Nuclear Instruments and Methods in Physics Research A 667 (2012) 5–10

Contents lists available at SciVerse ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/nima

On-machine non-contact dimension-measurement system with laserdisplacement sensor for vane-tip machining of RFQs

Y. Kondo a,n, K. Hasegawa a, H. Kawamata b, T. Morishita a, F. Naito b

a J-PARC Center, JAEA, Japan Atomic Energy Agency, 2-4 Shirakata-Shirane, Naka, Ibaraki 319-1195, Japanb KEK, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan

a r t i c l e i n f o

Article history:

Received 29 August 2011

Received in revised form

17 October 2011

Accepted 22 November 2011Available online 1 December 2011

Keywords:

Proton accelerator

RFQ linac

Machining

Vane-tip profile

On-machine measurement

02/$ - see front matter & 2011 Elsevier B.V. A

016/j.nima.2011.11.065

esponding author. Tel.: þ81 29 284 3133; faxail address: yasuhiro.kondo@j-parc.jp (Y. Kon

a b s t r a c t

We have developed a new on-machine non-contact dimension-measurement system using a laser

displacement sensor (LDS) for machining the vane tips of RFQs. The LDS was attached to a milling

machine, and the dimensions of a test piece (TP) with a modulated vane tip were measured to evaluate

the performance of the system. A longitudinal profile was measured with the LDS and it was compared

with the profile measured with a coordinate measuring machine (CMM). A transverse profile was also

measured. The required accuracy of dimension measurement was achieved using our system.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

For the fabrication of radio-frequency quadrupole (RFQ) cavities, itis very important to machine the vane tips accurately. Fig. 1 shows asemi-assembled (before assembling the upper vane) RFQ cavity; thevane tips with modulation are clearly seen in this figure. The accuracyof the dimensions and geometries of the vane tips are very importantto realize the design performance of the RFQ. To ensure the machin-ing accuracy after the final machining operation, the dimensionsshould be measured on a computer-numerical-control (CNC) millingmachine or measured using a three-dimensional coordinate-measur-ing machine (CMM). However, the vanes of RFQs in modern protonaccelerators are typically made of annealed oxygen-free copper (OFC),which is a very soft metal. Therefore, contact-type probes causeindentations at the measurement points, as shown in Fig. 2. Thediameter of the indentation is 0.3 mm. The surface electric field of theRFQ vane tip is high (typically 1.8 Kilpatrick); therefore, such damageto the vane tips should be avoided. Even if the damage by the contact-type probe is regarded as acceptable, it is difficult to performmeasurement at sufficient number of points to confirm the modula-tion profiles within a realistic time frame. The measurementof modulation profiles using a CMM with a contact-type probe isalso time-consuming. Thus, a non-contact-type measuring device is

ll rights reserved.

: þ81 29 284 3719.do).

desired that can perform measurement at many points in a shortperiod of time.

To this end, we have developed a new method for measuringvane-tip profiles on the CNC milling machine. In this on-machinenon-contact measurement system, a laser displacement sensor (LDS)is used to measure the vane-tip profiles, thus eliminating the need forany contact-type probes. It is difficult to measure the vane-tip profileaccurately using optical devices because the vane tip has a complexshape and a glossy metallic surface. However, an LDS can be used forthis type of measurement. We have developed this system for thefabrication of an RFQ to be used in the beam-current upgrade of theJ-PARC (Japan proton accelerator research complex) [1] linac. Similarto many modern proton linacs, the J-PARC linac uses a four-vane RFQ.The resonant frequency of the J-PARC RFQ is 324 MHz, and itaccelerates H� particles from 50 keV to 3 MeV.

