Lopez de Lacalle L.N., Lamikiz A. Machine Tools for High Performance Machining

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11 Micromilling Machines 375

Table 11.2 Application cases of micromilled moulds

Workpiece Mould for a surgical dosifier Mould for dental brackets

Photo

Material W.Nr 1.2343 (AISI H13) 55 HRC W.Nr. 1243 (AISI H13) 55 HRC

Tool Cylindrical micromill, z 2, Ø0.1 y Ø0.2

Cutting conditions N 40–60,000

rpm; F 15

mm/min; a

3–10

µm

Geometric figures Mould microchannels of straight

rectangular section 0.1

0.1

and 0.2

0.2

3D complex geometry with geo-

metrical details in the order of

0.2−0.3

Result 0.1

µm Ra, geometrical accuracy

µm, no burrs allowed

0.03–0.08

µm Ra, geometrical

accuracy +3

µm, no burrs allowed

Table 11.1 shows the current industrial capabilities of micromilling [18] for

certain simple geometries such as channels and ribs of straight rectangular sec-

tions, and circular holes and pillars in steel for moulds. Table 11.2 shows some

industrial cases of micromilled moulds.

In ultraprecision cutting with tools of monocrystalline diamond (it is extremely

hard and can be given a cutting edge of almost atomic-level sharpness) the produc-

tion of very sharp cutting edges is therefore one of the most important tasks speci-

fied for microcutting. Among the ultraprecision-machining components are

moulding tools made of non-ferrous metals – for example, for Fresnel lenses or for

roughness standards. Ultraprecision milling is more flexible than ultraprecision

Fig. 11.5 3 axis ultraprecision milling machine with diamond tools and some application cases

(courtesy of Fraunhofer IPT)

376 L. Uriarte, J. Eguia and F. Egaña

turning. For instance, single-point diamond fly-cutters allow the production of

grooves that can be crossed in corresponding angles to create columnar or pyrami-

dal structures. Materials for diamond-cutting are mainly aluminium, copper, brass,

nickel silver and nickel (see Fig. 11.5).

11.3 Miniaturised Machine Tools

In the literature we can find similar terms that can lead us to misunderstanding:

ultraprecision machines, micromachines, miniature machines and desktop ma-

chines among others. Up to now and in further sections we will be focussed on

ultraprecision machines, conventional size machines with submicron accuracy,

which can be employed to produce large parts with submicron accuracy and opti-

cal quality; for this purpose diamond tools are generally used, or to produce small

meso-micro parts; for this purpose carbide micromills are generally used.

The term micromachine has a more general meaning [8] than production

equipment, and it refers to any kind of microdevice able to do an action in a sense

closer to MEMS (micro electro mechanical systems).

Despite the vast volume of work and developments in the machine tool industry

for macro-machining, fairly limited research has been carried out in the field of

miniature machines. In general they are specially designed and built miniature

machine tools, integrated in a single platform (Fig. 11.6), which usually also inte-

grates assembly and inspection capabilities. Thus, it is more usual to find micro-

factories than independent miniaturised machine tools. A worldwide effort is cur-

rently underway to bring such microfactories to fruition. Such miniature machine

tools, however, remain unavailable commercially to-date. Most of the current

micromachining is done using the conventional-size machine tools, so this chapter

is devoted to those so-called ultraprecision machines.

Fig. 11.6 A series of miniature

factory units at Sankyo Seiki

11 Micromilling Machines 377

On the other hand there is a large market of desktop milling machines, which in

their external size could be similar to the previous miniaturised machine tools;

however they are specifically suited for meso-machining with low accuracy

(0.1

mm per slide as the standard). They are applied for printed circuit boards,

jewellery, etc.

11.4 Machine Drives

Drives for ultraprecision milling machines should be designed according to the

principles of precision engineering, the first one being deterministic design. Any

precision drive should fulfil the principles of minimising the Abbe error (see

Sect. 6.2), minimise the number of restrictions (kinematic or semi-kinematic de-

sign), and to act at any centre of action (centre of gravity, centre of inertia, etc.)

minimising the perturbations at the guiding system, a measuring system centred in

the goal position. The main objective to be achieved is the repeatability of the

movement, and by means of a further calibration, the final goal of precision.

