Machining vibrations
Encyclopedia
Machining vibrations, also called chatter, correspond to the relative movement 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
.
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.
Mathematical models make it possible to simulate machining vibration quite accurately, but in practice it is always difficult to avoid vibrations and there are basic rules for the machinist:
simulations compute workpiece and tool position on very small time scales without great sacrifice in accuracy of the instability process and of the surface modeled. These models need more computing resources than stability lobe models, but give greater freedom (cutting laws, runout, ploughing, finite element models). Time domain simulations are quite difficult to robustify, but a lot of work is being done in this direction in the research laboratories.
Two kinds of softwares are very promising : Time domain simulations which give not yet reliable prediction but should progress, and vibration machining expert software, pragmatically based on knowledge and rules.
and on trial and error
method to determine the best parameters. According to the particular skills of a company, various parameters are studied in priority: depth of cut, tool path, workpiece set-up, geometrical definition of the tool,…
When a vibration problem occurs, information is usually sought from the tool manufacturer or the CAD software retailer, and they may give a better strategy for machining the workpiece.
Sometimes, when vibration problems are too much of a financial prejudice, experts can be called upon to prescribe, after measurement and calculation, spindle speeds or tool modifications.
New Generation Tool Holders and especially the Hydraulic Expansion Tool Holders minimise the undesirable effects of vibration to a large extent. First of all, the precisely controlled of T.I.R to less than 3 micrometres helps reduce vibrations due to balanced load on cutting edges and the little vibration created thereon is absorbed largely by the oil inside the chambers of the Hydraulic Expansion Tool Holder.
Cutting tool
In the context of machining, a cutting tool is any tool that is used to remove material from the workpiece by means of shear deformation. Cutting may be accomplished by single-point or multipoint tools. Single-point tools are used in turning, shaping, plaining and similar operations, and remove...
. The vibrations result in waves on the machined
Machining
Conventional machining is a form of subtractive manufacturing, in which a collection of material-working processes utilizing power-driven machine tools, such as saws, lathes, milling machines, and drill presses, are used with a sharp cutting tool to physical remove material to achieve a desired...
surface. This affects typical machining processes, such as turning
Turning
Turning is the process whereby a single point cutting tool is parallel to the surface. It can be done manually, in a traditional form of lathe, which frequently requires continuous supervision by the operator, or by using a computer controlled and automated lathe which does not. This type of...
, milling
Milling machine
A milling machine is a machine tool used to machine solid materials. Milling machines are often classed in two basic forms, horizontal and vertical, which refers to the orientation of the main spindle. Both types range in size from small, bench-mounted devices to room-sized machines...
and drilling
Drilling
Drilling is a cutting process that uses a drill bit to cut or enlarge a hole in solid materials. The drill bit is a multipoint, end cutting tool...
, and atypical machining processes, such as grinding
Grinding (abrasive cutting)
Grinding is an abrasive machining process that uses a grinding wheel as the cutting tool.A wide variety of machines are used for grinding:* Hand-cranked knife-sharpening stones * Handheld power tools such as angle grinders and die grinders...
.
As early as 1907, Frederick W. Taylor described machining vibrations as the most obscure and delicate of all the problems facing the machinist
Machinist
A machinist is a person who uses machine tools to make or modify parts, primarily metal parts, a process known as machining. This is accomplished by using machine tools to cut away excess material much as a woodcarver cuts away excess wood to produce his work. In addition to metal, the parts may...
, an observation still true today, as shown in many publications on machining.
Mathematical models make it possible to simulate machining vibration quite accurately, but in practice it is always difficult to avoid vibrations and there are basic rules for the machinist:
- rigidify the workpiece, the tool and the machine as much as possible
- choose the tool that will excite vibrations as little as possible (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.)
