Vibrations of machining

Industrial context

Bond UGV <-> vibrations

The use of Machining High speed made it possible to increase the productivity and allowed the realization of parts hitherto unrealizable (fine walls in particular). N the other hand, the machines are less and less rigid vis-a-vis requests which are very dynamic. In many applications (long tools, fine parts,…), the appearance of vibrations is the factor more limiting for the productivity and obliges the machinists to reduce well the cutting speeds in on this side capacities tools or machine.

Defects and their origins

The problems of vibration of machining generally result in noise pollutions, degradations of surface qualities and sometimes of breakings of tools.

The independent sources of vibrations are of two types: forced vibrations and self-sustained vibrations:

the forced vibrations are mainly generated by the defects of offsetting stitches/tool/teeth, the interruptions of the cut (inevitable in milling for example), like by sources external with the machine.
the self-sustained vibrations are related to the fact that the thickness of a chip depends on the current location of the edge of cut compared to the part, but also of the position at the time of the preceding passage. Thus can appear vibrations which develop with each passage of tool until stabilizing itself on a level which is likely to denature the machined surface quality.

Research of the laboratories

Strategies “UGV”

The industrial and university laboratories largely studied the problems of vibration of machining. It is come out from it in particular from the specific strategies, in particular the interest to always machine a fine wall closest to its embedding to avoid to him bending, as well as the recommendation to limit to the maximum the length of the edge of cut in contact with the part.

Modelings

The modeling of the cutting pressures and the vibrations, although always predictive with much difficulty, must in the long term make it possible to simulate these problematic machinings.

The theory of the lobes

The multiplication of the models based on famous the Théorie of the lobes of stability, which makes it possible to find the best speed of machining, should in the long term make it possible to make these models rather robust and applicable to all the types of machining (turning, end milling, milling on side, drilling, correction).

Temporal models

Without a priori on the way in which the instability of machining appears and the form of the machined surface, of the models discretized in time, calculate the position of the part and the tool at every moment. These models are much greedier in computer's resources than the precedents, but they leave a greater freedom (laws of cut, re-heeling, deformations along the edge,…). They are also models more difficult to make robust numerically but much of efforts are deployed in this direction in the research laboratories.

Tracks

In addition to traditional “the theory of the lobes”, the use of tool with variable step, although not easily compatible with a reduced park of tool, very often makes it possible to reduce the vibrations to few expenses, it besides is proposed more and more by the manufacturers of tools.

Lastly, other tracks of research are promising, but at the price of important adaptations of the machine tools: continuous variation the speed of pin, controls active real-time,…

Methods used industrially to limit the vibrations

The traditional step

The usual method of development of the procedures of machining is still mainly based on the last experiment and a series of test-errors to choose the best parameters. According to the know-how of the company, various parameters are studied in priority: parameters of engagement of the cutting tool, strategy of machining, fixture, geometry and type of tool,… In the event of difficulty, it is current to ask council to the manufacturer of tools who will propose an alternative, or to the manufacturer of machine or software which will propose possibly more adapted strategy.

Sometimes, in contexts where the problems of vibrations are too penalizing, it can be called upon experts which prescribe, for example, after measurements and calculations, a speed of tool more adapted or a tool to studied variable step especially.

Limits of the new methods suggested

Vis-a-vis the industrial stakes, the commercial offer is relatively poor. To analyze the problems and to propose solutions, rare experts propose their services. Tools or tool holders miracles are proposed, but none convinced for the moment. Computation softwares of lobes of stability and their measuring instruments associated are proposed but, in spite of a good publicity, they remain very little used. Lastly, of the vibration pick-ups are often integrated into the machines, but are used for anything else only following the ageing of the machine or the wear of tool.

Random links: