review of proposals for amendments to the IS0 230 … · A review of proposals for amendments to the ... the working day, ... morning when the shop temperature was 10°C

A review of proposals for amendments to the I S 0 230 Standards

V. ~ h i l e s ' & D.M.S. ~ l a c k s h a w ~ i University of Northumbria, England I'T, Elland, England

Abstract

This paper summarises two years work done by a Craft EU project SMT4-25- CT98-5524 in the UK, Austria and Italy to review the range of machine tool test methods defined by the I S 0 230 series of standards It reviews 'points to be agreed' with a customer when tests are applied to a particular machine, as well as focusing upon the proposed 230-3 Thermal Tests (this is now an established standard) and 230-6 Diagonal Test procedures. A range of suggested improvements to test procedures has been submitted to I S 0 committees in the UK, in Italy, and in Switzerland.

1 Introduction

This project started out with the intention of examining known problems in the I S 0 230 series of Machine Testing standards'']. These standards define the range of performance criteria used to define machine performance and the established test procedures currently in use. A consortium of Machine Tool Builders and Users was established. with partner companies in Austria, Italy and the UK. Two I S 0 consultants were appointed from the start of the project (Dr Knapp, Switzerland, ISOTC39ISC2 Committee Member, and Dr Beltrarni, Italy, Chairperson of ISOlTC39 Machine Committee). Their task was to guide the project team on potential test procedure developments, and to guide the team on the correct way to input our suggestions into the I S 0 system.

Transactions on Engineering Sciences vol 34, © 2001 WIT Press, www.witpress.com, ISSN 1743-3533

C.B. Ferrari, Italy, are builders of 4 or 5 axis complex and accurate machining centres up to 3m long for use in Aerospace and other industries. KTW, Austria, are manufacturers of complex plastic moulding dies for use across the world. They are thus CNC users, having a wide range of CNC lathes, machining centres and grinding machines, with a high accuracy requirement on all their products. Datatronic, Austria, are suppliers and users of machine tool test equipment.

The general intention was to evaluate test procedures for testing large machines in the UK and a range of 'normal' sized machines across Austria and Italy. AB Manvin build machines with typical axis lengths of X=lOm, Y=3m, Z=2m, and Promil as users of these machines. Promil is also a supplier of machined components to Cincinnati and other UK companies, and precise knowledge of the capabilities of large machines is a key feature of their business. Cincinnati make "normal" size lathes and machining centres, although they have started building machines with traverses up to 3m during the project.

2 'Points to be Agreed'

An immediate problem the team faced was how to conduct similar and comparable tests in Austria, Italy and the UK.

It quickly became apparent that the "points to be agreed between the user and the supplier" in the IS0 230 test series left a wide margin of possible variation in test procedures and hence test results. This caused an in depth debate at the second project meeting to agree a common interpretation.

Agreement was reached on some of these points, and the results of our tests to further refine the required warm up procedures etc. form part of the final recommendations. These obviously have to be agreed with, or improved upon, by the world wide machine tool community.

3 I S 0 230:l - Geometric Test Procedures

Many of the test methods in this standard are very "traditional" and relate to longstanding measuring techniques e.g. a square and a dial gauge to measure squareness. AB Manvin often use a large granite square which is heavy to move, (obviously there are many smaller and lighter squares available) A granite square can weigh up to 1 tonne and AB Manvin frequently have to ship this heavy equipment around the world to align a machine eg., in Australia. Hence management of squares arriving on site when required is a serious problem. Also the definition of squareness in current use does not match measuring techniques introduced more recently eg., lasers, ball bars etc.


Laser . I lerrolo~' cind tlachine Perjormmce 235

3.1 Current Definition of squareness -

1 base line, aligned parallel to reference line

2 tolerance line, square to 1 3 tolerance line, square to 1

4 deviation from squareness 5 measured straight line or

motion

The current definition of squareness involves aligning one side of the square with the primary axis (1 above), and then using a dial indicator to measure errors (4) on the perpendicular axis. The squareness error is then given by the range between the highest and lowest dial indicator readings.

A further disadvantage of the square[21 is that it necessarily involves the use of a short length, say lm, of the primary axis which may not represent the true alignment of that axis through the measurement points.

The laser can be aligned with the whole axis motion, and the beam turned through a calibrated 90" angle. It is then used to create a "best-fit" line along the whole length of the secondary axis. The angle calculated then provides a "True" angle between the best fit lines of both axis motions. Angular errors are expressed in Arc Seconds, but it is now preferred that the error is expressed in mtdm as a more well known and more easily understood unit of measurement.

Z axis Sauareners

errors about each axis

the angle between the tv+o best fit straight lines

Proposed definition of Squareness


236 Lasev Iletvoiogy and \tuchrne Petjormance

3.2 Angular measurements

These were usually recorded directly after linear positioning measurements during this project. The machine axis being tested was therefore in a warmed-up condition, and a simple change of optics facilitated the angular measurement with a stationary angular interferometer and a moving angular reflector.

