PROSPECTS FOR IMPROVING MACHINING PRODUCTIVITY

OF LARGE CERAMIC OBJECTS FOR RADIO ENGINEERING PURPOSES

E. I. Suzdal’tsev,1 D. V. Kharitonov,1 G. A. Kharakhonov,1 A. G. Épov,1 and M. V. Nogarev1

Translated from Novye Ogneupory, No. 12, pp. 17 – 24, December 2011.

Original article submitted September 22, 2011.

Contemporary machining methods for ceramic materials are analyzed. Laser, hydroabrasive and high-speed

machining are considered. The possibility of using these methods for machining large complexly-shaped ce-

ramic objects having a double surface curvature is evaluated.

Keywords: glass ceramic, machining, laser machining, hydroabrasive cutting, high-speed machining.

The rapid development of science and technology in re-

cent years has led to a situation that objects for radio engi-

neering purposes made from ceramic and glass ceramic ma-

terials have high specifications with respect to shape accu-

racy in longitudinal and transverse sections, wall profile, and

surface quality. Recently considerable experience has been

accumulated for machining large ceramic objects or radio en-

gineering purposes, having double surface curvature, includ-

ing diamond grinding by programming and copying of the

internal and external surfaces of an object [1 – 4].

For example, studies carried out by us previously [3, 4]

have made it possible to determine the range of values of op-

timum diamond tool properties, cutting force, and direction

of feed for grinding large ceramic objects with double sur-

face curvature, which currently are used successfully in pro-

duction (Table 1). However, the steady increase in the level

of specifications for quality indices and machining produc-

tivity, development and creation of new materials, specify a

requirement for improving existing technology, equipment,

and tools.

As an example the average machining duration for one

ceramic object for radio engineering purposes having an in-

ternal diameter of 200 and height of 600 mm, by existing

technology (see Table 1) is 40 – 45 h, which is caused by the

considerable thickness of production allowances, and also a

requirement for reinstalling a component in a lathe in order

to provide the required geometry and contour. In view of this

it is desirable to carry out work for studying the possibility of

intensifying machining.

The problem of intensifying machining ceramic compo-

nents, including large objects for radio engineering purposes,

has received considerable attention in domestic and overseas

publications. In [5] it is proposed to introduce a systematic

approach to ceramic component machining. It includes the

following operations: choice of the required collection of

tools for machining a specific type of component, which

makes it possible to shorten the time for changing a tool and

overcome its dimensional nonconformities; development of

monitoring for a machine component geometry during inter-

Refractories and Industrial Ceramics Vol. 52, No. 6, March, 2012

424

1083-4877�12�05206-0424 © 2012 Springer Science+Business Media, Inc.

1FGUP ONPP Tekhnologiya, Obninsk, Kaluga Region, Russia.

TABLE 1. Machining Regimes for Large Ceramic Objects with Double Surface Curvature [3, 4]

Grinding parameters

Rough surface grinding Finishing surface grinding

inner outer inner outer

Component rotation rate, Vc, m/min 100 – 140 100 – 140 80 – 90 80 – 90

Cutting speed, Vcu, m/sec 35 – 40 50 – 60 35 – 40 50 – 60

Cutting depth, t, mm 0.5 – 1.0 0.5 – 1.5 0.02 – 0.30 0.02 – 0.30

Wheel feed, S, mm/rev 0.10 – 0.16 0.13 – 0.20 0.07 – 0.10 0.10 – 0.13

nal and external machining with the aim of considerably re-

ducing the number of removals (installations) of a compo-

nent from a lathe. The measures proposed are of mainly of an

organizational nature and do not resolve the problem of ac-

celerating grinding.

There is specific interest in using ultrasonic vibration

(USV) for intensifying grinding, proposed in [6, 7]. Accord-

ing to data of this work, grinding with application of USV is

intensified due to complex processes that occur in the cutting

area. It is well known that material breakage with abrasive

treatment occurs as a result of the occurrence of micro- and

macrocracks, propagating to some depth. By intersecting

they create a weakened layer, easily removed with repeated

action of the grain of a wheel. Ultrasound, loosening

microcracks, accelerates material dispersion. In view of this

there is an increase in penetration of lubricating and cooling

liquids (LCL), and there is an improvement in slurry removal

from the machining area. Presence of a radial USV compo-

nent leads to an increase in the length of path travelled by

abrasive grains, and an increase in grain turning angle. Due

to the normal component of USV an increase is provided in

the overall number of active grains. Grains acquire a devel-

oped microrelief with a considerable number of elementary

cutting edges. The cutting force and temperature are reduced.

