guide to centerless external cylindrical grinding – part ii of the reference work

27

After Chapter Fundamentals gave a short overview of centerless grinding, the fol-lowing explains it in more detail.

The grinding zone is the heart of centerless external cylindrical grinding. To achieve optimal roundness of the workpiece, specific geometric conditions are necessary in the grinding zone. If the acting forces and relationships are known, the roundness can be significantly improved.

Why, for example, does the regulating wheel determine the speed of the workpi-ece instead of the grinding wheel? Or why does the workpiece not jump out of the grinding zone? The following chapter will answer these questions. It also includes formulas for calculating important grinding parameters.

3

The grinding zone

Grinding zone geometry

Center shift Roundness Stability Height position Polygons Forces

28

3.1 Center shift

A special feature of centerless grinding is the inclined workpi-ece support on which the workpiece rotates. During grinding, the workpiece continuously loses in circumference, so its center incre-asingly moves into the grinding zone (Fig. 3-1).

This center shift is the reason why diameter-related infeed, not radius-related infeed (as in grinding between centers) is used in centerless external cylindrical grinding.

As the grinding zone into which the workpiece sinks becomes narrower and narrower, normally less infeed is required than the pure difference between initial diameter and final diameter would imply. On KRONOS machines with a grinding zone geometry soft-ware, this effect is taken into account and corrected automatically in the infeed program.

Fig. 3-1

Center shift of the workpiece

during the grinding process

InfeedInfeed

3 The grinding zone | Center shift

… space between grinding wheel,

regulating wheel, workpiece, and

workpiece support

29

3.2 Shaping process

Roundness of workpieces has a special significance in centerless external cylindrical grinding. Though the machined workpieces can have the same diameter at all points, they may still not be round. For better understanding, the following provides a definition of roundness:

3.2.1 Roundness

A workpiece is round if there is a point in its cross section (e.g. the center) from which all points on the circumference have the same distance. Ideally, this is a circle, but in practice this is never reached (Fig. 3-2). If the diameter of a workpiece is measured at various points, this cannot be used to infer its roundness, because roundness is not directly related to the diameter (Fig. 3-3).

Misleading: The workpiece in Fig. 3-3 is definitely not round. Nevertheless, its diameter is always the same. This is therefore referred to as so-called curve of constant width.

So, while the diameter can be determined using a two-point measuring method, the workpiece must also be rotated to measu-re its roundness (Fig. 3-4).

Fig. 3-2 Roundness of a circle

Fig. 3-3 Determining the roundness by the diameter

… more in the glossary

Shaping process | 3 The grinding zone

30

3.2.2 Determining the roundness error

Roundness errors arise during the grinding process due to a variety of influences such as geometric and dynamic factors.

To determine the roundness error (Rd), one calculates the diffe-rence between the smallest circumference (da) and the largest inner circle (di) (Fig. 3-4).

This calculation requires that the inner circle be located centrally to the circumference (Fig. 3-4).

The workpiece center required for this can be determined in several ways.

For correct roundness measure-ment, a roundness measuring de-vice should be used. Alternatively,a dial gauge [1] can be used in combination with a two-point support v-block [2] (Fig. 3-5). The v-block angle of a support v-block has a crucial significance for the measurement results.

Fig. 3-5

Determining the roundness during

rotation

Rd = da – di

Fig. 3-4

Workpiece center with the “Minimum circular ring zone“

method

↑ Section 3.2.4 Polygons

3 The grinding zone | Shaping process

[2] Two-point support v-block

[1] Dial gauge

31

Reducing the roundness error

During centerless external cylindrical grinding, the workpiece is both guided and machined using its outer surface. Therefore, it is not pos-sible to achieve absolute roundness in principle, resulting in con-stant width shapes. However, under suitable setting conditions, it is possible to reduce the remaining roundness error to a large extent. The results are in ranges that are comparable to those achieved by processes between centers.

3.2.3 Height position H and mounting angle β

The center line Z is the direct connection between the centers of the grinding wheel and regulating wheel. If the workpiece axis during grinding is on this center line Z, this is referred to as a zero height position of the workpiece (Fig. 3-6). If workpieces are ground

with this setting, they will not be round. If an elevation on the workpi-ece surface contacts the regulating wheel, a pit is ground exactly at the opposite side into the workpiece by the grinding wheel. An error (wave crest) on one side therefore also causes an error (wave trough) on the other side (Fig. 3-6).


Fig. 3-6 Height position equals zero

Wave crest Wave trough

32

During further revolutions, these two positions cannot cancel each other out, because they are diametrically opposite. The workpiece remains non-round.

To improve the roundness, the workpiece is moved out of the cen-ter and the support angle β is increased. In the so-called „grin-ding above center“, the contact points of grinding wheel and regulating wheel are not directly opposite each other anymore. The resulting center shift of the workpiece leads to a reduction of the circularity error, if its amount is smaller than the roundness

error that causes it (x < x1, Fig. 3-7). A limit of increase is achieved when the forces from the grinding wheel and regulating wheel start to act upward and the workpiece begins to lift off from the workpiece support.

