Riveting Process Monitoring with the Friedrich Nietcontrol
Quality Assurance for the Riveted Union
The Friedrich Nietcontrol is a control system with patented measurement method for radial point riveting machines.
optimal quality assurance = minimum number of actuations to perform + maximum operational safety
The Friedrich Nietcontrol checks all significant parameters of a riveted joint. The parameters are monitored by displacement distances and the riveting times. The relevant data is shown on the display and saved under a program number. Any faults and errors are detected and shown.
Displacement distances are the most important parameter for quality assurance. It means that the riveting machine can reproduce and very precisely remeasure the closing head height. It can also document and check the given tolerance compliances.
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 non-riveted 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 permanent 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 low measuring pressure and with the riveting die aligned in its longitudinal axis in order to avoid a deformation of the rivet during measuring. A NOK message is generated if the length of the non-riveted rivet (Z1) is not within the tolerance range. As the 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 rivet projection (H1) is measured on the non-riveted rivet. For the projection measurement the measuring instrument is arranged directly at the spring-mounted downholder. For adjusting the H0-level the downholder measuring system is compressed on a plane surface. As soon as the measuting inset and the riveting die are at one level the H0-level is reached. When the riveting spindle moves back the downholder will decompress so that the riveting die is then standing back behind the measuring inset. 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 low 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 of (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 joints. 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 non-riveted 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 (closing 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 radial point riveting machine with Delta-measurement (deviation riveting)
With Delta-measurement there will be riveted from the initial length (Z1 or H1) to a deviation value. The riveting operation is finished when the deviation value (ΔZ or ΔH) is riveted.
Friedrich-Nietcontrol with projection and length measurements
There is the possibility to equip a machine with length and projection measurement system. So rivet length and projection is measured as well as the presence of all components is checked. The riveting operation is finished when the final value (H2 or Z2) is reached.
Quality assurance in terms of rivet material properties
It is of paramount importance for quality assurance to identify the right rivet material. If components subjected 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:
- Measuring the force versus displacement curve of a riveted union
- Measuring the force versus time curve of a riveted union
- 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. These reproducible parameters like displacement and time can be measured with a very high degree of accuracy and, accordingly, be documented very well so they 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.
Representation of a riveting operation as a function of displacement and time
Displacement versus time diagram:
Curve 1: Material too hard
Curve 2: Material correct
Curve 3: Material too soft