In order to implement this method the authors of [7] pro-

posed fitting the end of a grinding spindle of a lathe with a

small ultrasonic head, which was realized by them in stan-

dard flat grinding machines. Flat specimens were machined,

rigidly fastened to the surface of the removable table. Use of

this method for machining ceramic components, having dou-

ble surface curvature, which as a rule during grinding does

have a rigidly fixed wall, may only lead to additional wall vi-

brations of a machined billet, and as a consequence to loss of

machining accuracy.

Development of laser technology has made it possible to

consider the potential of using laser radiation for removing a

production allowance from curvilinear ceramic surfaces [8],

including from objects with double surface curvature [9].

In [9] the outer and inner surface of a billet of glass ce-

ramic material was machined in two stages: in the first stage

the main allowance (~2.5 mm) it is removed in one to five

passes by evaporation of glass ceramic under the action of

powerful laser radiation (yttrium-aluminum garnet laser with

neodymium (ND-YAG) with power up to 2000 W with a

wavelength of 1.06 mm, and in the second stage the remain-

ing allowance (~0.05 mm) is removed by finishing grinding

using diamond wheels. Here, as the authors in [9] confirm,

introduction of this method makes it possible to reduce by a

factor of two the cost of machining components compared

with standard diamond tool grinding methods. In addition, in

[9] it is noted that after laser treatment at the surface of a

shell there is formation of a layer about 0.05 mm thick of

amorphous glassy (fuzed) material, enriched with Al, Mg,

and Ti, compared with the original materials which is re-

moved by grinding in the concluding machining stage. In ad-

dition, data have been provided for the effect of laser treat-

ment itself (without final diamond grinding) on glass ce-

ramic dielectric properties. It was noted that laser machining,

used in manufacturing objects for radio engineering pur-

poses, does not have an unfavorable effect on the dielectric

properties of glass ceramic.

In order to verify the applicability of the methods indi-

cated above for machining objects of glass ceramic OTM

357-0 an experiment was carried out for removal of material

from the surface of a disk of this material. An ytterbium fiber

device YLR-200-3M-WC was used with a power of 250 W

and with a wavelength of 1.06 mm. The laser beam move-

ment rate (scanning rate) was measured in the range

20 – 1300 m/sec. The depth of material removal with one

pass of the beam in relation to its movement is presented in

Fig. 1. It follows from the data presented that the slower the

scanning rate for a specimen surface, the higher it its thermal

effect, and consequently a greater depth of material removal.

Whereas with a rate of 200 m/sec it is possible to achieve

material removal to a depth of 0.1 mm, with an increase in

rate to 700 – 800 m/sec there is reduction in depth of re-

moval to 0.01 mm, and a further increase in rate leads to al-

most total absence of this.

A dependence presented in Fig. 1 makes it possible to

suggest that by varying such values as laser scanning rate, la-

ser head movement rate (analog of feed rate), and rate of

component rotation, it is possible to achieve the required ma-

terial removal over the surface of a component. However, it

is necessary to consider that the greater removal of material,

the rougher is the machined surface obtained (Fig. 2).

In specimens of glass ceramic, machined area by area,

with different scanning rates (see Fig. 2) it is clearly seen that

the slower the laser beam passage rate, the greater is surface

defectiveness. With machining at a rate of 200 and 300 m/sec

coarse droplets of fuzed glass are seen at it. This leads to a

requirement of further machining of a fuzed surface by tradi-

tional diamond grinding technology, as has been proposed

in [9].

Thus, use of laser machining methods for objects of glass

ceramic materials is promising in the rough machining

stages, when a considerable part of the production allowance

is removed.

Prospects for Improving Machining Productivity of Large Ceramic Objects 425

Fig. 1. Dependence of material removal depth on laser beam scan-

ning rate.

Another promising method for machining relates to

hydroabrasive cutting (HAC), which recently has achieved

popularity in various branches of industry, including aero-

space. The principle of hydroabrasive cutting is simple and

involves the fact that water with an abrasive powder (often

agate powder is used for this purpose) under a very consider-

able pressure (more than 400 MPa) through a cutter (jet)

head, having a nozzle diameter of 0.5 – 0.8 mm, is fed to the

surface of material being machined. The nozzle diameter

provides preparation of a cutting width of 1 – 3 mm and a

surface roughness Ra = 0.5 – 1.5 �m. An HAC device is

equipped with systems with numerical programmed control

(CNC) and this makes it possible to carry out shaped cutting

by a prescribed program (Fig. 3). Here the main distinguish-

ing feature of HAC technology is absence of a thermal effect

on a machined component, which makes it possible to pre-

pare objects with a minimum defective layer

In addition, the good quality of cut surfaces and absence

of spalling at the inlet and outlet of the abrasive hydraulic

suspension jet should be noted. This is indicated by photo-

graphs of sections of ceramic specimens of different materi-

als presented in Fig. 3. Consequently, HAC has a number of

considerable advantages compared with traditional cutting of

ceramic materials by diamond wheels. Data are presented in

Table 2 for cutting rate of typical materials by existing tech-

nology, proceeding from operating standards and with use of

HAC technology.

