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

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THE ROLE OF DYNAMICS IN THE MACHINING
PROCESS(MetalMAXTM Approach to Improving Milling
Cutting Performance)
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The Ideal Milling Process
Right first time
Ideal Milling Process
Low Cutting Forces
Unattended machining/High Process reliability
Long Tool Life
Elimination of benching
Stable Machining/ Low vibration
Optimum M/C utilization
Long Spindle Life
Max MRR/SGR
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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
• TATATAJ.Fox.1998
• Note None of the above is based on a sound
  scientific or objective approach.

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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

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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

Common factor in the above trends is the
increased importance of dynamic influences.
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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.

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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 someones point of view

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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
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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

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Stability Lobe Diagram
Process Damping Region
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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.

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Regeneration of Waviness
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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.

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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

Machine a part right the first time!
MetalMAXTM Hardware
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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!!!
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Measurement and Analysis
Frequency Analyser for Machine Tools
Data Acquisition and Machining Analysis
TXF
PCScope
Computation and Prediction
MilSim
Milling Simulation and Chatter Prediction
NC Integrated Spindle Speed Control
Non Automated CRAC Package

NC-
Verifying Performance
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FRF Measurement with MetalMAX Equipment
Schematic of Measurement Setup for TXF Tap or
Ping test.
Actual MetalMAX Equipment
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FREQUENCY RESPONSE FUNCTIONS (FRFS)
Flexibility
X-DIRECTION
Y-DIRECTION
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INFORMATION NEEDED TO GENERATE LOBING DIAGRAMS
FROM FRFS
Material/Tool Specification Orthogonal Meas.
File Cutting Limitations
Tool geometry Cutting Parameters
Material Parameters are reduced to 2 Cutting
Stiffness PD Wavelength
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Stability Lobe Plot 20 mm 3-fluted Tool in 30 kW
24 krpm Spindle
Process Damping Region
Torque Limit
Unstable
Chatter Frequencies
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Power Lobe Plot 20 mm 3-fluted Tool in 30 kW 24
krpm Spindle
Full Power
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Modal Parameter Estimation
Natural Frequency Modal Stiffness Modal Damping
Ratio
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Milling Simulation (Computer Model)
Cut Data and info.
Data loaded from TXF File
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Milling Simulation (Results)
Stability Lobe Diagram
Y-Displacement at 12,000 rpm
Chatter Frequency
Power Lobe Diagram
Y-Displacement at 11500 rpm
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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.

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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).

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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.

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Non-intuitive behavior Shorter not always
better.
FRF
Stability 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
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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
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Quickly identify Weak Component.
Tool Mode
Spindle Mode
Holder Mode
1-at tool tip
2-at tip of holder
3-at base of holder near spindle
3-at base of holder near spindle
1-at tool tip
2-at tip of holder
3
2
1
Spindle Side
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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.
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• 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.

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

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Tool Tuning Example30 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-out
90 mm stick-out
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Tests on KRYLE VMC
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Damping trials

• CL and Particle damping tested
• Harmonizer software used to record sound levels

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Stability Lobes Undamped
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Stability Lobes Damped
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Conventional Milling left to right Particle
damping, CLD
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Un-damped 6000 rpm
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CLD 6000 rpm
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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

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Webster Bennett VTL
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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

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Webster Bennett VTL
30 RPM
40 RPM
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• Presentation available
• on-line at
• www.mfg-labs.com
• click on Download to go to download area.