Link between high-speed machining and vibrations
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. Unfortunately, machine centers are less rigid because of the very high dynamic movements. In many applications, i.e. long tools, thin workpieces, the appearance of vibrations is the most limiting factor and compels machinist to reduce cutting speeds well below the capacities of machines or tools.Different kinds of problems and their sources
Vibration problems generally result in noise, bad surface quality and sometimes tool breakage. The main sources are of two types: forced vibrations and self-generated vibrations.- Forced vibrations are mainly generated by interrupted cutting (inherent to milling), runout, or vibrations from outside the machine.
- Self generated vibrations are related to the fact that the actual chip thickness depends also on the relative position between tool and workpiece during the previous tooth passage. Thus increasing vibrations may appear up to levels which can seriously degrade the machined surface quality.
High-speed strategies
Industrial and academic researchers have widely studied machining vibration. Specific strategies have been developed, especially for thin-walled work pieces, by alternating small machining passes in order to avoid static and dynamic flexion of the walls. The length of the cutting edge in contact with the workpiece is also often reduced in order to limit self-generated vibrations.Modeling
The modeling of the cutting forces and vibrations, although not totally accurate, makes it possible to simulate problematic machining and reduce unwanted effects of vibration.Stability lobe theory
Multiplication of the models based on stability lobe theory, which makes it possible to find the best spindle speed for machining, gives robust models for any kind of machining.Time domain numerical model
Time domainTime domain
Time domain is a term used to describe the analysis of mathematical functions, physical signals or time series of economic or environmental data, with respect to time. In the time domain, the signal or function's value is known for all real numbers, for the case of continuous time, or at various...
simulations compute workpiece and tool position on very small time scales without great sacrifice in accuracy of the instability process and of the surface modeled. These models need more computing resources than stability lobe models, but give greater freedom (cutting laws, runout, ploughing, finite element models). Time domain simulations are quite difficult to robustify, but a lot of work is being done in this direction in the research laboratories.
Paths
In addition to stability lobe theory, the use of variable tool pitch often gives good results, at a relatively low cost. These tools are increasingly proposed by tool manufacturers, although this is not really compatible with a reduction in the number of tools used. Other research leads are also promising, but often need major modifications to be practical in machining centers.Two kinds of softwares are very promising : Time domain simulations which give not yet reliable prediction but should progress, and vibration machining expert software, pragmatically based on knowledge and rules.
The classic approach
The usual method for setting up a machining process is still mainly based on historical technical knowhowKnowhow
Know-how is practical knowledge of how to get something done, as opposed to “know-what” , “know-why” , or “know-who” . Know-how is often tacit knowledge, which means that it is difficult to transfer to another person by means of writing it down or verbalising it...
and on trial and error
Trial and error
Trial and error, or trial by error, is a general method of problem solving, fixing things, or for obtaining knowledge."Learning doesn't happen from failure itself but rather from analyzing the failure, making a change, and then trying again."...
method to determine the best parameters. According to the particular skills of a company, various parameters are studied in priority: depth of cut, tool path, workpiece set-up, geometrical definition of the tool,…
When a vibration problem occurs, information is usually sought from the tool manufacturer or the CAD software retailer, and they may give a better strategy for machining the workpiece.
Sometimes, when vibration problems are too much of a financial prejudice, experts can be called upon to prescribe, after measurement and calculation, spindle speeds or tool modifications.
Limitations of the available methods
Compared to the industrial stakes, commercial solutions are rare. To analyse the problems and to propose solutions, only few experts propose their services. Computational software for stability lobes and measurement devices are proposed but, in spite of widespread publicity, they remain relatively rarely used. Lastly, vibration sensors are often integrated into machining centers but they are used mainly for wear diagnosis of the tools or the spindle.New Generation Tool Holders and especially the Hydraulic Expansion Tool Holders minimise the undesirable effects of vibration to a large extent. First of all, the precisely controlled of T.I.R to less than 3 micrometres helps reduce vibrations due to balanced load on cutting edges and the little vibration created thereon is absorbed largely by the oil inside the chambers of the Hydraulic Expansion Tool Holder.