1956/57, No. 11 325

ULTRASONIC ;MACHINING

1. TECHNIQUE AND EQUIPMENT

.Ey E. A. NEPPIRAS *) and ~. D. FOSKETT *).

The technique of using high frequency mechanicai vibrations for machining brittle materialshas assumed considerable importance in recent years. In this technique, a resonant 'electro-mechanical transducer is used to generate vibrations, at an ultrasonic frequency, which aretransmittei to the drilling tool through a mechanical focussing device designed to provide asufficiently intense vibration at the tool face. The actual cutting agent is an abrasive pouulerdispersed in a liquid.

The growing interest in this ultrasonic machining technique has made it necessary to obtainan accurate assessment of the potentialities of the method. The first part of this article, printedbelow, deals with th~fundameruals of the technique and gives a description of some ultrasonicdrilling machines deueloped.at the Mullard Research Lobortuories, The second part of thearticle, to appear shortly, gives an, assessment of the technique in terms of cutting speeds,accuracy and surface finish.

IntroductionThe first, mention of the possibility of using high

frequency electromechanical transducers for machi-ning operations is contained in a paper by Woodand Loomis in 19271). They showed how, by usinga piezo-electric crystal vibrator with a focusingdevice consisting of a tapered glass tube filled withwater, holes could be pierced in a glass plate heldagainst the end of the taper. It was not until thewar years (1939-45), however, that the techniquewas put to a useful purpose, when it was appliedon a limited scale for cutting and drilling preciousstones. Later it was realized that the principlecould be extended and applied to other brittlematerials, particularly metals and sintered carbides,some of which were found to be readily machinedby this method. In 1948 a patent was published 2)followed by a number of short articles and noticesin the American and British press describing somepractical results which had been obtained 3-7).

The obvious advantage of a reciprocating tool isthat the unidirectional motion permits the cavityproduced in the workpiece to follow closely theshape of the tool, provided that the tool is restrictedto removing particles of material which are small

*) Mullard Research Laboratories, Salfords, Surrey, England.1) R. W. Wood and A. L. Loomis, Phil. Mag. (7) 4, 417-436,

1927.2) British Patent No. 602801, 194.8, to Industrial Research

Corporation, U.S.A.3) S. G. Kelley, Materials and Methods 34, No. 3, Sept. 1951,

pp. 92-94.4) G. H. DeGroat, American Machinist 96, Sept. 15, 1952,

pp. 141-144.5) R. G. Woold, Machinist 97, 1601, Sept. 26, 1953.6) E. A. Neppiras, A, high-frequency reciprocating drill,

J. sci. Instr. 30, 72-74, 1953.7) E. A. Neppiras, Machining by high-frequency vibration

techniques, Research (London) 8, 29-34, 1955.

,534.321.9 :62~.95

compared with its own dimensions. In practice, thiscondition can be fulfilled by restricting the motionof the tool so that the chipping occurs on a micro-scopic scale, at the same time making the cuttingprocess purely an abrasive one, using the tool itselfnot as a cutting- device but merely to hammerparticles of abrasive powder into the work. Thetool, aided by the abrasive, impresses its image intothe work and it is therefore feasible to make cutsof any required shape by giving the tool the appro-priate form. Since reciprocating machines are inthis respect more versatile instruments than rotarydevices, the field of application of this type ofdrill is very broad: new types of machining opera-tions are possible, many of which had previouslynot been attempted. Ultrasonic frequencies are moresuitable than sonic frequencies for reciprocatingdrills not only because operation is silent but alsofrom the point of view of cutting speeds (seebelow) ..

Fundamentals of the technique

In carrying out a machining operation using thehigh-frequency reciprocating drill, the tool is pressedinto contact with the workpiece using a light pres-sure superimposed on the alternating motion. Anabrasive suspension is fed between the tool and theworkpiece. The wearing of the workpiece can bequalitatively explained simply as the result ofchipping caused by the abrasive grains being. crushedor ground against the 'York surface by the actionof the vibrating tool, the process involving actualcontact of the tool .with the abrasive particles andthe work.

326 PHILIPS TECHNICAL REVIEW VOLUME 18

There, are basically two types of operation which,can be successfully carried out with reciprocatingmachines, depending on whether the stock removalis by frictional forces (as in lapping or sizing anexisting hole, where the abrasive is rubbed overthe surface), or by hammer blows (as in directpiercing or, slicing operations). Since in both casesthe removal of material is achieved essentially by achipping action, the technique is limited to corn-paratively brittle materials and cannot be usefullyapplied to very soft or merely tough substances.OJi the other hand, the tool, which is also subjectto wear by chipping,' is best formed from a tough(not brittle) metal in which the abrasive grains.

