Somehow the Diamond Toolholder (a.k.a. Tangential Toolholder) came to my attention and I thought it sounded like a great idea. Uses a standard 1/4 inch HSS bit, easy to sharpen, easy to adjust height. So I put my money down.
The reality was a big disappointment: here it is on my 7×16 MiniLathe. The cross slide is fully retracted and I can only cut at radii less than 1/2 inch.
When I use a conventionally ground tool bit, I can reach the OD of this 3 inch faceplate.
Maybe if I had bought the left hand holder, I could get to a somewhat larger radius, but still not to the 1.5 inch radius of this faceplate.
I am not saying that this is a bad tool or a bad design, only that it is not suitable for the popular MiniLathes. If you buy one for use on your Southbend lathe, just be aware that the overhang from toolpost to cutting point is about 1.25 inch. What ever diameter you could cut with a conventional bit will be reduced by 2.5 inches. This is the “R8” model toolholder; the penalty is probably greater with the larger toolholders.
Interestingly, it works out better on my Sherline lathe.
Here I have it in Sherline’s standard holder for 3/8 inch bits and I can adjust the cutting point to center. I do not intend to use it here, however; the long overhang is likely to cause chatter.