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

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8 High Performance Grinding Machines 283

Fig. 8.3 KMT

Nano grinder. a Machine picture. b Axes configuration

A very special new machine architecture was presented in 2005 by the com-

pany KMT Lidköping

. The new concept is a very compact small machine, called

the Nano Grinder, in which the conventional combination of the rotating work-

piece and the longitudinal penetration displacement of the wheel is substituted by

two eccentric rotary axes, as shown in Fig. 8.3b. The machine is particularly ori-

ented to the external and internal grinding of bearing rings.

With this axes configuration the machine can perform internal and external cy-

lindrical grinding, of parts up to an 80

mm external diameter, with a very small

footprint. High accuracy is also claimed by means of the new architecture, sup-

ported by the use of linear motors and aerostatic guides.

8.2.1.2 Surface Grinders

In general-purpose surface grinders two general configurations are possible: “reci-

procating column” and “reciprocating table”. Similar to cylindrical, reciprocating

table is the preferred configuration except in cases of high length or weight work-

pieces, where reciprocating column is selected.

For both configurations a very common architecture, used in case of high volume

(height and width) parts, is the bridge-type machine. The grinding of machine tool

structures and components such as guideways is a typical application for bridge-

type grinders.

Among surface grinders, “creep feed grinding” machines must be remarked

upon. These machines, applied in very high material removal processes, in parti-

cular for difficult-to-machine materials such as titanium and nickel-based alloys

284 R. Lizarralde et al.

for aeronautic applications, present very compact and stiff architectures to over-

come the high grinding loads generated by such processes. Traverse column is the

most extensively used architecture, incorporating high power main spindles, and

special devices such as continuous dressing and very special coolant application

systems. An example of this configuration is presented in Fig. 8.4.

8.2.1.3 Vertical Cylindrical Grinding Machines

Vertical machines are a variation of cylindrical ones with some architectural simi-

larities with surface grinders. These machines are applied for the finishing operation

of big size parts, in particular big diameters with lower lengths, gaining positions in

the last years in the aeronautic and energy generation markets. The typical configu-

ration of vertical machines is presented in Fig. 8.5, a rotary table supporting the

workpiece, and a vertical column holding X and Z-axis and the grinding wheelhead.

Following the tendencies of the high productivity and flexibility of cylindrical

machines, vertical ones usually present multi-wheel heads, for different operations

Fig. 8.4 Creep feed grinding ma-

chine with continuous dressing

device

Fig. 8.5 Vertical grinding machine,

by Danobat

8 High Performance Grinding Machines 285

to be performed, including external and internal cylindrical grinding, surface

grinding and even turning in the most recent applications, as presented in Fig. 8.6.

8.2.1.4 Centreless Machines

In centreless machines there are two main configurations, “fixed work centre” and

“mobile work centre”, applied depending on the type of workpieces to be ma-

chined. In the latter case, the grinding wheel is fixed on the machine base, and the

regulating wheel and work-rest move to allow different workpiece diameters,

different set-ups and to perform the grinding infeed movement.

This configuration presents some variants in the movement of regulating wheel

and work-rest:

• Two displacement axes, one in the low carriage that holds the work-rest and

a second one between this carriage and the regulating wheel.

• In some machines the driving unit is unique and the two carriages can be

clamped or released, thus moving both together, to set up the workpiece, or

only the regulating wheel, for the infeed movement.

Fixed work centre configuration is mainly used for the grinding of long parts

and especially in through-feed grinding, in which the movement of the work-rest

makes very complicated the setting up of the workpiece loading system. The grind-

ing of long bars and pipes is a typical application of fixed work centre machines.

An unusual configuration in centreless machines is the one presented by com-

pany Nomoco

, with the grinding wheel located above the regulating wheel, as

presented in Fig. 8.7.

