Would this be the place to elaborate on the appropriate uses for carbide and HSS respectively?
Morning, Tim. I'll just add on to ttabbal's response.
The first thing I wanted to point out is that your lathes were meant for HSS, not carbide. Unless you have a high speed motor on them, HSS would be the first choice because carbide needs speed to cut well. I know you're aware of this but some new guy following this might not.
In addition to speed, carbide also benefits from rigidity and power, something a small lathe just doesn't have. Given these limitations, it might seem that carbide won't work but it will; you just need to choose carefully. This essentially means you should stick with positive rake tooling and keep nose radii small; both of these things keep cutting forces lower. You should also try to buy finishing inserts; these have very little space between the cutting edge and the chip breaker, which also lowers cutting forces. Whenever possible, use inserts with ground edges that are sharp; again, this reduces cutting forces. This narrows the field but still leaves thousands of choices so let me boil it down for you a little.
I have tried what seems like a lot of carbide tool holders and insert types. It took a long time to figure out what I told you above. If I had to choose a single tool holder and insert designation today, I would definitely recommend the SCLCX tool holders. These tool holders take CCMT and CCGT positive rake inserts and hold them at a 5 degree cant that allows the tool to both face and turn with the tool shank held perpendicular to the work piece. I would choose the smaller tool holder when given the choice, purely because the available nose radii include smaller radii with smaller tools. For me and my 11" lathe, 3/8" tool holders work best and I can get nose radii down to 0.008", which allows me to make smaller cuts for improved accuracy.
Inserts abound for these tool holders but assuming you buy a 3/8" tool holder, I would get CCMT and CCGT 25.205 inserts (the SCLCX tool holder will accept both types of inserts) and only go with larger nose radii if you have to. When buying CCGT inserts, look for the AK grade of inserts. These have a sharp ground edge that is uncoated and polished so they are very sharp. CCGT inserts are usually used with aluminum but they will also cut most other materials and are more useful than many hobby guys realize.
The other thing to know is that inserts are meant to be with the tool holder perpendicular to the work. However, they often work better when we turn the tool holder to positions that allow different edges to cut; this especially helps with finishes. You should play with this to see what works for you. If you watch videos of a CNC lathe with live tooling, you will see that the insert changes position to suit the contours of the part being cut and this also works for us; we just have to understand which edge should be in play for a given need. Play with this when you can.
There is a lot more to using inserts than I can put in this response. The minimum and maximum depths of cuts, speed and power requirements, impact of nose radii on accuracy and so on would take a book, one I'm not qualified to write. I have opinions on this subject that I sort of put in my Boring Primer but even that is not an authoritative work. Maybe one of the other guys might want to give it a shot.
Okay, time for the bottom line: For most work on your lathes, using the common hobby guy materials, I would try HSS first. If the tool does not cut well or dulls quickly, change to carbide and use as much speed as the diameter of the part allows. I rarely cut unknown metals so for me, 90% of my work is done with HSS. I can rough without problems and take very fine cuts to come in on size so HSS works best for me. I switch to carbide when cutting harder materials like stainless or medium carbon steels or tool steels. At times, I will rough tool steels with carbide and finish them with HSS; O-1 in particular benefits from this approach and will achieve a mirror finish at high speeds with a sharp HSS tool.
You have to play with this stuff to sort it out for yourself and your machines. That's what makes tooling choices fun. I hope this helps but if you have specific questions, lay them out and we'll try to be more helpful.