. embed themselves without chipping.The liquid medium holding the abrasive in suspen-

sion plays a three-fold role. It acts as a coolant forthe tool and workpiece, which would otherwiserapidly become very hot; by capillary \ action, itallows abrasive to flow to the work area and theworn material to escape; and it achieves !l> goodacoustic bond between the tool and abrasivé, allow-ing an efficient transfer of energy.Drilling is accompanied by violent cavitation of

the liquid between the tool and the work. Theaudible hiss of cavitation can generally be distin-guished above the noise of the actual grinding. Thecavitation occurs in-the form of streamers of bubblesoriginating from points on the tool and work.Experiments have shown that the general turbulenceproduced in the liquid by the motion of these bubblestreamers probably helps considerably in stirringup the abrasive mixture under the tool. In this way,by ensuring that broken abrasive is replaced byfresh material and at the same time removingabraded material from the work area, the cavitationaction results in an increase in cutting speed. In. fact, to a large extent, cutting rates are found tocorrelate with the observed cavitation intensity.The cavitation streamers actually consist ofmulti-

tudes of bubbles in a very violent state of agitation.'Bubbles of this sort have an erosive action on solidsurfaces and this effect almost certainly accountsfor some of the tool wear obtained. The work alsooften shows we~r markings, some of which appearto follow the pattern of the cavitation streamers.They appear to be channels cut into the materialmerely by the motion, of cavitation streamers,carrying abrasive grains with them. The streamersfollow fixed paths and the abrasive particles tendto cut comparatively deep furrows in these places.The wear produced in this way is not importantfrom the point of view of stock removal, but it does,of course, affect the quality of the surface finish

obtained and when this is an important conside-ration, these markings must either be removedafter formation or preve.nted from occurring. Thiscan be done by ensuring that the drilling operationsare carried out so rapidly that a stable streamerpattern is not allowed to form.

In the majority of the applications of this tech-nique . the three considerations 'of most practicalimportance 'are the cutting speeds, surface finishand machining accuracies obtainable. The mostimportant of these, the cutting speed, is to a largeextent dependent on the characteristics of thevil;>rator itself, e.g.' vibration amplitude and fre-quency, and on the static load between tool andwork. Before an efficient drilling instrument can,be designed, we must know how these factors arerelated'.

Experience shows thát cutting rates depend verymuch on the nature of the oscillatory motion of thetool. A series of drilling tests in glass showed thatunder many conditions of operation cutting ratesare approximately proportional to the square of theoscillatory amplitude. This is shown in fig. I, wherethe mean penetration rate is plotted against oscil-latory amplitude for each of four drills vibrating atdifferent frequencies, the experimental conditionsbeing otherwise identical in all cases.' However,experiments have also shown that, for operationswhich do not involve frictional forces (lapping)but only direct piercing of the material, there is apractical limit to the usable oscillatory amplitudes:little advantage is to be obtained in making thepeak-to-peak oscillatory amplitude of the tool verylarge compared with the grain dimensions of theabrasive 8). Moreover, whatever abrasive is used, ifthe impulse per blow is increased, a point is reachedwhere, because of spraying of the abrasive, it be-comes impossible to retain the abrasive paste underthe tool at sufficient concentration to allowanefficient cutting action to be obtained. Althoughthese restrictions apply only to direct piercingoperations, in order to produce a versatile machilletool capable of both pierci~g and lapping operationsa definite limit must be placed on the oscillatorymotion of the tool. '

Cutting rates increase somewhat less than pro-portionately with operating frequency, at ultra-sonic frequencies, for constant oscillatory amplitude

, '

8) A recent paper by D. Goetze ~J.Acoust. Soc. Amer. 28,1033-1037,1956) on cutting speeds in tool steel, shows anapproximately linear increase of cutting rates with ampli-tude. These results, however, refer to peak-to-peak am-plitudes up to 0.004", which are about twice the dimen-sions of the abrasives used. See also Part II of this article,to appear in the following issue of this Review.

1956/57, No. 11 ULTRASONIC MACHINING I 327.

The indications are that resonant piesomagneticl"]transducers (magnetostrictors) are likely to provethe most valuable of the available types for' thepresent application 11). Purely from the point ofview of power efficiency a low frequency is anadvantage, but there are also obvious advantagesin keeping the frequency in the ultrasonic range,from the point of view of inaudibility and to keepthe vibrator within reasonable limits, of 'size. It iswell known that piezomagnetic resonators are moreo

0~---;;o:-;:.o:;:;a:;:;a5~--o~.OO='----;::0.-;::OO:l::I:=5-----'0~.OO2inch easily constructed as efficient transducers at low- 5 ultrasonic frequencies than any other type of elec-

tromechanical transducer. Also, in an applicationof this sort, the transducers must be capable ofwithstanding long periods of use under ordinaryworkshop' conditions: piezomagnetic 'metallic mate-rials are robust ánd not easily damaged either byrough handling or by the rather high temperaturesto which they may he subjected in operation.Under specific pre-stressed conditions, certain re-cently-developed low-porosity piezomagnetic cer~-mics 12) (e.g, nickel-cobalt ferrites) and piesoelec-tric ceramics 13) (e.g. lead titanate-zirconates),though somewhat less robust, would also besuitable. Other types of electromechanical trans-ducer such as piezoelectric crystals are more fragileand temperature-sensitive.As the transducer, only longitudinally vibrating

piezomagnetic resonators need be considered here.The most economical form is a consolidated laminar