3.3 Straightness measurements

Thesewere then performed with a moving straighmess interferometer and a stationary straightness reflector fixed at the far end of the axis travel, straightness errors were recorded in two measurement planes. Machining centres were tested with short range straightness optics while the larger milling and boring machines have been tested with long range optics because of the longer axes involved. In both cases it appeared that for the most accurate results straightness slope error should be reduced to a minimum (i. e, around 0.0 1 Ornrn 1 1000mm for short range optics).

3.4 Squareness measurement

Test procedures have been refined using the laser interferometer. Considerable difficulty was encountered in obtaining repeatable squareness measurements when following the manufacturers recommended initial "slope error" setting of "align laser with axis travel to within .050mrn", and this requirement was revised to "within .010rnrn" to get repeatable test results. The laser manufacturer has subsequently modified their user instructions to include this requirement following these tests.

Values of squareness using traditional techniques (granite square and dial gauge) suitable for small machine axis lengths have been compared to squareness values from the laser interferometer for all axis lengths, and also with those squareness values from the diagnostic printout of the circularity plot while using a 150mm radius ballbar.

A second well established measuring technique is the use of a ball bar which is conventionally used to measure the machine table motions needed to create a circular motion. Squareness errors in axis motion will, however, change the resulting circle into an ellipse, and ball bar manufacturers provide squareness values as part of the normal test results.

3.5 Dynamic Straightness and Angular Laser Testing

The accepted laser test procedures involve taking measurements at a series of static positions along the axis length.

A series of dynamic tests have been conducted in the UK and Austria to investigate the relationship between conventional "static" test results, and dynamic readings which are taken in one continuous motion along the axis length.


These save a few minutes during each test, and can be useful in several situations, for example:-

1) Cincinnati perform these as a routine event on all the machines they make as a record of straighmess and angular errors.

2) In AB Marwin's situation large machines are assembled on site, and the installation engineers will have to adjust levelling jack screws to get the machine bed aligned correctly. This can take several hours, and involves several laser tests to confirm each adjustment. Hence time saving here can reduce machine installation times. Good correlation was achieved between dynamic tests and comparable static test results.

These dynamic tests were acceptable in BS3800 Part 2:1990, which has now been superseded by I S 0 230-2:1996, and it is thus suggested that these are re-examined by the I S 0 Committee.

4 I S 0 230-2 Positioning Accuracy & Repeatability

4.1 Points to be Agreed

As previously mentioned difficulty was encountered immediately on the "points to be agreed" between the manufacturer and the user prior to performing tests.

In general terms the warm up procedure is intended to establish thermal equilibrium across the machine structure but in practice this will change during the working day, depending upon:-

The work in^ cycle of the machine

Large batches - continuous running? "Chaotic" irregular

machine use'? Short or long

machining cycles?

The structure of the machine The size of the machine? Heavy cast structure? Lightweight, high speed machine? Materials used in structure? Design minimises thermal deformation? Linear scales or rotary encoders?

Thermal Compensation routines used?


238 Laser \ letroloa atzd l l c d r n e Performance

The points to be agreed between the supplierlmanufacturer and the user are as follows:

Specific Points (Extracted from I S 0 230-2: 1997) The maximum rate of environmental temperature gradient in degrees per hour for 12 hr before and during the measurements The location of the measuring instruments and the positions of the temperature sensors.

The warm-up operation to precede testing the machine.

( note:- the standard also suggests that alternatively you can use the minimum of moves required to align the instruments as a simple warm up routine. The machine should not have been used in the last few hours.)

The feed rate between target positions.

Comments

Severe difficulties were experienced recently in the UK when measuring a large machine on a Saturday morning when the shop temperature was 10°C initially, and the tests had to be repeated on another day when the heating had not been off all night. Thus tests on a Monday morning cannot be conducted if the workshop heating has been off all weekend, etc. The material temp. sensor has to represent the workpiece temp. thus it should be near the centre of the table ( on a machining centre), and the air temperature sensor close to the laser beam. Large machines should use 2 or 3 material sensors along the full axis length.

Our debate on the warm-up operation became polarised into two outcomes:

Small Machines (<2m axis length) - We adopted Cincinnati's procedure - Material temperature sensor has to reproduce the temperature of the workpiece, therefore it can be located on the table.

It is suggested for the first test series a 2-Step W arm-Up + 12 bidirectional runs at f=5000mm/min between the start and end positions of the travel. + 4 bidirectional runs at f=4000rnrnimin linear cycle and 5 seconds dwell time at the target positions.

Laroe Machines - It is very difficult to provide a general warm-up procedure - no recommendations made.

