When damping becomes necessary
The overhang ratio decides whether a conventional bar will work. As a working guide, a steel bar is comfortable to about three times diameter, a carbide or heavy-metal bar to about five, and a damped bar is required beyond that.
Chatter is not a gradual problem. Once the bar starts to vibrate, the surface finish breaks down and the bore becomes lobed, and increasing the feed or reducing the speed only trades one failure mode for another.
| Bar type | Comfortable overhang | Stiffness relative to steel | Use when |
|---|---|---|---|
| Steel | 3xD | 1x | Short bores, light finishing |
| Heavy metal (tungsten alloy) | 4-5xD | About 1.5x | General boring where a steel bar chatters |
| Solid carbide | 5-6xD | About 2-3x | Small diameters and higher speed |
| Tuned-mass damped | 6-10xD | Effective, not just stiff | Deep bores where no solid bar works |
How tuned-mass damping works
A damped bar contains a heavy internal mass supported on elastomer elements, tuned so that it moves out of phase with the bar's vibration and cancels it. This is different from simply making the bar stiffer, which is why a damped bar behaves well past the point where a solid bar, however rigid, starts to sing.
The tuning is set for a nominal overhang. Extending a damped bar beyond its tuned length reduces the effect, and shortening it changes the frequency, so damped bars are matched to a working length rather than used at any length.
Fine boring heads and diameter control
A fine boring head adjusts the cutting diameter by micrometer, typically on a 0.002 mm graduation, which allows a bore to be brought to size in controlled steps rather than by trial and error.
Balanced twin-cartridge heads cut on two opposed edges, which cancels the radial force on the bar and allows higher speed. Single-cartridge heads adjust in one direction only and are simpler, but they introduce a bending force that matters as overhang increases.
Choosing the bar for the job
Select a boring bar from the bore it must enter and the depth it must span, then check the overhang ratio against the bar type. A bar chosen for diameter alone will frequently chatter because the depth was not considered.
Where the calculation is marginal, a slightly larger bar run at reduced speed gives a better result than a slender bar run fast. If neither works within the bar families available, the geometry of the part or the order of operations is usually the thing to change.
Frequently asked questions
At what overhang do I need a damped boring bar?
Roughly beyond five times diameter for most work. A steel bar is comfortable to about 3xD, heavy metal and solid carbide extend that to 5 or 6xD, and the damping is tuned for a nominal overhang.
Can I extend a damped boring bar to any length?
No. A damped bar is tuned for a nominal overhang, so extending or shortening it beyond a modest range reduces the damping effect. Choose the bar for the working length you need rather than extending one bar for every job.
What is the difference between a single and a twin-cartridge fine boring head?
A twin-cartridge head cuts on two opposed edges, cancelling radial force and allowing higher cutting speed and better roundness. A single-cartridge head is simpler and adjusts in one direction, but the unbalanced force becomes significant as overhang increases.
Do you supply modular extensions with the boring bars?
Yes. Bars and extensions share a modular coupling so a system can be built for several depths. Because the coupling adds its own stiffness limit, we will confirm the assembled overhang rather than quote the bar alone.
Can you supply a boring system for a bore smaller than 10 mm?
Yes, in solid carbide micro-boring bars with through-coolant. At that size the limiting factor is usually the runout of the holder rather than the bar, so we normally quote the bar and holder together.