dynamic stability of grinding machines potentials and risks ......flexibility areas – amplifying...

Dynamic stability of grinding machines –

potentials and risks

planlauf GmbH, Aachen (Germany)

Thun, May 23rd 2014

Dr.-Ing. Severin Hannig

Dynamic stability of grinding machines – potentials and risks, Severin Hannig, May 23rd 2014

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planlauf GmbH – Measurement / Simulation of

Machine tools and Processes

Products and Services:

Measurement

Displacement / Cutting forces

Vibrations / Stiffness

Modal analysis / Frequency response

Geometry / Dyn. spindle concentricity

Analysis of floor and base

Long-term monitoring of machine tools

Simulation

Simulation of structural mechanics

Mechatronic simulations

Simulation of machining

Base analysis

Tool calculation

Design & development

Vibration exciter and damper

Measurement systems

Measurement and calculation software

measurement of

frequency response

modal analysis

simulation of machine tools

simulation of machining

measurements

simulations


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Structure

Dynamic stability and vibrations during grinding

Risks of dynamics in design of grinding machines

Possibilities for detecting causes of vibration

Potentials for avoidance of dynamical weaknesses

Summary


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Chatter vibration – a permanent risk in fine finishing

relative dynamic flexibility in X [µm/N]

0

100

200

300

400

500

600

frequency [Hz]

0,01

0,1

1

bending mode of the

clamped workpiece

Characteristic of Chatter

Self-energizing vibration

Caused by a resonance point of the

machine tool/workpiece/clamping

device

Vibration frequency depends on

properties of mass and stiffness

without any rotation speed dependency

Appearance slightly above the

dominant resonance of the machine

tool

Short wavelength partly visible as

surface waves, long wavelength only

measurable in the majority of cases

X

Y

Z

workpiece with chatter marks


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Self-excited vibration – the regenerative effect on workpiece side

Fc + Fdist Fc

Fc Fc

time T1

The mechanism of chatter

T1: Outer disturbance forces cause a

relative displacement between

workpiece and grinding wheel.

T2: The resulting vibration fades away

because of damping effects.

T3: After one revolution of the

workpiece the waviness causes a

change of the cutting forces.

T4: The deviation of the cutting forces

results in a new and increased shifting

which intensifies the forming of waves.

Note:

A similar mechanism is also possible

on grinding wheel side during grinding

and dressing!

time T2

time T3

time T4

grinding wheel

workpiece


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Flexibility areas – amplifying of the residual imbalance of the

grinding wheel

0

100

200

300

400

500

600

frequency [Hz]

0,01

0,1

1

Tilting of the

grinding unit in X

Characteristic of vibration

The rotary frequency of the grinding

wheel is located in an area where the

machine tool has a higher flexibility.

The increasing of the vibration level is

not proportional to the rising rotation

speed (resonance crossing).

No self-energizing or swing up.

The wave numbers on the grinded

workpiece can always be calculated

back to the rotary frequency of the

grinding whee,l respectively its

multiples.

Related problems:

Installation vibrations caused by

external excitation transferred via the

floor.

Vibration caused by internal excitation

from aggregates.

X

Y

Z

0

0,1

0

0,2

time [s]

-5

Abs. displacement of headstock in X [µm]

-2,5

5

2,5

rotation speed 1: 3.480 min-1

rotation speed 2: 4.140 min-1

relative dynamic flexibility in X [µm/N]


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Consequence of dynamic weak points of grinding machines

Consequence

Visible and/or measurable waviness on

grinded surfaces.

Abrasion/wave forming on the grinding

wheel.

Tolerance problems in FFT order

analysis.

Reduced life period of spindles,

bearings, guidance.

Questions

Forecast: Is the risk of vibration

calculable?

Classification: Is it possible to classify

vibration problems, e.g. point of origin?

Statistics: Which kind of vibration

problems frequently appear depending

on the type of machine tool?


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Structure



Possibilities for detecting vibration causes


Summary


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Vibration problems are complex and appear in various ways

– but can be classified by the point of origin

Workpiece unit:

Tilting/sliding of slides/housings/head stock

Bending/sliding/torsion of the spindle/chuck/workpiece

Self-deformation of the workpiece

Grinding unit:

Tilting/sliding of slides/housing/head stock

Bending/sliding/torsion of spindle/flange/grinding wheel

Plate oscillation of grinding wheel/tool

Other causes:

Unbalanced excitation

Geometrical runout

Pitch error of ball screw

Damages of bearings

Belt vibrations

Pump pulsation

…

Dressing unit:

