Indexable Boring Bar

I was trying to bore out a 13mm hole in brass by 40mm deep using my new boring bar with a indexable tip. As it was not cutting well I looked at the tip of the tool through a magnifying glass and noticed that the bottom of the tip was rubbing on the radius of the 13mm brass. As this must seem a dumb question, to get the tool to cut I will have to raise the tool above centre for clearance of the tip. Why does a index tip have no or very small rake or clearance, also the boring bar has three flats so I cant twist it so to gain slight clearance. Before I muck anything up would raising the tool above center work, any advice will be appreciated.

Boring bars have a specified minimum bore in which they will work to avoid this problem; presumably you're violating the limit? I think the minimum bore for my 16mm indexible boring bar is 20mm. On standard boring bars the insert is angled down more than on an external toolholder to minimise the minumum bore size.

Andrew

As Andrew says you probably need a 10mm or maybe even 8mm shank tool to bore a 13mm hole.

The reason that the common tips don't have much clearance is that the edge would be too fragile and chip easily.

You will find that as the bar dia gets smaller the angle that the tip is held at increases to give as much clearance as possible

3 Flats is unusual for  alathe boring bar, can you post a picture as it may be meant for  aboring head. If you do want to rotate it then make or buy something like this that is bored to fit the round of your bar

Edited By JasonB on 28/06/2016 19:01:49

Thanks for these posts, I understand what I've done wrong. Same old story, I need to spend more cash on this pastime.

Simple solution is to use High Speed Steel

Hello Robert,

The insert should be at centre height

Check the specifications for your boring bar, but yes the minimum diameter is generally limited by the front clearance of the insert rather than the diameter of the bar, see the small image at bottom right below.

Neil

As all above but will add if minimum tool hole diameter achievable, twice F in Neils is smaller than bore required the tool or tip will not rub if decent quality. Therefore setup to tool centre height.

You could pack the tool slightly high and would have even more clearance, not recommended but doable.

For brass it would be quite ok to raise the tool above centre height, and tilt the tip down to centre, giving negative top rake. Why bother, a HSS tool will have a sharper edge (needed for brass), 0 top rake. You can make one like this with the carbide tip from a circular saw. Ian S C