The machining accuracy of the vane tips of the J-PARC RFQ isrequired to be within 720 mm. According to the SUPERFISH [2]calculation, the frequency shift corresponding to a displacementof 20 mm of one vane tip is 0.2 MHz. This accuracy requirementis typical for � 300-MHz RFQs; however, careful machining isnecessary to achieve this accuracy. During the final machiningoperation of RFQ II for the J-PARC linac [3–5], we measured thedimensions of the vane tips on the milling machine after each cutexcept for the final cut. The measured values are fed back to offsetthe final cut depths and achieve the required machining accuracy.The dimensions were not measured on the milling machine afterthe final cut to avoid the damage described above. Details of thison-machine measurement are given in Section 2.

www.elsevier.com/locate/nimawww.elsevier.com/locate/nimadx.doi.org/10.1016/j.nima.2011.11.065mailto:yasuhiro.kondo@j-parc.jpdx.doi.org/10.1016/j.nima.2011.11.065

Fig. 1. Photograph of a semi-assembled RFQ cavity.

Fig. 2. Photograph of the indentation caused by the contact-type probe on theannealed OFC TP. The diameter of the indentation is 0.3 mm. The scale-like

patterns are marks made by the ball nose cutter.

Y. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 667 (2012) 5–106

The target of our new on-machine non-contact measurementsystem is to measure the ridge profiles of vane tips with sufficientaccuracy; i.e., the mean value of deviation from a profile mea-sured with highly accurate devices such as the CMM1 should bewithin 710 mm and the dispersion should be within 720 mm.Because the parameters of the J-PARC RFQs are typical for� 300-MHz RFQs, our system is applicable to dimension mea-surement of the vane tips of many RFQs in this frequency range.

1 Accuracy of CMMs is typically within a few mm.

In Section 2, the procedure of final machining of the vane tips ofRFQ II is briefly summarized. The details of the on-machine non-contact measurement system are presented in Section 3. Theresults of the test measurement are discussed in Section 4. Finally,in Section 5, a summary is presented.

2. Final machining of vane tips of J-PARC RFQ II

In this section, the procedure of final machining of the RFQ IIvane tips is described, because it is necessary to know the procedureto understand the need for the on-machine measurement system.

The RFQ II cavity consists of three longitudinally dividedunits, called units 1, 2, and 3. The vane lengths of each unit are1057.2 mm, 1053.6 mm, and 1061.3 mm. Each unit consists of twomajor vanes (upper and lower parts) and two minor vanes (leftand right parts), and the four vanes are brazed together. Ball nosecutters were used for machining the vane tips of RFQ II [3,4].

The final machining process of the RFQ II vane tips was carriedout as follows:

On-machine measurement before final machining (residualdimension: 0.2 mm)- first cut (residual dimension: 0.14 mm) - on-machinemeasurement

Fig. 3. Photograph of the vane-tip machining (upper figure) and the on-machinemeasurement with the contact-type probe (lower figure).

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Fig. 4. Target and measured residual dimensions of the vane-tip in the finalmachining process. The horizontal axes represent the number of cuts, and the

vertical axes denote the differences between the target or measured dimensions

and the design ones. The vane-tip dimensions were measured on the machine

immediately after each cut, and the target values of the final cuts were

compensated for by the measured dimensions. The dashed lines represent the

expected values of the final dimensions.

Fig. 5. Schematic drawing of the LDS measurement system.

Fig. 6. Photograph of the measurement setup.

Y. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 667 (2012) 5–10 7

- second cut (residual dimension: 0.08 mm) - on-machinemeasurement- final cut - no measurement

where residual dimension indicates the difference between thedimension after each cut and the design dimension. Fig. 3 showsphotographs of the vane-tip machining (upper figure) and on-machine measurement (lower figure) processes. To compensate forthe expansion of the spindle of the milling machine, the dimensionsof the vane tips were measured on the machine. After each cut wasfinished, the tool of the milling machine was changed from the ballnose cutter to a contact-type probe. The probe was moved to themeasurement point and touch the vane tip, as shown the lower partof Fig. 3. The dimensions can be derived from the position of the mainspindle. The accuracy of this measurement is within 10 mm. Theresults were fed back to adjust the depths of the final cuts. The on-machine measurement of RFQ II was performed at the followingpoints: For unit 1, two points on the radial-matching section, fourpoints at the top, and four points at the bottom of the modulationwere sampled. For units 2 and 3, ten points at the top of themodulation were sampled. One residual dimension was derived foreach vane by averaging the measured residual dimensions of thesampled points. The dimensions were manually read out and it tookabout 5 min to measure ten points. More points should be measured

