Boring bar tip height

I've bought a 6mm tipped boring bar with the tip set below the centre height, I've not used one before, all my bars are conventional. It occurs to me that I maybe shouldn't set the tip to the to the 16mm I set my turning tools too but should use a 10mm packer. Which is correct please?

5 weeks ago.

Boring bar setup.

Martin C

Edited By Martin Connelly on 14/04/2020 13:09:51

LOL, cheers Martin!

As the OP of the aforementioned thread I can confirm that setting them as recommended in that thread does work - and very nice holes they make, too.

Cheers Bob, good to hear, thanks for letting me know...

And set the cutting tip to centre height.

Indeed - they seem quite sensitive to getting that right.

I do follow this advice myself ie tool point at 9 o'clock, but a consideration of the geometry suggests that, other things being equal, the tip will cut at any "hour".

They are a in effect a negative rake tool and work best on centre height. Dropping the tool too low, particularly in a small hole, will make the bottom edge of the insert rub on the job. Result is rough finish and size. That is the reason the insert is mounted at the negative angle: to tilt that bottom edge of the insert away from the curvature of the hole it is boring.

Raising the tool higher has the effect of increasing the negative rake relative to the rotation of the job, plus means that cutting may not be soo good and definitely the depth of cut taken on the cross slide is different from the change in bored diamenter, by rules of Trig etc.

Despite their weird looks, on centre is where they are designed to sit.

That said, if you are using a small boring bar in a very large hole you can drop it below centre without rubbing the bottom edge of the insert. This in effect makes the top rake neutral instead of negative and can give a better finish in a light, chatter-prone hobby machine. But we are talking about something like a 12mm diam boring bar in a 75mm hole or the likes. For normal boring though, stick on centre.