Carbide boring bar and brass

[kmanuele]

Continuing my new lathe education: Thought I try boring a deep hole in my sacrificial brass rod. Don't have a boring bar with HSS, but one with carbide insert with those molded-in chip breakers. Thought I'd try it, not expecting great results.

First drilled out to 0.5", about 1.75" deep, then bored to ~0.69". Didn't want to go larger as the walls were getting thin.

I was surprised at the results, thinking that tool geometry vs. brass would be a problem. Good finish all the way in (didn't check ID all the way in, however).

The carbide bit is rotated down about 20 degrees. Maybe that helped?

 

[davidpbest]

The carbide bit is rotated down about 20 degrees. Maybe that helped?

Yea, altering that rake angle by 20° would certainly defeat the chip breaker geometry.

 

[jwmelvin]

Yea, altering that rake angle by 20° would certainly defeat the chip breaker geometry.

I imagine the 20° angle mentioned is the very standard configuration of increasing radial clearance by setting the insert with negative overall rake. That’s generally unavoidable if registering the flats on those insert boring bars. The chip breaker geometry seems to make up some positive rake at the cutting edge, but that will depend on the insert.

Brass is pretty forgiving in my experience.

 

[davidpbest]

I imagine the 20° angle mentioned is the very standard configuration of increasing radial clearance by setting the insert with negative overall rake. That’s generally unavoidable if registering the flats on those insert boring bars. The chip breaker geometry seems to make up some positive rake at the cutting edge, but that will depend on the insert.

Brass is pretty forgiving in my experience.

The OP didn’t say if he clocked the boring bar to the flat, but said "rotated down about 20 degrees". The insert shown in that bar is a CCMT which has a built-in relief angle of 7° and most boring bars with flats are designed to clock that insert at 15° off horizontal yielding an 8° relief for clearance. If over-clocked to 20°, that will change the attack angle of the insert, altering the cutting geometry effectiveness and rendering the chip breaker less effective.

 

[jwmelvin]

The insert shown in that bar is a CCMT which has a built-in relief angle of 7° and most boring bars with flats are designed to clock that insert at 15° off horizontal yielding an 8° relief for clearance.

Why wouldn’t that yield 22° relief?

Fully agree that the effective rake depends on the insert’s chip-breaker geometry and any clocking, including the default 15°, will matter.

 

[Video_man]

Brass doesn't like positive rake, that's why drill bits hog in. I turn and face outside with TPG inserts at 0 rake and bore pretty much as shown in the diagram posted above, to eliminate the hogging and chatter.

 

[davidpbest]

Why wouldn’t that yield 22° relief?

Fully agree that the effective rake depends on the insert’s chip-breaker geometry and any clocking, including the default 15°, will matter.

The relief angle varies as the ID increases. A high quality 1/2" boring bar with a 21.5 CCMT insert will have a minimum boring diameter of ~0.580" and at that ID, the relief angle will be just under 7°. As the ID increases, so does the relief angle. See below: 1/2" Boring Bar, CCMT 21.5 Insert, 15° Rake Angle, and notice the difference in relief angle under the cutting edge for a 0.580" ID versus a 3.0" ID.



@Video_man is correct that brass does not like positive cutting angles, so clocking the boring bar further than 15° will blunt the attack angle which alters the cutting angle from positive to more neutral.

 

[jwmelvin]

[mention]davidpbest [/mention] ah yes the bore curvature will matter. Doesn’t one want to run the tip of the insert at center height though?

 

[petertha]

I have some offshore label CCMT inserts that have positive rake relative to the bottom surface datum, so that would change the schematic picture too. Some inserts have zero rake ledge but almost immediately behind the nose have a positive ramp, I suspect related to chip forming. Not all of these are well documented, at least from some of the back alleys I shop at LOL

 

[davidpbest]

Not all of these are well documented, at least from some of the back alleys I shop at LOL

Stay out of back alleys my friend. It’s all inside. That insert is probably CCGT, not CCMT - even if the seller doesn’t say so.