0.06 inchr-----r:::--:--::-~r_-----r---____,Tool section-1l'":;

.5;

~ om. I-----t-----I---r(Qjc,c:.!2:g'"cf O.D2I-----t---;;(--J!<

1)1307

Fig. 1. Cutting rate as a function of oscillatory amplitude ~(peak-to-peak) for shallow cuts in glass, at four operatingfrequencies and for constant static load.

and constant static load. In effect this means thatthe velocity of the tool is of less importance indetermining cutting rates than the oscillatoryamplitude. The curve shown in fig. 2 is typical ofmeasured results.

Cutting rates are strongly dependent on the staticload imposed between the tool and workpiece andon the tool-face area 9), and also on the size andnature of the abrasive particles.

These questions will be discussed in greaterdetail in Part Ir of this article. At present, weare interested in the light they throw on therequirements to be fulfilled by the drill vibrator.This consists essentially of two parts: a transducerand a velocity transformer.

0.04inch

0.03 .>:.>

/-: .

Ir!

ao 5

Q130B

la-f

IS

Fig. 2. Cutting rate as -a function of operating frequency f,at ultrasonic frequencies, for a peak-to-peak amplitude of0..00125" and constant static load.

D) See A. Nomoto, J. Acoust. Soc. Amer. 26, 1081-1082, 1954and also Part II of the present article.

The transducer

stack driven into resonanc.e by passing a current,at the appropriate frequency, through a coil woundround it, the transducer being biased magneticallyby the application of a suitable polarizing field:the optimum biasing point is chosen to give maxi-mum magnetomechanical coupling in the transducermaterial and corresponds to a flux density about2f3rds of saturation in most piezomagnetic ma-terials. To achieve this flux density and also toprovide a path for the alternating flux, it is generally

2Qkcls

, , .

10) The term piezomagnetism is used to represent all reversible,nearly linear magnetomechanical phenomena in polarizedferromagnetic materials, to distinguish them from theirreversible, roughly quadratic effectsinunpolarized ferro-magnetics. The term magneloslriclion is used quite gene-rally to include both effects. See for example G. Bradfield,Acustica 4, 171-181, 1954, and the second paper referredto in note 12).

11) For further information on ultrasonic transducers and onultrasonic techniques generally, see for example, T. F.Hueter and R. H. Bolt, Sonics, Chapman and Hall,London 1955.

12) U. Enz, Die Erzeugung von Ultraschall mit Ferriten,Tech. Mitt. P. T. T. 33, 209-212, 1955. C. M. van derBurgt, Ferroxcube materials for piezomagnetic vibrators,Philips tech. Rev. 18, 285-298, 1956/57.

13) W. P. Mason, J. Acoust. Soc. Amer. 28, 1207-1218, 1956~

328 PHILIPS TECHNICAL REVIEW VOLUME 18

convenient to include a flux return path in theform of a laminated yoke of high permeability, low-loss material. A transducer of this type is shownin the sketch of fig. 3.

Fig. 3. Essentials of the vibration transducer as used for low-power ultrasonic drills. Y laminated yoke, P polarizing wind-ings, N nodal clamp, E energizing coil, V laminated vibratingmember, If tapped steel stub-holder. Thc vibrating member Vis À/2 in length.

The additional lamination structure complicatesthe mechanical design of bar-type structures andbecomes inconvenient if the transducer is operatedat a power level such that water-cooling is neces-sary. In such cases, a simpler but less efficient alter-native is the window type of construction which isalso widely used in ultrasonic work. These trans-ducers consist essentially of two laminated barsjoined at the ends. The stack is wound toroidallywith a single coil which carries both direct polari-zation and alternating drive currents. A typicalwindow-type transducer used in ultrasonic drills isshown in the photograph of fig. 4 (see also fig. 9).These transducers are not capable of generatingsuch high vibration intensitics as are the bar-typeunits under comparable conditions of drive 14).Nevertheless, because of their simple constructionand ease of cooling they are generally preferred tobar types in the larger models of ultrasonic machinetools.

14) H. H. Rust and E. Bailitis, Kritische Betrachtungen iiberdie linearmagnetostriktive Ultraschall-Erzeugung mittelstonpilzartiger Schwinggebilde, Akust. Beihefte, 1952,No. 2, pp. 89-90.