It may be concluded by analyzing Table 2 and Fig. 3 that

currently for ceramic materials there is no more economic

cutting method as an alternative to HAC. In addition, this

method is irreplaceable for the production of flat ceramic

components of complex shape, since it makes it possible to

prepare round, shaped billets, and billets with through holes.

The question of use of this method for machining compo-

nents having a body of rotation of double surface curvature

remains open. However, it may be suggested that knowing

the rate for broaching holes in each specific material, by reg-

ulating the billet rotation rate and movement of the head with

nozzles, machining of an outer surface of a billet is entirely

possible. Implementation of the HAC method for machining

the inner surface will be difficult due to the impossibility of

placing a cutting head within the cavity of an object being

machined.

In spite of the visible, at first sight, advantage of using

such methods as laser evaporation and hydroabrasive cutting

for machining large ceramic, the high price of equipment for

their implementation compels setting aside into the future

work on these methods. In view of this it is desirable to carry

out work for optimizing existing technology and moderniza-

tion of equipment currently available.

As noted above, previously we have established that op-

timum production regimes for machining large ceramic bil-

lets, having double surface curvature (see Table 1). At the

same time, the regimes described above do not provided the

maximum machine productivity. This is mainly caused by

the fact that the support of a turning machine is connected

with a kinetic movement mechanism of the machine, which

provides movement of a grinding head (installed on a sup-

port) along the surface of an object being machined only

with a constant prescribed rate S (mm/rev). In the majority of

cases the configuration of ceramic objects for radio engineer-

ing purposes is a hollow cone, within which the diameter

426 E. I. Suzdal’tsev et al.

Fig. 2. Effect of laser beam movement rate (indicated over specimens,

m/sec) on glass ceramic OTM 357-O specimen surface quality.

TABLE 2. Comparative Cutting Characteristics of Different Ceramic materials

MaterialMaterial

thickness, mm

Traditional cutting,

cutting speed, mm/min

Hydroabrasive cutting

cutting speed, mm/minbreakthrough time for hole

with diameter 1 – 1.5 mm, sec

Glass ceramic OTM 357 18 20 – 30 100 25

Quartz ceramic 18 30 – 40 120 15

SiC 10 10 – 15 8 40

Al2O3 10 15 – 20 5 30

changes over the height (from 400 mm to a radius of curva-

ture in the nose section of 5 – 10 mm), and correspondingly

there are many changes in the volume of material removed

for equal intervals of time, which may be considered by the

following equation:

Q = �dtSnc, (1)

where Q is productivity, mm3/min; d is billet diameter, mm; t

is cutting depth, mm; S is wheel feed, mm/rev; nc

is compo-

nent rotations, rpm.

Thus, during machining of these components there is

nonuniform material removal over an object height. For ex-

ample, in machining large billets (with a base diameter of

400 mm) with the grinding parameters: S = 0.1 mm/rev,

t = 0.5 mm, n = 100 rpm, we have material removal in the

skirt area (diameter 400 mm) in an amount of 6.3 cm3/min,

and in the nose area (with a billet diameter of 50 mm) this

value is reduced to 0.8 cm3/min. This difference in material

removal can only lead to occurrence of gradients of load ap-

plied to the body of a billet, and in fact they will be at a max-

imum in the skirt part of a component.

Proceeding from this and considering that a billet under-

goes application of maximum loads in the skirt area without

breaking, it is possible to increase the productivity of turning

machines due to providing uniform material removal over

the surface of a billet. This may be achieved due to a gradual

increase in the amount of wheel feed or component rotation

rate.

Calculated data are presented in Fig. 4 for the amount of

wheel feed and component rotation in relation to a change in

billet diameter, which may be recommended for machining

ceramic objects. The basis of the calculation was Eq. (1), in

which as a constant the value of maximum material removal

in the skirt area was used. For uniform material removal over

the whole surface of a billet it is necessary to provide an

eightfold increase in the value of S or value of nc, which is

quite a complex task. This is surely indicated by data ob-

tained by us previously [3, 4] about the effect of feed rate and

cutting depth on cutting rate and wheel force (Fig. 5). An in-

crease in feed rate above 0.3 mm/rev is undesirable since

there is a considerable increase in cutting force and wheel

pressure, which has a negative effect on machined surface

quality. An increase in billet rotation rate to a value above

400 rpm due to quite considerable weight and weight imbal-

ance both for itself and for production equipment leads to

occurrence of forced vibrations also worsens machining

quality.

