Download - Case Study-Chatter Issue
7/30/2019 Case Study-Chatter Issue
http://slidepdf.com/reader/full/case-study-chatter-issue 1/13
PROBLEM: CHATTER ISSUE
Intermittent chatter on crankshaft journal• Throughout wheel life (∅ 1066 mm to ∅ 750 mm)
• Occurs at all operational parameters
• On all the journals
1
7/30/2019 Case Study-Chatter Issue
http://slidepdf.com/reader/full/case-study-chatter-issue 2/13
R-120 Machine
7/30/2019 Case Study-Chatter Issue
http://slidepdf.com/reader/full/case-study-chatter-issue 3/13
System Inputs
3
Machine Tool
Machine Tool: Micromatic Grinding
Machine type: Rhino-R120 CNC
Wheel Speed: 60 mps
Work Speed: 90/65/50 RPM
Motor rating (hp): 22 kW
Coolant:
Type: Water soluble
Concentration: 4 % dilution ratio
Pressure: 3 bar
Flow: 120 Lpm
Direction:
aligned with direction of wheel
rotation
Nozzle type fixed
Dressing (Device/ tool/ spec.):Blade type (OC-2) : 2 needles
Cutting edges 2 needles
Work holding between centers, with Two steady rest
Driving Driving dog
Abrasive ProductCompany GNO
Specification: 38A54-3L8VS3 SB
Abrasive (type, size,
shape, content):white (friable) Al2O3
Grit Size: Combination of 54, 60, 80 (diff %)
Grade: L
Bond (type, content,
props.): Vitrified (VS3)
Structure (porosity): 8
Design: Straight
Wheel Surface speed: 60 mps
Size, shape, features: 1066 x 38.5 x 304.8
Work MaterialComponent Crankshaft
Type: SAE 1541
Hardness: 55 HRC
Case hardened depth: 1 mm
Size, shape, Geometry:
Stock 1.0 -1.2 mm On dia
Incoming Part Quality:
Input run-out 0.5-0.6 MM
Consistency:
Hardness ± 2 Hrc , on grinding
stock -0.050 mm
7/30/2019 Case Study-Chatter Issue
http://slidepdf.com/reader/full/case-study-chatter-issue 4/13
GRINDING LAYOUT
4
7/30/2019 Case Study-Chatter Issue
http://slidepdf.com/reader/full/case-study-chatter-issue 5/13
Chatter • Chatter, in grinding, is the undesirable vibration during the grinding process,
which results in marks on the surface of the work-piece or wheel or both
called chatter marks.• Basically caused due to two types of vibration:
– Forced vibration
– Self Excited Vibration
Ref.: Inasaki, Karpuschewski, Lee – Grinding Chatter – Origin and
Suppression, CIRP Annals, 2001.
5
Chatter Marks on a crankshaft journal
7/30/2019 Case Study-Chatter Issue
http://slidepdf.com/reader/full/case-study-chatter-issue 6/13
Measurement of chatter frequency
OD Chatter Impression (on sheet)
10 chatter
marks
6
7/30/2019 Case Study-Chatter Issue
http://slidepdf.com/reader/full/case-study-chatter-issue 7/13
Vibration Signals Captured
Peak at 85 Hz observed on the work-head
during grinding.
Motor Frequency=Motor RPM/60
=2553/60
=42.55 Hz
2x component= 2 x 42.55=85.1 Hz
Thus, 85 Hz peak corresponds to the
motor 2x component.
7
Free-running condition (on wheel head) – peak at 85 Hz
Peak at 85 Hz observed on the tailstock
during grinding.
7/30/2019 Case Study-Chatter Issue
http://slidepdf.com/reader/full/case-study-chatter-issue 8/13
Experiment to identify source of chatter
8
Work speed varied and Wheel speed constant:
Trial
No.
Work
Speed
(rpm)
Wheel
speed
(rpm)
Chatter
marks/rev.
Chatter
freq. (Hz)
Motor 2x
freq. (Hz)2 90 1407 65.2 97.8 93.8
3 70 1407 83.7 97.7 93.8
6 50 1407 115.8 96.5 93.8
15 40 1407 146.6 97.7 93.8
Work speed constant and Wheel speed varied:
Trial
No.
Work
Speed
(rpm)
Wheel
speed (rpm)
Chatter
marks/rev.
Chatter
freq. (Hz)
Motor 2x
freq. (Hz)
11 70 788 #N/A
9 70 959 53.1 61.9 63.9
7 70 1201 70 81.7 80.114 70 1323 74.4 86.7 88.2
3 70 1407 83.7 97.7 93.8
Chatter frequency is constant
at around 97 Hz indicatingsource is not from work-
drive side
Chatter frequency varies linearly
with change in wheel speed
indicating source is from wheel
drive side. Chatter frequency
matches motor 2x component.
This shows that the motor is the
source of chatter.
7/30/2019 Case Study-Chatter Issue
http://slidepdf.com/reader/full/case-study-chatter-issue 9/13
Conclusion and Action Taken
• Conclusion:
– The wheel-head motor 2x component of vibration is high, causing chatteron the component (internal forced vibration)
• Causes of high motor 2x component:
– 2-pole motor inherent characteristics may give rise to high 2x
component.
– High 2x component also may be due to soft-footing (weakness inmounting).
• Action Planned:
– Change the motor from 2-pole to 4-pole which will have lower inherent2x and overall vibration levels
– Increase the rigidity of the motor mounting assembly.
9
7/30/2019 Case Study-Chatter Issue
http://slidepdf.com/reader/full/case-study-chatter-issue 10/13
Changes made on existing machine
Drive motor changed from 2-pole (3000 rpm motor base speed at 50 Hz) to 4-pole (1500 rpm
motor base speed at 50 Hz) to decrease the vibration level (particularly the 2X component) of the
motor.
To maintain 60 mps throughout the wheel life (ø1066mm to ø780mm) with 4-pole motor, pulleys
were changed such that the 4-pole motor runs near its base speed to derive maximum power.
The motor mounting bracket was strengthened and made more rigid.
10
Old Design New Design
7/30/2019 Case Study-Chatter Issue
http://slidepdf.com/reader/full/case-study-chatter-issue 11/13
Signals captured after modification
11
Free-running condition on wheel head – overall vibration amplitudes are much lower –
peak at 25 Hz corresponding to motor 1x (1500 rpm/60=25Hz) but amplitude is low.
Signal captured on work-head during
grinding –
overall vibration levels are low- No chatter!
7/30/2019 Case Study-Chatter Issue
http://slidepdf.com/reader/full/case-study-chatter-issue 12/13
Final Status• Chatter was eliminated.
• Grinding Process Parameters: – Wheel speed: 60 mps
– Wheel diameter: 760 mm
– Wheel rpm: 1510 rpm
– Work Speeds: 96 rpm, 88 rpm, 76 rpm
– Dressing feed: 130 mm/min on OD and 131 mm/min on radius.
– Dynamic Balancing: unbalance below 0.3 um.
• Technical Outputs: – No chatter observed on the journal.
– Surface Finish = 0.5-0.6 um (Ra)
– Cycle Time: 17 min 05 sec
– Ovality: <= 3um
– Taper: < 3um
– Dimension: 83 .00 ± 4 um
– Cpk: 1.66 on thrust journal
12