Practical applications need a balance between contradictory requirements, but the

goal should be to move away from the minimum from the theoretical precision prin-

ciples. The next pages show examples of drives of existing micro-milling machines.

11.4.1 Conventional Ball Screw Configuration

Transmitting the movement to a linear carriage trough a ball screw is without

question the most common system in machine tools, just as in precision machines.

Ball screws enables a repeatability of 1

μm with ease, and even submicron repeat-

ability in the case of ultra high precision ball screws. In spite of this, no matter

how high the manufacturing accuracy of the screw is, the intrinsic nature of such

a system, in the end based on rolling elements, creates unavoidable perturbations

which can be clearly identified, even if they are submicrometric. This intrinsic

sources or error derive from the roundness and the uniformity of the balls, the

accuracy of the lead of the screw, the system for the recirculation of the balls and

the preload. Moreover, errors coming from the manufacturing and the assembly of

the machines must be added, which are the misalignment between the spindle and

the guideways, the capability of the design to cope with thermal expansions, the

bending of the spindle itself and several others. All of them affect the accuracy of

the movement of the linear carriage.

11.4.1.1 A Ball Screw Drive with a Floating Nut

In precision machines required for micromilling, where the final precision is the

main design criteria, it is necessary to isolate the movement of the carriage, defined

378 L. Uriarte, J. Eguia and F. Egaña

FEED DIRECTION

TRAVERSE DIRECTIONS

Fig. 11.7 “Floating nut” system to minimise the transmission of ball screw perturbations

by its guides, from the errors and perturbations due to the drive. The ideal situation

is that where the element in charge of transmitting the movement to the carriage, in

this case a ball screw system, creates no perturbation to the guiding elements of

such carriage, so that the guides become the elements that exclusively define the

trajectory of the moving elements.

One of the existing design solutions to achieve such objectives is the use of

a floating nut system. The principle behind the “floating” nut is that the flange that

joins the nut to the carriage is not completely coupled to said carriage; instead, the

flange is designed to be very stiff in the axial direction (the direction of the move-

ment) and under torsion, and very flexible in the other two directions perpendicu-

lar to the movement (Fig. 11.7).

Besides this, to improve its effect, the spindle has been one with a minimum

preload configuration, fixed on one side and in a cantilever configuration on the

other side. It is essential that the dimensions are correctly set during the design

phase to ensure that the stiffness in each direction is appropriate in order to avoid

affecting the dynamic response behaviour in comparison with the traditional solu-

tion with a nut tightly joined to the carriage.

11.4.1.2 A Ball Screw Drive with Intermediate Joints

In drives where there are space restrictions that avoid the use of the floating nut

system, errors of the spindle can be isolated by means of a ball screw with inter-

mediate joints. It consists of a traditional ball screw that is divided into several

sections, by purposely reducing its diameter in two points, so that a relative-joint-

effect is created. The points with reduced diameter are out of the range of the

11 Micromilling Machines 379

Free end

Carriage

clamping area

Reduced diameters

Fixed end

Fig. 11.8 “Intermediate joints” ball screw system

movement. Figure 11.8 shows the “ball screw with joints” concept. A cantilever-

fixed preloaded precision ball screw has been thus divided. It is also worth saying

that in order to maximise the “joint effect” the nut is fixed to the carriage in the

area close to the free end of the screw.

The correct dimensioning of the system is critical in this case as well, so that

the loose stiffness, which is obviously produced when reducing the diameter, is

minimised and does not affect the performance of the carriage. Concisely, the

stiffness under bending loads is reduced while, at the same time, trying to mini-

mise the loose of rigidity under axial and torsion loads. For example, in the appli-

cation case shown in Fig. 11.8, the bending stiffness has been reduced from

bending

160

N/mm (original screw) to K

bending

N/mm (reduced screw), which

means a reduction of 81% from the original figure. Meanwhile, the axial stiffness

has changed from K

axial

160

N/μm (original screw) to K

axial

130

N/μm (reduced

screw).

