sharp, you just have to unscrew and turn it a few degrees. And when you have finished one, you can yet turn it upside down ...
you have for your money ...
As seen on this upper picture, the square section of the bar helps stabilizing the tool on the tool rest of the lathe.
You can see that here, it is a version with a long (maybe tool long vs the section, I will have to shorten it) shaft and long handle for deep hollowing. I also have a shorter version.