Manufacturing

Reliable Connections Achieved by Monitoring Joining Forces

13th Paderborn Symposium on Joining Technology Mechanical Joining Adhesive Bonding29-30 November 2006

Dr. A. Kirchheim, R. Deuerling, A. Lehmann, Kistler Instrumente AG, Winterthur

H. Thommes, D. Hußmann, of the Laboratorium für Werkstoff- und Fügetechnik (LWF), University of Paderborn

Special Print920-351e-04.07

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Reliable Connections Achieved by Monitoring Joining Forces

Dr. A. Kirchheim, R. Deuerling, A. Lehmann, Kistler Instrumente AG, 8408 Winterthur, SwitzerlandH. Thommes, D. Hußmann, of the Laboratorium für Werkstoff- und Fügetechnik (LWF), University of Paderborn

PrefaceThese days industry in general and automobile manufactur-ers in particular are demanding more and more systems for monitoring joining processes such as resistance welding, clinching, self-piercing riveting and other assembly opera-tions. This paper draws on application examples from LWF Paderborn and other sources to present special systems for calibration and analysis of joining forces as the basis of reli-able process monitoring.

Despite joining and press-in techniques being in very com-mon use, non-destructive mechanical testing of the results is virtually impossible. However, analysis of the force-dis-placement characteristics typical of each method has proven effective time and again. In addition to using direct mea-surement, the joining forces can be detected indirectly using the proportional strain induced on the surfaces of machine tool elements such as clinching guns, on which piezoelectric strain sensors are very readily mounted. Integrated elec-tronics, compactness and robust industrial system design make these sensors very suitable for use in the production environment. This paper also shows how mounted strain sensors can be calibrated with very flat, internally preloaded force links.

Another way of boosting reliability is to simulate joining pro-cesses at the design stage, which will also be exemplified.

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Resistance WeldingThe critical welding parameters are time, current and weld-ing force. Perfect spot welding can only be achieved if the welding (electrode clamping) force has reached a required value before the welding current is turned on. The welding force is defined as the force with which the two electrodes press the parts to be joined together for example, sheet metal. Switching the current on too early, before the force has reached, for example, 90 % of its desired value, causes increased electrode wear and spatter. On the other hand to keep the cycle time as short as possible the current should be applied at the earliest possible moment.

Direct and indirect force measurements are a suitable means of monitoring and possibly also controlling and regulating the force applied during spot welding (Fig. 1):A) Periodic direct manual measurement (offline) of the

welding or clamping force with a force sensor or by introducing a test station with built-in force sensor

B) Indirect measurement of the welding force during weld-ing (online), for example using strain sensors in the pedestal or electrode holder

C) Direct measurement of the welding force during welding (online), for example using special force sensors inte-grated into the structure or directly in the drive motor

Welding force calibration transmitterType 9831C… (offline)

Welding force test transmitterType 9833C… (offline)

HighSens strain sensor/transmitterType 9232A…/9234A… (online)

Standard force sensors for integration (online)

Robot

Spherical bearing transmitter with integrated force transmitterType 9602A…/9602AA… (online)

OEM, End userEnd User

Offline

Motor with integrated force sensor (online)

Example: Electrode force measurement

Electrode clamping force

Online

Fig. 1: Examples of force measuring systems on welding robot

(C)(A)

(A)

(B)

(C)

(C)F

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Integrated force sensor

Time t [s]

Elec

trod

e fo

rce

F el [

kN]

Vol

tage

U [

V]

Wel

ding

cur

rent

I eff [

kA]

Force Current

Voltage

Fig. 2: Direct force measurement in the electrode (LWF)

Strain sensor

Time t [s]

Elec

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F el [

kN]

Vol

tage

U [

V]

Wel

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cur

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I eff [

kA]

Force Current

Voltage

Fig. 3: Indirect force measurement using strain sensor (LWF)

Fig. 4: Online and offline monitoring of clinching

To achieve a reliable welding process all parameters must be programmed into the control system before welding starts. Reference force sensors can be mounted between the elec-trodes to allow calibration of the correct welding forces.

Practical experience shows that the welding force can easily change from weld to weld. Large differences can arise if, for example, sheet metal is fed incorrectly or bent, so that the parts being joined do not lie flat on each other.

Figures 2 and 3 allow comparison of direct force measure-ment using an annular force sensor integrated into the elec-trode with indirect force measurement using a strain sensor mounted on the electrode arm.

Further evaluation of the force curves also allows conclusions to be drawn about the settlement of the weld spot during the welding process. In the case of spatter a significant force pulse can be detected. This is because, as a result of material being thrown out of the joining zone, the thickness of the weld spot is reduced, and this is manifested in a significant drop in force during or at the end of the weld interval.