to confirm the overall shape of the vane, for example, to determinewhether it is warped or not. However, an increase in the number ofmeasurement points increases the measurement time. The operationof the machine cannot be stopped for the long duration of a finalmachining process, because as the machine cools down, the expandedspindle shrinks back to its original dimensions, thus rendering thecompensation ineffectual. Acceptable interruption is about 1 h andtypical duration of interruption during a procedure of the finalmachining operation of RFQ II was less than 30 min.

Fig. 4 shows the target and the measured residual dimensionswith respect to the design dimensions. The target residual dimensionsdenote the offset value inputed to the milling machine, and themachine cuts the surface according to this residual dimension. At thefinal cuts, the target residual dimensions are compensated for bythe measured dimensions. The dashed lines in the figures representthe expected final dimensions. However, the final dimensions werenot measured on the machine to prevent damage to the vane-tipsurface due to the contact-type probe.

3. Apparatus for on-machine non-contactmeasurement system

In the previous section, the procedure of final machining ofthe RFQ II vane tips has been described. We will now describe theon-machine non-contact measurement system.

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Fig. 7. Linearity of the LDS measured at each measurement point on the TP. The ‘‘Top 1’’, ‘‘Slope 1’’, etc., correspond to the positions indicated in Fig. 5. The open squaresare the deviations between the displacements measured with the LDS and the movements of the LDS inputed to the CNC machine. The hatched areas represent the

required ranges for the measurement of the RFQ II vane-tip dimensions. The ‘‘Flat’’ shows the result of linearity measurement on the flat surface on a OFC block as

reference.

Y. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 667 (2012) 5–108

A laser displacement sensor KEYENCE LK-G50002 is used forthis system. The sensor head is LK-H022, and the controller isLK-H500V. This sensor head uses a red laser diode with awavelength of 650 nm and a power of 0.95 mW. The spot size is25 mm on the object to be measured, and the laser spot on theobject is viewed with a CMOS sensor. If the distance from the laserdiode to the object is varied, the position of the image of the laserspot on the CMOS sensor is also varied and this displacement canbe translated into distance. This sensor head was used because ithas a measurable range of 2073 mm, and with this range, we canmeasure the dimensions of the vane tips of many � 300-MHzRFQs. For example, the minimum bore radius of RFQ II is 2.11 mmand the maximum bore radius is 4.85 mm. Therefore, the width tobe measured is 2.74 mm (71.37 mm), and this range is muchnarrower than the measurable range of LK-H022. The maximumsampling frequency of LK-H022 is 392 kHz, but in the followingmeasurement, a default setting of 5 kHz was used. The nominalvalue of the linearity is 70.02% of the full range (6 mm), i.e.,71:2 mm, and the nominal value of the repeatability is 0:02 mm.Both values are obtained by measuring white diffuse-reflectivework pieces.

2 KEYENCE Co., 1-3-14, Higashi-nakajima, Higashi-Yodogawa, Osaka 533-8555,

Japan, http://www.keyence.com.

Fig. 5 shows a schematic drawing of the measurement setup.LK-H022 was attached to a holder and installed in the tool chuckof the main spindle of a machining-center FNC-1063 (MakinoMilling Machine Co. Ltd.4). Fig. 6 shows a photograph of themeasurement setup. A TP made of OFC shown in this figure wasmeasured to test the system. The length of the TP was 500 mm.

