That seems a tough order to me. A solid carbide bar and very sharp insert might be helpful, but RC50 and sharp carbide= limited insert life. I've had pretty good luck boring up to RC60 steel with one edge of a carbide endmill. You might try a 5/8 endmill if you have one that long. Regardless, that deep at that diameter in hard steel will be a challenge.
A sharp edge probably won't last in harder steel. I'd go with a very light rounding with a hone, as in just barely kiss the edge - that's if you're sharpening your own. An insert will likely already have edge prep.
And get your speed down. A good starting point with carbide would be 150-200 SFM. If it's not chattering or giving a bad finish due to vibration a solid carbide bar probably won't do anything to help you.
You mentioned deflection, but with a boring bar the deflection should be the same all the way through the hole. The exception is if you're getting tool wear and the bore gets smaller as you go due to increasing tool pressure. That's a tool wear problem though, not a bar stiffness problem.
I have no chatter and only a little vibration and the deflection is the same down most of the hole. Im getting a really good surface finish but am having a hell of a tome getting it to bite to cut.
Vinny, is this one off, or multiple ?
Is your tool tip on center? It needs to be pretty close or you'll have all sorts of deflection problems at small diameters.
You need to consider the quality of the carbide used as well.
I have bored thousands of holes in hardened steel with brazed carbide tools.
Typically a C6 grade will work, won’t find that at many discount retailers.
I have 12mm solid carbide shank boring bars, but have never tried hard turning with them. There are 12mm Chinese carbide shank boring bars on ebay if you look long enough, at prices which are realistic compared to the offerings from the big boys. These Chinese bars take CCMT 06 inserts and I even found a seller in Germany selling them singly in CBN inlay type. As I said, I have no experience with this combination, only external hard turning. If you have to go in this direction, put a thick shim on top of the bar to spread the pressure from the screws. Steel shanks should only be used up to 4 diameters, which would only be 2" in your case. Carbide shank tools can do your 3" depth ok, but you would have to be careful of swarf build up and be mindful of the increased brittleness of carbide shank tools. Even with carbide shanks, it is best to have the minimum tool stickout for the bore depth. All gibs should be adjusted to reduce any source of slop, and I would set the tip height about + 0.005" high.
Yes this is a one off deal . And yes as close as i can get it to on center.
I'm not sure i understand the problem here. You're getting a good surface finish. Is there a problem holding a finish dimension? A HSS cutting edge is supposed to be sharp, and cuts the material relying on that sharp edge. Conversely, if you look at a carbide insert up close it will look like it needs sharpening as compared to the HSS edge. The carbide doesn't so much cut the material as it rather unceremoniously plows its way through it. That's why it's difficult to take a really fine finish pass with carbide. The tool likes a big cut under pressure.
If you were experiencing excess chatter problems then yes, switching to the denser carbide shank might help. Otherwise, it sounds like things are working as to be expected.
My only experience with hard turning is OD work so tool flex was not a problem using cermet tooling rather then carbide. I suspect that a .75 bore X 4 diameters deep would not work well when hard turning, there are minimum parameters that must be met for it to work well.
what kind of inserts does the boring bar take?
Im having trouble holding the finish dimensions.