The accuracy of the welding force as a function of time is a crucial quality attribute with both pneumatically and electri-cally actuated welding guns. In modern robot control systems the welding gun is used with electric servomotors as the 7th axis, so that online measurement of welding force can be used not only for monitoring but also for controlling the welding process [3-7].

Clinching, Self-Piercing Riveting and Conventional RivetingMechanical joining methods such as clinching, self-piercing riveting and blind riveting are becoming increasingly well established in the sheet metalworking industry and are used in a good proportion of modern products. Force monitoring prevents defective components from getting into volume production and ensures consistently high product quality. It also cuts production costs by eliminating the need for sub-sequent manual rejection of defective parts.

In some sectors, such as car making, clinched connections have already displaced spot welding as the most popular joining process. This trend has spread from secondary auto-motive components to relevant assemblies such as doors, hoods, trunks and tailgates.

Clinching is a method of joining pieces of sheet metal and profiles of different thicknesses and materials by pressing them together into a die that forms a connection similar to a rivet. This method uses local cold forming to produce per-manent connections without additional or supplementary fastening devices. Its main feature is that it forms a positive connection from the material of the sheet metal to be con-nected.

One quality-determining parameter for monitoring the clinching process is the force necessary for plastic defor-mation. Kistler offers different measurement solutions for monitoring the clinching force curve.

In a similar way to the above-mentioned resistance weld-ing applications, with clinching it is also possible to check the clinching gun periodically with a calibration transmitter, document parameters and adjust as necessary in the course of servicing and maintenance. This approach also allows calibration of sensors for indirect or direct force measure-ment in the machine (Fig. 4).

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Punch displacement [mm]

Join

ing

forc

e [k

N]

Fig. 5: Joining force as a function of punch displacement for process analysis or monitoring of clinching (Laboratorium für Werkstoff- und Fügetechnik (LWF) of the University of Paderborn)

Fig. 6: Monitoring of the base thickness from a curve of clinching force (Böllhoff)

Fig. 9: Die blades not present [BTM]

Punch displacement [mm]

Join

ing

forc

e [k

N]

Reference curve

Defect curve

Die anvil defective

Metal broken out from die anvil

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•

Punch displacement [mm]

Join

ing

forc

e [k

N]

Reference curve

Defect curve

Die blade(s) not present

Die blades not located on die anvil

Rubber ring/die spring missing

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•

•

Continuous monitoring of the clinching curve with piezoelec-tric instrumentation from Kistler offers the option of indirect continuous checking of the long-term stability of a clinching unit consisting of drive and tool. This ensures the consistently high product quality synonymous with zero defect products. The elimination of any need for subsequent manual rejection of defective crimped connections also allows production costs to be reduced. Moreover, the process can be followed and documented.

Figure 5 shows a typical curve of clinching force against punch displacement for process analysis or monitoring.

Figures 6 and 7 show monitoring of the clinching force for the purpose of detecting a wide variety of defects, which may include:

Dimensional discrepancies or wrong number of sheet metal workpieces Level of clinching tool wearDetection of process defects such as punch or die frac-tureIncorrect clinching height setting on press

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••

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Strain transmitterType 9234A…

Fig. 7: Continuous monitoring of the curve of clinching force with piezoelectric strain transmitters. Typical defects detectable from curve of force against displacement [BTM].

Fig. 8: Metal broken out from die anvil [BTM]

Reference curve

Defect curve

Join

ing

forc

e [k

N]

Punch displacement [mm]

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Fig. 11: Measuring force for production monitoring during riveting

Fig. 10: Standard force sensor, directly integrated into clinching gun (top), and monitoring of the clinching force with CoMo View® ControlMonitor in BTM automatic clinching machines

Blind riveting is used primarily in positions only allowing access from one side. There are three different types of blind rivets:

Expanding RivetThe slit end of the tubular rivet is expanded by driving a notched pin into it.

Pop RivetThe pop rivet consists of a tubular rivet and a mandrel with tapered head. The rivet assembly is inserted into a hole drilled through the parts to be joined, then a hand or pneumatic tool is used to draw the mandrel into the rivet. This expands the end of the rivet and the mandrel is designed to eventually snap in a predetermined position.

Blind Rivet GunIn this case rivets with a female thread are inserted into the hole. The rivet tool is then introduced and drawn upwards to upset the metal under the workpiece. In this case as well the rivet force is also a critical process param-eter that determines the quality of the process. Figure 11 shows a ControlMonitor for riveting process visualization and production monitoring for a pneumatic rivet gun with integral force measurement.

