Presentation on theme: "THE ROLE OF DYNAMICS IN THE MACHINING PROCESS (MetalMAXTM Approach to Improving Milling Cutting Performance)"— Presentation transcript:
1 THE ROLE OF DYNAMICS IN THE MACHINING PROCESS (MetalMAXTM Approach to Improving Milling Cutting Performance)
2 The Ideal Milling Process Right first timeIdeal Milling ProcessLow Cutting ForcesUnattended machining/High Process reliabilityLong Tool LifeEliminationof benchingStable Machining/ Low vibrationLong Spindle LifeOptimum M/C utilizationMax MRR/SGR
3 Cutting Parameter Selection How do we choose our speeds, feeds and depths of cutThe Conventional ApproachHighly Experienced PlannerTechnological database from cutting tool supplierOperational Guidelines from machine tool supplierTATATA…J.Fox.1998Note: None of the above is based on a sound scientific or objective approach.
4 Conventional Approach Consequences of theConventional ApproachScrapped PartsExcessive “benching”Power tool life and tool failuresAccelerated spindle wearPoor process reliabilityUnpredictabilityAll of this results in wasted time and money
5 Trends exacerbate these problems Move to monolithic structuresBigger,deeper parts with high L/D ratios.Very Expensive, less margin for error.Greater opportunity to shineMove to Flimsier, lightweight partsMove to more exotic materialsCommon factor in the above trends is the increased importance of dynamic influences.
6 How can we scientifically select the cutting parameters to account for the system dynamics? Quickly obtain required dynamic informationUse this information to obtain optimum cut parametersQuickly verify cutting performance.
7 What is High Speed Machining? There are many definitionsCutting speed alone (tool maker viewpoint)Spindle speed alone (common for newcomers)Machining at speeds significantly higher thanconventional practice (machine shop view)OthersAll of the above definitions of high speed machining are correct from someone’s point of view
8 High Speed Machining (HSM) Definition From a dynamics perspective we define HSM as:“High-speed machining occurs when the tooth passing frequency approaches the dominant natural frequency of the system”Professor Scott Smith, UNCC, Charlotte NC
9 The Role of Dynamics in High Speed Machining HSM is greatly influenced by the dynamic characteristics of the machine-tool-work piece system.In HSM, upper limits are denoted by onset of “chatter”.Success in HSM depends heavily on the ability to recognise and deal with dynamic problems.Selection of an appropriate spindle speed and depth of cut is extremely important and not obvious
11 Chatter MechanismMost undesirable vibrations in milling are self-excited chatter vibrations.What mechanism is responsible for transforming the steady input of energy (from the spindle drive) into a vibration?The primary mechanism is “Regeneration of Waviness”.
12 Regeneration of Waviness *07/16/96Regeneration of WavinessThe force on any tooth is proportional to the chip thicknessEach tooth removes material from a surface generated by the passage of a previous tooth.Any vibration at the time that surface was being made results in a wavy surface.*
13 *07/16/96Process DampingChatter vibrations are inhibited at low speeds by “process damping”.Interference between the rake face of the tool and the tool path produces a net damping force.Dependent on surface velocity (spindle speed and cutter diameter) and flexible frequencies of cutter.*
14 The MetalMAX™ Approach Identify and isolate problems areasPredict dynamic behaviourAdjust to optimise.Measure and verifyOptimised? - if not back to step 1Move onMachine a part right the first time!MetalMAXTM Hardware
15 The package for dynamic/chatter prediction and control MetalMAX™The package for dynamic/chatter prediction and controlFrequency andFlexibility Measurement(Modal Analysis “Tap” Test)+Basic Cutting ParametersandCutting Theory=Predictions of Stable Depth of Cut limitsCutting Forces and DisplacementsDynamic Cutting AccuracyELIMINATE CHATTER!!!
16 Measurement and Analysis Frequency Analyserfor Machine ToolsData Acquisition andMachining AnalysisTXFPCScopeComputation and PredictionMilSim™Milling Simulation andChatter PredictionNC IntegratedSpindle Speed ControlNon Automated CRAC Package~NC-Verifying Performance
17 FRF Measurement with MetalMAX™ Equipment 4321EXCITATION(HAMMER)RESPONSE(ACCEL)Sensor Interface ModulePCAccelerometerSTRIKEHammerPower CableSensor CableSchematic of Measurement Setupfor TXF “Tap” or “Ping” test.Actual MetalMAX™ Equipment
18 FREQUENCY RESPONSE FUNCTIONS (FRF’S) Flexibility X-DIRECTION Y-DIRECTION
19 TO GENERATE LOBING DIAGRAMS FROM FRFS INFORMATION NEEDEDTO GENERATE LOBING DIAGRAMS FROM FRFSMaterial/ToolSpecificationOrthogonalMeas. FileCuttingLimitationsTool geometryCuttingParametersMaterial Parametersare reduced to 2:Cutting StiffnessPD Wavelength
20 20 mm 3-fluted Tool in 30 kW 24 krpm Spindle Stability Lobe Plot20 mm 3-fluted Tool in 30 kW 24 krpm SpindleProcessDampingRegionTorque LimitUnstableChatterFrequencies
21 20 mm 3-fluted Tool in 30 kW 24 krpm Spindle *07/16/96Power Lobe Plot20 mm 3-fluted Tool in 30 kW 24 krpm SpindleFull Power*
