Multi degree of freedom measurement of machine tool movements. W. Knapp, S.Weikert Institute for Machine Tools and Manufacturing (IWF), Swiss Federal

Transcription

1 Mult degree of freedom measurement of machne tool movements W. Knapp, S.Wekert Insttute for Machne Tools and Manufacturng (IWF), Swss Federal \ ^r Az. ca, we z'tz z w/: Agpr. graz. ca Abstract The standard measurement of a machne tool axs s realsed wth one set-up per one or two geometrcal devatons. The contourng performance s generally looked at n one coordnate plane of the machne. The measurng equpment ntroduced, the KGM-f- LONG, allows the measurement of up to sx degrees of freedom of a machne tool movement. It s based on the grd encoder of Hedenhan. The frst tests are made to look at the contourng performance of machne tools at hgher feed rates. The results when movng on a step n the XY plane are shown. 1 Introducton The measurement of geometrc devatons of a machne tool axs s carred out accordng to standards [1] and n practce wth one set-up per geometrc devaton. Only a few measurement devces offer the measurement of two geometrcal quanttes n one set-up. Mostly ths s for testng two straghtness devatons or ptch and yaw together. Latest draft standards on thermal dstorton due to runnng spndles suggest the smultaneous measurement of fve degrees of freedom [2]. Mult-degree of freedom measurements for testng lnear axes were suggested wth a straghtedge as the basc equpment [3], respectvely wth a laser beam as the measurement bass [4]. The ntended use of these systems s a statc

2 Laser Metrology and. Machne Performance 436 measurement,.e. the read-out of the sgnals s made as soon as the machne axs has stopped. The new system KGM+ LONG can be used n statc and dynamc mode. It s also applcable to test the contourng performance, e.g. accordng to ISO [5]. 2 The system KGM+ LONG The system KGM [6] s a two coordnate graduaton measurng system. It conssts of a two dmensonal grd encoder, n other words a two dmensonal lnear scale, and an optcal head for the detecton of the poston n two orthogonal coordnates. Gl, G2, G3: optcal heads for measurement n XY plane Tl, T2, T3: tactle probes for measurements n Z drecton A, B, C, X, Y, Z: components of moton accordng to ISO [1] TCP: tool centre pont Fgure 1: Confguraton of the KGM+ LONG used for smultaneous measurement of relatve moton n sx degrees of freedom. The system KGM+ LONG [7] s based on the grd encoder of Hedenhan. Wth three optcal readngs, Gl to G3 (see Fgure 1), the X, Y and C moton as the relatve moton between the tool sde and the workpece sde of the machne tool

3 Laser Metrology and Machne Performance 437 can be measured. Three tactle probes, Tl to T3 (see Fgure 1), measure the movement n Z drecton and gve the nformaton on the relatve Z, A and B motons. The optcal readngs are taken at a long grd plate, the tactle probes measure aganst a straghtedge. The resoluton for the measurements n the XY plane s down to 4 nm, the resoluton for the tactle measurements n Z s down to 20 nm. The accuracy of the measurements n XY s gven by ±1 jm, for the measurement n Z by ±2 p,m. These values are wthout any compensaton for the read-out. The resoluton and accuracy for the A, B and C evaluatons depend on the relatve dstance between the optcal heads, respectvely between the tactle probes. Z- components [jam] "j- - > Y-drecton [mm] 0 0 X-drecton [mm] Total form error: 12 m. Fgure 2: Form devaton of a 2 dmensonal ncremental grd plate used as a master. For the frst prncpal test of the system a dfferent set-up s used: Instead of a long grd plate a crcular grd plate of a dameter of 120 mm s used. The tactle probes are contactng the grd plate,.e. the grd plate s taken as plane master. Ths, of course, was checked, because for the standard use of the grd plate, the devaton from a plane s of no mportance. The grd plate used shows a

4 438 Laser Metrology and Machne Performance devaton from a plane of 12 m (see Fgure 2). For the evaluaton of geometrc devatons ths form devaton has to be compensated, furthermore the plate needs a reference poston n X and Y for correct compensaton. The maxmum feed rate for measurements wth ths system s 24 m/mn. A set of sx coordnates s read wth an frequency up to 5000 Hz. For the nterpretaton of the dynamc behavour of the machne tool just the dfferences between a low and a hgh feed rate are evaluated. For testng the dynamc performance of a machne tool the nfluence of the measurng equpment has to be known. An expermental modal analyss of the fxture of the optcal heads and the tactle probes showed afrstmode shape at 118 Hz (see Fgure 3). Ths mode arses at a maxmum extenson of the shaft for mountng thefxturen the blocked machne spndle. Thefrstmode s a bendng mode, manly n Y drecton. deformed structure^ un deformed structure deformed structure undeformed structure Frst mode: 118 Hz Shaft extenson: maxmum Fgure 3: Expermental modal analyss results for fxture of optcal heads and tactle probes of KGM+ system.

