life. In this research we are considering how to chatter stability predictions and detections and tool chatter control techniques in machine process. The aim of this research is finding out the most suitable technique to identify and prediction of tool chatter and in order to predict tool wear and increasing tool life in presence of chatter vibration.
Keywords: Chattering, Harmonic Vibration, Random Vibration 1. INTRODUCTION
Chattering also called machining vibrations, correspond to the reletive violent movement between the work-piece and the cutting tool. It’s happening at that time when operations are running in machine and corresponds to relative motion between the work-piece and cutting tool. These vibrations give results like the wave in machining surface. This chattering typically affects machining processes such as milling, drilling, grinding and turning operations.
Lathe is the basic machines which are used for producing many types of components of axial symmetry. Turning, facing, tapering, bevelling, and threading these are operations which has been done on lathe machine. Lathe machine only capable to work on cylindrical part. In this machine when operation has been running gives perfect surface finish initially but after some numbers of productions some types of variations occurs in machine such as tool variation, by these variations machine being vibrate.
These vibration frequently accompanying the machine processes and it’s effects to surface finishing. This improper surface finish is known as chatter.A chatter mark is an irregular surface of work-piece flaw left by grinding and continuous or regular mark left on long work piece in turning operation on lathe due to machining vibrations. Phenomenon of vibration in lathe machine is depends upon irrespective motion of wheel, cutting tool and work piece. It results poor surface
finish and conditions of tool wearing increased. Tool chattering is most common problem of machine vibration and it affects the operation directly. Many researchers have attempted to study and solute this problem of chatter mark. There are two techniques we can use for tool chatter suppression such as active damping and passive damping. But we don’t know which damping process is best for chatter suppression or reduce chattering.
In recent years found chatter mark mostly comes in the turning operations, it has seen in many work places. Turning is one of the most operation which is widely used in industry. In general, we will not able to predict when chattering will be in machine. The purpose of this study is prediction and identification of frequencies of chattering and also how to save work piece from chattering before being operation in machine. In this study we are using some types of predictive algorithms for chatter identification.
1.1 CHATTER MODELLING
Modelling is that step which is create a controller for detect the problem of system or counter chattering in machine. Many sources for the cause of vibration typically two basic type of vibration cause for chattering such as externally excited oscillations, it is caused by impulsive loading. Whenever operation is being on machine suddenly tool forced cut to work piece. This condition effects alignment of either work piece or tool. This disturbance
caused by impulse loading of tool externally. Another one is self-excited vibration which is mainly reason for chattering, this type of chattering also known as tool chatter.
Chattering occurs when improper relative motion between machine tool and work piece so that it effects relative cutting depth in order to cutting surface, this cutting briefly interrupted. It gives variety of reasons and also dependent of type of cut.
1.2 TOOL CHATTER MACHINE TOOLS There are two types of vibrations mostly occurs in machine such as self-excited vibration and forced vibration. When unbalancing of rotating element, in that time vibration generates vibration or force on tooth cutter it’s called forced vibration.
In this vibration the frequency of cutting tool vary due to large amplitude of vibration.
Self-excited vibration or chatter in most common type of vibration. Main reasons of tool chatter is regeneration and mode coupling.
Regenerative chattering mostly occurs when cutting tool cuts or finish the work piece at same place where as already operation has done, this cut produce regenerative waves. Cause of regenerative wave or vibration cutting tool doesn’t contact too many places of work piece and gives multiple regenerative chatter.
Mode coupling is depends upon the relative motion or vibration of cutting tool and work-piece. When axis of work-piece or plan of cutting tool goes to different direction there discontinue interaction between tool system and work-piece, then tool makes elliptic path of cut that results tool cut work piece irregularly, this is the coupled of vibration.
1.3CHATTER SUPPRESSION TECHNIQUES
Regenerative chattering occurs irregular interaction between tool and work-piece, these two independent entities cause of
We can easily identify self-excited chatter vibration because sensor signal already exists on the component. When vibration occurs on machine the chatter frequency fluctuate, that time these sensors invoke to chatter control program. It search highest stability and closest speed of spindle and gives that data which is necessary to re-stable spindle or how to reset for highest stability.
This method helps to simply calculate what favourable speed is using by stability lobe diagram, if this found speed not favourable then program gives command for reducing width of cutting to maintain this spindle speed. Stability diagram are not same for every machine.