However, height position and rotational speed are no guarantee for success, because other factors play a role as well.

Fig. 3-7

Reducing the roundness error

by grinding above center

Wave crest Wave trough

↑ Section 3.2.5 Stability index SI


The height position H is referred to

as above-center position H in the grinding-above-

center process as shown.

33

Instead of grinding the workpiece as described above the center line Z, it can also be ground „below center“. Good results are achieved also with this “grinding below center“. Both grinding methods have advantages and disadvantages, which are analyzed more closely in the following comparison:

3.2.4 Polygons

Each grinding zone adjustment has a typical roundness scenario. This means, the periodic waves on the outer surface always occur in the same degree of specification.

Grinding “above“ center Grinding “below“ center

In throughfeed grinding, workpieces with a conical regulating wheel shape diverge a frontal surface quality of the workpieces has no influence

Geometrically more stable areas

Low force action on workpiece support

Workpieces have a degree of freedom a less prone to dynamic influences

Workpiece can jump out of the grinding zone a workaround in throughfeed grinding: Application of an upper guide

Workpieces with a conical regulating wheel shape stay in one column in throughfeed grinding a workpieces do not tilt (important for rings)

Workpieces cannot jump out

The process is possible even at greater material removal rates

In throughfeed grinding, workpieces with a conventional regulating wheel shape also remain in one column

Few and small stable areas a very precise machine setting necessary

Workpiece has no degree of freedom a very susceptible to dynamic influences

… self-induced and externally induced vibrationsof machine and workpiece

↑ Chapter 6 Influencing factors

Tab. 3-1 Grinding above and below center

↑ Section 3.3.2 Height position H


34

These factors can be used to classify the roundness error in more detail. For this purpose, the periodic components of the roundness profile are eliminated by a so-called FFT analysis. The result of such an analysis is a roundness spectrum in which the amounts of the amplitudes of all harmonics are plotted. The harmonics are also called polygons in this context.

Figure 3-8 shows the superposition of polygons. A possible cause of the polygon shapes is the grinding zone geometry.

One variable when considering the polygons is the penetration depth e (Fig. 3-9). The lower the wave number of the polygons, the larger is generally the roundness error (Fig. 3-10).

… Fast Fourier Transformation

Fig. 3-8

Superposition of polygons

Fig. 3-9

Penetration depth e

+ =

Polygon of 3 Polygon of 35

Fig. 3-10

Relationship of wave number and

roundness error


35

But how are polygons formed? Let us imagine a grinding zone for plunge grin-ding (Fig. 3-10a). The workpiece rotates and the grinding wheel is fed forward. This creates a new lateral surface (Fig. 3-10b). If it comes in contact with the workpiece support, the original work-piece center will shift. The workpiece moves in the direction of the workpiecesupport, whereby the infeed changes (Fig. 3-10c). A new size of the outer sur-face is created (Fig. 3-10d). If this then comes in contact with the workpiece support, the effect occurs again. Ho-wever, not only by contact of the new la-teral surface with the workpiece support does the center shift occur, but also by contact with the regulating wheel. This in turn changes the infeed again, resul-ting in another roundness error. In total three roundness errors emerged by a half workpiece revolution, which in turn cause further roundness errors.

3.2.5 Stability index SI

The stability index SI provides information on whether a polygon of a certain order improves (is reduced) or worsens (becomes more pronounced) during grinding.


Fig. 3-10a

Fig. 3-10b

Fig. 3-10c

Fig. 3-10d

36

SI(z) = positive a polygon is reducedSI(z) = zero a polygon is not reducedSI(z) = negative a polygon grows

A geometrically stable setting is reached, if the stability index of all considered polygons is positive. In practice, usually the polygons from 2 to 30 are considered.

However, one or more polygons often have negative values. The polygon with the most negative stability index (hereafter called w) should be re-flected the most – from a purely geometric point of view. But tests and energy considerations reveal that the energetically more favorable lower polygon orders with negative SI are dominating. So, there is no direct correlation between the degree of stability and the final working result.

For example, a geometrically unstable setting with w > 15 is preferableover a stable setting with w < 15, because the influence of low polygon orders on the roundness is considerably higher.

The stability index is calculated according to Reeka as follows:

… polygon with the most negative

stability index

Formula 3-1

Calculation of the stability

index according to Reeka

Fig. 3-11

Stability index SI vs. polygon order z

z

More on ϕ: ↑ Fig. 3-13


… polygons with few waves

37

Here, z is the number of corners of the polygon to be calculated. So the formula has to be calculated individually for each polygon. The results can then be represented, for example, in a column chart.

3.2.6 Stability cards

With the formula for the stability index and the subsequent eva-luation in a column chart (Fig. 3-11), only one geometric configura-tion can be investigated at a time.

