1

Dealing with thermal errors in heavy duty machine tools

Gorka Aguirre gaguirre@ideko.es

Precisiebeurs 2015

Veldhoven, NL

Dealing with thermal errors in heavy duty machine tools 2

IK4-IDEKO

Dealing with thermal errors in heavy duty machine tools 3

Outline

Large heavy duty milling-boring machines

Thermal error management

Thermal Error Compensation System

General considerations

Dealing with thermal errors in heavy duty machine tools 4

Large heavy duty milling-boring machines

Main thermal issues

Heavy duty: high heat generation, spindles can rate up to 88kW

Large machine: error amplification with increasing workspace

Applications

Heavy duty milling and boring

Large parts, high precision

Oil&gas, wind energy, aeronautics, etc.

Large machines

Vertical travel up to 8m

Longitudinal travel up to 60m

Multiple spindle/quills with automatic changer

Dealing with thermal errors in heavy duty machine tools 5

Thermal error management strategies

Temperature control

Minimize temperature variations in the machine

Design for thermal error reduction

Minimize errors at TCP generated by temperature changes

Thermal error compensation

Compensate remaining errors at TCP

Dealing with thermal errors in heavy duty machine tools 6

Temperature control

Heat sources

Bearings around spindle area

Ambient temperature, not controlled

Hydrostatic bearings

Hot chip falling against machine

Motors

Pumps in cooling circuit

Minimize temperature variations in the machine

In practice

Heat in bearings is dominant effect, by magnitude and dynamics.

Ambient temperature and other effects appear less relevant

Dealing with thermal errors in heavy duty machine tools 7

Temperature control

Cooling units

Minimize temperature variations, but limited by high required power.

Dangerous, cooling circuits can be a disturbance source

Might be too expensive

Minimize temperature variations in the machine

50 100 150 200 250 300 3500.3

0.4

0.5

0.6

0.7

0.8

0.9

1

MINUTESG

RO

WT

H

0 RPM3000 RPM

Dealing with thermal errors in heavy duty machine tools 8

Design for thermal error reduction

Minimize errors at TCP generated by thermal changes in the machine

Application of precision design principles

Machine design is focused on stiffness and cost

Precision is important, but it comes next

Focus on avoiding bending errors

Linear errors are 'easy' to compensate

Bending errors are more difficult to compensate at TCP

Bending errors change tool orientation

Dealing with thermal errors in heavy duty machine tools 9

Thermal error compensation

Estimate and compensate thermal errors during machine operation

Thermoelastic machine model

Relates thermal error with temperature field, spindle speed, position in workspace, etc.

Can be based on simulations (FEM) and/or measured data

Implementation

Experiments/simulations to characterize model

Simulation model running on CNC/PLC

Impact

Low cost solution, big improvements are possible

Risk of lack of robustness, machine operation is highly variable and only a very limited range of conditions can be analysed during implementation.

input data

thermoelastic model

CNC compensation

Dealing with thermal errors in heavy duty machine tools 10

Summary

Temperature control is limited by the high generated power and the risk of creating fast temperature changes. Advanced control strategies are needed

Design for thermal error reduction is very limited, since machine architecture is defined by other aspects, such as cost and workspace.

Thermal error compensation can provide relevant performance improvement with low implementation cost, but robustness must be ensured

Dealing with thermal errors in heavy duty machine tools 11

Thermal Compensation System

Robust and effective compensation method:

Improve machine accuracy, never make it worse

Minimize machine occupation time:

Fast machine characterization method

Multiple heads, one 8h shift per spindle head / quill

Simple to implement

By machine operator

Automated operation

Flexible

Compatible with main CNC systems

Full range of machines/spindles/quills

Main objectives

Dealing with thermal errors in heavy duty machine tools 12

Identification of main thermal effects

Multiple sources for thermal variations

Heat generated by tool rotation, in motor, bearings and gears

Heat generated in the cutting process

Cooling fluids in structural elements

Cooling fluid in tool

Ambient temperature

Local heating by contact with hot chip

Temperature variations in the workpiece

- ambient

- cutting process

Temperature variations in the pressurized air in linear scales

Dealing with thermal errors in heavy duty machine tools 13

Machine state monitoring

As a general rule, place them in structural elements, near the heat sources

- Advanced numerical methods, like Singular Value Decomposition can be used on simulation or measured data for optimizing t hese locations

Continuous monitoring of the thermal condition of the machine

Direct temperature measurement at selected key points is the ideal solution

Need to identify:

- Most relevant effects to be compensated

- Optimal sensor locations for each effect

Good sensor location enables simple compensation models.