11.4.2 Friction Drives

Friction drives have been commonly used in ultra high precision machines such as

coordinate measuring machines (CMMs). Their main advantage is the reduction of

the error sources such as clearances, lead errors, ball-recirculation errors etc. The

only “death area” of this particular drive comes from the deformation of the ele-

ments in the transmission, which can be regarded as non-existent if the system is

designed properly. On the other hand it is a pretty simple drive from the concept

point of view, and although commercially available solutions are uncommon, both

their design and their assembly are not challenging at all.

Nevertheless, it shows important drawbacks, namely low stiffness, low damp-

ing, low load capacity and low capability for reduction. If one wishes to increase

the pushing force, the preload between the wheel and the pushing rod must be

considerably increased, thus creating problems dealing with deformations and

stress in the contact between elements.

380 L. Uriarte, J. Eguia and F. Egaña

Driving pulley

Pushing rod

Friction shaft

Preload pulley

Fig. 11.9 Friction drive for a micromilling machine

Swing to this, their application is limited to movements requiring extremely

high precision and softness, low pushing force and low speed, as in the mi-

cromilling machines.

In a friction drive (Fig. 11.9), the rotation motion of the servomotor is trans-

formed into the lineal advance by means of the interaction between a friction

wheel and a pushing rod. The rotation of the servomotor is transferred to the fric-

tion wheel either directly or through a belt and pulley system. The pushing rod

transmits the lineal movement to the carriage trough, a flexible coupling which

absorbs the misalignment between rod and carriage. The contact and rolling be-

tween the rod and the friction wheel are ensured by means of an adjustable preload

system, which acts over the pushing rod through the preload pulley.

11.4.3 The Linear Motor

There are many advantages of linear motors that make them suitable for precision

applications:

• The accuracy, resolution, and repeatability of the system to be moved is deter-

mined by the feedback measurement system.

• Regarding the stiffness on the movement direction, the advantage of the linear

motors is that they do not introduce additional flexibilities to the system because

11 Micromilling Machines 381

the transmission is direct and no additional components are needed as in the

case of rotary motion that “transmission elements are needed” to convert rotary

motion to a linear one. Then on the movement direction the achievable stiffness

of an axis driven by a linear motor is dependent on the mechanical structure to

be moved and the control strategy implemented.

• Without mechanical transmission components, there is no backlash and also

there is no wear.

• For ultraprecision applications the best option is an air core or ironless linear

motor. The winding itself has no iron in it, thus the names “air-core” or

“ironless”. This type of linear motors are more suitable for precision applica-

tions due to the fact that they have no attraction forces and no cogging.

The next paragraphs show an example of an ultraprecision linear slide by using

a linear motor (Fig. 11.10). It is composed of a six hydrostatic pad bearings slide

using the slide’s own weight as a preloading force, an air core linear motor in

order to have no attraction forces, and a linear scale as measurement system.

The lineal slide is supported over hydrostatic bearings, using an integrated lin-

ear motor acting just at the centre of gravity of the moving slide, an optical linear

scale Heidenhain

LIP 401, with a travel range of 100

mm with 0.1

μm of resolu-

tion with 4

μm of separation between lines, the linear speed is 5

mm/s, and there is

g of acceleration and stiffness values of 45 and 20.5

N/μm in vertical and lateral

directions, respectively.

The stiffness of the slide is fully dependent of the bearing design. The slide is

not flat and has two different slopes on its contact surface having six flat capillary

hydrostatic bearings, four on the lowest slope side and two on the highest one.

Figure 11.11 is a section of the 3D model in which the moving part (A) is shown

in dark and the fixed part (B) is represented darker.

