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Centerless external cylindrical grinding

Manual

2nd editionJune 2016

Liability disclaimer:Subject to technical and other changes.All information and recommendations have been compiled up to the time of printing to the best of our knowledge and belief and perceived as correct. Nevertheless, any liability for errors in the information is excluded.

Authors: Dipl. Technical Editor (FH) Kristian Conrad Sven Stoll Dipl.-Ing. Karsten Otto Copyright: Schaudt Mikrosa GmbH Saarländer Strasse 25 04179 Leipzig · Germany sales@schaudtmikrosa.com www.schaudtmikrosa.com

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Contents

Preface 7

Fundamentals 9

1.1 What is centerless external cylindrical grinding? 10 1.1.1 Functional principle 10 1.1.2 Infeed grinding and throughfeed grinding 12 1.2 Advantages and disadvantages 14 1.2.1 Advantages over grinding between centers 14 1.2.2 Disadvantages over grinding between centers 15

Machine concepts 17

2.1 Classification according to inclination of the machine bed 18 2.2 Classification according to grinding zone 18 2.2.1 Moving grinding zone 19 2.2.2 Stationary grinding zone 20 2.3 Spindles 22 2.3.1 Spindle bearing principle 22 2.3.2 Wheel mounting 25 2.3.3 Spindle bearings 25

The grinding zone 27

3.1 Center shift 28 3.2 Shaping process 29 3.2.1 Roundness 29 3.2.2 Determining the roundness error 30 3.2.3 Height position H and mounting angle β 31 3.2.4 Polygons 33 3.2.5 Stability index SI 35 3.2.6 Stability cards 37

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3.3 Grinding zone geometry 40 3.3.1 Tangent angle γ� 40 3.3.2 Height position H 41 3.3.3 Angle at the contact points 41 3.3.4 Relative support angle βrel 42 3.3.5 Grinding zone geometry during infeed grinding 42 3.3.6 Grinding zone geometry during throughfeed grinding 44 3.4 Forces in the grinding zone 46 3.4.1 Forces on the grinding wheel 47 3.4.2 Forces on the workpiece support 48 3.4.3 Forces on the regulating wheel 49 3.4.4 Consideration of the forces 50

Infeed grinding 53

4.1 General principle 54 4.2 Process control 54 4.2.1 Grinding from solid stock 56 4.2.2 Grinding with additional functions 57 4.2.3 Oscillation 58 4.2.4 Influence of the workpiece geometry 58 4.3 Grinding zone layout 59 4.3.1 Undercuts and spacers 59 4.3.2 Stop 61 4.3.3 Initial grind 63 4.4 Grinding of end surfaces 64 4.4.1 Inclination of the grinding wheel 65 4.4.2 Inclination of the workpiece support 66 4.4.3 Axial plunging in straight plunge with Z-axis 67 4.5 Removal of workpieces 68

Throughfeed grinding 69

5.1 General principle 70 5.2 Shape of the regulating wheel 71

Contents

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5.2.1 Conical regulating wheel shape 74 5.2.2 Symmetric regulating wheel shape 77 5.2.3 Numerically controlled regulating wheel shape 79 5.2.4 Stagnation point offset 80 5.3 Process control and wear development 82 5.3.1 Multiple passes 83 5.3.2 Single-stage process (swivelling the regulating wheel) 84 5.3.3 Multiple-step process (profiling of grinding wheel) 85 5.3.4 Conicity of the grinding zone 86 5.4 Guidance of workpieces 88 5.4.1 Guide jaws and gibs 88 5.4.2 V-block workpiece support 90 5.4.3 Upper guide 91

Influencing factors 93

6.1 System parameters 94 6.1.1 Infeed grinding 94 6.1.2 Throughfeed grinding 95 6.2 Control variables 97 6.2.1 Grinding wheel rotational speed nS and Grinding wheel circumferential speed vS 98 6.2.2 Regulating wheel rotational speed nR and Regulating wheel circumferential speed vR 99 6.2.3 Workpiece rotational speed nW and Workpiece circumferential speed vW 100 6.2.4 Infeed ae and infeed rate vfr during infeed grinding 102 6.2.5 Feed s and feed rate vfa during throughfeed grinding 105 6.2.6 Number of grinding passes U and grinding time ts 106 6.2.7 Sparking-out revolutions Ua and sparking-out time ta 107 6.3 Indicators 108 6.3.1 Speed ratio qS 108 6.3.2 Relative stock removal rate Q´W 109 6.3.3 Equivalent cutting thickness heq 111

Contents

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6.3.4 Grinding ratio G 113 6.4 Process design 114 6.4.1 Strategy for specification of reference values 114Symbols and units 117Glossary 121Index 127Bibliography 133

Contents

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We at Schaudt Mikrosa GmbH have decided to describe the most relevant information about the centerless grinding process. Due to the wealth of information and the limited space, however, we do not claim to be exhaustive.