The so-called stability cards graphically depict multiple results regarding the specificity of polygons at specified grinding settings. They allow a quick view of the optimal grinding settings in each case.

This requires that the information content of the stability cards be reduced compared to the column charts shown in Fig. 3-11. Gene-rally only the polygon with the most negative SI is considered. In doing so, it is often tried to represent both its stability index and the number of corners in the same chart.

Advances in computer performance have made it possible to cal-culate cards for the current case at hand. This allows optimising grinding zone and grinding process even further. In the machines by Schaudt Mikrosa GmbH, a software is used that calculates and outputs a current stability card to the operator already when ente-ring data into the machine control.

Over time, several types of stability cards have been developed, with usage and evaluation options that partly greatly differ from each other. The following table provides a small overview of some stability cards:

↑ Formula 3-1


38

Universal stability cards

• Universallyusable a therefore kept generic and not very meaningful

Reeka typeprovides information about:

• Stableandunstableareas a bright areas are geometrically stable areas

• NumberofcornerswiththeworstSI(w) a indicated as a number in the unstable areas

as a function of:

• Tangentangleγ

• Supportangleβrel Note: Graphic has been calculated for dR/dS=0 .8

Applies only to a specific size ratio of grinding wheel to regulating wheel

Meis typeprovides information about:

• Stableandunstableareas a dark areas are geometrically stable areas

as a function of::

• TangentangleγS

• TangentangleγR

Suitable for examining the stability during throughfeed grinding

Well suited for examining the wear of the grinding wheel and regulating wheel

Note: Graphic has been calculated for β=30°

Applies only to a specific support angle β

Tab. 3-2

Universal stability

cards


39

Specific stability cards

• Arecalculatedspecificallyforthecaseathand a therefore very accurate

Grindaix type (Cegris)provides information about:

• Stableareas(blue)andunstableareas (grey)

• Largenumberofcorners(bright)and low number of corners (dark)

as a function of:

•ϕ1 (also α)

•ϕ2 (also βG)

Mikrosa type (Heureeka)provides information about:

• Stabilityindexa color

as a function of:

•ϕ1

•ϕ2

• OptionallyheightpositionHandsupport angle β or grinding wheel diameter ds and regulating wheel diameter dr

Evaluation:

• Yellow,greenandblueareasarestable

-10 0 10 H 3010

20

30

βrel

50

• Redareasareinstableandshouldbeavoided

Tab. 3-3 Specific stability cards

More on ϕ: ↑ Fig. 3-13


40

3.3 Grinding zone geometry

An accurate understanding of the grinding zone geometry is ne-cessary to optimally control the grinding process and counteract roundness errors. The following chapter therefore focuses on the angular relationships in the grinding zone and their calculation.

3.3.1 Tangent angle γ

The center line Z is the direct connection between the centers of the grinding wheel and regulating wheel. The two wheel ra-dii form the two angles γS and γR towards the workpiece center above this center line. From these two angles results the tangent angle γ (Fig. 3-12).

The tangent angle γ depends on the: • Diameter of the grinding wheel dS• Diameter of the regulating wheel dR• Diameter of the workpiece dW• ValueoftheheightpositionH

↑ Section 3.2.2 Determining the roudness error

Fig. 3-12

Angle relationship in the grinding

zone 1: Tangent angle

3 The grinding zone | Grinding zone geometry

41

3.3.2 Height position H

The contact points of the workpiece on the grinding wheel and regulating wheel are decisively determined by the height position H of the workpiece. If the height position is changed, γS and γR as well as γ itself change also. This means there is a direct rela-tionship between the tangent angle γ and the height position H (Fig. 3-12).

3.3.3 Angle at the contact points

For efficient calculation of the grinding zone, the four parameters γS, γR, β and H are reduced to the two variables ϕ1 and ϕ2:

The angle ϕ1 captures the contact points of the workpiece with the grinding wheel [ 1 ] and the workpiece support [ 2 ].

In contrast, the angle ϕ2 represents the relationship between the workpiece and the grinding wheel [ 1 ] and between the workpiece and the regulating wheel [ 3 ] (Fig. 3-13).

↑ Section 3.2.3 Height position H and support angle β

Fig. 3-13 Angle relationship in the grinding zone 2: Contact points

Grinding zone geometry | 3 The grinding zone

42

3.3.4 Relative support angle βrel

The angle βrel is the difference bet-ween the support angle β and the inclination angle of the center line Z. βrel is relevant during throughfeed grinding and/or grinding with lowered regulating wheel.

3.3.5 Grinding zone geometry during infeed grinding

To grind the workpiece “above center“, there are two ways to po-sition the workpiece above the center line Z: by raising the work-piece or lowering the regulating wheel.