22 Modal Parameter Estimation Natural FrequencyModal StiffnessModal Damping Ratio
23 Milling Simulation (Computer Model) Data loaded from TXF FileCut Dataand info.
24 Milling Simulation (Results) Stability Lobe DiagramY-Displacement at 12,000 rpmChatter FrequencyPower Lobe DiagramY-Displacement at rpm
25 Limitations of Approach Critically dependent on cutting stiffness and process damping wavelength.Once established for a particular grind of tool and material then will produce accurate predictability.Will change after tool wears.1/4” diameter tool is practical lower limit of effective measurement.Improvements currently being developedIn worse case an indirect measurement approach can be applied.Measurement of dynamics performed under static conditions.Measurements can be made at speed with non-contact sensor.Most advance and current spindle designs have good dynamic repeatability and consistency.
26 An Example of Benefit Obtained Spar Mill Cutting with 1.25” Diameter indexable mill with 2 inserts.Initial Conditions (5 mm depth, max. full dia.):21,500 rpm, 0.11 mm chip load, 118 mins. per load machining time.Getting chatter when cutter becomes fully immersed, lowered chip load to attenuate damage to part.New Conditions:24,000 rpm, .2 mm chip load, 62 mins. per load machining time.BenefitsSavings: $35 per load.Approximate 50% increase in machine capacity (near 50% reduction in machining time per load).
27 Other Benefits of Easy Dynamic Measurement Rapid dynamic measurement can quickly identify many conditions.Non-intuitive behavior.Most flexible mode may not be the most likely to chatter.Quickly identify which component is producing the most flexible mode.Identify when stiffness or damping is loss.Quickly detect changes or compare performance.
29 Most Flexible Mode May not Cause Chatter. Standard 3/4” Mill in SF HolderLong 1” Mill in Collet HolderMaximum Dynamic FlexibilityCritical for Chatter
30 Quickly identify Weak Component. Spindle ModeTool ModeHolder Mode1-at tool tip2-at tip of holder3-at base of holdernear spindle3-at base of holdernear spindle2-at tip of holder1-at tool tip321Spindle Side
31 Detecting Problems after “Events” Spindle loss bearingpreload. Subsequentmeasurements confirmthat there was nopreload.Same Tool and holderon two different machines,spindles of different age butstill in “good” condition.
32 It determines whether chatter is or is not present. It does this by “listening” to the cut and suggesting alternative spindle speeds that harmonise the “good” and “bad” vibrations, producing constant chip thickness.Knowledge of the spindle speed is essential.Spindle speed components generally dominate the audio spectrum unless chatter is very severe.Other audio sources are related to spindle speed, bearing passing frequencies, air-oil hiss, etc.Correct setting of threshold maximizes sensitivity.
33 Trial and Error Example using Harmonizer® 10,000 RPMCorner Cut raw audio signal.10,000 RPMFrequency content with filters4 Fluted 25 mm diameter Carbide End-Mill in Collet holder with maximum speed of 10,000 rpm
34 Trial and Error Example 8393 RPMCorner Cut raw audio signal.8393 RPMFrequency content with filters.4 Fluted 25 mm diameter Carbide End-Mill in Collet holder with maximum speed of 10,000 rpm
35 Trial and Error Example 10,000 RPMFrequency Content with no filters.8393 RPMFrequency content no filters.4 Fluted 25 mm diameter Carbide End-Mill in Collet holder with maximum speed of 10,000 rpm
36 Tool TuningWith knowledge of the dynamics we can exploit the behaviour to our advantage.From a previous slide we know length is critical, sometimes shorter is not better.We can many times select holder and tool geometry to produce best performance at maximum speed.
37 Tool Tuning Example: 30 kW, 24,000 RPM Spindle with 20 mm 3-Fluted tool Full Power 30 kW12 mm depth of cutNot full Power 30 kW4 mm depth of cut70 mm stick-out90 mm stick-out
47 Tap Test ResultsFour dominant modes identified from tool; 870 Hz, 2500 Hz, 3500 Hz, 4500 HzAccelerometer recordings during turning at 30 rpm show excitations at 3500 Hz and 4500 HzIncreasing the spindle speed to change the cutting frequency reduced the excitation at the tool tip