The practical limit, referred to earlier, to the usableoscillatory amplitudes for machining purposes istoo large to he attained at the vibrating face of thefree transducer. In the transducer, the limit is setby saturation effects and by the danger of fatiguingthe piezomagnetic material under the largealternating stresses set up in it. It is, however,possible to increase the amplitude at the tool bydirecting the vibrational energy on to it through amechanical focusing device (velocity transformer).In this way maximum cutting rates can be achieved.

The velocity transformer

The transducer-transformer-tool system whichforms the basis of the ultrasonic drill is essentiallya resonant mechanical transmission line, two ormore half-wavelengths long. The velocity trans-former itself generally consists of a tapered metalstub, of suitable high fatigue strength and highmechanical Q, which is rigidly bonded to the trans-ducer face. The transformer stubs used in ultra-sonic machine tools are generally designed to beresonant at the transducer frequency, as this notonly simplifies the design calculations but alsoallows interchangeable stubs to bc used withoutchanging the distribution of nodal planes in thesystem or its resonant frequency. The importanceof this arises from the fact that for the purpose ofsupport it is necessary to provide a rigid clamp, andto avoid excessive damping this should be locatedat a displacement node; clearly the latter shouldremain fixed, independent of the stub used andits loading.

The resonant length of tapered metal stubsdiffers from that for parallel-sided members anddepends on the form of the taper and the trans-

91286

Fig. 4. Transducer-transformer-tool system used on the high-power ultrasonic drill (input power 2 kW, frequency,.._,20 kc/s), The slotted ring clamps the final N2 stub to theupper mounting stub.

19.56/57, No. 11 ULTRASONIC MACHINING I 329

formation ratio required. Calculations are simpli-fied if the stub section is made to change exponen-tially. In practice therefore this is often done andthe design of stubs of this form has been workedout in some detail (see below and 6)). The increasein vibration amplitude g is always exactly equal tothe root of the inverse ratio of the end areas forany taper, provided that the maximum lateraldimension of the stub is small in relation to thewavelength. In other words, in any continuousresonant loss-free stub of any numbe~ of J../2sections, the product area X ç2 is constant at allanti-nodal planes and the transformer provides anincrease in vibration intensity only at the cost ofareduction in the effective drive fase area.

The equation giving the motion of a tuned stub of varyingseetion can be obtained by analogy with the acoustic trans-former (or acoustic horn) of which the general equation relatingthe velocity potential tp with distance x along the horn and. ,time t is

1 02rp 02rp orp d2' ~ = .."...+ " .-d In A,c ut uX- uX x

where c is the velocity of sound in the medium and A thecross-section at x. Assuming the vibrations to he simplyharmonic, we can write 02rpJOt2= - w2rp, where t» is the angularfrequency, and by substitution in equation (1) we eliminatethe time variable and obtain

d2rp drp d w2

dx2 +d;'·dxlnA+~rp=O.

Th!s equation, which holds for any law of taper, is true onlyif the largest diameter of the horn is small compared with thewavelength of the vibrations in it and if the "flare" of thetaper is .not greater than a critical value. For the particularcase of the exponential taper, the solution is simple. Here,if A = Au exp (-yx) where Au is the area of the stub at thewide end and the exponent y defines the flare of the taper,

d2rp ydrp w2

dx2 -'"dx'" +~ rp= 0,

or, since the particle velocity v = - drp/dx,

d2v dv w2

dx2 -y dx +~v = 0,

of which the general solution is

v = (Kl cos wx/c' +K2 sinwx/c/) ~xp (yx/2),where

Thus the effect of the taper is to increasethe effective velocityof sound waves in the material by the factor 1/(I~y2c2J4w2)'/ •.It can be seen that the velocity is real only when y is less

than 2wJc: this is the critical value referred to above, for theexponential case. Transmission ceases when the flare becomesgreater tha.n this value; the horn is an effective filter and thiscondition determines the cut-off frequency. A consequenceof this increase in the effective velocity of sound is that thelength corresponding to the fundamental resonance of a taperedstub of this form is greater than that of a uniform rod and isgiven by 1= c'n/w = ).IJ2.

The constants Kl and K2 in equation (2) are determinedfrom the appropriate boundary conditions. Considering thefundamental resonance, v = Vu at x ='0, where Vu is the velo-city of the transducer face, and the strain is zero at the freeend where x = I = c'n/w. These conditions give

so thatv = Vu [coswx/c' - (yc'/2w) sinwx/c/] exp (yx/2) .. '(4)

For the half-wavelength stub, at x = I, the particle velocityVI = -Vu exp (yl/2), so that '

-Vl/Vu = (AuJAJ!' = exp yl/2 = a, say,

where Al is the area of the stub at x = I. Thus, for a ï.f2resonant stub the particle velocities, and therefore amplitudes,are increased at the small end, as compared with the largeend, in the ratio of the square root of the inverse area ratio.The quantity c mentioned above is a useful design param-

eter. The effective velocity of the sound waves may beconveniently expressed in terms of the ratio a of the particlevelocities at the small and large ends. Since y = (2/1) In a andc' = wiJn, equation (3) may be re-written as

c' = c/[I-(c/c/)2(ln a)2/n2]'!',which rearranged gives

(1) c' V (In a)2-= 1+-c n (5)