Tilting/sliding of the dressing slides / housings

Bending/sliding/torsion of the dressing spindle

Self-deformation of the dressing tool

Basic machine:

Tilting/weaving of columns

Installation vibration of the machine bed


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Vibration problems appear in all types of grinding machines

– but the causes are unevenly distributed

Percentaged distribution of chatter causes:

Other (unbalancing/runout)

Vibration of the dressing unit

Vibration of the workpiece unit

Vibration of the grinding unit

Vibration of the basic machine

Statistics: Metrological problem analyses of 62 grinding machines of different sizes and types:

16 External cylindrical grinding machines (between centers)

4 Internal cylindrical grinding machines

11 Portal or column grinding machines (Large designs)

9 Surface or profile grinding machine (with single- or double-spindle)

12 Centerless grinding machines

10 Other grinding machines (gear / blade grinding etc.)

percentage of vibration causes [%]

0 10 20 30 40

7%

46,5%

17,4%

9,3%

19,8%

Note:

Vibration problems with multiple

causes were also classified in more

than one field.


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Overlayed vibration problems can appear during grinding and dressing -

- but in common cases there is only one problem during grinding

Chatter vibration appears:

Chatter vibration is caused by:


0 10 20 30 40 50 60 70 80

Conclusion:

Because of higher cutting forces, the chatter vibration appeared during grinding in about 78% of considered cases.

In about 38% of considered cases, the work piece deviance is caused by more than one dynamic weak point.

Statistics: Metrological problem analyses of 62 grinding machines of different sizes and types:

during grinding

during dressing


0 10 20 30 40 50 60 70 80

three sources

two sources

one source

77,6%

22,4%

3,3%

34,4%

62,3%


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Sliding/tilting of the

grinding unit

Bending of the spindle-

bearing-system

Tilting of spindle and

flange/grinding wheel

Plate oscillation of the

grinding wheel

General risks of vibration of a grinding unit

Sliding/tilting of the grinding unit

Bending of the spindle-bearing-system

Tilting of spindle and flange/grinding wheel

Plate oscillation of the grinding wheel

0 10 20 30 40 50 60 70 80

percentage of vibration caused by grinding unit [%]

X

Z

Y

X

Z

Y

X

Z

X

Z

Statistics: Evaluation of 42 grinding machines, where grinding units caused the chatter vibration:

40,4%

45,2%

9,5%

4,9%


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General risks of vibration of a workpiece unit

Sliding/tilting of the workpiece unit

Bending of spindle/chuck/workpiece

Self-deformation of the workpiece

Sliding/tilting of the

workpiece unit

Bending of the whole unit (spindle/chuck/workpiece

and sleeve in some cases)

Self-deformation of the

workpiece

0 10 20 30 40 50 60 70 80

Statistics: Evaluation of 14 grinding machines, where workpiece units caused the chatter vibration:

14,3%

71,4%

14,3%

X

Z

Y

X

Z

X

Z

percentage of vibration caused by workpiece unit [%]


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General risks of vibration of a dressing unit

Sliding/tilting of the dressing unit

Bending of spindle-bearing-system

Self-deformation of the dressing tool

Sliding/tilting of the dressing unit Self-deformation of the dressing tool

(dressing roll / plate / diamond etc.)

0 10 20 30 40 50 60 70 80

Bending of spindle-bearing-system

Statistics: Evaluation of 8 grinding machines, where dressing units caused the chatter vibrations:

50%

50%

X

Z

Y

Z

X

0%

percentage of vibration caused by dressing unit [%]


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S-shaped self-deformation of a crankshaft

Installation vibration with a phase

displacement between both sides

Tilting oscillation of the tool side in

antiphase with the workpiece side

Design-specific risks: cylindrical grinding machines,

grinding machines for crankshafts and camshafts

Overswinging, caused by differences

in mass and inertia between tool and

workpiece side, is caused by:

Fast change in direction of the slides

due to the cam shape

High mounting above the machine bed

Similar natural frequencies of the tool

unit and workpiece unit

Natural vibration of a clamped

crankshaft is problematic at:

Crankshafts with a high stroke (weak

profile)

Clamping concepts without distortion

(joints in chuck)

First grinding without stationary support

A waviness at bearing position A can be

transferred to bearing position B via

contact of a stationary support.