5 Laser Metrology and Machne Performance Expermental results Frst experments wth the modfed KGM+ LONG are carred out on a vertcal machnng centre, where all movements are on the sde of the tool. The prncpal desgn of the machne tool under test s shown n Fgure 4. The nomnal path s a step n the XY plane, wth each of the straght lnes beng 20 mm long. Nomnal path: step n XY plane, each straght lne s 20 mm long Fgure 4: Confguraton and desgnaton of axes of the machne tool under nvestgaton.

6 440 Laser Metrology and Machne Performance The nomnal feed rate s m/mn, the measured velocty s gven n Fgure 5. The horzontal plane n Fgure 5 shows the nomnal path, the vertcal axs shows the velocty n functon of the programmed contour. Acceleratons and deceleratons at the start and end ponts of the straght lnes are obvous. 1 0) 5 "U <D 0) -30 Y-Axs [mm] X-Axs [mm] Programmed feed rate: m/mn Fgure 5: Measured velocty along the programmed path. Fgure 6 shows the contourng performance n the XY plane. The upper part of Fgure 6 gves the straghtness devaton from thefrststraght lne, the 20 mm parallel to Y. The mddle part s the straghtness devaton from the contour parallel to X, and the lower part s the straghtness devaton from the last 20 mm of the contour, whch are agan parallel to Y. The devatons are up to ±30 Jm. If the measurements are taken n sngle ponts after the machne has stopped, the postonng accuracy along the contour, here n X and Y drecton, could be evaluated. The nterpretaton of Fgure 6 s dffcult, because the devatons n X and Y do not clearly reveal the source(s) of the devatons. Therefore the devatons n Z, A and B drecton are evaluated.

7 Laser Metrology and Machne Performance 441 X X.22 X X 'c X t st rt j^""'" ^~ " 1 l H t, ; ; T T r measured trajectory, 4. / 4- J- l... J ; ; \ ^t ^ L \ t - r programmed path" J~ *- r r r - 1! Y-Axs [mm] T T 1 ~ P\ + FMWrmmoH octh s~s~*~~~~~\, \ \_^ x[ \ 1 /s 1 \ \ /S \ T \ T / 1 \ [ + \^^ v; \ _1 V'. I T fneastred-trajeetory X-Axs [mm] l l l T r r _[ measured Jraectory [_ /\ // // \ /^ ^s^ r / \^----\-\----\-~^^-\-/-----\ prpgfammet-pmrt,» t Y-Axs [mm] end Nomnal path: step n XY plane, each straght lne 20 mm Programmed feed rate: m/mn Relatve devaton to feed rate of 0,1 m/mn Maxmum straghtness devaton wthn ±30 jm long Fgure 6: Path devatons n XY plane of vertcal machnng centre.

8 442 Laser Metrology and Machne Performance Z-devatons [jam] Y-Axs [mm] X-Axs [mm] Nomnal path: step n XY plane, each straght lne 20 mm Programmed feed rate: m/mn Relatve devaton to feed rate of 0,1 m/mn Maxmum devatons wthn Jm long Fgure 7: Devatons n Z drecton of vertcal machnng centre. Fgure 7 shows the devatons square to the nomnal path of the machne,.e. n Z drecton. The largest devatons, n the magntude of 40 Jm, occur on the straght lne parallel to the X axs, durng acceleraton and deceleraton. Ths mght be caused by a B rotaton of the movng arm. The two straght lnes parallel to Y show a sgnfcant dfference n the Z devatons: the second lne, whch has a larger offset to the column (Z axs of machne tool) shows are larger Z devaton. The larger offset,.e. the dfferent X poston, ncreases the effect of a B rotaton when movng parallel to Y.

