Byron Knapp, Dave Arneson, Dan Oss, and Mel Liebers

Professional Instruments CompanyHopkins, Minnesota, USA

Ultra Precision Oil Hydrostatic Spindle Design and Metrology

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A presentation given at the 10th International Symposium of Measurement Technology and Intelligent Instruments

KAIST, Daejeon, Korea 01 July 2011

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Professional Instruments CompanyHopkins, Minnesota, USA

Byron Knappbknapp@airbearings.com

Professional Instruments Company7800 Powell Road

Hopkins, Minnesota, USA

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Snow-covered Professional Instruments headquarters, January 2011.

Ultra precision oil hydrostatic spindle

Some industrial applications

demand high precision and high

load capacity spindles.

A bi-conic oil hydrostatic spindle is

ideal for the rigorous demands of

precision machining and grinding.

This presentation describes an oil

hydrostatic spindle that features:

• nanometer-level error motion• high load capacity• excellent dynamics• high crash resistance

Background

Design

Metrology

4

Benefits of oil hydrostatic spindles

Machine tool spindles commonly

use rolling element bearings.

Some potential advantages of oil

hydrostatic bearings (compared to

rolling element bearings):

• lower error motions• high damping• no wear

Background5

K Wasson. “A Comparison of rolling element and hydrostatic bearing

spindles for precision machine tool applications.” Proceedings of ASPE

Conference Precision Bearings and Spindles. June 2007.

Photo credit: SKF

rolling elements

oil hydrostatic

Comparison of two types of compensation

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Hale, L, Donaldson, R, Edson, S, and R Thigpen. “Hydrostatic bearings

designed for POGAL.” Proceedings of ASPE Conference Precision

Bearings and Spindles. June 2007.

orifice compensation

with pockets

step

compensation

Design

Step at the outer edge of the

bearing area provides restriction.

• higher average pressure• tighter clearances• high stiffness• shear film heating

Orifice or slit prior to bearing

pocket provides restriction.

• average bearing pressure less than half of supply pressure

• pockets affect error motion

Comparison of two rotor configurations

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“H” style rotor

bi-conic rotor

Design

“H” style uses basic geometric

elements with separated axial and

radial bearings.

Bi-conic design offers several

potential advantages:

• high bearing efficiency (load capacity for a given volume)

• high structural stiffness• balance of radial, axial and tilt

load capacities• design simplicity

Bi-conic design with ring restrictor and pockets

8

Kane, NR, Sihler, J. and AH Slocum. “A hydrostatic rotary bearing with angled

surface self-compensation.” Precision Engineering, 2003;27(2): 125-139.Design

Bi-conic with step compensation

Bi-conic with step compensation:

• no pocket “print-through” to error motion

• higher oil film pressure• increased bearing area• improved bearing stability• high stiffness• no complicated up-stream

restrictors

9Design

20 µm

oil films

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Non-influencing, non-contact air seals

Design

Bi-conic modal analysis

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• Kistler 2,000 N modal hammer• Kistler 50 g K-shear accelerometer• HP Dynamic Signal Analyzer

Metrology

12Metrology

Outstanding dynamic response!

Ideal axis of rotation

An axis of rotation with pure

rotation θ about the Z reference

axis.

Rotation θ is the intended function

of the spindle.

Five other basic degrees of freedom

exist: 3 linear and 2 angular.

ANSI/ASME B89.3.4M-2010. “Axes of Rotation: Methods

for Specifying and Testing.” ASME: New York (2010).Metrology

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Radial motions

Pure radial degrees of freedom as a

function of θ in the X and Y

directions.

ANSI/ASME B89.3.4M-2010. “Axes of Rotation: Methods

for Specifying and Testing.” ASME: New York (2010).Metrology

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Tilt motions

Angular degrees of freedom as a

function of θ about the X and Y axes.

ANSI/ASME B89.3.4M-2010. “Axes of Rotation: Methods

for Specifying and Testing.” ASME: New York (2010).Metrology

15

Axial motion

One degree of freedom as a function

of θ in the Z direction.