The ManufacturerIUser always wants the test performed in the minimum time, so Cincinnati's normal test feed rate of F4OO was adopted.


5 I S 0 230-3 Evaluation of Thermal Effects

The position of the 2000mrn or 360' measurement travel to be regarded as the normal working area

I if relevant ' Position of the slides I or moving

components which are not under test. Dwell time at each target position. Location of fxst and last target positions.

This standard suggests three basic tests:

Normally the centre of the axis motion.

These should be positioned so as to minimise errors, generally in the mid position of each axis.

5 seconds dwell was agreed.

"the ends of the normal working length of the axis" was agreed since some machines (eg Ferrari) use an overtravel at the end of a table motion to perform tool changes

a> An environmental test to examine the effect of the User's factory environment upon the machining accuracy. This could be over 4hrs, 8 hrs, 24 hrs, or a few days. Test duration to be chosen to meet customer's requirements. b) A test which involves running the spindle for four hours, followed by a one hour cool down period, to evaluate the effect of spindle bearing heating and motor heat upon the machine structure. c> A test involving continuous axis motion of the machine to evaluate the effects of axis motor heat, ballscrew heat, and guideway heat upon the stability of the table accuracy.

Considerable testing has been undertaken on these new test proposals, following detailed suggestions from Dr Beltrami, and debate among the partners to the project.

5.1 Environmental tests

These have been conducted in the University of Northumbria, at KTW in Austria, in the Technical University of Graz and at AB Manvin. The machine is 'powered up' but all axes are stationary and the spindle is stopped during the test.


They, in general, show surprisingly large "Drift" of the machine datum positions over time, with severe effects arising:

From opening doors to bring goods into a factory From windows being opened From factory heaters going on and off

User companies are largely ignorant of these errors and need considerable education to make them aware that they will N z a c h i e v e the accuracies quoted for their machines unless they provide a stable environment. The supplier should issue guidelines for acceptable conditions to obtain the best accuracy from a machine. It is then the users responsibility to provide these conditions. It is very difficult to provide a temperature controlled environment but major improvements can be made by avoiding direct draughts onto machines from doorways, windows or heaters.

5.2 Spindle running tests

The recently issued standard (March 2001) suggests two possible test procedures:-

1) Spindle run at a % of maximum speed continuously.

i i) Spindle run through a repeating spectrum of spindle speeds.

Detailed tests of both suggestions have been conducted, and obviously higher speeds generate more heat, and thus more distortion. Dr Beltrami has suggested that the test procedure is modified to allow the use of contact probe systems (eg dial gauges) to measure the spindle mandrel positions[31. This will involve stopping the machine every few minutes to move it into contact with the probes, and it is felt to be a more representative test procedure because it includes repeated stopping and starting of the spindle. I S 0 230-3 allows for a simple test with dial gauges, and also includes figures showing the set up.

It is thus recommended that the I S 0 230-3 test should adopt 75% of the maximum spindle speed with a one minute pause every ten minutes to allow for contact probes to be used. It is considered that the provision of a tw-o possible spindle speed options does not provide a simple and clear test, and will promote confusion which will delay adoption of this test.

5.3 Axis motion tests

The current test requires the machine to be moved from end to end of its axis motion for four hours, stopping while the position of each end of the axis motion


is measured in the positive and negative directions every five minutes, using a laser or similar equipment. Tests have been conducted exactly as suggested, but several suggestions for inlprovements have been made, particularly by Dr Be1trami:-

Measuring the axis reversal at each end of the motion is irrelevant - reversal is measured in 230-2 in a clearly defined procedure

Companies do not want to spend up to a day testing each axis, so Dr Beltrami has suggested an accelerated test procedure taking one hour, or two hours to conduct.

Tests were conducted on machining centres at UNN to initial 230-3 suggestions, ie traverse rnachme continuously along axis at a high feedrate then measure results once every 5 minutes, measuring positive and negative directions of motion at each end of the table.

6 I S 0 230-4 Circularity testing

Circularity tests have been used for many years now, and can provide a great deal of information about the condition of a machine in a very short time. The equipment is readily available from several suppliers, and there are now several suggestions about possible further applications of the tests:

They could be used to provide satisfactory squareness values when aligning large machines['] 0 Circular accuracy will not be the same across the whole area of the table, and customers should be aware of this, tests should thus be conducted at two or three positions along the major axis

Recommended test speeds are low, and higher speeds could be used to save time

6.1 Points to be Agreed

It was necessary to define precisely how tests were performed, to ensure that comparable tests were reformed across the project countries.

a) Warm up procedure prior to testing the machine

b) Test parameters

~ r a z University performed a series of tests with and without warm up procedures. The addition of a warm up procedure did not significantly affect the results.

a) Diameter of Test ball bart?- 150mm normal, tests of up to 600mm radius have been successfully undertaken b) Feedrate? 270.680 , 100. 3000, and 5000 have been


used. c) Contouring Direction- both directions used in all tests.

d) Machine axes used?- usually X/Y, but XIZ and YIZ also done.

e) Location on table8?- Normally tests are done in the centre of the table, but extensive tests have been done at various positions across machine tables. f ) Temperature- not considered g) Data capture method- as per Renishaw or Heidenhain equipment h) Machine compensation used?- ALL normal compensation methods must be used during the test.