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Integration of piezoelectric force sensors into a rivet gun and the process control system (CoMo View) makes it pos-sible to respond to more stringent production requirements. The process of assembly involving permanent connection of components used in the production process can be moni-tored and documented. This prevents further processing of defective components and their eventual incorporation in volume production. CoMo View has a web server with integral 145 mm touch screen and can therefore be used for handling and monitoring the different riveting processes.

The big advantage is that production processes can be monitored from anywhere. The ControlMonitor is supplied in a sturdy desktop case. The advantages of integrated process monitoring are obvi-ous:

Continuous checking of joining operation Good and bad parts distinguished immediatelyComplete riveting process can be documented Easy integration into existing LANRemote monitoring and diagnostics if service needed Multilingual, intuitive operator guidanceQuick exportation of process data in different formats Software for data export included in accessories

Typical application sectors include the manufacture of automobile parts whose documentation is mandatory and applications in which the rivet connection cannot be visually inspected.

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Figure 10 shows another example, in which a force sensor is integrated directly into the clinching gun. Here as well the process can be depicted and analyzed effectively using the force-displacement curve. In this case the process was monitored with the CoMo View ControlMonitor.

Mandrel breakload

Mandrel head penetration

Course (mm)

Pulli

ng s

tren

gth

(daN

)

Riv

et b

ody

com

pres

sion

Cla

mp

forc

e

�

Punc

h fo

rce

(N)

Displacement (mm)

Clinching force as a function of displacement

Bending sheet Positioning Filling die Forming undercut metal on die

Clinching, basic process

With 1,1 mm thick sheet metal

With 0,9 mm thick sheet metal

With 125 % flow curve

With 75 % flow curve

Fig. 10: FEM simulation of different thicknesses of sheet metal being connected by clinching

Simulation of Joining Process

Another way of boosting process reliability is to simulate joining processes at the design stage. DEFORM is a finite ele-ment analysis program for simulating joining processes such as clinching or self-piercing riveting. Forming, heat treatment and machining processes can also be simulated.

Features of Simulation Tool DEFORM™:2D and 3D FEM simulations on standard PCs or note-booksUser-friendly, intuitive user interfaceMaterials database with approximately 200 recordsWizard for fast simulation of forming and joining pro-cessesProblem-free CAD importation using STL (3D), or IGS or DXF (2D), interface

FEM simulation allows quick comparison of different sheet metal thicknesses, materials, etc., as shown in the following diagram.

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Fig. 11: Determination of factor of safety against shear failure (top), correlation of micrograph geometry with simulation (bottom)

Summary

The various sensor systems that have been presented are sufficiently sensitive and accurate to provide good and reli-able force signals that can be used to analyze and monitor the process involved in many of today's joining methods, such as spot welding, clinching, self-piercing riveting and conventional riveting. The sensors respond sufficiently quick to always allow perfect acquisition of the complete process curve, even when cycle times are extremely short.

Robust, largely overload-resistant and absolutely stable characteristics are not only essential for continuous, reliable monitoring of industrial production, but also form the basis for comprehensive documentation and online monitoring of the joining processes with the process control systems presented.

Kistler also offers a program package for the increasingly important simulation of manufacturing processes from the start of production.

References1. Kirchheim A., Lehmann A., Staub R., Schaffner G., Jeck N.:

Kraftmessung beim Widerstandsschweissen, Treffpunkt Widerstandsschweissen. DSV Conference, Düsseldorf, 26.-27.5.2004

2. Kirchheim A., Lehmann A., Schaffner G., Staub R.: Einfache Überwachung von Fügeprozessen, SMM. Die Industriezeitschrift für die Praxis, 38 (2002)

3. Habermann, S.: Effiziente Applikation am Industrieroboter. VDI-Z 145(2003), No. 4, pp. 24-25.

4. Speck J.: Modulares Automatisierungssystem für Servo-Schweisszangen. Schweissen&Schneiden 54 (2002) vol. 8, p. 422.

5. Vollrath K.: Wirtschaftliche Karosseriefertigung durch AC-Servoschweisszangen. der praktiker, 5/2003, pp. 152-155.

6. Kirchheim A., Schaffner G., Staub R., Jeck N.: Elek-trodenkraft als wichtige Prozessgrösse beim Wider-standschweissen. Schweissen&Schneiden 53 (2001) vol. 9, pp. 635-637.

7. Kirchheim A., Lehmann A., Schaffner G., Deuerling R., Jeck N.: Kraftüberwachung optimiert Widerstands-schweissen und verwandte Fügeprozesse. Grosse Schweisstechnische Tagung 2005, Essen, 13.-14. Septem-ber 2005

Source: FhG IWU Chemnitz, Joining and Prototyping Dept.

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