The distance Xn from the large end to the displacementnode mayalso be expressed in terms of a. From (4,), theparticle velocity is zero at the point Xn given by

WXn yc' In acot"?' = 2w = --;:' . . . (6)

and since this is positive, Xn < ).1/4.The plane of maximum particle velocity is found by dif-

ferentiating (4) with respect to x and equating to zero:

~:= Vu~ exp (Y;)[ - (~~+ !;) sin ~~]. . .

This is zero for x = 0 and x = ).1/2 = I. The planes ofmaxi-mum particle velocity are therefore at the extremities.The strain e(x) in the exponential stub is proportional to (7),

since s = og/ox which is proportional to ov/ox. Also, for thestress distribution a(x), we have

(7)

. og (YX) . wx,a(x).=Ee(x)=Eox=Kvuexp "2 sm7' ..

where K is a constant involving y, c', wand the Young'smodulus E. Differentiating with respect to x and equating to

(8)

(2)zero we get

oa(x) w (YX) [YCI

• WX WX]-- = Kv - exp - - sm-+ cos-,-Ox u c' 2 2w . c' c= 0,

(3)or, since yc'/2w = (In a)/n,

WXacot-c'

In a(9). .....

n

which gives the position of maximum' stress Xa in the expo-nential stub. The negative value of cot wXa/c' implies thatXa> ).1/4, and comparing (9) with (6) we see that x« and Xn arein fact eqnidistant from the rid-point of the stub (see fig. 5).

The required drilling tool may be screwed orbrazed to the end' of the stub, the stub generallybeing chosen so that the dimensions of its free end

330 PHILIPS TECHNICAL REVIEW VOLUME 18

are comparable with the tool size. Consequently, toaccommodate a large range of tool sizes it is con-venient to provide a range of matching trans-

- formers having different transformation ratios andtherefore different' end diameters. Typical trans-former stubs are shown in fig. 5 with drilling toolsattached.

a002 a.004 inch -s.3:/ kg/mm2

------~>- --_.:..--------Fig. 5. Half-wavelength velocity transformer stubs with toolsattached, showing corresponding amplitude and stress .dis-tributions (measured).

:A considerable increase in efficiency can be ob-tained, and the product Ae2 at the work-face canbe greatly increased, by employing multiple massivecoupling stubs and transformers, in which an areadiscorrtirïuity exists at (displacement) anti-nodaljunctions (fig. 6). In this way the distribution ofvibrational energy in the whole system is re-arrangedin such a way that a bigger fraction of the totalenergy is stored in the less lossy parts, i.e, in thevelocity transformer(s). The' efficiency of sucharrangements depends on the stubs remaining very

__il () a.DO/inch ~-a--~ a.rotioch5 o-_il __1T' 5kg/mm2

I__ 1'1. 2

Fig. 6. Transducer-transformer system with step disconti-nuities at antinodal junctions, showing amplitude and stressdistribution (measured). Displacement nodes and antinodesare indicated by N and A respectively.

nearly resonant under all practical loading condi-tions (so that the stress transmission across the areadiscontinuity is small). The mechanical Q of theunloaded system increases and the magnetomechan-ical coupling coefficient 15) k is reduced in such a

way that the product Ikl2Q remains approximatelyconstant. Because of its constancy over a widerange of conditions this product is a very valuabledesign parameter. When resonant matching stubsare used, the product Ae2 referred to the transducerface is proportional to (lkI2Q)2 for constant drivingconditions if the stubs can be assumed loss-free.If not, then this product decreases with increase inmass of the .ooupling stubs, being very nearly in-versely proportional to the total mechanical damp-ing of the system. In practical cases, therefore, thereis of course a limit to the vibration intensity ob-tainable at th~ working face under free-runningconditions, but this will be very high when the stubsare of low-loss material, oorreeponding to a highmechanical Q of the system.

91310

Two other measures may be mentoined by which, undercertain conditions.vdrilling efficiencymay be increased abovethat obtainable with simple continuous stubs.

