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Another old wives tale. There are plenty of modern carbide inserts and tooling with configurations that flat work on manual machines. Way too many of us have been doing it successfully for far too long now to keep this up. I have inserted tooling that'll take skim cuts in the tenths leaving beautiful finishes on my 13" Rockwell without high speeds or power.This is especially true on lathes that do not have enough speed/power to use carbide properly.
IMO you're better off using HSS rather than carbide to do chamber work. This is especially true on lathes that do not have enough speed/power to use carbide properly. This is rather complicated in regard to the rake angle, but carbide in general prefers fast speeds and more often than not you want to cut your chamber at slow speeds, so HSS is desirable for that. You could grind a tool bit blank to form a small boring bar, those work fine. You can also get HSS inserts from Arthur Warner as Bob Pastor has long recommended (Maker of Viper's Venum cutting oil). Not a bad option if you don't know how to grind a tool bit:
If you are asking about what boring bar to use, I have to guess you're not very competent with a lathe, so whatever you do, take your time and most of all be safe. In the case of chambering where you're basically sneaking up on your dimension from .001"-.003" at a pass, that is not enough for carbide to cut cleanly and carbide needs to turn fast in order to leave a clean finish. There are so many factors involved here I just can't over emphasize to be safe no matter what you do.
Believe it or not, there was machining before carbide inserts. We cut all kinds of steels with HS , because that's what we had. You had to actually know something about tool geometry, it wasn't as simple as removing a worn insert and putting a new edge to the work. Tough to cut alloys have been around for a long time.I work with 4140 quite a lot at work. We use Kennametal tools with carbide inserts.
Even, or because of, the feeds & speeds we use, we go through a fair number of inserts. Even with coolant.
Not sure how HSS would hold up against it.