Boring Bar Question

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Earl

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This evening I was looking thru one of my tool boxes and discovered a brand new, small boring bar. It has a carbide insert and the bar is marked SO8K-SCLCRO6. I have no idea where it came from but it looks as if it could be useful. Looking closer at it, I noticed that the carbide insert is not in line with the flats on the bar. I would guess that the cutting edge of the insert is about 100 degrees down from vertical. I would expect that it would be about 90 degrees from vertical (perfectly horizontal). My other bars with flats on them have perfectly horizontal cutting surfaces. When I use the round bars that I have, I tilt the cutting edge slightly up from horizontal. They seem to work better for me that way. I became more confused when I was looking at a list of aloris tool holders and saw one with a boring bar with the same configuration. I am now on my third lathe and third mill and I have never noticed boring bars like this before tonight. Can someone please enlighten me?

Thanks,
Earl
 
without seeing a picture, i'm assuming the extra angle is for clearance, so that the tool doesn't rub on the work.
 
ok that makes sense. at 90 degrees, the clearance angle would be the angle on the insert. I have been enlightened. Thank you.
 
Agreed that this is a negative radial rake bar. The negative rake provides relief under the cutting edge of the square-edged insert.
 
Boring bars, and other tooling as well, are generally somewhat tolerant to cutting angles from positive through neutral to negative, at least to some extent, and depending on the material and machine and setup rigidity. The import brazed carbide boring bars are often ground wrong as purchased and rub on the bore when set level. They really should be reground correctly, but in a pinch simply turning them down a bit to a negative rake will work surprising well on many jobs.
 
If it takes a square edged insert, then the insert will typically have twice the life. Because it will have two sides to use, you can flip it over and use the other side. Can save some $.
 
That's a positive rake boring bar he has there. Uses CCMT (and others) screw down style inserts.
 
If the bar is used with the flat ground into it as a reference, the insert is held with a negative rake. At least the one I saw is when I looked this bar up.
 
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