This guide is intended to give a quick introduction to a complex subject. It summarizes as a compact reference the most important relationships in centerless grinding. However, with its comments, formulas, tables and diagrams, it should also be of help to expe-rienced grinders.

The guide does not have to be read linearly and therefore often contains references to further information in other chapters. A quick access to key information is provided by the glossary, the section on formula symbols and units, as well as the index in the Appendix.

Centerless external cylindrical grinding has established itself in the past as a successful grinding process mainly because of its high productivity.

Also in the future, further increases in productivity can be ex-pected, in particular through the use of CBN. With new machine concepts and advances in grinding technology, ever smaller geo-metrical and positional tolerances become possible. Centerless grinding is already increasingly used in small series batches. Ho-wever, it requires comprehensive process knowledge by the ma-chine operator and setup technician.

Your Schaudt Mikrosa GmbH

… only called cen-terless grinding hereunder

… Cubic Boron Nitrite: Extremely hard grinding abra-sive for highspeed applications to reduce cycle times

Preface

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The following chapter provides in brief form the fundamentals of centerless external cylindrical grinding. It should help to gain a quick overview of the many relationships. Exact details of the different variables can be found in further chapters.

1

Fundamentals

Centerless external cylindrical

grinding

Introduction Relationships Fundamentals Workpiece support Regulating wheel Grinding wheel

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1.1 What is centerless external cylindrical grinding?

Centerless grinding is a method of grinding cylindrical bodies. It is used for mass production of cylindrical rollers, tapered rollers, jet needles, pump pistons, hydraulic valves, journal crosses, drills, etc.

1.1.1 Functional principle

The workpiece is not clamped between two centers, which is why it is referred to as centerless grinding. Instead, the workpiece lies in the so-called grinding zone between the grinding wheel and the regulating wheel on a workpiece support. The workpi-ece is rotated and stock is removed by the spinning grinding wheel and regulating wheel (Fig. 1-2).

Fig. 1-1

Selection of some products

of centerless ex-ternal cylindrical

grinding

1 Fundamentals | What is centerless external cylindrical grinding?

11

Fig. 1-2 Principle of centerless ex-ternal cylindrical grinding

The grinding pressure of the grinding wheel [1] pushes the work-piece [2] against the regulating wheel [3] and the workpiece support [4]. The rotating regulating wheel provides the backpres-sure necessary for grinding.

The regulating wheel and grinding wheel have the same direction of rotation but different rotational speeds. The regulating wheel rotates at a lower speed, thereby slowing the workpiece.

In addition, the regulating wheel supports the workpiece and determines during throughfeed grinding, by its inclination in its rotary axis and rotational speed, the throughfeed speed of the workpiece.

The rotating workpiece lies on the workpiece support during grin-ding. The support influences the quality of the grinding process due to its thickness, the support angle β and the material used for the contact surface.

What is centerless external cylindrical grinding? | 1 Fundamentals

[1] Grinding wheel [2] Workpiece

[3] Regulating wheel[4] Workpiece support

... also referred to as positioning rail

12

1.1.2 Infeed grinding and throughfeed grinding

A distinction is made in centerless external cylindrical grinding between two methods: Infeed grinding and throughfeed grin-ding.

1 Fundamentals | What is centerless external cylindrical grinding?

Infeed grinding Throughfeed grinding

Roughing Finishing Sparking-out

Application area: Workpieces with stepped or profiled lateral surfaces The workpiece is inserted from the top or the side into the grinding zone and positioned by an axial stop. This is achieved by a slight inclination of the regulating wheel (0.1°–0.2°).