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Tilting of the portal Weaving of the slide

Design specific risks: Portal and vertical grinding machines

Tilting of column/portal

Natural vibration of workpiece/table

Weaving of the slide

Other causes

0 10 20 30 40 50 60 70 80


Statistics: Metrological problem analysis of 42 large machine tools in portal-, gantry- or floor standed column layout

with vertical slide

9,6%

50%

21,4%

19%

The weaving of the slide/grinding head

is caused by:

Wide throat depth of the slide

Heavy tool heads / grinding heads

Small/weak profile of the slide

X

Z

Y

X

Z

Y


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Structure





Summary


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Analysis of machined surface Stiffness analysis of machine tool

Expenditure of time

Vibration analysis of processes

Complete, detailed modal analysis Weakness analysis / measures

Metrological process and machine analysis:

systematic detection of dynamic weak points

Complete examination:

Measurement on site: 1 - 2 Days

Pre-evaluation: instantly

Profitability report: 2 – 5 Days

Of these:

Machine downtime: 1 – 2 Days

Results after: 4 – 7 Days

Total: 4 – 7 Days

Dyn. flexibility

Frequency

Shaker excitation

complex chatter image

X

Y

Z

X

Z

Y

Vibration acceleration

Dyn. flexibility

Frequenz

X

B

C

Evaluation of enhancement measures:

A: Process changing

B: Component stiffening

C: Damping of machine structure

D: …


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Shaker: Important measurement system for acquisition of the

„dynamical fingerprint“ of a machine structure

Vibration exciter (Shaker):

Easy relative fixation between grinding

wheel and workpiece side

Relative excitation of the machine

structure close to the process with a

static preload

Flexible choice of the excitation

spectrum

Application:

Reproducible measuring of the quasi-

static and dynamic flexibility behavior

of machines

Seperation of the flexibility-shares of

grinding wheel and workpiece side

Continuous excitation and force

acquisition for modal analysis

Measurement system

Shaker in different sizes


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TDA – Acquisition of non-stationary processes in time domain

Direct time domain analysis (TDA):

Non-linear and non-stationary

processes (swing up, timed changes of

vibration direction) can not be analysed

with methods of the frequency domain

(operational vibration / modal analysis).

The measured vibration accelerations

are integrated two-fold and displayed

as vibration displacement.

Generally, a simultaneous

measurement with multiple triaxial

sensors is necessary.

0

Zeit [s]

5

10

15

20

0

1

2

-2

-1

0

2

4

-2

-4 Dis

plac

emen

t [µ

m]

Acc

eler

atio

n [m

/s²]

two-fold

integration

Direct display of the vibration

movements per measuring

point in 3 directions

X

Z

Y

[TDA – Time Domain Analysis]

Acceleration signal of triaxial sensor

Displacement


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Evaluation of vibration processes starts with the grinding results:

Proper estimation of FFT-order-analysis

0,1

0,2

0,3

0,4

0,5

0,6

0,7

[µm]

20 40 60 80 100 120 140 160 Wave order n

Imbalance of

grinding wheel?

Bending grinding

spindle?

Sliding of

X-slide?

Bending

workpiece? Side bands?

workpiece

roundness

Challenges:

Fine wavinesses often have

amplitudes within the scope of

roughness.

Low tolerance limits are often

exceeded by a mutliple of the wave

order of the actual problem.

The speed of wheel rotation necessary

to differentiate chatter problems from

imbalances/bearings of the grinding is

often not documented.

frequencies the workpiece rotation

speed needs to be documented.

In order to estimate the exciting of

waviness shares the future installation

has to be considered (e.g. the amount

of rolling elements of a bearing)


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Calculational concept- and machine analysis:

Early detection and avoidance of dynamical weak points

Not very time-consuming:

Time for calculation of an entire

machine takes:

about 5 – 7 days

Machine simulation:

FE-modeling and flexible multiple-

body-simulation of the machine

Consideration of all components of

structure, spindles, guideways,

bearings and the machine installation

Linking of the drive control

Calculation possibilities:

Static stiffness

Frequency response functions

Modal analysis / natural frequencies

Consideration of the base

Wall thickness optimization

Rating of tuned-mass dampers

Example of a surface grinding machine:

Working space (X/Y/Z): 800/2000/600 mm³

X-Axes

Guidance: 2x2 INA RUE 35 D

Ball screw: Steinmeyer 10.40.7,5.4

Fixed bearing: INA ZKLF 3080

Engine: Siemes 1FT6086-1AF71-2AH1

Y-/Z-Axis

Guidance: INA RUE 45 D (Y = 2x3, Z = 2x2)

Ball screw: Steinmeyer 20.50.9.4

Fixed bearing: INA ZKLF 3590

Engine: Siemens 1FT6108-8AF71-4PG4

Main spindle

Front: 4x spindle bearing FAG B7020C.UL

Rear: Cylinder roller bearing FAG NN3018.ASK

Engine: Siemens 1PM6138-2VF81-1BR1

Materials

Machine bed: Epument 145B

Column, X-slide, table slide: GG25

All other parts: Steel

Machine installation: 12x Isoloc UMS5-ASF/30


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Static stiffness calculation: load in X-direction

Shares of the individual components in

the overall deformation

FX

X

Z Y

Bed 2,1%

Column 15,6%

X-slide 43,6% Table slide0,5%

Grinding spindle

10,8%

Guideways

12,5%

Ball screws 14,9%

X-slide, the column and the X-ball screw have

The largest shares of the deformation.