4. Results of test measurement

4.1. Linearity of LDS

To evaluate the basic performance of our on-machine mea-surement system, test measurement was performed. First, thelinearity of the LDS was confirmed. In the following measure-ment, the longitudinal direction of the TP was defined as thez-axis, the vertical direction was y-axis, and the horizontaldirection was x-axis, as shown in Fig. 5. For the linearitymeasurement, the x- and z-positions were fixed at each measur-ing point and the y-position of the LDS was scanned within themeasurable range of the LDS. Then, the distance of motion of the

3 This machining center was used for the test measurement; RFQ II was not

machined with this machining center.4 Makino Milling Machine Co. Ltd., 3–19 Nakane 2-chome, Meguro-ku, Tokyo

152-8578, Japan

http://www.keyence.com

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Fig. 8. (a) Comparison between the LDS and CMM measurement results of thetransverse profile of the modulation of the TP. The open circles are the dimensions

measured with the LDS, and the open squares represent those measured with the

CMM (left axis). The closed circles indicate the deviations between the LDS and

CMM measurement (right axis). (b) Histogram of the deviations. The mean value

of the deviation is 6 mm.

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Fig. 9. Measured transverse profiles of the vane tip. The open circles are thedimensions measured with the LDS and the open squares are those measured with

the CMM (left axis). The solid circles are the differences between the LDS and

CMM measurement (right axis). The deviations are within 715 mm in the regionof �2 mm to 2 mm from the ridge of the vane.

Y. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 667 (2012) 5–10 9

LDS inputed to the CNC machine and the displacement measuredwith the LDS were compared. The measured points were twopeaks, two centers of the slopes, and one bottom of the modula-tion, they are denoted in Fig. 5 as ‘‘Top 1’’ to ‘‘Top 2’’.

Fig. 7 shows the results of the linearity measurement. Thehorizontal axis of each figure is the y-position of the LDS, and thevertical axes are the deviations. The deviation is calculated bysubtracting the movement of the LDS inputed to the CNC machinefrom the measured displacement obtained by the LDS. The scanswere started from the largest y-values, and therefore, the devia-tions are zero at the maximum y-values. The ‘‘Flat’’ indicates theresult of linearity measurement on the flat surface of an OFC blockmeasured as reference. The hatched areas represent the requiredranges of the vane tips of RFQ II to be measured. In these ranges,the deviations were within 710 mm at ‘‘Slope 1’’, ‘‘Slope 2’’, and‘‘Bottom’’; this is sufficiently high to achieve the target measure-ment accuracy. However, the deviations were from �35 mm toþ0 mm at ‘‘Top 1’’ and ‘‘Top 2’’. The measured linearities wereworse than the nominal value because the vane tip has a complexshape and metallic luster. Especially, the linearity was not good atthe top of the modulation; this is because the vane behaves as aconvex lens at the top of the modulation, that is, if the axis of thelaser is not exactly perpendicular to the vane, the position of theimage of the laser spot on the CMOS sensor changes drasticallyowing to a small change of the displacement. Precise tuning of thelaser axis is difficult, and therefore, the performance of the systemwas evaluated including this imperfection.

4.2. Longitudinal profile measurement

Next, a longitudinal profile of the two periods of modulationwas measured with the LDS, and the measured profile wascompared with that measured with the CMM. In this measure-ment, the x- and y-positions of the LDS were fixed on the ridge ofthe modulation and the z-position was varied with a 1-mm pitch.The pitch of the CMM measurement was also set as 1 mm. Theorigin of the y-axis was set at the bottom surface of the TP. InFig. 8a, the profiles measured with the LDS and CMM are shown.The open circles are the dimensions measured with the LDS, andthe open squares are those measured with the CMM; they appearalmost overlapped. The solid circles represent the deviations,which were calculated by subtracting the dimensions measuredwith the CMM from those measured with the LDS. Fig. 8b shows ahistogram of the deviations. The mean value of the deviations is6 mm. If the values measured at the top of the modulation aretruncated, where the linearity was found to be insufficient asdiscussed in Section 4.1, the dispersion of the deviations from themean value is within 720 mm.

