Quality Assurance for the Riveted Union

The Friedrich Nietcontrol is a control system with patented measurement method for radial point riveting machines.

Minimum number of actuations to perform + maximum operational safety = optimal quality assurance

The Friedrich Nietcontrol controls and checks all significant parameters of a riveted union. By measuring the displacement distances and the riveting times all parameters can be monitored. All relevant data are shown on the display and saved under a programme number.

Faults and/or errors are pinpointed and the place where they originated is shown. Displacement measurement is the most important parameter for quality assurance in connection with riveted unions. The displacement of the riveting spindle and, thus, the rivet height, the projection and the rivet head height can be reproduced at any time and re-measured correctly to the hundredth at the riveting workstation. The compliance with tolerances specified on drawings can be checked and documented by the machine.

The Friedrich QAPV (Quality Assurance and Process Visualization) software continuously records the measured values provided by the control and uses them to calculate the machine capability as well as the process capability.

Friedrich riveting machine with length measurement

What is measured is length (Z1) of the unriveted bolt rivet and length (Z2) of the finish riveted bolt rivet. The measuring instrument for length measurement is arranged directly at the riveting spindle and transmits the measured values to the Friedrich Nietcontrol once every millisecond. Measurement takes place on a per-manent basis during the riveting operation. Programmable time slots make it possible to simultaneously query whether the material properties meet the requirements.

Riveting operation and length measurement

The Friedrich Nietcontrol causes the riveting spindle to advance to the rivet at measuring pressure and with the riveting die aligned in its longitudinal axis in order to avoid a deformation of the bolt rivet during measuring. A NOK message is generated if the length of the unriveted rivet (Z1) is not within the tolerance range. As the bolt rivet was not deformed during measuring it can be replaced by a new one. If length (Z1) is within the specified tolerance range the riveting motor is started and the riveting spindle works at the programmed riveting pressure until the finished dimension (Z2) is attained. If the finished dimension (Z2) entered in the control is within the tolerance range and if the riveting operation takes place within the specified time slot the riveting result is OK. The riveting spindle moves back to home position and the riveting die is again aligned in its longitudinal axis.

If however the measured finished dimension is outside the specified tolerance range or if the riveting operation does not conform to the time slot set, a NOK message is generated.

Friedrich riveting machine with projection measurement

The bolt rivet projection (H1) is measured on the unriveted rivet, while the rivet head height (H2) is measured on the finish riveted rivet.  For the projection measurement the measuring instrument is arranged directly at the spring-mounted downholder.  For adjusting purposes the downholder measuring system is compressed on a plane surface. As soon as the downholder and the riveting die are at one level the measuring system is set to zero. When the riveting spindle moves back the downholder will decompress so that the riveting die is then standing back behind the downholder. The projection value is shown on the display.

Riveting operation and projection measurement

The Friedrich Nietcontrol causes the riveting spindle to advance to the rivet at measuring pressure and with the riveting die aligned in the longitudinal axis. As the riveting spindle advances the downholder measuring system is preloaded until the die touches the rivet. The measurement (H1) takes place at low pressure so that the bolt rivet is not deformed. At the same time the projection measurement is used to check for the presence of all components of the future riveted union. A NOK message is generated if a part is missing. The missing part can be put in place subsequently. The projection measurement thus makes it possible to avoid undesirable faulty riveting operations. If the projection dimension of the unriveted rivet is within the specified tolerance range the riveting motor is started and the riveting spindle works at the programmed riveting pressure until the finished dimension is attained. If the finished dimension entered (rivet head height H2) is attained and if the riveting operation takes place within the specified time slot, the riveting result is OK. The riveting spindle moves back to home position and the riveting die is again aligned in its longitudinal axis. If however the measured finished dimension (H2) is outside the specified tolerance range or if the riveting time does not conform to the time slot set, a NOK message is generated.

Friedrich-Nietcontrol with projection and length measurements

The riveting machine is equipped with two measuring axes. The operator is thus enabled to set the priorities which are the most important ones for his quality assurance. For example the overall length of the unriveted bolt rivet can be measured with priority 1 in the length measurement while the finish riveted rivet height is given priority 2 in the projection measurement. The selected priorities can be allocated at discretion to different riveting points.

Quality assurance in terms of rivet material properties

It is of paramount importance for quality assurance to identify the right rivet material. If components sub-jected to extremely high loads were riveted using aluminium rivets instead of steel rivets, this mistake might have disastrous consequences. It is therefore important for the riveting machine to positively identify a wrong rivet material.

The following methods for identifying and examining material properties are known:

1. Measuring the force versus displacement curve of a riveted union
2. Measuring the force versus time curve of a riveted union
3. Measuring the displacement versus time curve of a riveted union

As measuring the force during riveting is a very complex matter, very often methods 1 and 2 are not suited for quality assurance purposes because it is almost impossible to determine the force in highly dynamic riveting processes. It is therefore more convenient to use the "hard" parameters to examine the material properties. The "hard" reproducible parameters like displacement and time can be measured with a very high degree of accuracy and, accordingly, be documented very well. These parameters are suited for quality assurance. The displacement of the riveting spindle is measured with a precision of a hundredth of a mm by measured-value transmitters and recorded. The Friedrich Nietcontrol Software subdivides the time line into millisecond intervals. Theoretically it would be possible to plot a permanent curve with any desired number of time slots. In practice, however, one time slot is sufficient.