Height position of the workpiece

=

Simple setup

Tangent angle changes due to grinding wheel wear and regulating wheel wear


Fig. 3-14

Angle βrel

Tab. 3-4

Raising the work-piece support

Also in plunge grin-ding, the regulating

wheel is slightly inclined. However,

since the inclina-tion is very small, it is neglected in the figures, and

the lines of contact on the workpiece are considered as points of contact.

In Tab. 3-4, the height position H

corresponds to the height position of

the workpiece Hw.

↑ Chapter 4 Infeed grinding

43

Tab. 3-4 Continued Raising the work-piece support


Height position of the regulating wheel

Ideal engagement conditions during angular infeed grinding

Tangent angle remains constant despite grinding wheel wear (the angle changes by regulating wheel wear)

Center height remains constant during dual grinding. Correction for H are made by changing HR

All formulas apply also to grinding “below center“.

Set of formulas:

Tab. 3-5 Lowering the regulating wheel

44

3.3.6 Grinding zone geometry during throughfeed grinding

The inclination of the regulating wheel by the angle α and the hyperbolic shape of the regulating wheel generated by dressing results in continuously changing geometric conditions during throughfeed grinding. Therefore, the grinding zone geometry du-ring throughfeed grinding is considered separately.

γS, γR, βrel, dR and dW change along the grinding zone width de-pending on the dressing model used. The result is that “stability zones“ are passed along the grinding zone. Software tools should therefore be used to design the geometry during throughfeed grin-ding.

It is important to ensure that geometrically stable areas (positive SI) are present at least in the outlet area. In the inlet area, geome-trically unstable areas (negative SI) can be accepted, since they can be ground out again in the further course of grinding.

↑ Chapter 5 Throughfeed

grinding


Set of formulas:Tab. 3-5 Continued

Lowering the regulating wheel

When lowering the regulating wheel, the value of HR is

negative.

Observe the sign!

45


Fig. 3-15 Inclination of the regulating wheel during through-feed grinding

Fig. 3-16 Relationship between degree of stability, wave number and grin-ding wheel width applies to an inclination angle of αR= 4° in the exa-mined workpiece diameter range0 60 120 180 240 300mm

5 36 32 30 26 24 18

-0.06

0.00

0.06

Stab

ility

ind

ex S

I

Wave number

Width of grinding wheel bs

stable

instable

Workpiece diameter dw = 30 mm

dR Exit

dR Entry

γ0

γ1

β0

β1

H1

H0

46

3.4 Forces in the grinding zone

As already described in Section 3.2.3, the height position H and the support angle β influence the roundness. To optimize it speci-fically, precise calculation of forces is useful.

As with other machining methods, the forces depend to a large extent on the selected cutting conditions. The decisive forces act on the three contact points of the workpiece:

1. Workpiece–Grinding wheel2. Workpiece–Workpiece support3. Workpiece–Regulating wheel

Tangent forces: FtS, FtR, FtANormal forces: FnS, FnR, FnAHorizontal forces: FxS, FxR, FxAVertical forces: FyS, FyR, FyAResultant forces: FS, FR, FA

↑ Section 3.2.3 Height position H and support

angle β

Fig. 3-18

Forces in the grinding zone

Fig. 3-17

Contact points

3 The grinding zone | Forces in the grinding zone

47

The friction force relationships at the individual contact points provide the information on the composition of the forces in the normal and tangent directions. As can be seen in Fig. 3-18, the re-sultant forces can also be transformed into vertical and horizontal components. The following rules apply:

The magnitude of these forces is influenced by the grinding zone geometry and the machining conditions:

3.4.1 Forces on the grinding wheel

The tangential force FtS on the grinding wheel during grindingis the desired force. This is why it is called the main cutting force.

Influences by grinding zone geometry

Influences by machining condi-tions

• Diameterofgrindingwheel,workpi-ece, and regulating wheel

• TangentangleγS, γR and support angle β

• Inclinationofthemachinebed(θ)

• Powerappliedtothesystem(PS, PR, infeed amount ae)

• DeadweightGoftheworkpiece

• Contactconditionsatthethreecontact points

… also referred to as friction coeffici-ent µS , µA and µR

Tab. 3-6 Influences on the forces in the grinding zone

Forces in the grinding zone | 3 The grinding zone

48

FtS... Tangential force on the grinding wheel [N]PS... Drive power on the grinding wheel [W]vS... Circumferential speed of the grinding wheel [m/s]

The friction force ratio at the grinding wheel, also called cutting force ratio µS, provides information about the friction conditions in the contact zone between the grinding wheel and workpiece. It mainly depends on the type of cooling lubricant used. The table below summarizes common values for the cutting force ratio:

3.4.2 Forces on the workpiece support

The forces on the workpiece support cannot be determined solely by power measurement. Accurate determination is possible only by force sensors that are integrated into the workpiece support.