1') Useful practical mechanical transforming systems canbe designed having diseontinuities of area in planes other thanthe displacement antinodes. In particular, a }./2stub eonsistingof two Af4 cylindrical sections of different areas (see fig. 9) givesan amplitude transformation in the inverseratio of the end areas(i.e. a greater ratio than the corresponding smoothly-taperedtransformer). However, the alternating stress developed in atransformer of this sort is greater than for tapered stubsgiving the same drive face amplitude, so that mechanicalfatigue troubles may be encountered. The use of these doublequarter-wave stubs is therefore restricted to transformingeomparatively small end-face amplitudes.2) As mentioned above, matching stubs must generally be

chosen so that the end dimensions are comparable with thetool size. Also a tool loading somewhat below optimum isnormally used, in order that small changes in tool face areaand loading conditions do not cause too severe mismatchingof the amplifier. It is sometimes possible to go some waytowards increasing drilling efficiency, however, (without thenecessity of increasing the unloaded Q of the system) by merelyincreasing the cutting area of the tool in relation to the trans-former end area. (In order to take advantage of this, however,the whole electromechanical system has to be designed forone given operation.) In this way the radiation loading dueto the liquid medium and the 'workloading are both increaseddirectly. If the tool area is considerable the radiation lossfrom the free part of the upper surface may be eliminated byproviding acoustic pressure-release'material (sponge rubber or:a similar material) on this surface. .When using this arrangement it is important to ensure

that the dimensions of the tool (its thickness especially)are such that no severe flexural modes can be excited.

15) For the definition of the material constant k, see for exam-ple the paper by C.M.van der Burgt mentioned in note 12).Apart from .this k, other coupling coefficients may bedefined which also take into account the nature and di-mensions of the vibrating system, viz. kY/2 (for a Af2resonator), ky/2,mctal (for a }./2 laminated metal resona-tor, < ky/2, owing !_oeddy current losses) and an overallcouplingcoefficient k (for a composite system, e.g. resonanttransducer + resonant transformer(s) + tool). It is thislast, k, with which we are concerned here.

strument is operated at aninput power level sufficient-ly low that external coolingof the transducer is notneeded, while in the largermachine it is necessary towater-cool the transducer.Most of the drilling workdescribed in this paper wascarried out with one or otherof these machines.The low-power drill is

illustrated in fig. 7. Thisdrill is fed from an oscil-lator supplying 50 W atabout 20 kc/s. The transducer (fig. 3) consists ofa ! inch squarc stack of laminations of nickelwhich have been given suitable magnetic propertiesby an appropriate heat treatment. The laminationsare bonded together by an insulating cement andfixed to the lower end of the stack is a steel-bushwhich is tapped to serve as a holder to accommodatea range of velocity t.ransformers. The vibrating

Fig. 8. a) Block schematic diagram of drill vibrator drive system for the lower-power drill. 0 variable frequency oscillator,A power amplifier, Tr step-down output transformer, + and - terminals for D.e. supply for polarizing field, Vtransducer, Y yoke.b) In the high-power drill a blocking capacitor C and an L.F. choke L are added, whereby drive and D.C. polarizing supplycan be fed to a single winding on the transducer.

1956/57, No. 11

Ultrasonic drillsAs examples of the ap-

plication of the design prin-ciples outlined above, twoultrasonic machining m-struments developed andproduced by the MullardResearch Laboratories willbe described - a small in-strument capable of small-scale drilling work whichuses a laminated bar-typetransducer, and a full-sizedmachine tool for generalworkshop use which usesa laminated window-typetransducer. The smaller in-

ULTRASONIC MACHINING I 331

Fig. 7. Low-power ultrasonic drill (input power 50 W, frequency ",-,20 kc/s; Mullardtype E 7682).

member is clamped rigidly at its centre (which is anode) and its upper half is surrounded by the ener-gizing coil wound on a former which, to avoidmechanical damping, does not make physicalcontact with the vibrating laminations. To drive thetransducer a conventional oscillator and poweramplifier combination is used. A block diagram ofthe set-up is shown in fig. 8. The output from the

g b 91)12

332 PIIILIPS TECHNICAL REVIEW VOLUME 18

amplifier IS matched, through a step-down trans-former designed to handle the chosen frequencyefficiently, to the transducer load, which can bevaried as necessary merely by adjusting the numberof turns on the energizing coil. Polarization for thetransducer is provided by completing the magneticcircuit of the transducer with a yoke of high per-meability laminations supported sufficiently closeto, but not touching, the vibrating member and onwhich is wound a separate coil carrying directcurrent from a suitable low-voltage supply asshown in fig. Sa. The velocity transformers, expo-nentially tapered, which amplify the vibrationsobtained at the transducer face, have been shown infig. 5. The required drilling tip can be screwed orsoldered to the end. The complete drill head ismounted in a precision drill stand with arrange-ments to enable the drill to rest on the work withthe necessary light pressure.

The transducer employed in the larger (2 kW)machine is built from nickel laminations of thewindow type described above. Biasing and drivecurrents pass through the same winding (see fig. Sb).A square stack (V) of these laminations (jig.9) isbrazed to a tapered mounting stub (E) and is subse-quently consolidated into a solid stack using a resincement. The complete transducer is supportedby a nodal flange at the lower end of the cooling

Q1313

Fig. 9. Essentials of the high-power ultrasonic drill. S low-friction ball slides, J water jacket, L leads to transducerwindings, R rack and pinion mechanism, W water connections,V transducer, B J,.j2 mounting stnb, TI" double J.f4 cylindricalvelocity transformer, T tool.

