Boring bars for boring heads

petertha

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I have a set of M42 0.500" shank Borite bars like these. I'm happy with how they cut. They can be dressed on the flat for sharpness. But I'm curious why they don't make a flat on the shank so the set screw has something to register on vs. obliquely tangent to the shank. This has always bugged me. Yes, I suppose it gives you 'adjustability', but I would prefer 'repeat ability'. They have a similar boring bar set with carbide inserts, same deal, no flats. Dumb question but why?

On a side note I bought an offshore conventional boring bar with CCMT, turned the shank down to 0.500" & preserves the flat so the insert is maintained at correct angle. I'm seriously thinking of just buying more & making my own set at various lengths & shanks.

Any other brands out there I should consider
 

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Are those cutters on plane with the center of the shaft?
They appear to be better suited mechanically than the brazed carbide.
Would you consider milling some flats?
Probably a price point decision why they don't??
 
Janderso, the shanks are hardened like the rest of the tool so the flat would likely have to be ground, not (easily) milled. Possible, but pita. I'm considering having it done by a buddy, hence the question.

JimDawson, that's the usual reason I hear. What does 'properly' mean? Isn't essentially the same as boring in a lathe where the vast majority have flat side shanks so the insert is maintained at designed orientation? I can see the potential to tweak the rotational position but how would you measure a few degrees relative to the boring head? When the time comes to re-dress to restore the edge, chances are very good one would maintain the same flat orientation. If one has the insert style, I assume the rake has been optimized via the chosen insert, or, get a different insert of the same family. But on the Borite insert style, again no flat. Attached pic of D'Andrea boring shank with flat (or where the cutter is in fixed rake orientation, same difference relative to insert orientation). This seems to be common practice on high end boring heads.

My beef with no flat on the shank is when you have a precision fit in the boring head (which can be an expensive tool), then any potential marring of the set screw against the shank means its going to be difficult to remove the cutter & potentially scouring the tool hole. I see way more advantages to a ground flat vs. un-ground.
 

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My beef with no flat on the shank is when you have a precision fit in the boring head (which can be an expensive tool), then any potential marring of the set screw against the shank means its going to be difficult to remove the cutter & potentially scouring the tool hole. I see way more advantages to a ground flat vs. un-ground.

I can see the concern for raising a burr with a screw but I have never marred the shank of a boring bar in a boring head, nor have I had one with a round shank slip. I use Criterion heads, Borite cobalt and inserted tip bars and Micro 100 carbide bars and have never had a problem so raising burrs is not a realistic concern for me. The reason for flats, in my view, is speed. You can lock a boring bar in the head without thinking about it or removing the head from the mill so it's fast to change bars. Again, this is not important to me in a hobby shop. At the end of the day, if you want flats then buy bars with flats or have them ground in.

JimDawson, that's the usual reason I hear. What does 'properly' mean? Isn't essentially the same as boring in a lathe where the vast majority have flat side shanks so the insert is maintained at designed orientation?

I'm not sure I agree with the "vast majority" part of this. Many/most high end solid carbide boring bars for the lathe do not have flats on them. Flats are more common on zero lead bars or positive lead bars meant for roughing, although this is not a uniform thing in the industry. All my solid carbide Circle Machine bars do not have flats and I much prefer shanks like this because I can alter the rotational position of the bar if I choose to. Most of us use inserted carbide bars with the insert oriented as the factory intended but when the relief angle of the insert allows for it, a degree or two of positive rake can reduce radial cutting forces enough to improve accuracy significantly. Sometimes I have to do this when I have only a tiny bit more to take off to come in on size and the nose radius won't allow me to take a small enough cut. A few degrees of rotation can allow me to dial in a cut that the nose radius normally wouldn't allow and this can be the difference between coming in on size or starting over again. If the bar had a flat on it, that option would not be possible when using a boring bar holder with screws to lock the bar down.

Inserts are usually meant to be used with the tool holder in a fixed position but this is not written in stone. This is especially true with turning tools on the lathe. Quite often, the best position for an insert is to turn the tool holder so the insert cuts with the side or end cutting edge. With boring bars, the same is true rotationally; a degree or two can make a huge difference.
 
JimDawson, that's the usual reason I hear. What does 'properly' mean? Isn't essentially the same as boring in a lathe where the vast majority have flat side shanks so the insert is maintained at designed orientation? I can see the potential to tweak the rotational position but how would you measure a few degrees relative to the boring head?

Many times I tweak the rotation of a boring bar in a boring head to solve chatter or get a better angle for the particular material. I set the rotation by eye. I've never had a problem with raising a burr on one with the set screw. I don't like the flats on boring bars because sometimes I want to change the angle. I only own one boring bar with a flat on it and have never used it, all the rest of my bars are round shank.
 
OK good info. My hand wringing is for naught. I'll stick with what I have. Actually I got thinking about this some more. There is nothing saying the flat ground on a brand new boring cutter face is exactly centered relative to the shank. Its probably close but if its plus or minus some thou, then the effective rake will be something different anyways even when aligned to the boring head lateral axis.

So give me some guidance examples of rotational tweaking. I assume you start with the cutter face +/- aligned. Then you hear chatter. Then what. Increase the angle a bit for ductile materials? Something different for ferrous or harder materials? Do you ever go negative? Whats a min/max, like 5-deg? Are you mostly looking at finish or are there audible clue something is amiss? (Like I say I've had very nice results with the Borite M42's just wondering out loud.
 
Normally I start with what looks correct, then adjust from there as needed. Sometimes this even means pulling the tool out and reshaping it to get different relief angles. On occasion I will go negative. Just depends on the ''feel'' of the work. Sometimes you get visual clues, but mostly it's audible. There is no min/max, only what works.

Having said all that, most times chatter can be reduced or eliminated while boring by reducing the rpm, increasing the feed rate, or where possible take a heavier cut, in other words loading the tool a bit more.
 
So give me some guidance examples of rotational tweaking. I assume you start with the cutter face +/- aligned. Then you hear chatter. Then what. Increase the angle a bit for ductile materials? Something different for ferrous or harder materials? Do you ever go negative? Whats a min/max, like 5-deg? Are you mostly looking at finish or are there audible clue something is amiss? (Like I say I've had very nice results with the Borite M42's just wondering out loud.

When using a boring head, I don't do much rotational tweaking myself. I tend not to do that because it affects my depths of cut. When the tool face is oriented on center as it should, the feed screw is accurate to the thousandths. If I alter it then I cannot accurately anticipate the change. Of course, you can measure it and wing it, which is what you must do when altering your angle.

With a boring head, the cutting forces are still there and you have to account for them. Therefore, when you encounter chatter the same rules apply - reduce speed and/or increase feed.

What we also have to account for is centripetal forces and that can actually come in handy. When I have a critical bore to hit, I will try to do it on my Sherline mill if I can fit it. The reason is because it has infinitely variable speed and speed is a critical parameter when boring because of that pesky centripetal force. When I am reeaally close to final size, I know I usually cannot rely on the feed screw to dial in what I need so I increase my speed without changing the depth of cut. Centripetal force will act on the mass (the cutting tip) to increase my depth of cut in micro-steps. This is not a calibrated thing. You have to make a speed adjustment to see what it does but on the Sherline, an increase of as little as 25-50 rpm can bring me in on size when I'm just a few tenths away. This is really handy thing to know.

On the lathe, yeah, I use rotational adjustments a lot when boring with cobalt bars but that is not what you're asking.
 
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