Damped Boring Bar

A damped boring bar, sometimes also called a tuned bar, has a hollow body with an inertia mass suspended between rubber bushings, one at each end. The mass of inertia is surrounded by a distinctive oily liquid. In deep hole machining, boring bars are commonly used. In the boring process, vibration is prevalent due to the low stiffness of a long bar. Vibration damping can result in higher productivity and a better surface finish. To improve the efficiency of boring operations, a dynamic vibration damper with variable stiffness can be added inside the bar to reduce vibration. We call it a damped boring bar. When vibration starts, the damping system absorbs the vibration energy and minimizes it. For high L/D ratios, damped bars are used. This article explains the use of different types of damped boring bars.

How damped boring bars work?

The preset damped boring bars are adjusted to a specific L/D ratio. If you want to use such a bar with a different L/D ratio, you have to re-tune it. Re-tuning basically means shifting the internal mass of inertia axially by turning the screws out of the rod. By making trial cuts and turning the screws to the values defined by the manufacturer of the instrument, this is achieved. For several years, passive methods of vibration control have been used in architecture.

Thanks to the use of tungsten carbide rods, longer overhangs can be bored. In addition, vibration can be reduced by using particulate shock absorbers. There are also damped boring bars made of a composite material. The composite material can increase the natural frequency, static stiffness, and damping factor of the bar. Machining experiments have shown that the amplitude can be reduced by approximately one-third compared to the results for a conventional boring bar.

Advanced Damping Technology

Modern damped boring bar types combat vibration thanks to a damping unit inside the bar. When the cutting forces cause the stick to vibrate, the damping mechanism, which consists of a heavy mass supported by rubber spring elements, vibrates. The oil inside the unit increases the damping effect. The energy of the vibrations is dissipated as heat, and the vibrations diminish quickly.

Another type of damped boring bar uses a combination of a conventional steel body with a pressed carbide core for optimal vibration damping. In terms of damping and torsional rigidity, this mixture offers comparable performance to solid carbide devices, but with a much better price-performance ratio due to the reduced amount of carbide required. In the steel shaft of this damped boring bar, the carbide core is embedded and the coolant will pass through it and emerge directly over the cutting edge, a function that further improves process efficiency and the ability to machine with higher cutting data while providing a high-quality surface finish.

Damped Boring Bars and Turning Applications

Turning applications, especially ID turning, typically involve a number of tool overhangs. The hole lengths are deep in many of the components concerned, requiring bar overhangs four to fourteen times the diameter. Therefore, tool selection and correct application are critical to the result as internal turning is particularly sensitive to vibration tendencies. Building a damping mechanism as close to the cutting edge as possible is beneficial because it provides the fastest response to any vibration tendencies.

Normal steel boring bars are sufficient for overhangs up to 4x the diameter. Solid carbide rods can be used for overhangs of six times the diameter. Damped boring bars are required in areas with long overhangs. Damped steel bars for overhangs up to ten times the diameter and carbide reinforced up to fourteen times the diameter.

However, grooving and threading generally have less overhang potential. In addition, all machine tools have their own individual specifications and stability lugs - frequency areas where machines respond during machining. Therefore, it is important to develop standard vibration-damping tools to operate in the largest possible frequency range, one being a damped boring bar.

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