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THE ROLE OF DYNAMICS IN THE MACHINING PROCESS (MetalMAX TM Approach to Improving Milling Cutting Performance)

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Presentation on theme: "THE ROLE OF DYNAMICS IN THE MACHINING PROCESS (MetalMAX TM Approach to Improving Milling Cutting Performance)"— Presentation transcript:

1 THE ROLE OF DYNAMICS IN THE MACHINING PROCESS (MetalMAX TM Approach to Improving Milling Cutting Performance)

2 Ideal Milling Process Long Tool Life Elimination of benching Optimum M/C utilization Long Spindle Life Max MRR/SGR Unattended machining/High Process reliability Right first time Stable Machining/ Low vibration Low Cutting Forces The Ideal Milling Process

3 Cutting Parameter Selection How do we choose our speeds, feeds and depths of cut The Conventional Approach Highly Experienced Planner Technological database from cutting tool supplier Operational Guidelines from machine tool supplier TATATA… J.Fox.1998 Note: None of the above is based on a sound scientific or objective approach.

4 Consequences of the Conventional Approach Scrapped Parts Excessive “benching” Power tool life and tool failures Accelerated spindle wear Poor process reliability Unpredictability All of this results in wasted time and money

5 Common factor in the above trends is the increased importance of dynamic influences. Trends exacerbate these problems Move to monolithic structures Bigger,deeper parts with high L/D ratios. Very Expensive, less margin for error. Greater opportunity to shine Move to Flimsier, lightweight parts Move to more exotic materials

6 How can we scientifically select the cutting parameters to account for the system dynamics? Quickly obtain required dynamic information Use this information to obtain optimum cut parameters Quickly verify cutting performance.

7 What is High Speed Machining? There are many definitions Cutting speed alone (tool maker viewpoint) Spindle speed alone (common for newcomers) Machining at speeds significantly higher than conventional practice (machine shop view) Others All 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

10 Stability Lobe Diagram Process Damping Region

11 Chatter Mechanism Most 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 The force on any tooth is proportional to the chip thickness Each 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. Regeneration of Waviness

13 Process Damping Chatter 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 Machine a part right the first time! MetalMAX TM Hardware The MetalMAX™ Approach Identify and isolate problems areas Predict dynamic behaviour Adjust to optimise. Measure and verify Optimised? - if not back to step 1 Move on

15 MetalMAX™ The package for dynamic/chatter prediction and control Frequency and Flexibility Measurement (Modal Analysis “Tap” Test) + Basic Cutting Parameters and Cutting Theory = Predictions of Stable Depth of Cut limits Cutting Forces and Displacements Dynamic Cutting Accuracy ELIMINATE CHATTER!!!

16 ~ Measurement and Analysis TXFPCScope MilSim™ Frequency Analyser for Machine Tools Data Acquisition and Machining Analysis Milling Simulation and Chatter Prediction Computation and Prediction Verifying Performance NC Integrated Spindle Speed Control Non Automated CRAC Package

17 FRF Measurement with MetalMAX™ Equipment Schematic of Measurement Setup for TXF “Tap” or “Ping” test. Actual MetalMAX™ Equipment EXCITATION ( HAMMER ) RESPONSE ( ACCEL ) Sensor Interface Module PC Accelerometer STRIKE Hammer Power Cable Sensor Cable

18 FREQUENCY RESPONSE FUNCTIONS (FRF’S) X-DIRECTION Y-DIRECTION Flexibility

19 INFORMATION NEEDED TO GENERATE LOBING DIAGRAMS FROM FRFS Tool geometry Cutting Parameters Material Parameters are reduced to 2: Cutting Stiffness PD Wavelength Material/Tool Specification Orthogonal Meas. File Cutting Limitations

20 Stability Lobe Plot 20 mm 3-fluted Tool in 30 kW 24 krpm Spindle Process Damping Region Unstable Torque Limit Chatter Frequencies

21 Power Lobe Plot 20 mm 3-fluted Tool in 30 kW 24 krpm Spindle Full Power

22 Modal Parameter Estimation Natural Frequency Modal Stiffness Modal Damping Ratio

23 Milling Simulation (Computer Model) Cut Data and info. Data loaded from TXF File

24 Milling Simulation (Results) Stability Lobe Diagram Power Lobe Diagram Y-Displacement at 12,000 rpm Y-Displacement at rpm Chatter Frequency

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 developed In 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. Benefits Savings: $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.

28 Non-intuitive behavior: Shorter not always better. FRFStability Map 3 flute carbide 3/16” diameter ball-nose tapered end-mill with 5/8” shank 6.9” overall length 3 flute carbide 3/16” diameter ball-nose tapered end-mill with 5/8” shank 6.3” overall length

29 Most Flexible Mode May not Cause Chatter. Long 1” Mill in Collet Holder Standard 3/4” Mill in SF Holder Maximum Dynamic Flexibility Critical for Chatter

30 Quickly identify Weak Component. Spindle Side at tool tip2-at tip of holder 3-at base of holder near spindle Tool Mode Holder Mode Spindle Mode 1-at tool tip2-at tip of holder 3-at base of holder near spindle

31 Detecting Problems after “Events” Spindle loss bearing preload. Subsequent measurements confirm that there was no preload. Same Tool and holder on two different machines, spindles of different age but still 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 RPM Corner Cut raw audio signal. 10,000 RPM Frequency content with filters 4 Fluted 25 mm diameter Carbide End-Mill in Collet holder with maximum speed of 10,000 rpm

34 Trial and Error Example 8393 RPM Frequency content with filters RPM Corner Cut raw audio signal. 4 Fluted 25 mm diameter Carbide End-Mill in Collet holder with maximum speed of 10,000 rpm

35 Trial and Error Example 8393 RPM Frequency content no filters. 10,000 RPM Frequency Content with no filters. 4 Fluted 25 mm diameter Carbide End-Mill in Collet holder with maximum speed of 10,000 rpm

36 Tool Tuning With 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 kW 12 mm depth of cut Not full Power 30 kW 4 mm depth of cut 70 mm stick-out90 mm stick-out

38 Tests on KRYLE VMC

39 Damping trials CL and Particle damping tested Harmonizer software used to record sound levels

40 Stability Lobes: Undamped

41 Stability Lobes: Damped

42 Conventional Milling left to right; Particle damping, CLD

43 Un-damped 6000 rpm

44 CLD 6000 rpm

45 Webster & Bennett VTL Initial Spindle speed 30 rpm 3mm DOC Tap Tests on Component, Ram & Tool Deflection of Ram recorded during turning Excitation of Tool reduced by increasing spindle speed

46 Webster & Bennett VTL

47 Tap Test Results Four dominant modes identified from tool; 870 Hz, 2500 Hz, 3500 Hz, 4500 Hz Accelerometer recordings during turning at 30 rpm show excitations at 3500 Hz and 4500 Hz Increasing the spindle speed to change the cutting frequency reduced the excitation at the tool tip

48 Webster & Bennett VTL 30 RPM 40 RPM

49 Presentation available on-line at: click on “Download” to go to download area.


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