Different machines have their different stability lobe diagram. This diagram totally depends upon the machine condition. For cutting force signal dynamometer is mostly used for chatter monitoring but calculation of chatter frequency may be difficult. Very well performs this technique if single dominant of natural works on structure but in reality more than one frequencies of structure are involved in this chattering. In spindle speed regions where convergence may be poor there stability lobes overlap to each other and gives multiple structural modes to chattering.
This procedure need to trigger for chatter instability which help to identify problem and take corrective action. This type of activity may be detrimental action to the life of machine tool. Vibration control during machining process has to take very critical strategy to eliminate or suppress this type of vibration and chattering. The aim of this study is suppress chattering by reduce relative displacements between tool and work piece and also calculate speed and frequency of chatter vibration it’s very necessary for chatter suppression.
2. PROBLEM DEFINITION
technique for above mentioned domains simultaneously.
(d) Vibration signal are such a complicated noisy signals so it is very difficult to identify frequencies of tool chatter. Presently suitable signal processing technique has been developed for study noise vibration signals or tool chatter frequencies.
(e) Effects of process damping has not been taken for experimental view and also for prediction of tool chattering.
Cobalt and molybdenum with steel. HSS has high hot hardness to withstand at elevated temperature. Hardness, wear resistance and 3 to 4 times higher cutting speed as compare to carbon steel. Most commonly used HSS have following compositions. We select 18-4-1 HSS i.e.
18% tungsten, 4% chromium, 1%
vanadium with a carbon content of 0.6 - 0.7%.
Table1: Recommended tool geometry for single point cutting tool
Figure 1: Solver parameter for setup analysis
Calculated Machining Conditions N=Spindle speed: 1250 RPM Q=Metal removal rate: 12cm3/min Tc= Cutting time: 0.02 min
Cutting Forces Calculation Ft = Tangential force: 141.6 N Ff = Feed force: 35.85 N Fr = Radial force: 162.53 N R = Resultant force: 218.52 N Power Calculation
Ps= Horsepower at the cutting tool:
0.28KW
Pm = Horsepower at the motor: 0.35KW T = Torque: 1.16N-m
These calculations are based upon theoretical values and are only intended for planning purposes. Actual results will vary. No responsibility from Kennametal is assumed.
Above data is for turning of herdned Mild Steel alloys using HSS.For each experiment new insert is used .insert is mounted on indexible tool holder in which two inserts can be mount but we have used only one insert mounted over tool holder. Vibration was measured with accelerometer (LABVIEW:A8531 Probe)in mm/sec. Below is the image of experimental conditions.
RunOrder Speed (rpm) Feed
(mm/tooth) Doc (mm) V
rms (mm/s) Tool Wear (mm)
1 636.60 0.150 2.50 0.001314 0.133
2 795.80 0.150 2.50 0.001960 0.139
3 795.80 0.200 2.00 0.002148 0.17
4 716.20 0.175 2.67 0.001189 0.153
5 636.60 0.150 2.00 0.001614 0.131
6 795.80 0.150 2.00 0.001178 0.135
7 716.20 0.175 2.25 0.001180 0.148
8 716.20 0.175 2.25 0.001180 0.149
9 636.60 0.200 2.50 0.001556 0.162
10 636.60 0.200 2.00 0.002453 0.159
11 582.33 0.175 2.25 0.001669 0.141
12 850.07 0.175 2.25 0.001875 0.163
13 795.80 0.200 2.50 0.002274 0.172
14 716.20 0.175 2.25 0.001180 0.146
15 716.20 0.133 2.25 0.001231 0.128
16 716.20 0.175 2.25 0.001180 0.148
17 716.20 0.175 2.25 0.001180 0.151
18 716.20 0.175 2.25 0.001180 0.143
19 716.20 0.175 1.83 0.001289 0.139
20 716.20 0.217 2.25 0.002184 0.179
Table 4: Random vibration data set for HSS tool 3. RESULTS AND DISCUSSION
3.1 BOUNDARY CONDITION FOR TOOL
HOLDING AND FORCE
IMPLEMENTATIONS
Thrust and cutting forces are applied on the edges are 200 N and 500 N respectively as shown in figure. 75% tool
part is fixing and 25 % tool is act as a cantilever for machining. Fixing of beam is reduced to 50% and analysing the same for the random vibration and stress generation aspects for the different modes.