Application area: Cylindrical, non-profiled workpieces such as rings, bars, pins and pipes The workpieces run continuously through the grinding zone by axial feed force. This is made possible by the incli-nation of the regulating wheel about its rotational axis. To ensure that, despite this inclination, all workpieces have contact across the entire wheel width, the regulating wheel has a hyperbolic shape (fig. above).

Tab. 1-1

Infeed grinding and throughfeed

grinding

13

↑ Chapter 4 Infeed grinding

↑ Chapter 5 Throughfeed grinding

↑ Section 4.4 Grinding end surfaces

Infeed grinding Throughfeed grinding

The grinding wheel determines the shape of the workpiece and therefore has a corresponding profile. Also the regulating wheel is profiled according to the workpiece contour in order to support the workpiece. Several seats of a workpiece can be ground simultaneously and without interfering with each another. If the grinding zone is wide enough, even multiple workpieces can be in the grinding zone. Special case: Angular infeed grinding (even end surfaces can be machined)

The rotational speed and the angle of inclination of the regulating wheel together determine the feed rate of the workpieces. The workpieces contact each other with their end surfaces until they pass through the last section of the grinding zone. At this point, the circumferential speed of the regulating wheel is high-est, because its circumference is largesthere (fig. above). The machining process is extremely economical, because the feeding of the machine is carried out during grinding. Infeed motions are necessary only inso-far as they are required to compensate the wear of the grinding wheel. Special case: Throughfeed grinding of tapered rollers and cambered cylindrical rollers

What is centerless external cylindrical grinding? | 1 Fundamentals

… feeding themachine withnew blanks

Tab. 1-1 Continued

Infeed grinding and throughfeed grinding

14

1.2 Advantages and disadvantages

Each grinding method has its advantages and disadvantages. The comparison below explains the specific advantages of centerless external cylindrical grinding over conventional grinding between centers.

1.2.1 Advantages over grinding between centers

High productivity: Clamping of workpieces is unnecessary. This results in very low secondary times during insertion and remo-val of workpieces. The secondary times are usually much smaller compared to clamping between centers. In centerless throughfeed grinding, one workpiece immediately follows the other. The secon-dary time for feeding is therefore very low.

Stable support of the workpiece: The workpiece is supported by the regulating wheel and the workpiece support over a substan-tial part of its length or over its entire length. This is extremely advantageous particularly for longer workpieces with very small di-ameters. Vigorous rough grinding is possible without considering bending and torsional stresses. Harmful deformities by the grinding forces do not occur. As a result, the stock removal rate can be increased. During finish grinding, the stable support of the work-piece allows very precise grinding, i.e. even workpieces with lowtolerances are machined reliably.

Double precision: The adjusted infeed amount corresponds to the diameter reduction of the workpiece during centerless grin-ding. When grinding between centers, however, the infeed amount is radial. This means that at the same infeed amount the diameter is reduced by approximately double the amount. The basic accura-cy of the process is thus much higher in centerless grinding. Infeed errors, caused, for example, by grinding wheel wear or thermal

… time which cannot be used for grinding itself, but

is necessary

1 Fundamentals | Advantages and disadvantages

… a grinding pha-se, such as finish grinding; more in

the glossary

15

displacements in the machine, are only half as large.

Reduced preparatory work: Preparation of the workpiece by making the centerings, as required for grinding between centers, is unnecessary.

Lower allowance: In centerless grinding, the workpiece seeks its rotary axis at the beginning of machining according to the exi-sting lateral surface. Since the highest points on the workpiece come in contact first here, the largest diameter possible in the respective condition is ground and finished in centerless grinding. This means that the allowance for centerless grinding can be less than for grinding between centers, as the required allowance de-pends there on the centering position relative to the outer surface.

1.2.2 Disadvantages over grinding between centers

Regenerative effect: This effect may cause unstable behavior during the shaping process. However, the relationships have been well known for some years and can be controlled by choosing a suitable grinding zone geometry.

Problems with coaxiality/ concentricity: There is no concen-tricity of the outer surface to centering holes that may exist on the workpiece. This may cause problems if subsequent steps are working again from the center. This effect can be influenced with the KRONOS dual.

↑ Section 3.3 Grinding zone geometry

… entire machi-ning allowance of a workpiece

Advantages and disadvantages | 1 Fundamentals

… more about coaxiality and concentricity in the glossary

16

Lower flexibility: Changeover to another type of workpiece is – due to handling of partly rather large grinding and regulating wheels – quite time-consuming. At least in plunge grinding, a workpiece support customized for the workpiece is required.