7 D1S:ISO 230-6 Diagonal displacement te~ts[''~'

This test is a development from the ANSIIASME B5.54 standard which calls for four body diagonals to be measured at approximately the same linear spacing intervals as conventional positioning tests. It also recommends that the test programme is finalised to provide even steps between the two end point positions that the laser optics can actually measure on a particular machine. This recognises that it is not usually possible to measure the full length of the "theoretical" space diagonal

There can easily be some confusion about which diagonal is being measured, and Dr Beltrarni has suggested that a "PYP" naming system is adopted for each diagonal. This is explained in the table:-

/ Positive X direction / Represented by / P Negative Y direction and / Represerlted by / N Positive Z direction / Re~resented hv / P

We agree with this simple test description and recommend its adoption.

7.1 Case Study 16'

A considerable amount of testing to this draft standard was achieved in Austria, Italy and the UK. It is worth reporting the results of one significant example, the testing of a large machining centre (1.3M cube) which had been installed in the UK to produce aerospace components. This machine had been tested to B5.54 before being delivered from the USA to London, and the aerospace customer required that it was re-tested on installation to confirm its accuracy.

The detailed report on this test shows that there was a close correlation between the diagonal tests performed in the USA and the same tests conducted in London. The mean Linear Displacement Accuracy (ANSI definition of accuracy) deviation for the diagonals in the USA was .025mm and in London it was


Laser 11etroiog and \liiciirr~e Per.foim7nce 243

.0275mrn ie an average difference of ,0025nlm. Individual diagonals did show more differences, see below:

I From I To I LDA I LDA I

The major change from B5.54 is the inclusion of face diagonals in the I S 0 230-6 test procedure and the opportunity was taken to measure four face diagonals on this machine. Ball bar tests were also taken to provide a comparison with the squareness figures calculated from the face diagonals:-

Diagonal 1 Diagonal 2 Diagonal 3 Diagonal 4

7.2 Squareness Results

+X+Y-Z -X+Y-Z -X+Y+Z XX+Y+Z

There is a very close correlation between the two methods of squareness testing, especially considering that the ball bar measured over a diameter of 300mm while the diagonals were over l m in length.

Method Ball Bar Test Face Diagonals

8. Conclusions

-X-Y+Z +X-Y+Z +X-Y+Z -X-Y-Z

Suggestions have been submitted to the I S 0 Secretariat to update the definition of squareness to reflect the use of lasers etc. Dynamic straightness and angular tests have been satisfactorily achieved and suggestions for their adoption submitted to the I S 0 procedures. Suggested warm-up procedures for 'normal' sized ( IS0 10791) machining centres have been submitted for consideration. Satisfactory warm-up procedures for large machines have been found not to be practical and a suggestion for routine statistical analysis to demonstrate lack of thermal drift of the normal positioning test has been submitted. A range of environmental tests have been conducted, which demonstrate the large size variations that occur on machine tools due to changes in the workshop environment.

XY Squareness .015mm/m .0 1 6 d m

USA ,024 .039 ,024 ,012

XZ Squareness .002mm/m .003mm/m

UK .03 1 ,021 ,015 ,043


A standardised spindle warm-up procedure has been proposed after extensive evaluation of various test procedures. A suggested two hour axis traverse warm-up procedure has been suggested to test axis thermal drift. Extensive ball bar testing has been undertaken to confirm use of ball bar for assessing squareness measurement, particularly using ball bars with up to 600mm radius on large machines. Body diagonals have been confirmed as a valuable method to assess overall machine accuracy. Face diagonals have been confirmed as demonstrating accurate squareness measurements. A system for clearly identifying diagonal measurements has been proposed.

9. References

[ l ] I S 0 230 parts 1-4 series of standards [2] Squires B. Circular Tests on Large Machines, Technical University of

Graz Seminar Nov 2000.

[3] Beltrami 0. A deeper investigation into thermal drift of a linear axis, Lamdamap 1999.

[4] I S 0 230-6 Draft Standard [ 5 ] Knapp W. Interim checks for machine tools, Lamdamap 1997. [6] Morris T. Diagonal Displacement Tests, Technical University of Graz

Seminar Nov 2000.


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review of proposals for amendments to the IS0 230 … · A review of proposals for amendments to the ... the working day, ... morning when the shop temperature was 10°C