ANSI/ASME B89.3.4M-2010. “Axes of Rotation: Methods

for Specifying and Testing.” ASME: New York (2010).Metrology

16

Radial motion measurement

Measurement of the pure radial and

tilt at a particular axial location.

ANSI/ASME B89.3.4M-2010. “Axes of Rotation: Methods

for Specifying and Testing.” ASME: New York (2010).Metrology

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Tilt motion measurement

Can be calculated from two radial

measurements at specified axial

locations…

or two face measurements at

specified radial locations.

ANSI/ASME B89.3.4M-2010. “Axes of Rotation: Methods

for Specifying and Testing.” ASME: New York (2010).Metrology

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Axial motion measurement

Sensor placed collinear with the axis

of rotation.

ANSI/ASME B89.3.4M-2010. “Axes of Rotation: Methods

for Specifying and Testing.” ASME: New York (2010).Metrology

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Separating ball and spindle error

Radial motion measurements include out-of-roundness of the master and radial motion of the axis of rotation.

For nanometer-level spindles, the ball out-of-roundness can be significant.

Without separation of ball out-of-roundness, the spindle error can appear better or worse.

Ball: 9 nm Spindle: 8 nm

Ball and spindle: 14 nm

Metrology20

Reversal

Reversal techniques are theoretically the simplest way to remove artifact error.

Donaldson reversal for radial motion measurements; Estler reversal for face measurements.

Both require exact artifact reversal.

Potential errors from debris, handling, and remounting.

Metrology21

Multiprobe method

Sensor orientation angles affect harmonic suppression.

Asymmetric orientated measurements reduce harmonic suppression.

Orientation angles of 0.000°, 99.844°, and 202.500° give good separation results to 225 UPR. xEm ArtifactA

sincos yxEm ArtifactB

sincos yxEm ArtifactC

E Marsh. Precision Spindle Metrology. 2nd Edition.

Destech Publications: Lancaster, PA (2010).Metrology

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Lapped master

25 mm diameter ball of monolithic design for uniform circular stiffness.

Provision for balancing.

Flat mounting flange with six mounting screws.

Out-of-roundness is 9 nm as measured with a capacitive sensor and 150 UPR filter.

23Metrology

jig-ground locating

hole (3x)

jig-ground indexing

hole (4x)

toroidal

pilot

24Metrology

Error separation tooling

Bi-conic spindle error measurement

MCS brushless DC motor with cooling jacket and 1,024 line count encoder

Fixed displacement pump provides inlet pressures up to 70 bar with light spindle oils.

Uses 2 lpm oil (Mobile Velocite #6, viscosity 10 cSt) at 40 bar.

25Metrology

Spindle metrology setup

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• Lion Precision SEA software

• Lion capacitive sensor

• Lion Elite amplifier

• Custom target and error separation

• Stiff structural loop for testing

Metrology

Radial error measurement

27Metrology

Axial error measurement

28Metrology

Machining results

29Metrology

Retrofit workhead of CNC cylindrical grinder (Parker Liberty) :

• Hardened 440C test piece, 75 mm diameter• Better than 0.25 µm out-of-roundness

0.22 µm

Conclusions

Conclusions30

• Design and metrology techniques of an ultra-

precision oil hydrostatic spindle are described.

• This spindle ideal for the rigorous demands of

precision machining, hard turning, and grinding.

• Nanometer-level error motions, high load

capacity, excellent dynamics, and high crash

resistance are realized.

Byron Knappbknapp@airbearings.com

Professional Instruments CompanyHopkins, Minnesota, USA

Sewon Eng. LTDwww.sewoneng.net

Seoul, Korea

Contact information:

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10th International Symposium of Measurement Technology and Intelligent Instruments

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Bonus features.

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33Bonus

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Grinding wheelhead

Bonus

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Grinding the cone

Bonus

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Continuing work: lathe retrofit

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Goals: demonstrate advantages of precision hydrostatic

spindle retrofitted into a standard lathe for hard turning.

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Stiffness and load capacity (40 bar bearing inlet pressure).

Bonus

39Bonus

3 decimal places on the angle?

Zeiss Contura G2 Inspection Report

40Bonus