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



R-120 Machine



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



GRINDING LAYOUT

4



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



Measurement of chatter frequency

OD Chatter Impression (on sheet)

10 chatter

marks

6



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.



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.



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



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



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!



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

case study-chatter issue

Documents