Cooling lubricant class µS according to Ott

CL 1 Water without lubricant 0.6

CL 2 Emulsion with 10% mineral oil in the concentrate 0.5

CL 2.5 Emulsion with 20% mineral oil in the concentrate 0.47





CL 5 Pure grinding oil with additives 0.3

Tab. 3-7

Cooling lubricant classes and

associated cutting force ratio µS


49


The factors that influence the friction coefficient at the workpi-ece support µA include:

• Materialpairingofworkpiecesupportandworkpiece• Lubricationconditions(dependingoncoolinglubricant,nozzle and workpiece width)• Roughnessofthefrictionpartners• Circumferentialspeedoftheworkpiece• Infeedrate

The friction coefficient µA varies within wide ranges of about µA= 0.12–0.4

3.4.3 Forces on the regulating wheel

These forces determine the circumferential speed of the workpi-ece. The value of the friction coefficient of the regulating wheel µR, depends on the material and the binding of the regulating wheel, among other factors.

FtR... Tangential force on the regulating wheel [N]PR... Drive power on the regulating wheel [W]vR... Circumferential speed of the regulating wheel [m/s]

The sign of this friction coefficient depends on the machining si-tuation, depending on whether the workpiece is driven or slowed down by the regulating wheel.

µR = positive a deceleratingµR = negative a accelerating

FtA...tangential force on the work-piece support [N]

Type of regulating wheel µR

Rubber-bound regulating wheel (grain 150) 0.34

Cast steel regulating wheel 0.17

Tab. 3-8 Typical values for µR

50

3.4.4 Consideration of the forces

Consideration of the forces may be useful for the followingreasons:

Loss of contact to the workpiece supportThe angle of the absolute forces between workpiece–grinding wheel (FS) and workpiece–regulating wheel (FR) must not be grea-terthan180°(Fig.3-18).Upwardforcescanbepreventedbyredu-cing the height H or by pressure rollers.

Avoid transverse forces on the workpiece supportThe horizontal forces (FxA) acting on the workpiece support should be low or ideally be even zero. This prevents bending of the work-piece support in the direction of the grinding or regulating wheel. If the friction coefficient µA is known, the support angle β is calcu-lated using the following formula:

Reducing signs of wear on the workpiece support Reducing the friction force acting on the workpiece supportwill extend its service life.


51

Rotation of the workpiece during idlingTo prevent initial grind issues of workpieces, they may be rotated by the regulating wheel before grinding (idling). This requires that the friction force on the workpiece support (µ0A·FnA) be lower than the friction force on the regulating wheel (µR·FnR). This means:

Here, µ0A is the static friction of the workpiece support. For heavy workpieces, a start-rotation device may be used.

Slippage between the regulating wheel and the workpieceThe radial contact pressure of the regulating wheel, FnR, is normal-ly 1.5 to 4 times greater than the tangential cutting force FtS of the grinding wheel. This is why only the regulating wheel determines the rotational speed of the workpiece. To minimize the risk of slipping, the friction coefficient must be high enough for continuous workpiece rotation on the regulating wheel. Measures: Increase infeed ae and decrease H.


53

In external cylindrical plunge grinding, the workpiece is ground by advancing the grinding wheel. For this purpose, the grinding wheel and regulating wheel have a negative profile of the workpiece, which is generated by means of dressing tools.

A workpiece can be ground in a single plunge in spite of different diameters, cham-fers and fillets. If the grinding zone is wide enough, multiple workpieces can be machined simultaneously. This is called double or multi-production.

By tilting the workpiece or the grinding spindle axis, machining an angle to the work-piece axis is possible. This allows the machining of end or lateral surfaces. Find out more on the following pages.

4

Infeed grinding

Infeed grinding

Profiling Roughing and finishing Stop Tilting the grinding wheel End surfaces

54

4.1 General principle

In external cylindrical plunge grinding, also called crossgrinding, the regulating wheel is inclined only minimally (e.g. 0.1 … 0.2°). The rotation of the regulating wheel creates an axial force acting on the workpiece. This moves the workpiece against an axial stop where it is accurately positioned.

The workpieces are loaded into the grinding zone from the side, with integrated gantry, or from the top with an external gantry while the grinding wheel is in the base position. Then the grinding zone closes by the infeed slide and the workpiece is ground. At the end of the process the grinding wheel retracts back to the base position and the workpiece can be changed.