Thus, an increase in productivity during machining of

large ceramic billets is possible due to a compromise in-

crease in feed rate and component rotation. An example is

presented in Fig. 6 for implementing the proposed method

Prospects for Improving Machining Productivity of Large Ceramic Objects 427

Fig. 3. Examples of implementing hydroabrasive cutting for plates of different ceramic materials.

for a billet recently machined with a height of 1000 and base

diameter 320 mm. Initially an increase in productivity was

achieved only as a result of a gradual increase in feed rate.

Then on S reaching a value of 0.27 mm/rev there was a grad-

ual increase in component rotation rate to 240 rpm. No rec-

ommended properties are exceeded, but productivity in-

creases considerably. An estimate of the increase in produc-

tivity is possible by calculating the time required fro machin-

ing the whole surface of a component for one pass:

T = L/(Snc), (2)

where T in time, min; L is billet length, mm; S wheel feed,

mm/rev; nc

is component rotations, rpm.

Calculation by Eq. (2) taking account of data in Fig. 6

showed that only due to a gradual increase in wheel feed rate

and component rotation is it possible to shorten the duration

of one pass by 30 – 35% (from 100 to 65 – 70 min). In spite

of such a marked effect, this implementation is only possible

with use of CNC machines when knowing the size of a com-

ponent and the maximum permissible material removal, a

control program is developed that also controls the drives.

In the case of using of using copying lathes, undoubtedly

there is considerable help from using a device proposed by

us in [10], consisting of a traditional lathe with a copying

system and a pressure sensor placed on the grinding head,

connected with an analytical device, controlled in turn by the

lathe drive (drives for support feed and object rotation). Here

the support of the turning lathe is fitted with its own separate

from lathe movement mechanism kinematics, which makes it

possible to change support movement rate, and consequently

the grinding head along an object surface.

Installation of a pressure sensor on the grinding head

makes it possible to monitor the load applied a machine com-

ponent during the whole machining process. An analytical

device compares the signal from the pressure sensor with the

value applied to it, which is determined by calculation and

depends on the material, machined component wall thickness

and dimensions, and it feeds a command to the support drive

mechanism (or the object rotation drive) providing move-

ment of the grinding head along an object surface at a con-

stantly prescribed pressure on the machined object wall. The

scheme proposed makes it possible to shorten the number

grinding head passes over an object surface, and the overall

duration of machining by approximately 30 – 35%.

In spite of such a marked effect, productivity may be in-

creased even more. This becomes possible with use of

high-speed machining methods (high-speed cutting), which

recently have been used extensively in machining various

objects of ferrous and nonferrous metals [11 – 14]. The basis

of high-speed machining theory is the fact that in some nar-

row cutting rate range an increase leads to a reduction in cut-

ting force. This is represented more clearly by Solomon

curves (Fig. 7) obtained over 30 years of the last century by

Carl Solomon, who established that the cutting temperature

does not depend uniformly on cutting rate, but has a specific

extreme nature. This is caused primarily by the fact that in

small cutting cross sections in this range the main mass of

428 E. I. Suzdal’tsev et al.

Fig. 4. Calculated data for the dependence of feed rate nd and com-

ponent rotation rate S on billet diameter for providing Q = const.

Fig. 5. Dependence of cutting force Py (a) and wheel pressing � (b )

with different feed rates (shown on the curves, mm/rev) on cutting

depth t.

Fig. 6. Recommended feed rate parameters (1 ) and component ro-

tation rate (2 ) on billet length with the condition Q = const; 3) com-

ponent contour.

heat is concentrated in the chip, not managing to be trans-

ferred to a billet, and therefore there is heat redistribution in

the cutting area. A reduction in cutting cross section with an

increase in cutting rate leads to a marked reduction in cutting

force, acting on an object being machined.

Proceeding from this on the basis of high-speed machin-

ing (HSM) the following rule is applied: a marked increase

in cutting rate due to high spindle rotations and high momen-

tary feed, is possible with a small cutting cross section. This

principle, as already noted above, has been realized in ma-

chining various metals [11, 12, 14].

In contrast to metallic materials, machining of ceramics

is carried out with diamond grinding wheels. In machining

shells of rotation with double curvature grinding is carried

out over the outer and inner surfaces with different diamond

wheels using grinding heads with a maximum spindle rota-

tion frequency of 5000 rpm.