Fig. 8.6 Detail of vertical grinding machine

with three tools: surface grinding, internal

grinding and turning, by Danobat

286 R. Lizarralde et al.

Fig. 8.7 Nomoco

centreless vertical wheel configuration with gauge

With this configuration the workpiece setting up can be simpler and the possi-

bility of applying a measuring gauge in the workpiece area is also claimed, as

shown in Fig. 8.7b.

8.2.1.5 Combined Configurations Integrating Grinding and Turning

The discussion about the frontiers and overlapping between turning and grinding

was launched in the 1980s supported in the development of “hard turning” proc-

esses (see Sect. 7.2.1.1) and more accurate lathes that were able to compete in

some applications with grinding, and at the same time, the development of su-

perabrasives for grinding wheels boosted the development of high removal rate

grinding processes, allowing the complete machining of parts, roughing and fin-

ishing, in grinding.

This discussion, together with the growing tendency of the industry towards the

complete machining of parts in one set-up has promoted the development of lathes

that integrate grinding devices and grinding machines that incorporate turning

tools, drifting the discussions towards the question of which solution is more suit-

able. Some of the most important grinding machine manufacturers have material-

ised this tendency into new models such as the Studer

S242, shown in Chap. 1,

Fig. 1.4, besides the above mentioned vertical machines that usually integrate

turning tools.

8.2.2 Materials Applied in Structural Parts

The main characteristic that differentiates grinding machines from any other ma-

chine tool is the required high accuracy. Specifically, this is the accuracy repre-

sented by accurate positioning, accurate movements, and the avoidance of any

8 High Performance Grinding Machines 287

perturbance: vibration, static or thermal deflections, stability and repeatability. For

this purpose, special care is taken in the selection of the components and the de-

sign of the machine structure, searching for compact architectures, with high stiff-

ness and dynamic stability.

The same accuracy and stability criteria have been historically applied to the

selection of the constructive materials. Cast iron is the clear leader, applied in

more than 80% of the machines, as explained in Chaps. 1 and 2.

Although some manufacturers decided to use steel instead of cast iron the better

damping characteristics of this material is the principal motivation for its massive

use in respect to steel.

Nevertheless, grinding manufacturers are the most active among the machine tool

manufacturers in the search for alternate materials. Polymer concrete, in several

slight variants, and natural granite are the two materials mostly used after cast iron.

Polymer concrete (also known as mineral casting, in particular in Germany) is

a material composed by a polymeric resin, usually epoxy based, filled with different

Fig. 8.8 Cylindrical grinding

machine with pure granite bed,

y Overbec

-Danoba

Table 8.1 Comparative values: polymer concrete vs cast iron and steel

Characteristic Polymer concrete Cast iron Steel

Density (kg/dm

) 2.3 7.15 7.85

E-Modulus (kN/mm

) 30–40 80–140 210

Tensile strength (N/mm

) 10–15 150–400 400–1,600

Compressive strength (N/mm

) 110–125 600–1,000 250–1,200

Bending strength (N/mm

) 25–35 250–490 –

Relative damping 0.02–0.03 0.003 0.002

Thermal conductivity (W/K·m) 1.3–2 50 50

Specific effective heat capacity (KJ/kg·K) 1 0.5 0.5

Thermal coefficient of expansion 14–16 11 12

288 R. Lizarralde et al.

sized stone particles in specific percentages. Variations in this distribution lead to

different material denominations, such as the Granitan

, registered by the company

Studer

. The advantages of polymer concrete in comparison with cast iron are:

• Technically, a much better damping ratio, a better thermal response and similar

or better stiffness achievable in the parts. Table 8.1 presents the comparative

values between polymer concrete, cast iron and steel.

• From the manufacturing side, a proper design of the structural parts allows to

include into the structural elements components such as guideways, or fluid and

electric conductions, obtaining better assembly and set-up times.

Natural granite is less extended than polymer concrete, but in the last years it

has grown in applications, especially in very high precision machines. The main

virtue supporting its use is the stability along time, besides the mechanical charac-

teristics achievable. An example of a cylindrical high precision machine bed made

by granite is presented in Fig. 8.8 and in Chap. 6, Fig. 6.15a.

