Throughout the world, machinists have to deal with the presence of problematic vibrations on a daily basis. Most recently, the design and development of anti-vibration tools, otherwise known as tuned or damped tools, has been applied to the boring bar.
One of the most common turning applications is the boring of components, a function also known as internal turning. The most widespread tools used for this type of machining operation are boring bars. Boring bars’ shapes enable them to operate through previously drilled holes, and to efficiently enlarge and accurately profile holes according to their specific requirements.
Situated inside these tools is a damping mechanism that consists
of a heavy mass that is supported by a rubber spring element
containing oil to increase the required dampening effect
The correct application of a boring bar enables a bore’s internal profile to be machined according to specification, an accurate hole diameter to be achieved and the required surface finish quality to be realised.
There are three common types of boring bars offered: solid steel, solid carbide and anti-vibration. The maximum overhang for solid steel boring bars is up to 4xBD. This limitation is due to the fact that machining with a longer length of steel shank (more than 4xBD) can induce unwelcome vibrations due to the elasticity and characteristics of the steel.
In order to limit the vibration on a higher overhang of more than 4xBD and up to 6xBD, the use of solid carbide boring bars is recommended. Solid carbide boring bars represent an excellent, highly efficient option for boring applications of up to six times the tool’s machining depth. This capability is attributable to solid carbide possessing a coefficient of elasticity that is three times higher than that of steel.
However, when the machining of high overhangs of more than 6xBD is required, even the use of a solid carbide shank can cause vibrations. Therefore, in these cases the use of solid carbide can be somewhat limited.
Recommended insert geometry for anti-vibration use is a
positive rake angle, as this shape exerts a lower tangential
cutting force when machining.
Deep turning solutions for machining high depth to diameter internal applications include special anti-vibration boring bar systems with a ‘live’ vibration dampening system located inside the tool body.
Recent developments by Iscar have seen the release of Whisperline anti-vibration boring bars, which have been designed to significantly reduce vibrations when working with a high overhang from 7xBD to 10xBD.
Situated inside these tools is a damping mechanism that consists of a heavy mass that is supported by a rubber spring element containing oil to increase the required dampening effect.
In addition, the system contains other elements which help to further reduce vibrations. The reactive damping mechanism comes into action during machining with high overhang work depths and acts as a counter to vibrations.
The anti-vibration damper effect is applicable for large depth of cut and high feed rates, allowing for continuous, efficient machining.
Coolant supply is also available direct to the cutting edge, increasing the insert’s tool life by reducing temperature and also improving chip control and chip evacuation. These factors enable meaningful increases in productivity to be achieved, improvements in surface quality on high overhangs to be attained, scrap levels to be reduced and users’ profitability to be enhanced.
Correct insert geometry is a very important factor when using anti-vibration boring bars. The most recommended insert geometry for successful anti-vibration use is a positive geometry insert with a positive rake angle, as this shape exerts a lower tangential cutting force when machining.
Choosing the appropriate nose radius of the insert is also a vitally important consideration. A lower nose radius is recommended as this configuration significantly reduces the cutting forces, due to the lower contact between the insert and workpiece, which helps to limit and reduce vibration. A greater nose radius creates much larger radial and tangential cutting forces that can produce unwelcome vibrations.
Chip Breakers & Secure Clamping
Inserts with the benefit of appropriate chip breakers are recommended for improved chip evacuation, as the production of long and curled chips can cause a range of problems when working with long overhang tools. In addition to increasing vibration during machining, long and curled chips are liable to spoil or damage the surface quality of the workpiece.
Appropriate chip breakers ensure excellent chip control and the creation of small chips which can be evacuated more easily with the help of the coolant supply. An additional, very important factor in reducing vibrations is ensuring the clamping stability of the anti-vibration boring bar. Secure clamping helps users to achieve the correct workpiece dimension, which results in excellent workpiece surface quantity and assists in avoiding vibrations; clamping length should be 4xBD.
Using anti-vibration turning tools in conjunction with the appropriate approach can provide effective solutions for reduction and even elimination of vibrations.
APMEN Cutting Tools, Apr 2017