Process reliability in the Screwdriving Technology
A Machine Capability Study is the testing of a machine in regards to its suitability for a special production- or tightening task. The Machine Capability Study (Cmk) can be done in the Laboratory or by using variable factors.
Contrary to the Machine Capability Study, a Process Capability Study (Cpk) can only be performed directly on the assembly line and under inclusion of all influencing factors of the screwdriving process.
By stating a Cmk-value, a clear statement in regards to the machine capability is possible. The deviations from the nominal value as well differing machine variations are acquired by this capability index. The larger the average deviation of a machine, the higher is the possible share of defective parts when using this machine in the production.
For screw-assemblies, torque or the angle is the deciding medium. In order to examine merely the influence of the screwdriver on the production process, we use a reproduced test-setup to perform 50 measurements under optimal conditions without a break. The result is then recorded using a suitable reference system.
We compare the machine capability study (Cmk) with the required tolerance values (upper and low limit) and the standard deviation.
A machine capability study is documented in a detailed protocol with statements in regards to all relevant data.
A Cmk-value of 1.67 (this value is frequently demanded for many different applications) testifies, for example, that 99,99994 percent of the assemblies are within the allowable tolerances. This means that the fault rate is 0.6 parts per million In short, from 1 Million screw-assemblies, 0.6 assemblies can be outside of the allowable limits.