Fig. 10. Mullard high-power drill mounted on a standard typeof machine-tool base, with moveable work table.

jacket (J) and may be raised and lowered alonglow-friction ball slides (S) by the rack and pinionmechanism (R). The weight of the transducerprovides too great a static load for many drillingoper ations, and may be counter-balanced by meansof an adj ustable lever and weight system (not shownin fig. 9).

The complete drilling head is mounted above awork table closely resembling that of a standardmilling machine. This enables the work to be movedaccurately along the three rectangular coordinateaxes, thus giving the machine some of the qualitiesof a jig borer.

Tools are carried at the lower end of a secondvelocity transformer (Tr) which screws on to themonnting stub. A photograph of the completemachine is shown in jig. 10. A close-up of a taperedvelocity transformer and tool is shown in fig. 11.

Applications

The most valuable applications of the vibrationmachining technique arise from the fact that holesand patterns of intricate shapes can be cut in anybrittle material.

1956/57, No. 11 ULTRASONIC MACHINING I 333

Materials prepared by sintering techniques areoften difficult to machine by conventional methods.Normally, sintered parts are pressed to shape be-fore firing, so that the least possible amount ofmachining has to be undertaken on the sinter.Ultrasonic machine tools enable all manner of irrtric-ate shapes to be cut successfully in the fully-sinteredstate. Applications include the cutting of holes, slotsand depressions in ceramics and sintered carbides.

Images and patterns may be simply and rapidlyproduced by the new method, the tool being merelythe reverse impression of the required image, pro-duced in a suitable metal. Since the chipping of thework is un such a small scale, extremely fine detailcan be reproduced accurately. For working glass,ceramics and materials of comparable hardness,many impressions can be made from a single toolbefore tool wear results in any appreciable loss ofdefinition in the image. The technique can of coursebe used to make dies for medallions or coins, insintered carbides or die steels. In such materials,tool wear is always considerable so that it is neces-sary to use both roughing and finishing tools.Fig. 12 shows photographs of cuts in tungstencarbide and glass by this technique.

Sapphire, ruby and agate are machined veryrapidly; consequently the time required for themanufacture of jewel bearings for watches and

Fig. 11. Close-up photograph of a velocity transformer andtool fitted to the high-power drill.

91290

91291

Fig. 12. Holes and bas-relief patterns of various shapes cut insintered tungsten carbide (above) and glass (below).

precision inst.rumeuts can be reduced considerably.In such applications, the major operatien consistsin cutting out small discs from thin sheet material.As the required dimensions are very small, it ispossible and indeed for economic reasons necessaryto use a multiple tool capable of blanking out alarge number of discs in a single operation. The toolactually used in a typical job of this sort consistedof a cylindrical piece of mild steel rod J" long,i" diameter, having 35 holcs each 1.6 mm diam-eter drilled into it longitudinally. To ensure aparallel-sided cut, and to avoid chipping at theedges as the tool penetrates the work, the sapphiresheets are fixed rigidly to a glass backing plate andthe tool is allowed to penetrate not only the workbut also some distance into the backing material.In this way discs have been cut to a dimensionalaccuracy of ±O.OOI inch on diameter in abont oneminute from sapphire sheets 1 mm thick.An important application is in the manufacture

of transistors, since the materials used, germaniumand silicon, are brittle enough to be easily machinedby this method. For slicing bulk crystals, the mosteconomical technique is to use a multiple slicing

334 PHILIPS TECHNICÁL REVIEW VOLUME 18

tool in the form of a set of parallel blades' fixedrigidly in a supporting frame with the required .separation. For cutting out the small discs requiredin transistor manufacture, a multiple blankingtechnique is used, similar to that described above.This is a valuable production technique, a consi-derable advance on ~hè use of rotary diamond-impregnated tools.

Considerable difficulties exist in applying con-.ventional techniques to drilling very small 'holesand depressions in brittle materials. This applies'even to ro~nd holes. Experiments have shown thatextremely small holes, down to at least 0.007 inchdiameter, can be cut on the vibration machineprovided suitable precautions are taken. To applythe very light static loading required, the drill ispreferably made immovable and the load appliedby supporting the workpiece on a guided. platform,itself supported on 'a light spring. The load can inthis way be set by adjusting the compression ofthe spring. In cutting small ,round holes where thecircularity is important it is an advantage to rotatethe workpiece during the drilling. There is no ad-vantage in rapid rotation and a rate of a few revo-lutions per, second is adequate. The rotation alsohelps considerably in circulating the abrasive underthe tool. As examples of the drilling of small holesto close tolerances we may mention the manufac-ture of diamond wire-drawing dies and, again, intransistor manufacture.

