Tips/Techniques Use of boring bar - really basic question



Ultra Member
I'm a newbie when it comes to using a boring bar, I have successfully enlarged a few holes arguably without the best surface finish, I basically "winged" it.

So what is the preferred way to do this:
1) For positioning, assume I'm looking from the tail stock end towards the chuck, with 12 O'clock being at the top, where should the cutting tip be 9, 3 ? O'clock?
2) Chuck rotation direction forward (CCW), or reverse (CW)
3) Spindle RPM?


9ish or 8:30, ccw, and speed is set by insert recommendations. Or 4*cutting speed for material/hole diameter. With carbide possibly up to 4x.

I’d like more people to recommend the angle…


Well-Known Member
I like the cutting edge slightly above center. The bar is going to deflect a little from the force of cutting, if it is a little above center, then the deflection reduces the depth of cut (a small amount). If you start on center, then the deflection causes a deeper cut (and increases the load on the tool, giving a bit more deflection) - not much.


Ultra Member
Premium Member
Also, depending how small the hole is, you might have to have the boring bar positioned higher than normal, and then pivot the boring bar to get the tip of the cutter on center, (or slightly above center to compensate for the bar flexing). Doing this will provide clearance below the cutting tip to prevent rubbing. If you follow my drift!


Super User
Premium Member
The rigidity of the boring bar plays a role here too. As the cutter starts to bite, the bar will deflect downwards, increasing the depth of cut. Therefore slightly raise the point of contact (2-5 thou) and ensure the cutting edge is really sharp. Cuts should be less than 5 thou on light machines. The bore should be smooth and chatter free. Slow and easy on the feed, and plenty of oil if you are having trouble.

I usually cut with the tool at 9 o'clock with the chuck turning anticlockwise, but for visibility in deep holes, I prefer the tool used inverted at 3 o'clock so I can see down the bore as its cutting.


Ultra Member
Premium Member
Its potentially another one of those 'it depends' type answers. If you are using neutral or slightly negative rake inserts and/or the boring bar pocket is milled with negative effective angle, raising it up can go the wrong way for effective cutting or finishing. And the opposite is also true. Yes the boring bar may torque under load, giving it a slightly different angle, but for insert style bars, are you second guessing what the toolmaker already factored into the insert datum angle? It usually pays to do a test cut. Personally I start at zero (9:00 position) & just give the toolholder adjustment a quarter turn either way to see if one setting is better than the other. I know my steel cutting inserts behave a bit different than the aluminum, but they aren't too far off neutral last time I checked.