4.2 Process control

The grinding cycle consists of several consecutive steps, which is why it is also called a multi-stage grinding process. In multi-stage grinding, the task is to remove the grinding allowance of a workpiece in the shortest time possible, i.e. cost-efficiently, whi-le maintaining the required roughness of the workpiece. This can be achieved if the grinding process is divided into several stages

4 Infeed grinding | General principle

... that can also be the regulating wheel slide (X4-

axis)

Fig. 4-1

Grinding cycle during infeed

grindingX 1 X 1

X 1 X 1

1. Base position 2. Grinding 3. Base position

55

(e.g. roughing, finishing and sparking-out) A detailed division of these stages can be found in the following figure:

A short grinding time can be realized only by an increased stock removal rate QW or a higher plunging rate in the roughing pha-se. The quality is achieved in the finishing and sparking-out pha-ses. No further infeed occurs during sparking-out. The workpiece is then ground only by releasing the elastic deformation of the

Process control | 4 Infeed grinding

Fig. 4-2 Positions of the grinding cycle

↑ Chapter 6 Influencing factors

Fig. 4-3 Forces occuring during the infeed process

t

X1.W

X1.0

X1.1

X1.2

X1.3 X1.4

Air

gri

ndin

g

Roug

hing

2

Stock + safety

Part load position

Ro

ughi

ng 1

Final dimension

Sem

i fini

shin

g

Fini

shin

g

Spar

king

-out

FX1.0 FX1.1 FX1.2 FX1.3 FX1.4 FX1.5

t

Fts vfa

PsQ’w

56

grinding wheel and the machine. This improves the roundness and surface.

The infeed amounts and infeed rates can be defined for each of these individual processes in the control of the machine. But it should be noted that much heat can be generated at a high cutting rate, resulting in expansion of the workpiece. After cooling, its diameter may be less than desired.

4.2.1 Grinding from solid stock

The so-called grinding from solid stock is used for cutting large allowances. Reduction of the diameter moves the center of the workpiece towards the regulating wheel. In the process, the pro-jection of the workpiece over the workpiece support usually redu-ces and a risk is incurred of grinding into the workpiece support. In addition, the height position changes. To counter the shifts, infeed is applied by both the grinding wheel axis X1 and the regulating wheel axis X4. The switching points of the axis X4 are reached at the same time as those of the axis X1.

4 Infeed grinding | Process control

... a large amount of stock is remo-

ved

Fig. 4-4

Compensating the center shift

by advancing the X4-axis

X1 X4

... hence good cooling should be

ensured

57

So it is possible to influence the angle of the center shift.

Opening the grinding zone on both sides also provides a favora-ble loading and unloading situation, allowing the workpiece to be changed on the regulating wheel side without any contact. To do this v-nests have to be applied on the workpiece support to hold the workpiece in position while loading and unloading the part. During grinding the regulation wheel is pushing the workpiece away from the v-block.

4.2.2 Grinding with additional functions

The additional functions provide ways to influence the operation in more detail:

On the one hand, it is possible to store different workpiece speeds for each switching point. This provides a way to influence the chip-forming process.

On the other hand, there is a retraction function (temporary ope-ning of the grinding zone) and/or an intermediate sparking-out function. The aim of this is to take the load of the system, which can lead to a reduction of grinding time in particular for flexible shafts. However, this process control is rarely used in practice, because the support by the regulating wheel of this kind of work-pieces is stiff enough.

Process control | 4 Infeed grinding

↑ Section 3.2.3 Height position H and support angle β

58

4.2.3 Oscillation

Oscillating plunge grinding is possible also on centerless grinding machines. Prerequisite is axial relative movement between the work-piece and the grinding wheel besides radial movement. In the KRO-NOS S series, this is realized by the Z-axis of the grinding wheel‘s cross slide system. The oscillation rate and the oscillation path can be variably programmed depending on the workpiece length and workpiece geometry. In most cases, the path lies between 0.2 mm and 5.0 mm, while considerably higher values are possible.

The advantages of oscillating plunge grinding are: • Improvementofsurfacequality• Increasingthespecificstockremovalrateandreducingthegrin- ding wheel wear when grinding extra-hard materials

4.2.4 Influence of the workpiece geometry

If several diameters are to be ground on a workpiece simultaneously,the regulating wheel requires different diameters as well. This re-sults in different circumferential speeds of the regulating wheel. Theoretically, the result would also be different workpiece speeds. But obviously this is not possible in practice. The result is a single speed. Which one this is, depends mainly on the diameter/width ratio and the allowance of each workpiece diameter. At the diame-ters that do not determine the speed, differential speeds result by the slippage occurring there, which cause increased abrasion of the regulating wheel. These differential speeds can also cause irregula-rities in the rotation of the workpiece, resulting in roundness errors.

On the grinding wheel side, the same effect causes, on the one hand, cutting speed differences and, on the other hand, different specific stock removal rates. Points where the cutting performance istoohighmaybepronetothermaldamagetotheworkpiece.Using

4 Infeed grinding | Process control

… friction between the regulating

wheel and workpi-ece is lost and the

workpiece slips through

59

the example of grinding of engine valves, the following demons-trates in principle how widely the value of Q’w varies with the work-piece geometry. To prevent thermal damage to the border zones, it is important to ensure good cooling lubrication or to reduce the feedrate.

