Measurement of the
Dimensions of Piston Rings.
Piston rings have to be manufactured to very close tolerances. The two critical
dimensions are the thickness (known as the width) and the size of the gap when
the ring is closed to its working diameter.
These machine uses pneumatically operated probes to measure the thickness at up
to four locations around the ring, a fifth probe measures the size of the gap in the
ring. Variable-reluctance contact probes are used as these are the only type found
to give the required measurement resolution of 0.1 micron and able to ignore the
thin film of oil on the rings.
The existing machines all had obsolete measurement electronics which were
difficult to set-up. Complicated cam timers were used to control the pneumatic
walking beam, which transferred the rings through the machine, raised and
lowered the probes, and triggered the measurement electronics.
The electronics and cams were replaced with a digital control computer, programed
in a multitasking high level control language. The system allows all aspects of the
machine timing to be easily tuned for the optimum balance between accuracy of
measurement and production rate. The system also calculates the mean and
standard deviations for a batch, which, together with other production statistics,
are output to a small panel printer.
The systems have been very popular with the machine operators; ease of set-up
being a key feature, with which the operators were closely involved at the design
stage.
Nine of these systems have been converted over a period of three years, the first in
1995. They have proved to be very reliable, with maintenance overheads reduced
to a fraction of the original. Various software updates were installed up to 2002.
These systems are a good example of how a computer system can be added onto
an old machine and give it a new lease of life.
