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In Focus - Archivo Febrero 2008
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01.02.2008 -
Induced vibrations occur in nearly all areas of industry.
Examples are hammers, presses and rollers in the metal and steel industry, crushers and disintegrators in the stone processing industry, weaving machines in the textile industry, cutting tools in machining centers, fans and pumps in labs and cleanrooms.
All of these examples have one thing in common: the generation of vibrations that can be disturbing to people or damaging to machine components.
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What are vibrations?
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Oscillating suspension pins
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Vibration, or oscillation, comes from the Latin verb ‘to swing’ or ‘oscillate’ (oszillare). Oscillation describes a periodically repeated process.
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Undamped harmonic vibration
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The figure shows an undamped harmonic vibration with the elongation (vibration displacement), the amplitude and the period of vibration (T).
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Free damped vibration
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This figure shows the temporal course of a free damped vibration. Nearly all vibrating systems are subject to damping. Therefore, they always require an external driving force for sustained vibration at a constant amplitude.
Corresponding to the compliance frequency response of the overall machine/toolholder/tool/workpiece clamp/workpiece system, the cutting force produces vibrations that can affect the surface quality and the tool life.
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There are two types of vibration: externally induced and self-induced vibrations.
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Self-induced vibrations
Vibration systems in which the energy supply is controlled by the vibration process itself execute self-induced vibrations. A typical example of this is the vibrating strings of a violin. The vibrations are caused by the fact that the static friction between the bow and the strings is greater than the sliding friction, and the sliding friction decreases as the differential speed increases. Another example is the sound produced by a glass when the edge is rubbed.
Self-induced vibrations are practically always non-linear; otherwise - if the excitation period were unlimited - the amplitudes would increase exponentially, therefore destroying the vibration system.
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Self-induced vibrations in metal cutting machining centers
Chatter vibrations in the milling process are self-induced vibrations from the cutting tool and the machining centers. The cause of the chatter process is the change in the cutting depth during the machining process. Vibrations of the cutting tool at its natural frequency are excited by a disruption. As a result of these vibrations, a milling tooth produces a wave-shaped pattern on the surface of the workpiece. The next tooth cuts into this wave-shaped pattern and the vibrations are excited in the cutting tool at exactly the natural frequency, since the cutting depth changes with the natural frequency. Chatter can be affected by variations in the cutting speed (Vc), cutting depth (ap), feed rate (f) and cutter geometry.
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Externally induced vibrations
In a forced, or externally induced vibration, the system is propelled by an external force (which usually itself is periodic).
An example is a child’s swing, which always receives a push at the highest point. All mechanical clocks execute forced vibrations; the external force in this case comes from the balance wheel. Musical instruments also execute forced vibrations, for example by the hammering or plucking of strings, the air flow in wind instruments or the percussive stroke on a drum.
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Externally induced vibrations in metal cutting machining centers
Externally induced vibrations in machining centers can occur as a result of imbalance, flank impacts of gears, imbalanced rolling-contact bearings or the frequency of hydraulic pumps. Transmissions through the foundation or an interrupted cut are also possible causes. During milling, a periodic excitation is caused by the interrupted cut of the individual teeth.
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Examples in everyday situations
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The Tacoma Narrows Bridge
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Galloping Gerti
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A suspension bridge over Puget Sound in Tacoma, Washington (USA) was opened in July 1940 and only months later received extensive press coverage. After its completion the Tacoma Narrows Bridge, with a span of 853 meters, was the fifth longest suspension bridge in the world. The narrow width gave the bridge an elegant appearance and it was generally considered architecturally perfect.
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Beginning of the movement
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Soon after it was opened for traffic, it became obvious that the bridge was very sensitive to crosswinds. This is not unusual for suspension bridges – it is an inherent design weakness. However, the Tacoma Narrows Bridge – also known as “Galloping Gertie” – was especially susceptible to winds due to its low weight and narrow design. The bridge not only moved from side to side, but also up and down in wave-like motions along the longitudinal axis.
The Tacoma Narrows Bridge soon became known as "Galloping Gertie" and many motorists avoided the bridge, accepting long detours. But there were also the adventurous, who went to Tacoma on the weekend just so they could take a "rollercoaster" ride on the bridge.
The engineers were aware of the problem, of course, and they attempted to stabilize the structure with additional reinforcements. This, however, did not produce the desired effect. The bridge was then watched closely and even a camera team was constantly at the site to record the movements of the bridge surface.
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The bridge in torsional mode
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On November 7, 1940 a moderate wind with a speed of 68 km/h arose over Puget Sound, once again causing the bridge to vibrate slightly. This time, however, the movements became increasingly larger, resulting in ever greater twisting of the bridge surface.
At about 11 a.m. Joe Arlington and his wife attempted to drive across the bridge despite the threatening situation, but their car was flung against the railing by the movements of the bridge. They left their car and ran as fast as they could to the safety of the bank. They had just reached solid ground when several steel cables broke, the pavement crumbled and the entire middle section of the bridge collapsed into Puget Sound with a deafening crash.
