Yes, indeed- thanks Mike for the explanation. I suppose the biggest thing that stood out to me is the "designed" minimum depth of cut. It really stood out to me and made me realize I have SO much left to learn (as if I didn't already know that). But for me, I am regularly trying to sneak up on a size- taking .001" or sometimes .000x" depths of cut. The insert tools are so frustrating to me because, I assume, the tool pressure isn't right. It's like they'll just rub the surface and not cut anything. I advance the cross slide and still nothing happens. I advance some more and then- bam, I've taken a .005 cut. For this small work that I'm trying to do (and especially because I have considerable wear in my lathe) I think that a properly ground HSS bit is exactly what I'm looking for.
Now the reason I was looking for REX stuff is because I do a fair amount of cast iron stuff in my tops. I've heard that REX tooling is superior when cutting stuff like stainless or cast iron. Now, it's unclear to me if I need REX aaa or REX 95, but my take on it is that the 95 version is the harder of the two, and therefore perhaps the better option.
[EDIT]- I would add that my Tormach Super Fly with the SEHT1204AFFN-X83 inserts do an exceptional job on my mill. So yes, I agree that inserts absolutely can work! I just found it interesting when they started talking nose radius and minimum depths of cut.
The key reason you're having trouble with light cuts is because an inserted tool does not like to take light cuts. The reason for that is because radial cutting forces are very high with light cuts and the tip is being pushed out of the cut. At minimum, the lightest finishing cut an insert can take is about 1/3 - 1/2 the nose radius of the cutter, while roughing cuts need to be at least 0.005 - 0.010" larger than the nose radius at minimum if you want to avoid chatter. So, you need the right nose radius on the insert. Complicating this is the fact that in order for the chip breaker to work as intended, you need to take a deep enough cut. This is typically around 0.020" deep or more for it to work as intended for most inserts.
You need to know how your tool/insert like to cut in the material you're working with. For example, a SCLCR tool with a CCGT 25.51 AK insert (this has a nose radius of 0.016") in 6061 has a minimum roughing depth of cut of about 0.02 - 0.03", and a minimum finishing depth of cut of about 0.005 - 0.008". If you try to finish with a shallower depth of cut, the insert won't cut properly. If you can find a depth of cut that works well, that insert will consistently cut that depth pass after pass after pass. Knowing what your finishing depth of cut must be, you can rough until you get to, say, 0.016" from final diameter and dial in the finish pass. You will need to slow your feed rate to about 1/2 the nose radius in inches per minute but it will take that cut with a decent finish.
This is not as complicated as it sounds. Once you figure out how your roughing and finishing pass requirements, you just dial it in and make your cuts. The insert will cut consistently if all your cutting conditions are dialed in.
On the other hand, HSS is much less fussy but your tool geometry makes a big difference. I won't go into that here but in my experience, you will have far fewer issues with HSS.
I was wrong. Rex AAA is also 5% cobalt, not 8%. I think you're on the right track going with cobalt for cast iron. It is a superior cutting tool. I prefer Cleveland Mo-Max cobalt (5%) and Super-Mo-Max (8%) for this. I normally turn cast iron with a brazed carbide tool, actually, and finish with a cobalt tool. If turning a rough piece, use an old HSS tool to break through the cast surface, then switch to a very sharp brazed carbide tool with a small nose radius on it to continue roughing it in. Then change to a sharp HSS to finish it. Keep your speeds low - back gear if you have it - and it will go well.