Process capability and equipment reception
The capability of the technological process represents its ability to make parts with characteristics within tolerances.
W. A. Shewart defines the ability of the process as its property to ensure a field of variability of a certain measurable characteristic smaller than the specifications.
A stable technological process is capable if the natural spread obtained under known technological conditions and defined by the difference between UCL (upper control limit) and LCL (lower control limit) is less than the spread given by the difference between USL (upper specified limit) and LSL (lower specified limit).
The standard that regulates process capability designates a process that is capable of meeting two conditions: stability + Cpk> = 1.33.
Cpk = min ((X̄-LSL) / 3s, (USL-X̄) / 3s), where X̄ represents the average of the measured values.
The process capability reflects the influence of all factors in the 6 M category.
In modern processes, equipment induces an increasing influence, therefore, it is critical to determine separately the capability of the equipment (machine), ie what is identified with Cmk.
The calculation formula is similar to Cpk. Cmk reflects the strict influence of the car, for this reason, Cpk (which includes the influence of the other 5M) will always be smaller than Cmk.
Cmk is calculated by making lots of parts maintaining 5 of the 6 fixed categories (man, environment, material, method and measurement) the only variable being the car.
This means that in order to have a capable process, the Cmk must be with a class superior to the accepted Cpk.
For example, to get a Cpk = 1.33, we should have (at the reception of the car) a Cmk of at least 1.67.
Failure to measure the capacity of the machine at its reception as well as periodically, will lead to negative financial effects due to a weak process capability induced by many defective parts.
Of course, during the use of the equipment, the neglect of preventive maintenance will induce a decrease in the capacity of the machine and the process, with the harmful consequences of defective parts, uncontrolled stops and overtime.
So, do not neglect the determination of Cmk at the reception of the car (initial and after each major overhaul) and do not forget about preventive maintenance if you want to have peace of mind in the production process and make a profit!
W. A. Shewart defines the ability of the process as its property to ensure a field of variability of a certain measurable characteristic smaller than the specifications.
A stable technological process is capable if the natural spread obtained under known technological conditions and defined by the difference between UCL (upper control limit) and LCL (lower control limit) is less than the spread given by the difference between USL (upper specified limit) and LSL (lower specified limit).
The standard that regulates process capability designates a process that is capable of meeting two conditions: stability + Cpk> = 1.33.
Cpk = min ((X̄-LSL) / 3s, (USL-X̄) / 3s), where X̄ represents the average of the measured values.
The process capability reflects the influence of all factors in the 6 M category.
In modern processes, equipment induces an increasing influence, therefore, it is critical to determine separately the capability of the equipment (machine), ie what is identified with Cmk.
The calculation formula is similar to Cpk. Cmk reflects the strict influence of the car, for this reason, Cpk (which includes the influence of the other 5M) will always be smaller than Cmk.
Cmk is calculated by making lots of parts maintaining 5 of the 6 fixed categories (man, environment, material, method and measurement) the only variable being the car.
This means that in order to have a capable process, the Cmk must be with a class superior to the accepted Cpk.
For example, to get a Cpk = 1.33, we should have (at the reception of the car) a Cmk of at least 1.67.
Failure to measure the capacity of the machine at its reception as well as periodically, will lead to negative financial effects due to a weak process capability induced by many defective parts.
Of course, during the use of the equipment, the neglect of preventive maintenance will induce a decrease in the capacity of the machine and the process, with the harmful consequences of defective parts, uncontrolled stops and overtime.
So, do not neglect the determination of Cmk at the reception of the car (initial and after each major overhaul) and do not forget about preventive maintenance if you want to have peace of mind in the production process and make a profit!