Machining of the outer surface is performed with grind-

ing wheel 1A1 with a size of 200 � 6 � 5 � 76 mm, and the

maximum cutting rate is 52 m/sec, which specifies its high

speed. A further increase in rotation rate for a wheel 200 mm

in diameter is restricted by the limiting speed of 50 m/sec set

by plants manufacturing these wheels. In view of this it is un-

desirable to carry out further work for increasing the machin-

ing rate for an outer surface.

Machining of an inner surface is carried out successively

by diamond wheels of shale 1A1 with a diameter of 100 mm,

and 32 mm, and spherical head diameter of 4.5 mm. maxi-

mum cutting rate is correspondingly 26, 8.3, 2.35 m/sec, and

it is extremely low for intensifying internal grinding of a bil-

let. It is required to increase the maximum rate as a minimum

by a factor of two to five. It is not possible to do this by

changing the gear ratio of V-belt transmission of the grinding

heads.

Emerging from this complicated situation it is possible to

use high-speed electric spindles (for example, from the firm

Jager type Z80-M530.03.98 with a shaft rotation frequency

up to 30,000 rpm and a power of 4.5 kW). Introduction of

them into production makes it possible to increase the grind-

ing head rotation rate, and correspondingly abrasive wheels,

which provides the possibility of machining the internal sur-

face at increased rotation rates for a shell billet, and feed

rates. Theoretical calculations have shown that use of

high-speed electric spindles makes it possible to increase the

amount removed Q during machining with a wheel 100 mm

in diameter by a factor of 1.7, and with use of a spherical

head 4.5 mm in diameter by a factor of three.

However, for successful implementation of HSM it is

necessary to provide balancing of a production scheme and

reduce vibration. This is caused primarily by the fact that

with a high rotation rate a rigid production system is trans-

formed into a class of elastically-deformable and dissipative

characteristics, which impose considerable limitations on its

operating regimes [14]. Therefore, in order to implement the

prescribed high-speed cutting regimes it is necessary to re-

turn once more to the question of increasing the rigidity of

fastening billets and centering them in production equip-

ment.

One version for increasing stiffness may be installation

of additional stiffness supports over the length of an object

being machined [15]. Supports are placed on fittings both for

the internal and external machining, and the number of them

is determined proceeding from the relationship

N = L/D, (3)

where N is number of additional fixing assemblies; L is billet

length, mm; D is billet diameter, mm.

Implementation of this technical solution has led to a sig-

nificant reduction in billet wall deformation, and the error of

wall thickness was ±0.02 mm, which in turn markedly im-

proved the quality of the object obtained. At the same time,

in spite of a favorable result, operation of additional fixing

assemblies is connected with some difficulties. Primarily this

relates to the fact due to different production factors, such as

scatter of shrinkage values during firing, presence of an ellip-

soidal nature, billet warping during firing, etc., geometric

billet dimensions are not constant values. Therefore in in-

stalling such objects on a drum for internal machining it is

extremely difficult to select the size of an additional wall fix-

ing assembly for an object, as a result of which there is no to-

tal wall fixing. In installing an internally machined object on

a mandrel for external machining the same difficulties arise

due to the fact that machining of the inner surface of an ob-

ject occurs with some proportion of deviation with respect to

thickness and prescribed shape, and the additional fixing as-

semblies used provide an increase in stiffness of an object in

a limited part of its surface.

It is possible to simplify fixing for the whole thickness of

the wall of a machined component with use as a fixture of

rubber caps, placed between the drum (with internal machin-

ing) or a mandrel (with external machining) and the surface

of a component which are filled with water under pressure

[16]. Before the start of machining water is fed to a rubber

chamber under pressure, excluding defect generation in the

billet material (0.2 – 0.3 of the material ultimate strength in

bending). Supply of water to a rubber chamber leads to the

situation that it expands and uniformly supports a component

Prospects for Improving Machining Productivity of Large Ceramic Objects 429

Fig. 7. Dependence of cutting force on cutting speed (Solomon

curves) for different materials: 1 ) heat-resistant steel; 2 ) aluminum;

3) fibrous metallic materials.

being machined, as a result of which there is a significant re-

duction in component wall deformation, there is pressure re-

distribution along the grind head on the whole surface of a

component being machined, which in turn facilitates compo-

nent machining and markedly improves quality.

Thus, it is possible to dare to assert that HSM is the most

promising direction in the field of machining ceramic materi-

als, and carrying out work in this area will make it possible to

increase productivity considerably during machining of large

ceramic objects with improvement in surface quality.

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