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Concept
Machining vibrations
Summary
In machining, vibrations, also called chatter, are the relative movements between the workpiece and the cutting tool. The vibrations result in waves on the machined surface. This affects typical machining processes, such as turning, milling and drilling, and atypical machining processes, such as grinding.
A chatter mark is an irregular surface flaw left by a wheel that is out of true (off-center) in grinding, or regular marks left when turning a long piece on a lathe, due to machining vibrations.
As early as 1907, Frederick W. Taylor described machining vibrations as the most obscure and delicate of all the problems facing the machinist, an observation still true today, as shown in many publications on machining.
The explanation of the machine tool regenerative chatter was made by Tobias. S. A. and W. Fishwick in 1958, by modeling the feedback loop between the metal cutting process and the machine tool structure, and came with the stability lobes diagram. The structure stiffness, damping ratio and the machining process damping factor, are the main parameters that defines the limit where the machining process vibration is prone to enlarge with time.
Mathematical models make it possible to simulate machining vibration quite accurately, but in practice it is always difficult to avoid vibrations.
Basic rules for the machinist for avoiding vibrations:
Make the workpiece, the tool and the machine as rigid as possible
Choose the tool that will least excite vibrations (modifying angles, dimensions, surface treatment, etc.)
Choose exciting frequencies that best limit the vibrations of the machining system (spindle speed, number of teeth and relative positions, etc.)
Choose tools that incorporate vibration-damping technology (with structure damping using high damping material in the joint areas and with mass dampers using a counteracting force to stabilize the motion).
The use of high speed machining (HSM) has enabled an increase in productivity and the realization of workpieces that were impossible before, such as thin walled parts.
Official source
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