Fig. 11.10 Precision drive by linear motor preloaded by the carriage weight

382 L. Uriarte, J. Eguia and F. Egaña

Fig. 11.11 Precision drive by linear motor. a Geometry verification. b Bearing calibration.

c Slide error measurement. d Loading test by a “human weight”

11.4.4 New Tendencies: Hydrostatic Screws

An alternative existing in the market with a very good performance is the use of

hydrostatic leadscrews (Fig. 11.12), which can be complemented also with hydro-

static support bearings. They have better performance than ball screws and clearly

can compete with linear motors. The only drawback is the need of a hydraulic

system for the oil supply. Their main advantages in comparison with ball screws

and linear motors are as follows:

• Wear-free as no contact when in use, so no loss in accuracy, even when used

for long periods under full load at maximum speed.

• Friction-free and no “stick slip” effect even at low speeds.

• No backlash when the direction changes, only the change of inertial force shall

be considered.

• Translates even the smallest turning movement.

• No oscillation due to ball circulation which produces no fluctuations in fric-

tional torque.

11 Micromilling Machines 383

• Higher axial stiffness similar to ball screws.

• Great improvement in damping, resulting in better surfaces and tool life.

• Also suitable for high speeds and acceleration.

• A cost-effective alternative to the linear motor, with high accuracy and much

less heat created.

• Much lower losses and the absence of permanent magnets means no problems

with metal chips.

• Much lower cooling power than linear motors.

11.5 Guideways

Although drives and guideways are presented in separate paragraphs, it is unthink-

able to afford each of them in a separate way from the design point of view. Kine-

matic designs, no friction, no wear and no backlash are the key issues to achieve

the required repeatability.

11.5.1 Special Rolling Guides Configurations

The use of cylindrical roller bearings without recirculation is moderately common

in the motion of the linear carriages of micromilling machines. The absence of

recirculation systems eliminates the related perturbations, at the expense of higher

size and space demands. If submicrometric accuracies are to be achieved, solu-

tions with recirculation become unthinkable and highly unlikely.

The embodiment of choice is 2 V-guides in opposition, in a semi-kinematic

configuration with the rollers contacting two faces thanks to a constant preload.

The preload force shall be calculated to fit the application and the related loads

Fig. 11.12 Hydrostatic lead screw by Hyprostatik®. a Screw detail. b Comparison of perform-

ance in relation with linear motor slides; the good damping performance can be observed

384 L. Uriarte, J. Eguia and F. Egaña

that the carriage is suffering during operation. Anyway, when high precision is

the primary objective, a “constant preload” solution must be chosen above “con-

stant displacement” solutions. Constant displacement solutions are easier to ap-

ply, but, on the contrary, create higher dispersions in repeatability due to the

irregularities of the rolling surfaces. Constant force preloads can be achieved in

different ways, e.g., created by the weight of the system itself in the case of

horizontal carriages, by means of counterbalancing masses in vertical carriages

(Fig. 11.13) or by spring-based locking systems that show well-known behav-

iours. With this configuration the following stiffness values have been achieved:

450

N/μm in the perpendicular and lateral directions of the carriage. The

achieved straightness of the carriage is +

0.2

μm in 150

mm for a vertical slide.

11.5.2 Aerostatic and Hydrostatic Guides

Hydrostatic and aerostatic guides are widely applied to precision feed tables,

since their motion is accurate due to the lack of friction and wearing. Besides, the

so- called averaging effects of the fluid (oil or air) average the motion errors. In

the case of aerostatic guides, where load is supported over a thin film of air, the

low viscosity and compressibility of air obliges the use of very high manufactur-

ing and assembly tolerances. Typical gap values can be in the order of 10

μm for

a standard air pressure supply of 6

bar flowing in a continuous way to the atmos-

phere. Their main advantage is the lack of friction, which assures a fine and pre-

cise movement even at low or high speeds. Their drawbacks are related with the

low load capacity and the complexity of the design and manufacturing. These

Fig. 11.13 Guiding by rolling elements. a Kinematic concept. b Preload by the carriage weight

in a horizontal slide. c Preload by counterbalancing masses in a vertical slide