4.3 Grinding zone layout

4.3.1 Undercuts and spacers

In plunge grinding, the grinding wheel moves radially into the work-piece. Grinding is possible simultaneously with straight or shaped grinding wheels or with a grinding wheel set. (see Fig. 4-6)

Grinding zone layout | 4 Infeed grinding

Fig. 4-5 Variation of Q’w by the workpiece geometry © WZL Aachen

z

1 2 3 4 5 z position [mm]

Q‘W

[mm

³/mm

s]

0

2

4

6

8

10

7,20

1,34

20 30 40

60

UndercutsSince length tolerances may occur on workpieces, the grinding wheel is slightly wider than the corresponding workpiece seat. On the regulating wheel, it is the opposite: It is narrower than the corresponding workpiece seat.The regulating wheel and workpiece support are undercut at tran-sitions from one diameter to another. This ensures a stable and process-reliable position of the workpiece in the grinding zone.

SpacersOpen spaces are important for better drain of cooling lubricant – the regulating wheel supports the workpiece only at essenti-al points. Rings between the individual regulating wheels create spaces through which coolant and chips can be carried away.

Polypenco ringsSo-called polypenco rings are used to compensate for any une-venness between the wheels and the spacers. Compared to rings made of cardboard, they have much better resistance to cooling lubricant and do not change their size. But their width of about 0.5 mm must be taken into account in setting up the grinding zone.

4 Infeed grinding | Grinding zone layout

↑ Fig. 4-6 Undercuts and

spacers

↑ Fig. 4-6 Under-cuts and spacers

Polypencoring

Undercut

Spacer

Fig. 4-6

Undercuts and spacers

… plastic rings

61

4.3.2 Stop

The stop‘s function is to fix the workpiece in the axial direction in the grinding zone. The handling system is usually programmed to insert the workpiece at about 0.2 mm distance to the stop. Dif-ferent stops are necessary, depending on the workpiece shape; essentially three types can be distinguished:


1. Stop at the workpiece support

This type of stop is used if the zero point for the longitudinal dimension of the workpieces is not on the outside, but in the middle of the workpiece. Depending on the run-out of the face touching the stop, a longitudinal movement of the workpiece occurs in the grinding zone. This causes errors on contour elements (chamfers, fillets, cham-fers, cones, etc.) depending on the size of the longitudinal movement.

2. Point stop

A point stop is preferably used for workpieces with planar faces. It should be positioned exactly in the center of the workpiece. This will ensure that there is no axial movement of the workpiece and the axial run-out during grinding is not copied from the stop surface to the face.

Tab. 4-1 Stops

62


3. Surface stops

a) Surface on surface

On this mechanically very sturdy stop, the workpiece rests with its face. It is used mainly when only cylindrical parts of a workpiece are to be ground or the workpiece‘s longitudinal dimensions are relative to the face. If end surfaces or slants are to be ground, it must be ensured that the stop is 100% perpendicular in two planes to the workpiece support. As in general the stop is adjustable, this requirement is very difficult to realize. For face grinding operations, this possibility is therefore used only if the workpieces have no fixed center (such as rings, tubes or workpieces with a hole in the center) or to machine very large end surfaces. Because the stop surface and the work-piece surface contact each other, even small angle errors at the stop result in significant longitudinal movements of the workpiece in the grinding zone.

b) Point on surface

This type of stop is only used when the workpieces have ball-like or conical frontal surfaces or centers. As the work-piece only has a small point of contact with the stop, hardly any lengthways movement is caused during grinding. This means that the quality of the frontal surface even with the existence of small angle errors at the stop is very good.

Tab. 4-1 Continued

Stops

63


4.3.3 Initial grind

In centerless grinding, stable positioning of the workpiece is par-ticularly important. The workpiece support and the grinding and regulating wheel include a negative profile of the workpiece that is being ground.

The initial grind situation must be considered especially for pro-filed workpieces with different allowances on the diameters. It must be tested where the grinding wheel comes in first contact with the workpiece. To ensure it is stable in the grinding zone, it should be ground at several points simultaneously if possible.

The regulating wheel system has to be assessed as well. If, for example, only a small point of the regulating wheel has contact at the beginning of the grinding process, the regulating wheel willnot be able to stop the workpiece.

It must absolutely be avoided to start grinding on chamfers or end surfaces. Some parts require special measures due to their shape in order to lie stable in the grinding zone when grinding starts:

Pressure roller and hold-down deviceIn the event that only certain areas of a workpiece are ground, pressure rollers are used. They push the workpiece against the workpiece support and regulating wheel. This increases the fric-tional force between the workpiece and the regulating wheel. The workpiece diameter not to be ground forms the guide basis, and any roundness errors here are transferred to the areas to be ground. Therefore it must be ensured that the roundness of the guide basis is better than the roundness that is to be achieved with the grinding process.

Pressure rollers are used also for workpieces with large end sur-faces or for workpieces with large unbalance. They are often pus-

… the instance of first grinding contact

64

hed by means of spring force, and they are mostly driven passively by the connection between regulating wheel and workpiece.

