Boring bar rake.

[tom_d]

Thank you all for your helpful replies.
I'm sorry my explanation was not completely clear. I have tried to post a photograph to clarify things, but for some reason the site won't allow me to.
I'll try all your suggestions and see how I get on.
Thanks again.

Your photos show up in your profile, accessed by clicking on your name. If what I see is the bar in question, then it looks like, as Illinoyance pointed out, the insert was positioned to provide clearance under the cutting edge. This tool as shown, and baring any chip breaker forming a cutting edge, is set for a substantial negative rake when used for facing operations, cutting from the lathe center of rotation and feeding the tool to the OD of the part. If the tool was used for boring a hole with the front cutting edge tipped up as it is, then the chips would be thrown out against the finished diameter. Rotating the tool to eliminate that problem might lead to clearance problems, especially when boring smaller holes.

[Mike Burch]

Thanks, Tom. I didn't know the photos could be accessed that way.
Yes, one picture is indeed an end-on view of the boring bar in question, and the other is of the label on the container, giving the type number.
The tool is definitely intended for use as a boring bar, not as a facing tool. (Which is not to say it can't be persuaded to multi-task, of course.) The cutting edge (or tip) is tipped down, not up. It's on the right of the picture.

[MattiJ]

The tool is definitely intended for use as a boring bar, not as a facing tool. (Which is not to say it can't be persuaded to multi-task, of course.) The cutting edge (or tip) is tipped down, not up. It's on the right of the picture.

If you used it as a facing tool then you would really have very negative rake angle (unless you rotate the tool)
mcgyver was already referring to this side rake vs. back rake in his post. Used as a boring bar it has neutral rake angle and if used for facing it would have very much negative rake angle.

"Back" rake and "side" rake angles are confusing at best

[BCRider]

This the picture?



This bar seems to suffer from the same issue as many other import small diameter boring bars for the geometry issues. A couple of folks on You Tube have produced videos on how to grind the tips to enhance the geometry and reduce the cutting forces by improving the nasty rake issue.


Now if the tip does have a strong chip breaker groove on the upper side as mentioned by others this may not be the issue it seems. The rake can be in the chip breaker groove. But if it's a flat upper face then it's STRONGLY negative and would benefit from some modification as per this video.

OK, I was going to post a link here. I seem to recall that at LEAST two different popular machinists on YT have posted about how to grind this sort of boring bar to improve the geometry of the top. But do you think I can find ANY of them now? GRRRRR...... Anyone recall who it was that did such a video? I'm pretty sure I've seen it more than once by different folks.

The take away from the videos was that the top surface should be ground down so the insert becomes strongly wedge shaped so that when the tip of the cutter is at center height the top of the carbide is level. That at least gives you a neutral top rake. I've done this to a couple of my own cheap short boring bars and it did in fact make them cut much more cleanly. They cut to size more predictably (less spring away from the cut) and leave a nicer surface finish.

My "litmus paper" test for this is to make a relatively light cut then back out the cutter without lifting it out of the cut. If it's still cutting quite strongly during the removal then the tip was strongly loaded and sprung the bar heavily. Now boring bars will spring anyway. But what I like to see is more of a light skim than a fairly heavy grooving when retracted. It's a bit of judgement call I know.

By the way, during the roughly 10 minutes of frustration over trying to find the video discussing boring bar cutter geometry I ran across this one on making a boring bar tool post.

https://www.youtube.com/watch?v=hluRHxvsgGE

And an even simpler one;

https://www.youtube.com/watch?v=BGwMAHILOXI

If you go back and look at my own holder you'll see that I went with the idea of splitting and using clamping screws instead of the set screws used by these two videos. The reason being I didn't want to chew up the boring bars. But there's no doubt that simple screws with perhaps some buttons of brass down the holes first would work just as well.

[MattiJ]

This the picture?



This bar seems to suffer from the same issue as many other import small diameter boring bars for the geometry issues. .

And it's not any better on Sandvik or Kennametal boring bars if you want to cram CCMT0602 size insert in 10mm hole. Albeit they have also better options to offer (at 50x cost!)

[mattthemuppet]

can't see what the problem is - the chip breaker provides the back rake (?) and the tilt of the insert provides the clearance. The bigger issue is trying to use a relatively blunt insert (CCMT) on a thin bar with a small lathe - you'll get spring and chatter before you can get a deep enough cut to make the insert work properly. Stick a CCGT insert on that instead and you'll be much happier, I certainly was when I did the same for my boring bars.

[Mcgyver]

I did the following sketch to demonstrate boring bar rake angles. commonly I think the emPHASis is on the wrong sylLABle....it might help some of the newer guys. The depth of cut is exaggerated to make it visible

the angle shown in bc's pic above with the frontal head view really doesn't matter that much - unless your withdrawing the tool to face or cutting an internal groove. The only reason for the tip to the far right of this photo to be at the centre line is for dimensional control via the crossfeed screw. Otherwise it could be straight up and down and it wouldn't change the rake angle of the boring operation.

best boring bar (yeah I know, define what best is lol) is hss brazed to carbide - lowest cutting force (via correct geometry) on the most rigid bar