9 Laser Metrology and Machne Performance 443 B-devatons [jm/m] 0 50, " ^ ^ j - " "jl :art ^ r " ehl Y-Axs [mm] X-Axs [mm] Nomnal path: step n XY plane, each straght lne 20 mm Programmed feed rate: m/mn Relatve devaton to feed rate of 0,1 m/mn Maxmum devaton wthn ±130 pm/m (±26 arcsec) long Fgure 8: Path devatons n B drecton (rotaton) of vertcal machnng centre. Fgure 8 shows the B rotaton when movng on the nomnal path. The B rotatons on the two straght lnes parallel to Y show smlar behavour of the arm of the machne tool. The dfferent offset n X gves the dfferent effect n Z drecton, as assumed when dscussng Fgure 7. The large B rotaton when movng parallel to X are caused by the acceleraton forces n X. The Y gudeway s desgned wth a small extenson n X and mght be the weakest pont n respect to prevent a B rotaton. The vbratons of B do not show up n the devatons of Z. Therefore t can be concluded, that these vbratons are around an axs that s near the tool centre pont, e.g. the spndle n ts bearng. In Fgure 9 the A rotaton s drawn as a functon of the nomnal path. The acceleratons n the straght lnes parallel to Y cause large A motons of up to

10 444 Laser Metrology and Machne Performance 320 (Jn/m (64 arcsec). Ths does not show up n the Z devatons, obvous the rotaton s around the tool centre pont, whch corresponds to the symmetry of the machne tool under test CO c g 0 *4-«<c -0 o "U Y-Axs [mm] X-Axs [mm] Nomnal path: step n XY plane, each straght lne 20 mm Programmed feed rate: m/mn Relatve devaton to feed rate of 0,1 m/mn Maxmum devaton wthn ±320 (m/m (±64 arcsec) long Fgure 9: Path devatons n A drecton (rotaton) of vertcal machnng centre. 4 Outlook The system KGM+ allows the smultaneous measurement of the sx degrees of freedom n statc and dynamc mode. A frst verson of the system shows the applcablty n practcal tests of machne tools. The next steps are the realsaton of the KGM+ LONG for testng lnear axes, the measurement of dynamc parameters of the drves and the NC control, e.g. reversal test, step-response-test [8], and the expermental verfcaton of computer smulatons related to path devatons at hgh feed rates [9].

11 Laser Metrology and Machne Performance 445 Acknowledgements Ths project s supported by the Swss Commsson for the Promoton of Appled Research (KTI project ) wth one of the man partners beng Hedenhan (CH) AG, Schwerzenbach. References 1. ISO 230-1:1996, Test code for machne tools - Part 1: Geometrc accuracy of machnes operatng under no-load or fnshng condtons. 2. ISO/DIS 230-3, Test code for machne tools - Part 3: Evaluaton of thermal effects, clause 5 Thermal dstortons caused by rotatng spndle. 3. Treb, T., 1987, Error budgetng, Annals of the CIRP, 36/1: A T-Tortrr %) Q T \^ 1QOA T ao*»f m<aoc''f<=»rr»»r»t' nof~rrr»p»r»f fr\r fact" I. A JLUU.AA 2, A.»_,., '*, JL., ^^NJ, " J «C*UXX.». A A AWkXO UU V^A A AX^A A V ' JLt.AOl~l.Ul.AAWl.lt. ~. W A, 1 V*-._» V calbraton of machne tools, Amercan Socety for Precson Engneerng, Proceedngs Volume 14: ISO 230-4:1996, Test code for machne tools - Part 4: Crcular tests for numercally controlled machne tools. 6. Measurng systems for machne tool nspecton and acceptance testng, February 1997, Dr. J. Hedenhan GmbH, Traunreut. 7. Braasch, J., Kalmc, C, Wekert, S., 1997, Vorrchtung zur Erfassung der Poston von zwe Korpern, Schwezersche Patentanmeldung Nr. 2279/ Haas, R, 1994, We genau arbetet Ihre CNC-Maschne? De Dynamk m Brennpunkt, Technk Report 9A, September: Rehstener, F., Wekert, S., Rak, Z., 1998, Accuracy optmzaton of machne tools under acceleraton loads for the demands of hgh-speedmachnng, Proceedngs of the ASPE Annual Meetng, St. Lous, Ms., Oct, 25-30,