A .useful technique for drilling in places whichwould normally be inaccessible is to couple thevibrations from the transformer to the tool througha flexible coupling, in the form of a thin wire of amaterialof suitable high fatigue strength and lowloss characteristics. This technique also allowsthe work to be carried out at considerable distancesfrom the transducer when suitablè wire is used.Experience shows that this technique is ,a veryconvenient one for drilling small holes rapidly whenthe operation must be done manually and whenpositional accuracy is not important. The vibrationsare transmitted, with very little attenuation, largelyas axial vibrations with only a small shear compon-ent, even when the wire is bent so sharply thatthe radius of curvature is small compared with thewavelength. This suggests an interesting applica-tion of this technique: the drilling of curved holes- holes with axes bent in any desired way andof any required section. Many actual cuts of thissort have been made in glass.

A field of some promise for ultrasonic techniquesis in dental drilling. Investigations have been madein this direction 'using the above-mentioned wire

transmission lines in conjunction with remote trans-ducers and also using miniaturized transducers.'the vibration machining technique is likely to

have its greatest application in the field of diemaking, since the materials used - hardened steels,sintered carbides and diamonds '_ are brittle enoughto be machined quite rapidly by these methods.Die making by conventional methods is generallya very protracted process involving lengthy periodsof lapping, much of which must be done manuallyusing expensive diamond powder and diamond-impregnated tools. Moreover, dies of any complexitymust generally be made up in a number of segmentalsections and are therefore inherently weak. Byusing the ultrasonic technique these difficulties areovercome. The most complex shapes can be cut outcomparatively rapidly in one piece, using inexpen-sive boron-carbide abrasive, except where diamondsthemselves are to be cut. During the investigationsrecorded in this article, a great number of holes ofall shapes have been cut in tungsten carbide. Ex-trusion dies and wire-drawing dies require a taperinglead-in and exit portion which can readily be producedon the ultrasonic machine using an appropriate toolof tapering section. However, in large carbide dieswhere stock removal is considerable, machiningtime can be saved by sintering the carbide blankroughly in the required form (but undersized on allinternal dimensions] and using the vibration tech-nique merely for sizing. If this is done it may bepossible to reach the required dimensional toler-ances in only one operation.The second part ofthis article will deal in some de-

tail with cutting speeds, accuracy and surface finish.

Summary. This article gives an introduetion to the techniqueof drilling by means of ultrasonic vibrations. The actualcutting action consists of a continuous chipping of the workby. abrasive particles in suspension, fed between tool andwork. A vibration frequency in the ultrasonic region (.......20kc/s) is used, both for silent operation and in order to getreasonable cutting rates. Piezomagnetic (magnetostrictive)transducers are particularly well suited for generating thevibrations. Two types of laminated metal transducers are _described - a bar-type and window-type. These are used intwo ultrasonic drills developed by the Mullard Research Labo-ratories, the former in a small 50 W drill and the latter in alarger water-cooled 2 kW -model, The associated velocitytransformers are described in some detail: these are resonantstubs, fixed to the transducer and holding the tool, which serveto amplify the vibrations at thé tool face. Ultrasonic drillingcan be applied only to hard, relatively brittle materials. Amongthe applications are cutting, the drilling of holes of anycross-section and the production of has-reliefs, in such mate-rials as glass, tool steel, _sintered carbides, ceramics andprecious stones. Using a mechanical transmission line of flexi-ble wire it is possible to drill holes in otherwise inaccessibleplaces; even curved holes can he bored. Some of the otherapplications mentioned are the manufacture of blanking diesand wire-drawing dies and of precious-stone bearings forwatches and instruments. The use of ultrasonic machiningin transistor manufactore and for dental, drilling are alsomentioned. A second article will discuss in more detail cuttingspeeds, accuracy and surface finish.

1956/57, No. 11 335

HERBICIDE MANUFACTURE IN THE PHILIPS-ROXANE FACTORY AT AMSTERDAM

The firm of Philips-Roxane markets a number of insecticidesand other chemicals for agriculture and horticulture. One ofthese is the herbicide 2,4,5 trichloro- O-CH2-crXJHacetic acid (see inset formula) referred Ito as 2,4,5-T, used to combat weeds 1). /'CIThe above photograph shows the plant 11 Iin which this chemical is synthesized. Cl ""--ti'Hexachlorohexane is changed into a Cldichlorophenol in the autoclave just visible at bottom left of

the photograph. The 2,4,5-T is 'produced in two further stages:thereaction of the dichlorophenol with monochloroacetic acidand the replacement of a certairi H atom in the benzene ring bya chlorine atom. By reaction with suitable alcohols (apparatusto left of steps) the 2,4,5-T is then made into an ester, whichis the form in which it is used.

I) See Philips tech. Rev. 16, 356, 1954/55; 17, 295, 1955/56.