- Sensors can be replaced by model complexity, risk for robustness

Dealing with thermal errors in heavy duty machine tools 14

Compensation model

Thermoelastic machine model based on experiments on each machine

Preferred situation

- Temperature readings and axis position as only input to the model

- Model structure, the simpler the better

- Temperature sensor location defines model accuracy

p(t): estimated error at point of interest T

m(t): Temperature readings from m sensors

Sm

(s): Transfer function relating temperature and growth K(x

1,x

2,...): Correction factor for machine axis position x

1,x

2,...

Alternatives

- When rotating elements not accessible for probes

- State observers using CNC info, such as spindle speed, power

Dealing with thermal errors in heavy duty machine tools 15

Machine characterization

All effects to be compensated need to be characterized experimentally

Time and feasibility are the main issues

- Tool rotation heat is simple to excite, few hours to stabilize

- Ambient temperature cannot be controlled, and time cycle is of a full day

- Effects of structural cooling follows other effects, no need to excite separately

- Workpiece heating, effects of cutting fluid and hot chip are in general very

difficult to analyse.

In the general case:

- Tool rotation heat is always analysed, following predefined speed profiles

- Ambient temperature can be considered optionally, but requires longer testing and variation might not be sufficient in function of the weather

Dealing with thermal errors in heavy duty machine tools 16

Experimental setup

Software in external laptop

Data acquisition from CNC/PLC and displacement sensors

Generate movement program for CNC

Automatic model fitting

Automatic generation of compensation tables

Hardware

Several measuring points within workspace

- RAM and quill positions

- Spindle orientations

Thermally stable measurement targets

Dealing with thermal errors in heavy duty machine tools 17

Implementation in machine control

Data acquisition

Temperature from embedded sensors

Machine axis positions

Variety of CNC

Heindenhain Siemens Sinumerik Fanuc Fagor Automation

Compensation in CNC/PLC

Application on CNC PC

Embedded code in PLC

Dealing with thermal errors in heavy duty machine tools 18

Spindle head tests

Thermal errors are measured within workspace

- Three main orientations of the main spindle

- Full range of the ram

Dealing with thermal errors in heavy duty machine tools 19

Compensation results: spindle head

All results are normalized to the maximum measured value

Dealing with thermal errors in heavy duty machine tools 20

Quill tests

Thermal errors are measured within workspace

- RAM and quill are aligned with the same axis.

- Several combinations of both axis are measured

Dealing with thermal errors in heavy duty machine tools 21

Compensation results: quill

All results are normalized to the maximum measured value

Dealing with thermal errors in heavy duty machine tools 22

General considerations

Do not be too ambitious, focus on dominant errors:

Smaller effects are more difficult to characterize, and

Considering them might affect robustness

Compensation model structure

The simpler the better, improve sensor location before adding complexity

Temperatures and machine position as only inputs to the model

FE can be useful

Improve design and cooling elements (Minimize bending deformations!)

Find optimal sensor location (structural elements, near heat source, near linear scales)

FE not for compensation model, experiments are needed if high precision is required

Dealing with thermal errors in heavy duty machine tools 23

General considerations

Careful with cooling systems, their goal should be to:

Keep machine temperature constant, not necessarily at 20ºC

Damp transient effects

Keep in mind industrial feasibility

Minimize machine occupation time

Simple procedure to be implemented by operators

Be aware of control requirements/limitations

Special applications with extreme requirements

When possible, re-think the manufacturing steps (CAM)

Add many more sensors, use some math tricks (e.g. POD) to find best correlating ones

Take time to properly characterize the effect of ambient temperature

Ideko-IK4ren irudi bateratua lortzeko formatuak 24

Thank you!