Figure 2: Basic Geometry and Boundary Conditions (75% of the tool is fixed) 2A: tool geometry with selected tool signatures as per design
2B: Thrust and cutting load implementation on the tool 2C: Harmonic Response geometry domain
2D: Random Vibration for PSD
Figure 3: Basic Geometry and Boundary condition implementation (50% Length of the tool fixed)
3.2 STRESS AND DEFORMATION ANALYSIS, A STATIC RESULT
Deformation is very small for the 200 N and 500 N force conditions and the deformation is moreover in the XZ plan not in XY plan so it is not in the critical
and serious category.Shear stress reaches 322.5 MPa maximum which is bearable and Von-Mises stresses (maximum ~2100 MPa) are quite small as compare to hartzien stress or local contact stresses.
Figure 4: Stress distribution and results output(75 % length is fixed) 3A: Deformation along X-Direction
3B: Maximum Shear Stress on tool profile
3C: Shear Stress on XY plane
3D: Von-Mises Stress Distribution along the tool profile
Tool material is HSS and it is very rigid and stiffness (AE/L) is huge so that the deflection in x axis is quite small and it's around 0.75 micrometre as far as magnitude concern.
Figure 5: Stress profile of the tool (50 % fixed length) Stress distribution along the cutting edge
is reached the maximum value up to 1200 MPa. This value is quite high as we
considered the flank wear. But this is stress is quite less than the bearing stress of high speed Steel named as axial stress.
Figure 6: Deformation profile of the tool (75% fixed length)
Figure 7: Stress Distribution plots for HSS Tool (50% length fixed) Equivalent von-Mises stresses are reached
up to 2086 MPa, which is quite high as the flank wear considerations.
4. STRAIN ENERGY DISTRIBUTION AND LIFE ESTIMATION OF THE TOOL Total energy bust is near or around the cutting edges and at the end of the jigs
and fixture, where the strain energy due to banding is maximum.Factor of safety of the tool when 75 % fixing is used is around 1.84 which is satisfactory for the tool selection.
Figure 8: Factor of Safety and life of tool (75% tool length id fixed) 4A: Strain Energy Distribution across
the tool
4B: Factor of Safety of the tool
4C: Life of the tool in particular loading
Failure opportunity of the tool is prominent at the cutting edges known as flank wear and at the fixture end named as brittle fracture of the tool base.
If the fixing length reduced from 75% to 50%, there is not much significant changes in equivalent von-misesstresses.
Factor of safety of the tool is around 1.82 which is falls in quite comfortable zone.
If the fixing length increases from 50% to 75%, factor of safety of the tool for the flank wear increases but, not significantly.
4.1 MODAL ANALYSIS
Modal analysis of the structure is the study of the dynamic properties of the systems infrequency domain. Modal analysis was done with asingle-input, multiple-output (SIMO) approach, that is, one excitation point, and then the response is measured at many other points.
Figure 9(A, B, C, D): Modal Analysis and stress distribution results (75% Fix) In this analysis the stress distribution in
different modes are shown. Basically it’s a mode-superposition method, is a linear dynamic-response procedure which evaluates and superimposes free-vibration mode shapes to characterize displacement tool patterns. The purpose of a modal analysis is to find the shapes (Eigen
Vector) and frequencies (Eigen Value) at which the structure will amplify the effect of a load. The stress developed in the tool with different modal frequency is not varying much with the mode shapes, but the third mode “4C” shape is dangerous and critical against the banding.
Figure 10: Modal analysis for dynamic loading (50 % Fixed)
Figure 11: Harmonic Response of the tool (75 % tool length is fixed) For the given continual cyclic load will
generate sustained cyclic response known as harmonic response in a structural system. Harmonic response analysis gives us the ability to predict the continual dynamic behaviour of tool structures,
thus permittingus to verify whether or not our designs will successfully overcome resonance and fatigue effects of forced vibrations. 14.1 Hz frequency means its approximant 900 RPM of motor speed gives 484.6 MPa principal stresses in tool.
Figure 13: Random Vibration (75 % tool length is fixed)
Figure 14: Random Vibration for 50 % fixed length
Figure 15: Random Vibration input data for 50 % tool length fixed
indication to reduce the vibration induced by manipulating the free distance of cutting tool from the fixed point and consider it superior than other parameters.
3. The stress and the deformation are increasing with cutting speed and the deformation in x-direction and z-direction are symmetric for e.g. (von-mises stress percentage increase 20% for speed 40-125 rpm, 25% for speed 125-180 rpm and 36% for speed 180-540 rpm).
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