No grinding in opposite direction: Grinding in the opposite direction is not possible. The frictional force on the regulatingwheel is not sufficient to overcome the friction of theworkpiece support as well as the cutting force on the grindingwheel.

… grinding wheel and workpiece run in opposite

directions at their contact point

1 Fundamentals | Advantages and disadvantages

17

To meet the various demands for the centerless external cylindrical grinding, diffe-rent machine concepts are in use. The major ones are mentioned below and their advantages and disadvantages are explained briefly.

2

Machine concepts

Machine types

Machine bed Cross slide Spindles Bearings

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2 Machine concepts | Classification according to inclination of the machine bed

Horizontal Angular Vertical

•Mostusualmodel

• Goodaccessibilityduringdressing

• ApplicationatMIKROSA

• Suitableforheavyworkpieces, as a part of the weight is shifted to the regulating wheel

• Avoidanceofabrasionasworkpiece is supported while it is being turned

• Specialconstruction

• Advantagesinthehand-ling of heavy workpieces

• Only„grindingunderthecenter“ is possible

• Highercontactpressurebetween workpiece and regulating wheel

2.1 Classification according to inclination of the ma- chine bed

When machines are classified according to their bed inclination,three types can be distinguished:

2.2 Classification according to grinding zone

Another feature of classification is the grinding zone. A distinctionis made between machines with moving and stationary grinding zone. The latter includes also special cases such as dual grinding and the cross slide principle.

Tab. 2-1

Classi-fication

according to inclina-

tion

19

Classification according to grinding zone | 2 Machine concepts

2.2.1 Moving grinding zone

In this principle, the grinding spindle head is permanently con-nected to the machine bed. All three movable slides (X1, X4, B1) are located on the side of the regulating wheel. The system is thereforealsocalleda„3-slide system“.

Compact design Very high rigidity of the grinding wheel side Cost-effective production since no complex infeed slide is required for the grinding wheel side

Reduced rigidity of the regulating wheel side by several slide guides

Handling must be tracked with high effort, because the work-piece support is moving due to grinding wheel wear

B1B1

X4-

+X4

-

+

X1-

+X1

-

+

--X2X2

+ Z2Z2

+

-

X3-+

X3-+

Z3

-

+ Z3

-

+

Fig. 2-1 Moving grinding zone

20

2 Machine concepts | Classification according to grinding zone

… machines con-nected in series

Fig. 2-2

Stationary grinding zone

2.2.2 Stationary grinding zone

The workpiece support is stationary or minimally swivellable (B1-axis) in the center of the machine. Both the grinding wheel and the regulating wheel have their own axis slides by which they can be moved to the workpiece support.

Handling need not be carried along a advantageously especially with interlinked machines

Feeding during throughfeed grinding can be absolutely linear a important for machining of long workpieces

Good accessibility a quick and flexible changeover possible Reduced rigidity of the grinding wheel side, because the grinding spindle is not firmly connected with the machine bed

More time-consuming and thus more costly Large space requirement

B1B1X4

-

+X4

-

+

X1-

+X1

-

+

X2X2+

-+

-

+

-

Z2Z2

X3-+X3-+

Z3

-

+ Z3

-

+

21

The cross slide principle and dual grinding are considered special cases of the stationary grinding zone:

Cross slide principle

In this machine concept, the grinding wheel and regulating wheel are arranged, together with their drives, on cross slides. As the name suggests, these slides can be moved both along the X-axis and the Z-axis. The dressers are fixed in the center of the machine.

Dressing at workpiece level results in higher dressing precisi-on

The machine with its 4 axes has the same functionality as conventional designs with 7 axes

Cross slides need more space, resulting in a wider machine a restricted wheel widths

No CD dressing possible

Fig. 2-3 Cross slide principle

X4-

+X4

-

+

X1-

+X1

-

+

+

-+

-

Z4-

+ Z4-

+

Z1Z1

2 Machine concepts | Classification according to grinding zone Classification according to grinding zone | 2 Machine concepts

… dressing during the grinding process

22

2 Machine concepts | Spindles

Dual grinding

Dual grinding combines grinding between centers and centerless grinding on one machine. The workpieces are first ground between centers and then placed in the grinding zone for centerless grin-ding.