8.2.3 Main Components

This section will focus on the components that have a significant influence on the

machine behaviour, in particular driving systems and guideways.

8.2.3.1 Driving Systems

Since the strong development of electronics, electric motors and controls, the most

expanded driving system in machine tools and, consequently in grinding, has been

that composed by a ball screw driven, by means of a belt transmission directly

operated by an electric motor. This is the most standard configuration, applied in

horizontal and vertical axes, to drive the workpiece and wheelhead.

Nevertheless, this basic configuration has experienced several significant de-

velopments, oriented to improve the performance of the system in concordance

with the market increasing demands. Beyond the general improvements achieved,

in terms of higher speeds available, obtained through new lower-friction materials,

surface coatings applied and new ball screw designs, the most remarkable con-

cerning grinding process requirements, are:

• Hydrostatic ball screws. One of the disadvantages of ball screws concerning

their use in grinding is the metallic contact between the two moving parts con-

nected by the ball screw. On the one side, this is because any geometric or sur-

face error in the system can be directly translated to the contact between part

and grinding wheel, and finally “copied” into the ground surface. On the other

side, this is due to the absolute lack of damping introduced by that metallic

contact. Hydrostatic technology has been developed for ball screws using the

experience and principles applied over the years in hydrostatic guides. The

8 High Performance Grinding Machines 289

balls between screw and nut are replaced by a pressurised oil film, avoiding the

metallic contact between both elements, as shown in Fig. 8.9.

Hydrostatic ball screws are mainly applied in the grinding of eccentric parts,

like cams and crankshafts. In these eccentric parts the process is performed by

means of the synchronisation of the part rotation with the forward and back-

ward displacement of the grinding wheel. Hydrostatics improves the quality

and smoothness of the grinding wheel movement and also presents a signifi-

cantly better life for the system, due to the lack of contact wear.

• Isolation of the driven component from the ball screw system in the traverse

axes. This solution is performed to avoid the transmission of perturbances from

the grinding wheel side to the workpiece or vice versa. By means of this sys-

tem, the axial performance of the ball screw and the whole system remains in-

variable, but the traverse directions are decoupled. There are several different

solutions, by means of sliding units or fluid chambers, the solution patented by

Toyoda

being one of the most advanced.

Linear motors were dedicated in their beginnings to machine tools in high

speed milling, supported by their ability to reach significantly higher speeds and

accelerations, compared to traditional transmissions. The evolution of linear mo-

tors towards smaller sizes, more efficient constructions and ironless motors which

minimise the cogging effect, have opened their application range, and in the last

years many grinding machine manufacturers are selecting them for their high

Fig. 8.9 Scheme of hydrostatic screw

(company Hyprostatik

)

Fig. 8.10 High precision

driving solution: hydro-

static guideways plus

a linear motor

290 R. Lizarralde et al.

precision applications, combining them with hydrostatic guides to configure ultra-

high precision machines without any mechanical contact and high dynamic re-

sponse machines, as presented in Fig. 8.10.

8.2.3.2 Guideways

The demands for guideways in grinding machines are: smooth movement, high

repeatability and positioning accuracy and providing of static stiffness and damp-

ing for a stable, vibration free process.

Historically, slide guideways have been the most widely applied system. The

advantages of slide guideways are the large contact surface, that provides very

good stiffness and good damping. Hand scraped surfaces and special low friction

materials such as Turcite

are among the machine elements that have been the

subject of maximum care for the manufacturers. On the other side, their main

disadvantage is their relatively high friction that penalises the positioning accuracy

and repeatability, in particular in reversing movements due to stick-slip.

Nowadays, slide guideways are still the most used system, although the com-

petitors are increasing.