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The collapse
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It turns out that the wind exerted a periodic force on the bridge, which corresponded exactly to the natural frequency of the bridge. It was discovered that it takes a very small force to cause a bridge (or any other structure) to collapse if the external force matches the natural frequency.
The phenomenon of the natural frequency is also the reason why a column of soldiers never marches in step when crossing a bridge. Today we also know that certain components can be made to collapse even by transmitting a tone with the corresponding frequency. The same effect is at play when an opera diva shatters a glass.
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Signal analysis
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Vibrations also occur in technical toolholding systems. They can be responsible for chatter marks on the workpiece or for premature wear of the tool (cutting edge blowouts).
Vibrations of toolholders are measured by means of signal analysis or the pulse hammer method. The results can be used to conduct a frequency analysis for each type of toolholder.
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Vibration damping and TOTAL TOOLING from SCHUNK
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TOTAL TOOLING
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Toolholders from SCHUNK’s TOTAL TOOLING line feature outstanding vibration damping, which significantly improves the surface quality and extends the life of the tool and spindle.
Large selection of precision toolholding systems
The TOTAL TOOLING line from SCHUNK offers several precision toolholding systems with outstanding vibration damping properties, in particular TENDO, TRIBOS-R, SINO-R and the toolholders of the MEGA series from BIG.
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The TENDO hydraulic expansion holder is a universal toolholder for precision machining in which the clamping force is transferred to the tool shaft by means of hydraulic oil. The hydraulic system provides excellent vibration damping and therefore ensures optimum workpiece surfaces. Micro-blowouts on the cutting edge of the tool are prevented, for significantly longer tool life and reduced costs. Wear on the spindle is also reduced. This system optimizes set-up times, making it possible to change or adjust tools easily with a simple Allen wrench – genuine added value for smaller companies.
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TRIBOS polygonal clamping technology is a further patented in-house development from SCHUNK that covers a broad spectrum of different size and system variants. TRIBOS can handle everything from micro-machining with shaft diameters starting at 0.3 millimeters to rough machining. The areas of utilization range from the automotive industry to precision engineering and medical technology. TRIBOS-R scores especially high in this sector with excellent vibration damping. TRIBOS-R gets its damping properties primarily from damping inserts in the hollow chambers around the clamping diameter. This extends the tool life and, above all, produces considerably better machining results.
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The SINO-R universal toolholder is suitable especially for rough machining. SCHUNK designed SINO-R as a universal toolholder based on expansion technology. The design is innovative and absolutely price-optimized. In terms of quality, flexibility and cost savings, these universal toolholders offer convincing features as compared with conventional toolholders such as Weldon, Whistle Notch or ER collet style toolholders. Designed for vibration damping and equipped with a reinforced expansion sleeve, the power chuck is a significant improvement with respect to reliable performance and power transmission.
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Precision collets from BIG of Japan round out the line of toolholders with vibration damping properties from SCHUNK. The design principle of a groove-free clamping nut reduces machining noises and achieves excellent surface qualities also in rough machining.
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12.2008
Quick jaw change brings flexibility and substantial savings
Flexibility in production is a crucial competitive advantage. Only companies that can respond to demand with flexible yet absolutely precise production can supply their customers rapidly with a constantly increasing variety of products without needing to build up expensive and inefficient stocks.
 más…
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11.2008
Pneumatics or mechatronics? … It all depends!
To date, pneumatically driven gripper modules have been used primarily in gripper technology. Recently, however, more and more electric grippers have been coming onto the market. Both competing systems have powerful arguments in their favor.
más…
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10.2008
High-Performance-Cutting (HPC)
Hard on the outside, clever on the inside: Toolholder systems developed especially for HPC that have strong rigidity as well as good vibration dampening provide clear benefits for HPC.
más…
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09.2008
Cleanroom
The importance and the use of cleanrooms in assembly and handling are constantly increasing.
más…
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08.2008
Clamping Force / Torque
We are frequently confronted with torques on a daily basis, even though we do not recognize them as such.
más…
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07.2008
Convenient robot or attractively priced spindle gripper?
A significant potential for increasing efficiency in modern machining processes is the automated loading and unloading of the machines.
más…
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06.2008
Polygon clamping technology - round pegs in polygonal holes
The principle behind polygon clamping is surprisingly simple. The know-how that makes it work is however highly advanced.
más…
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05.2008
Packaging
A package for everything
Packaging is a universal function that is required in practically every industry and meanwhile is so important that it has become a separate industry itself.
más…
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04.2008
Magnetic clamping technology has strong attraction
Workpiece clamping with electroermanent magnets reduces set-up times by up to 50 percent and brings about significant advantages especially in the manufacture of large parts.
más…
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03.2008
Quick-change systems speed up production
Changing systems for components such as grippers and tools add a high degree of flexibility to production processes
más…
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02.2008
Vibration damping
Improves surface quality and reduces wear on the tool and spindle.
más…
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In Focus - 2013
In Focus - Archivo 2012
In Focus - Archivo 2011
In Focus - Archivo 2010
In Focus - Archivo 2009
In Focus - Archivo 2008
In Focus - Archivo 2007
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