Static stiffness in X-direction:

kXX,REL = 17,9 N/µm

kXX,TOOL = 18,1 N/µm

kXX,WP = 2117,9 N/µm


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Optimization of component weight and stiffness

by wall thickness optimization

Objective:

Decrease in weight of component but

with equal stiffness

Increase of the stiffness with equal

weight of component

Example machine column:

Through a weight-neutral optimization

of the column‘s wall thickness, the

stiffness of the surface grinding

machine can be increased in X- and Y-

direction by about 3 to 4%.

With a deformation share of 16 - 21%

(X- and Y-direction) this represents a

quite a good improvement.

Additional (minor) Iimprovements

require considerably more material

(cmp. variation 2: +285 kg).

60

k [N

/µm

]

kXX,REL

20

10

0 kYY,REL kZZ,REL

30

V0 (origin state)

V1 (equal weight)

V2 (increased stiffness)

V3 (decreased weight)

mass of column

1800

600

300

0

900

m [k

g]

1200

16,1 16,9

35,6

16,6 17,5

36,1

16,9 17,9

36,4

15,4 16,1

34,5

1515 1515

1190

+3% +4%

+1%

Starting points of the optimization:

Front wall Ridges Side walls Rear wall Cover / bottom

1800


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Optimization of the static machine stiffnesses

Variation 0

Origin state

Material: GG 25

Variation 2

Base plate: +40 mm

Square bar: 220 mm

Material: GGG40

Variation 1

Base plate: +40 mm

Material: GGG40

Objective:

Systematic increase of static stiffness

of components with largest shares in

the overall deformation

Example X-Slide:

Variation 1: The enhancement of the

base plate by about 40 mm with a

simultaneous change of material from

GG25 to GGG40 causes a

considerable gain of stiffnesses of

between 4% (Z) and 33% (X).

Variation 2: The increase of the

square bar dimensions to

220 mm appears to be a good

compromise between disturbance

outline and stiffness. It increases the

stiffness in grinding-sensitive direction

(X) by about another 12 %.

60

kXX,REL kYY,REL kZZ,REL

60

20

10

0

30

Variation 0

Variation 1

Variation 2

17,9

26,6

30

Relative static stiffnesses

17,6

22,2 25,3

18,1 18,8 18,9

k [N

/µm

]


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Optimization of the dynamic machine rigidity

Damping of the X-slide:

The square bar also executes a

nodding vibration in X-direction

because of the increased mass in the

statically enhanced variation 2.

Variarion 3: With a tuned mass

damper at the square bar the

maximum dynamic flexibility can be

reduced by about 70% compared to

variation 2.

G [µ

m/N

]

GXX,REL GYY,REL GZZ,REL

0 Frequency [Hz] 20

0,01

40

0

-1,0

0

-180

60

-90

80

Real part [µm/N]

Phase [°]

Dynamic flexibility [µm/N]

0,1

1

Variation 3

Position of the tuned

mass damper

GXX,REL

0,5

0,25

0 GYY,REL GZZ,REL

0,75

Variation 0 (origin state)

Variation 1 (statically enhanced)

Variation 2 (statically enhanced)

Variation 3 (dynamically optimized)

Max. dyn. flexibility

1,21

0,38 0,49

0,15

1,42

0,18

0,78 0,84

0,71

0,18 0,18 0,18

1


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Summary

The quality of a workpiece can be reduced by a multitude

of different vibration influences.

However, not all vibration problems appear equally often.

Certain machine types are, depending on the design, more

or less prone to chatter vibrations, whereas the cause is

often found in the same assembly group.

Nevertheless, well-tried measurement strategies for

vibration and stiffness acquisition allow for a quick and

systematic cause analysis.

State-of-the-art simulation methods help in early detection

and avoidance of dynamical weaknesses.


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If you are interested in this topic:

You can see an animated 3D-view of the

presented machine simulations and modal

analysis with our free software,

planlauf/VIEW,

You can download the software and this

presentation at:

www.planlauf.com

Thank you for your attention!

dynamic stability of grinding machines potentials and risks ......flexibility areas – amplifying...

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