4.3. Transverse profile measurement

A transverse profile of the vane tip was also measured. Fig. 9shows transverse profiles measured with the LDS and the CMM.The open circles represent the dimensions measured with theLDS, and the open squares are those measured with the CMM. Thesolid circles represent the deviations between the LDS and CMMmeasurement. In the region from �2 mm to 2 mm from the ridgeof the vane, the deviations are within 715 mm. This accuracy issufficiently high to confirm the transverse profile of the vane.

4.4. Three-dimensional scanner

Fig. 10 shows the demonstration of an LDS as a three-dimen-sional scanner. The LDS was scanned in the longitudinal (z)direction, and the scans were performed along seven scanninglines. The measured range of x-direction was from 0.0 mm to

3.0 mm, and the pitch was 0.5 mm. The read-out rate of theLDS was set to 10 Hz; the data is processed at a rate of 5 kHz inthe LK-H500V but read out every 500 samples. The LDS wascontinuously moved along the z-direction. The number of points

Fig. 10. Demonstration of the LDS as a three-dimensional scanner. The LDS was scanned along the z-direction, and the scanning lines are distributed from x¼0.0 to 3.0 mmwith a pitch of 0.5 mm.

Y. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 667 (2012) 5–1010

along one scanning line was 1.33 points/mm, i.e., the movingspeed of the LDS along the z-axis was 7.5 mm/s. The data wereaveraged over 256 samples, and thus, the y-dimensions are thevalues averaged over the z range of 0.38 mm. The number ofpoints in one scanning line was 435 points. With this setting, thetime required for scanning one line was 43.5 s. If a 1-m vane ismeasured with a 1-mm pitch, the number of points is 1000 andthe data for one line can be taken in 100 s. The read-out rate canbe increased up to the sampling frequency. However, if themoving speed is increased, the position of the LDS should be keptstopping while reading the data. The setting of the LDS and themilling machine should be carefully tuned to achieve the bestperformance.

5. Summary

An on-machine non-contact dimension-measurement systemwith an LDS was developed to measure the dimensions of RFQvane tips. The longitudinal profile of a modulated test piece wasmeasured and compared with the profile measured with a CMM.The absolute value of the deviation was 6 mm, and the dispersionwas 720 mm; these values meet the requirement of this system.

A transverse profile was measured with an accuracy of 715 mmin the region of 72 mm from the ridge. The speed to measure thelongitudinal profile of one vane tip with a pitch of 1 mm is0.01 m/s. This speed and number of points are much higher thanthose of the contact on-machine measurement of J-PARC RFQ II;they were typically 0.003 m/s and 10 points/m, respectively.Improving the linkage between the data-acquisition system ofthe LDS and the motion of the milling machine remains an issue.We will use this system for the on-machine measurement of ournext RFQ to be used in the beam-current upgrade of theJ-PARC linac.

References

[1] T. Koseki, et al., Challenges and solutions for J-PARC commissioning and earlyoperation, in: Proceedings of IPAC’10, 2010, pp. 1304–1308.

[2] J.H. Billen, L.M. Young, POISSON SUPERFISH, Technical Report, LA-UR-96-1834,1996.

[3] T. Morishita, et al., Fabrication of the new RFQ for the J-PARC linac, in:Proceedings of IPAC’10, 2010, pp. 783–785.

[4] T. Morishita, et al., Vane machining by the ball-end-mill for the new RFQ in theJ-PARC linac, in: Proceedings of LINAC2010, 2010, pp. 518–520.

[5] T. Morishita, et al., Vacuum brazing of the new RFQ for the J-PARC linac, in:Proceedings of LINAC2010, 2010, pp. 521–523.

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On-machine non-contact dimension-measurement system with laser displacement sensor for vane-tip machining of RFQsIntroductionFinal machining of vane tips of J-PARC RFQ IIApparatus for on-machine non-contact measurement systemResults of test measurementLinearity of LDSLongitudinal profile measurementTransverse profile measurementThree-dimensional scanner

SummaryReferences

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