In the case of so-called top-heavy workpieces, hold-down de-vices are used (Fig. Function of pressure roller and hold-down de-vice). They are usually operated by a spring and hold the top-heavy workpiece on the workpiece support. They also improve the initial rotation of the workpiece because the hold-down device increases the frictional force between the workpiece and the regulating wheel.

Grinding of end surfaces

Besides the lateral surfaces of a workpiece, its end surfaces can be ground at the same time. There are three different methods, which are considered in more detail in the fol-lowing sections.


Fig. 4-7

Function of pressure roller and

hold-down

Hold-down devicePressure roller

Lateral surface

End surface

… also called circumferential

… also called shoulder surfaces

Fig. 4-8

Lateral surfaces and end surfaces

4.4

65

In end surface grinding, it is important to take care of the allo-wance distribution, besides the initial grind situation. Grinding starts on the lateral surface of the workpiece. Only when the work-piece is stable in the grinding zone, contact with the end surface is allowed. It should be ensured that the radial allowance is at least 1.2 times as large as the axial allowance.

4.4.1 Inclination of the grinding wheel

• FeasibleonKRONOSS• Loweringtheregulatingwheelrequired• RequiresZ-axisforgrindingwheel

No differential speeds of the workpiece at the regulating wheel (as under 4.4.2)

Differences in cutting speed

Grinding of end surfaces | 4 Infeed grinding

Fig. 4-9 End surface grinding by tilting the grinding wheel by 15°15°

X+ZX+Z

The grinding wheel axis is located at an angle to the regulating wheel axis.

In the axial plunging process depictedhere, the X- and Z-axes are advanced diagonally interpolating. This allows finish-machining the lateral and end faces in one plunge, resulting in time and precision benefits. The grinding contact length on the shoulder is lessthan in straight plunge grinding so that the risk of thermal damage is re-duced.

66

4.4.2 Inclination of the workpiece support

The axes of grinding wheel and regulating wheel are aligned par-allel to each other. However, the workpiece support is manufac-tured at an angle:

The grinding wheel and re-gulating wheel are dressed according to the angle of this inclined position. As a result the diameter of the regulating wheel changes along the work-piece axis and different cir-cumferential speeds occur. As this should theoretically lead to different workpiece speeds, which is not possible, this me-thod may cause an unstable po-sition and slippage, especially for long workpieces.

Use: Possible on all KRONOS machines, because infeed movement is necessary only in the X direction

No Z-axis required Can be realized on all KRONOS machines

Special workpiece supports necessary Differential speeds of grinding wheel and regulating wheel Dressing and correction programs usually complex

4 Infeed grinding | Grinding of end surfaces

Fig. 4-10

End surface grin-ding by tilting the

workpiece support by 6°

6°

XX

67

4.4.3 Axial plunging in straight plunge with Z-axis

The axes of grinding wheel and regulating wheel are aligned par-allel to the workpiece axis. Axial plunging machines the shoulder surfaces here as well by moving the grinding wheel along its Z1-axis in addition to the radial infeed movement.

In this grinding process, it must be observed that, due to dressing and wear, the grinding wheel contour moves in the -Z and -X directions. This means that the size bb is getting smaller and smaller. When the value falls below aminimum, the grinding wheel must be newly profiled.

Use: On all machines with X- and Z-axis:KRONOS S; KRONOS M 250 with Z1-axis

• RequiresZ-axisforgrindingwheel• Contourstobedressedrequireinclinationofthedressingtool

No differential speeds Profile shift during shoulder dressing Risk of overheating when grinding

Grinding of end surfaces | 4 Infeed grinding

Fig. 4-11 End surface grinding by axial plunge of the grin-ding wheel

bb

2.

1. X X

Z Z

68

4.5 Removal of workpieces

There are three ways to unload workpieces from the grinding zone:

4 Infeed grinding | Removal of workpieces

Clearing movement Pusher Grippers

The regulating wheel is moved away from the work-piece support, the workpiece falls into the resulting zone and is carried away by a conveyor belt.

X4X4

An axial ejector pushes against the workpiece, the workpiece slides across the workpiece support and exits the grinding zone.

The grippers remove the workpiece.Usuallythesearedouble grippers that remove the finished workpiece while inserting a new blank at the same time.

Veryfast

Check required that the workpiece has been carried away properly or whether it is still in the machine (requires additional time)

Risk of a crash by dropped workpieces

Workpieces can be damaged by falling down

Position of workpieces has to be checked for subsequent reworking

Large parts possible, which are impossible with clearing movement

Workpiece may be damaged when ejected

Check required that the workpiece has been carried away properly or whether it is still in the machine (requires additional time)

Little/no damage to the workpiece

Position of workpieces need not be checked for subsequent reworking

Not necessary to check that the workpiece has been carried away properly

Somewhat more time-consuming

Tab 4-2

Removal of workpieces

guide to centerless external cylindrical grinding – part ii of the reference work

Engineering