Commercial linear guideways have gained their market share in the last

years, due to the great efforts of the manufacturers towards the minimisation of

their limitations. The main advantages of these systems are the low friction that

allows good positioning and repeatability, even in short stroke and reversal

movements, the good stiffness achievable and the easy assembly and maintenance

operations. On the negative side, the low friction and purely metallic contact be-

tween parts rebounds in very poor damping values.

Several configurations of linear guideways are being applied, form the compact

recirculating units applied in general-purpose cylindrical and surface machines, to

Fig. 8.11 Self-compensating membrane valve

8 High Performance Grinding Machines 291

the needle cages applied in high precision machines, centreless machines, dressing

units and heavy-duty creep feed surface grinding machines.

Finally, non-contact systems are applied in high precision machines. Hydro-

static or hydrodynamic systems collect the benefit of the other systems, in terms of

very smooth and accurate positioning, high stiffness and high damping, and long

operating life, with the sole disadvantage of the high cost of manufacturing and

operation.

Hydrostatic systems are the subject of much research in several ways, which

searches for more controllable, more flexible, adaptable and easier to manufacture

elements. Among the various research lines are the following:

• Self-compensating valves. These systems replace the typical capillary restric-

tors that provide a fixed pressure drop, by valves that are able to absorb and

compensate variations in the pocket pressure, stabilising the system for the new

conditions. The scheme of these valves can be seen in Fig. 8.11.

• Self-compensating coupled bearings. In this type, coupled bearings (on one side

and the opposite side) are connected to each other, allowing the system com-

pensation and avoiding the use of any external elements, such as capillaries, to

generate the necessary pressure drop. This solution is much easier to manufac-

ture and set up, requiring also lower maintenance. A scheme and a prototype of

this type of systems are presented in Fig. 8.12.

Very high precision machines and also heavy weight machines configure the

main market of hydrostatic guideways.

8.2.4 Wheel Dressing Systems

Wheel dressing is the process that regenerates grinding wheel geometry and to-

pography, removing ground material chips that penetrate in the cavities of the

porous bonding, removing also the most worn abrasive grits and sharpening the

rest of the grits, renewing the cutting capabilities of the wheel.

Fig. 8.12 Hydrostatic self-compensated coupled bearings

292 R. Lizarralde et al.

8.2.4.1 Conventional Dressing

This process is performed by means of the interaction between diamond grits

with the grinding wheel surface, grits and bonding. These diamond grits are

structured in different distributions and the interaction is performed by different

strategies and devices, configuring the various dressing systems existing in the

market:

• The original and most basic dressing is the single-point process, performed by

a simple diamond tip. It can provide a very accurate profiling of the wheel, and

is the simplest system, not requiring any special device. On the negative side,

the dresser has a short life due to the small size of the tip.

Variants of this single-point dresser are the long diamond blade and needle

diamond dresser tools. Both are also simple static elements, and their difference

in relation to the single-point is that they present several diamond grits, or

a bigger diamond plate, providing in both cases a longer life for the dresser.

Single-point dressers are nowadays used mostly in very cheap, simple, gen-

eral-purpose machines or to obtain complex profiles in low to medium produc-

tion applications.

• The roller disc dresser (Fig. 8.13). A metallic disc with diamond grits plated on

it. This is the next step in terms of productivity and it is necessary for dressing

harder abrasives such as CBN. Besides the pure advantage of the longer life

provided by the higher surface of the diamond, the roller allows variants in

terms of dressing strategies, playing with the disc diameter and the combination

of wheel and dresser speeds, searching for the optimum condition for each

application.

Concerning the configuration of the system, the dresser disc is a simple de-

vice conformed by a motor driving a precise spindle that holds the disc. The

whole dresser is usually fixed in the workpiece side of the machine (workhead

or tailstock body in case of cylindrical grinding and table in surface grinding)

to compensate any geometric or thermal error between the wheel side and

workpiece side.

Fig. 8.13 Disc dresser. a Vertical machine. b Disc integrated in workhea