Boring bars, lathe vs mill applications

I use Bokum boring tools (HSS) for both lathe and mill work, they are easily set on center, and made in both shank type and a screw on style, one great advantage that they have is that they have back rake on the cutting edge, so they cut much more freely than the usual type. They are form ground, so that they are sharpened only on top of the cutting face.
 
Thanks for all your comments! It sounds like the real variable here is the wide variation in quality of brazed carbide tooling. That would explain the range of experiences with the stuff.
 
If you haven't tried a cobalt boring bar in a boring head before, you should give it a try. They cut really well and definitely finish finer, and since the top of the cutting edge is on the tool centerline they tend to also be more accurate in use. Bokum and Borite are the two best brands I know of, although there are sure to be others. I think LMS sells a set for a reasonable price.
 
Yes, LMS has a set that will fit my boring head. The current price is just under $87.

How does one re-sharpen these? To keep the cutting edge on the centerline you don't want to grind the top so I assume you go after the side. But there's only so much "meat" there before you get down down to the shank of the tool. Curious minds want to know :).
 
You hone the end and deburr the top. My 3/8" set is now over 20 years old and is fully functional and looks new. It takes very little honing to resharpen these things.

EDIT: I use Borite bars. Bokum bars are another story. @benmychree can tell you about those.
 
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For those awful brazed-carbide cheapos, how large a hole must be drilled beforehand? So far I've broken two of the set of six I bought a few years back. Purportedly 1/4", but they do not enlarge holes that are 1/4" or 3/8" in diameter - just rub and eventually snap.
 
I prefer at least 1/8" clearance, more if possible. You need enough room to get the chips out.

EDIT: I wanted to add that a boring bar does not like to cut on two edges at the same time. For example, when you drill a starter hole for a blind bore and try to enlarge the hole, there is an inverted cone of material at the bottom. Taking a cut on the wall and then running the tip of the bar into that extra material at the bottom is usually not a good idea. That can snap a solid carbide bar in pieces due to the sudden rise in tangential forces as you hit that stuff on the bottom. Therefore, it is wise to flatten the hole to final depth first, then start cutting on the wall. I learned this little tidbit by breaking a $350.00 solid carbide boring bar off in a piece of aluminum.
 
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Makes sense - the through-hole is under the boring bar shaft, after all.

Figured out the problem I was having: the back of the cutter was rubbing, but not enough to interfere with cutting. Boring bar got about as deep as the head, then wouldn't go any further, even after being withdrawn and the hole cleared out. Turns out the rubbing gradually bent the boring bar shaft inwards, just enough so that the head was rubbing the bottom of the hole, instead of the cutter cutting it.

So, 1/2" minimum it is!
 
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