Good coaxiality from the centering to the outer surface by grinding between centers

Short grinding time and high accuracy by subsequent center-less grinding

Stable support by the regulating wheel Complex design with additional axis and spindle Reduced accessibility and wider machine Increased setup effort

2.3 Spindles

The spindles for the grinding and regulating wheels are essential core components of the machine. Since the design of these spind-les has a significant impact on the grinding process, its bearing and support principle are part of the next sections.

2.3.1 Spindle bearing principle

The spindles of the grinding and regulating wheels are supported either on one side (floating bearing) or on both sides (gantry be-aring):

… having a com-mon axis

23

Gantry bearing

The body [9] is supported in two bearing bushes [10] that are located next to the grinding wheel on both sides. The spindle be-arings [6] ensure optimal rotation.

The drive is provided externally via the v-belt pulley [7]. It trans-fers the rotation to the inserted torsion bar [8] that drives the spindle itself. Such a drive is referred to as free of lateral forces. Forces as they arise during tensioning of belts thus do not act on the spindle bearings.

If a wheel mount [2] is used, it is pushed with the mounted wheel onto the spindle. The spindle nut [5] fixes the wheel support by a means of a pressure ring [4] and the clamping cone [3] (Fig. 2-4).

[1] Wheel[2] Wheel mount[3] Clamping cone[4] Pressure ring[5] Spindle nut[6] Spindle bearings

[7] V-belt pulley[8] Torsion bar[9] Body

[10] Bearing bushes

Fig. 2-4 Gantry bearing

Spindles | 2 Machine concepts

24

More rigid than floating bearing Suitable for large grinding widths Wheels can be mounted on the spindle directly and without a wheel mount

Time-consuming wheel change

Floating bearing

The shaft [2] is supported only in one bearing bush [6] at several points. The spindle bearings [7] ensure optimal rotation.

The drive is provided via the v-belt pulley [8]. It transfers the rota-tion directly to the spindle. A disadvantage of this type of power application is that tension forces from the belt act directly on the rear spindle bearing.

Fig. 2-5

Floating bearing

[1] Wheel[2] Shaft[3] Wheel mount[4] Clamping cone[5] Spindle nut

[6] Bearing bush[7] Spindle bearings[8] V-belt pulley

2 Machine concepts | Spindles

25

If a wheel mount [3] is used, it is pushed with the mounted wheel [1] onto the spindle. The spindle nut [5] and the clamping cone [4] fix the wheel mount (Fig. 2-5).

Quick wheel change Unsuitable for large grinding widths Always requires an additional wheel mount Lower rigidity

The regulating wheel runs usually in gantry bearings for reasons of stability. If it is provided with a floating bearing, it often receives an additional support bearing.

2.3.2 Wheel mounting

The grinding or regulating wheel is either mounted directly on the spindle (direct mounting) or fixed previously on a so-called wheel mount (Figs. 2-4 and 2-5).

If multiple wheel sets are used in a machine, the use of wheel mounts is preferable. This does not require replacing each wheel individually, but only the wheel mount is replaced instead. The setup times for changing a workpiece type can thus be reduced. A floating spindle bearing always requires a wheel mount.

2.3.3 Spindle bearings

Both rolling bearings and plain bearings are used as spindle bea-rings. Some bearing types are more and others less suitable, de-pending on the application and cost. The table below gives a brief overview of the types of bearings:

Spindles | 2 Machine concepts

26

Rolling bearings Plain bearings

Hydrodynamic bearings

Hydrostatic bearings

Cost-effective, since no oil generator necessary

Maintenance-free

High speeds possible

Good damping

High rigidity

Wear-free continuous operation

Maintenance and wear-free

Very high rigidity and damping

Must be replaced if worn

Low damping

Higher price than rolling bearings

Deficit in the lower speed range Note: Regulating wheel can be used only from approx. 35 rpm

Very high price

Conclusion: Advantage-ous at high speeds, more cost-effective than plain bearings

Conclusion: Goodprice/performance compromise

Conclusion: Exceptfor the price, the bestsolution

Low noise

Adjustment of the grinding spindle bearing limits the usable speed range

Strong heat generation at higher speeds

Extensive gasket

Maintenance-free

Tab. 2-2

Spindle bearings

2 Machine concepts | Spindles

The grinding zone