the role of dynamics in the machining process (metalmax tm approach to improving milling cutting...
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THE ROLE OF DYNAMICS IN THE MACHINING PROCESS
(MetalMAXTM Approach to Improving Milling Cutting Performance)
Ideal Milling Process
Long Tool Life
Eliminationof benching
Optimum M/C utilizationLong Spindle Life
Max MRR/SGR
Unattended machining/High
Process reliability
Right first time
Stable Machining/
Low vibration
Low Cutting Forces
The Ideal Milling Process
Cutting Parameter SelectionHow 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.
Consequences of theConventional 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
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
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.
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
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
The Role of Dynamics inHigh 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
Stability Lobe DiagramStability Lobe Diagram
ProcessDampingRegion
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”.
• 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.
bfKF s
Regeneration of Waviness
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.
Machine a part right the first time!
MetalMAXTM 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
MetalMAX™The package for
dynamic/chatter prediction and controlFrequency and
Flexibility Measurement(Modal Analysis “Tap” Test)
+ Basic Cutting Parametersand
Cutting Theory=
Predictions of Stable Depth of Cut limitsCutting Forces and Displacements
Dynamic Cutting AccuracyELIMINATE CHATTER!!!
~
Measurement and Analysis
TXF PCScope
MilSim™
Frequency Analyserfor Machine Tools
Data Acquisition and Machining Analysis
Milling Simulation andChatter Prediction
Computation and Prediction
Verifying Performance
NC IntegratedSpindle Speed Control
Non Automated CRAC Package
FRF Measurement with MetalMAX™ Equipment
Schematic of Measurement Setupfor TXF “Tap” or “Ping” test.
Actual MetalMAX™ Equipment
4
3
2
1EXCITATION
(HAMMER)
RESPONSE
(ACCEL)
Sensor Interface Module
PC
Accelerometer
STRIKE
Hammer
Power Cable
Sensor Cable
FREQUENCY RESPONSEFUNCTIONS (FRF’S)
X-DIRECTION
Y-DIRECTION
Flexibility
INFORMATION NEEDEDTO GENERATE LOBING DIAGRAMS FROM FRFS
Tool geometry
CuttingParameters
Material Parametersare reduced to 2:Cutting StiffnessPD Wavelength
Material/Tool Specification
OrthogonalMeas. File
Cutting Limitations
Stability Lobe Plot20 mm 3-fluted Tool in 30 kW 24 krpm Spindle
ProcessDampingRegion
Unstable
Torque Limit
ChatterFrequencies
Power Lobe Plot20 mm 3-fluted Tool in 30 kW 24 krpm Spindle
Full Power
Modal Parameter Estimation
Natural FrequencyModal StiffnessModal Damping Ratio
Milling Simulation (Computer Model)
Cut Dataand info.
Data loaded from TXF
File
Milling Simulation (Results)
Stability Lobe Diagram
Power Lobe Diagram
Y-Displacement at 12,000 rpm
Y-Displacement at 11500 rpm
Chatter Frequency
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.
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).
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.
Non-intuitive behavior: Shorter not always better.
FRF Stability Map
3 flute carbide 3/16” diameter ball-nose tapered end-mill with 5/8” shank6.9” overall length
3 flute carbide 3/16” diameter ball-nose tapered end-mill with 5/8” shank6.3” overall length
Most Flexible Mode May not Cause Chatter.
Long 1” Mill in Collet HolderStandard 3/4” Mill in SF Holder
Maximum Dynamic Flexibility
Critical for Chatter
Quickly identify Weak Component.
Spindle Side1 2 3
1-at tool tip 2-at tip of holder 3-at base of holder near spindle
Tool Mode
Holder Mode
Spindle Mode
1-at tool tip2-at tip of holder3-at base of holder near spindle
Detecting Problems after “Events”
Spindle loss bearingpreload. Subsequent measurements confirm that there was nopreload.
Same Tool and holderon two different machines,spindles of different age butstill in “good” condition.
• 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.
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
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
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
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.
Tool Tuning Example:30 kW, 24,000 RPM Spindle with 20 mm 3-Fluted tool
Full Power 30 kW12 mm depth of cut Not full Power 30 kW
4 mm depth of cut
70 mm stick-out 90 mm stick-out
Tests on KRYLE VMC
Damping trials
• CL and Particle damping tested• Harmonizer software used to record
sound levels
Stability Lobes: Undamped
Stability Lobes: Damped
Conventional Milling left to right;Particle damping, CLD
Un-damped 6000 rpm
CLD 6000 rpm
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
Webster & Bennett VTL
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
Webster & Bennett VTL
30 RPM
40 RPM
Presentation available on-line at:www.mfg-labs.comclick on “Download” to go to download area.
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