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Tool Overhang Causing Vibration Blues?

12. August 2014


Tool overhang results in vibration, which can be countered by light cutting and an optimized tool approach. But whenever a tool enters a bore, it is subjected to deflection forces, resulting in vibration. Challenges of long overhang operations can be countered, however, with six simple steps.

​​​​​​Productivity improvement is often focused on the capability of the cutting edge. But as component features change, the way the cutting edge is presented to the machined part needs to be taken into account. Overhang—the distance from the tool holder flange at the spindle-end to the cutting edge—results in vibration tendencies, which can be countered by light cutting and an optimized tool approach. But whenever a tool enters a bore, it will be subjected to deflection forces, subsequently resulting in vibration tendencies.

​​Many current internal turning, parting and grooving, boring, and milling operations, though, need longer tool reach. Until recently, long-overhang applications have been forced to sacrifice machining performance to increase productivity. Tool chatter leads to machining security risks, component quality issues, excessive noise levels, poor tool life and even scrapped components. Therefore, cutting data well below the limits of the cutting edge is often applied, resulting in higher machining costs and longer throughput times. Many of the challenges of long overhang operations can be countered when taking these 6 tips into consideration.

1. Cutter selection and toolpath
The very nature of the milling process causes vibration tendencies through intermittent cutting action. With long tool reach, an increasing number of milling operations are prone to vibration tendencies. A lot can be done to counter vibration-inducing tendencies, such as applying the right milling cutter and toolpath. But when tool overhang exceeds three times the diameter of the adaptor, additional measures are needed to achieve results on par with today’s standards.

2. Increase tool reach with milling adapters
In combating vibration tendencies, added parameters have been developed to minimize vibration amplitudes, thereby achieving smoother metalcutting at higher machine rates. Vibration cannot be entirely eliminated, but it can certainly be reduced to levels where it does not pose any threat to process or results. Advanced simulation methods and equipment—along with measurement systems and a better understanding of structural dynamics to counter the forces acting on the tool—have made this possible.

For milling, new standard adaptors provide tool reach without compromising performance. These adaptors have been developed for minimizing the amplitude of vibrations typical to two different areas of overhang.

The productivity increase achieved by the new system quickly pays off the adaptor investment. These milling adaptors allow increases in axial-depth-of-cut as well as higher feed rates. This has led to considerably better productivity and longer tool reach.

3. Applying dampened boring bars to internal turning
Internal turning involves a wider range of tool overhangs. The bore length is deep on many of the components involved, demanding boring bar overhangs from four times to 14 times the diameter. The selection and correct application of tools are crucial, as internal turning tends to be sensitive to vibration tendencies. By building the dampening mechanism as close to the cutting edge as possible, an operator can quickly respond to any vibration tendencies.

All machine tools have their individual specifications and stability lobes—frequency areas within which machines react during machining. It is therefore important that standard vibration-dampening tools are developed to operate smoothly within as large a frequency area as possible.

There are three reasons for applying a dampened boring bar in internal turning: to maintain tolerance and surface finish levels, to minimize machining time through the smallest number of passes and to achieve competitive, economical machining rates. Productivity and security are the key terms here, as many components are subjected to competitive pressures in manufacturing.

4. Modular tooling couplings
ISO-standard Coromant Capto modular tooling couplings and serrated locking (SL) interfaces play a major role in providing the most suitable solutions. The SL quick-change and setting functions have been developed to improve setting and performance, and to make a large assortment of indexable inserts available for use.

Large boring bars have Coromant Capto C10 interfaces for internal turning of holes larger than 3.94 inches (10 mm) at overhangs of up to 10 times the diameter, providing high-quality holes at high metal-removal rates. The quick-change function allows boring bars to be set up quickly with high accuracy.

5. Using high-pressure coolant
Applying high-pressure coolant can play a significant role in internal turning. Chip control and evacuation are improved with the use of coolant-nozzles behind the cutting edge. Unique nozzles direct jets accurately at the cutting zone, facilitating chip breaking and transporting chips out of holes and onto chip conveyors. Adoption of quick-change tool holding on turning centers, for example, has been accelerated with the use of higher coolant pressures. These have connections and supply built in, whereas the tool-change time is considerably longer with external piping. Optimized cutting units have the nozzles pre-set and fixed, reducing the set-up time on the machine.

6. Choosing the right turning method
The right internal turning methods can make a substantial difference in performance, security and results. One example is Sandvik Coromant’s three-pass method, where part of the programming involves using a diameter larger than that required. Using this method, a finishing operation can be completed to a close tolerance in five minutes, rather than thirty minutes or more. Operators achieve this dramatic reduction in time by eliminating unnecessary small cuts.

Conclusions

As components grow in size and complexity, companies are tasked with discovering innovative and dependable methods to machine them. Even more refined tooling expertise will be required to meet future demands and regulations. And though vibration can never be completely eradicated, companies like Sandvik Coromant will continue to improve in vibration dampening for even longer overhangs on even bigger, more difficult-to-machine components.

This article was reprinted with the permission of Sandvik Coromant and also appears on their website here: http://www.sandvik.coromant.com/us/news/pages/increase-your-productivity-in-long-overhang-operations.aspx

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