Table of contents
1. Cmk (equipment capability index) introduction:
Definition: Cmk is a parameter often used in the German automotive industry. It is the abbreviation of "Machine Capability Index", which is called the critical machine capability index. It only considers the influence of the equipment itself, and considers the deviation between the average value of the distribution and the standard central value. Since only the influence of the equipment itself is considered, other factors should be strictly controlled during sampling, and interference from other factors should be avoided as much as possible. The calculation formula is exactly the same as Ppk, but the sampling is different.
The equipment capability index is an index that reflects the process in continuous production, generally abbreviated as Cmk. It is a short-term capability index, also known as initial process capability. Long-term process capability can also be estimated based on Cmk results. For example, the general specification Cmk of the automotive industry requires ≥1.67 (5 sigma), while Cpk requires ≥1.33 (4 sigma), which is generally an offset of 1 sigma.
A machine performance study is the test of a machine's suitability for a particular production task. The Machine Capability Index can be used to measure the effectiveness of process improvement efforts. It is used to study the position of machine performance relative to tolerance limits.
Cmk refers to the capability index of the machine (equipment) (m refers to the machine or equipment). The object of the research is equipment, which is expressed as the processing consistency of a certain equipment processing a certain part under a certain tolerance.
2. Cmk (Equipment Capability Index) Concept:
The CP (or Cpk) process capability index refers to the actual processing capacity of the process in a controlled state (steady state) within a certain period of time. It is the inherent ability of the process, or it is the ability of the process to ensure quality.
The process referred to here refers to the process of the comprehensive action of five basic quality factors such as operator, machine, raw material, process method and production environment, that is, the production process of product quality. Product quality is the comprehensive performance of the various quality factors in the process.
CPK: emphasizes the inherent variation of the process and the actual inherent ability;
CMK: considers short-term discrete, emphasizing the influence of the equipment itself on quality;
CPK: the analysis premise is The data obeys the normal distribution, and the process is controlled;
CMK: used for new machine acceptance, new product trial production, after equipment overhaul, etc.;
CPK: at least 1.33
CMK: at least 1.67 is a qualified
CMK. Sampling 10 groups of 50 samples within one hour
CPK takes 25 groups of at least 100 samples at an appropriate frequency when the process is stable and controlled
3. Application timing of Cmk:
Under what circumstances should the application timing of Cmk test the machine capability? In many cases, the machine capability index test may be required, but in these three cases, the test is most needed:
① During the acceptance process of new equipment;
② When the process capability is monitored during the production process, "process capability "Abnormal situation, when analyzing and taking measures to find out whether it is the cause of the equipment;
③ used as one of the basis for acceptance after equipment maintenance, that is, before and after the overhaul of the equipment; and when carrying out the inspection of the equipment itself, or related tooling and fixtures (or tools) for technical transformation, in order to evaluate the effect of transformation.
Other calculation Cmk situations:
Cmk Practical Application
Acceptance of new equipment to assess whether the performance of newly manufactured equipment meets key performance requirements
Equipment activation to assess whether there is any degradation of key performance after transfer or overhaul
Material changes (such as localization, replacement of suppliers), evaluation of materials Impact on key performance
New process (for example, the connection was realized by welding before, and now it is screwed), evaluate whether the new implementation method meets the short-term key performance requirements, and if necessary, the impact of material limit conditions needs to be considered Newly designed parts to evaluate
its Whether the key performance is met, the influence of material limit conditions needs to be considered when necessary.
The first two types are widely used (in a narrow sense, they are directly related to equipment). In practice, the production process and equipment department will always encounter the situation that CmK is not enough. A considerable part of the reason is that the material/process and design engineering are wide-ranging (possibly cost The administrative order under the down requirement, the ability or awareness of the designer and the reviewer, etc.). But the requirements for producing products are strict, especially for automotive/medical products.
Therefore, in many cases, no matter how the equipment or process parameters are adjusted, or even after the purchase of more sophisticated equipment, the CmK still cannot be passed, and the production can only be continued temporarily by way of concession release (cumbersome documents and approval process) + 100% inspection. This is a contributing factor to the irascibility of process and equipment employees.
4. Preliminary preparation and requirements of Cmk:
Measurement system: meet the requirements of measurement system analysis MSA, which is the premise of Cmk. MSA is just playing hooligans by asking for Cmk.
People: fixed personnel (operating qualifications) and shift machine:
fixed machine tooling and parameters such as speed
fixed hardware and parameters of the support system, such as cooling water temperature, pressure and flow rate
equipment and tooling positioning, prohibiting adjustment (often called fine-tuning)
materials :
For the same batch of semi-finished products and materials, it is recommended to record the material batch and key attribute information to facilitate problem analysis.
Method: In the current production process, no additional processing should be done for this batch of test materials.
Ring:
Short-term fixed (or relatively small compared to the scope of control beating) ambient temperature, air pressure, humidity, electric field, magnetic field environment, etc.
Five, Cmk test requirements:
For Cmk, what we care about is the capability of the machine itself, and we should try to eliminate the influence of other factors during the sampling process. Therefore, the
test requirements should meet the following points:
①The sampling data should be continuous in the shortest possible time (reduce environmental Influence),
②The same operator (to reduce the influence of human factors),
③Adopt standard operating methods (methods),
④For the same processing materials (same batch of raw materials),
⑤Only assess the deviation of the machine itself.
(The operator must be skilled and competent; the measuring tools must meet the requirements and have been calibrated; the same batch of incoming materials, preferably the same mold; the equipment and products cannot be adjusted during the test).
In terms of calculation methods, the number of samples can be based on the actual situation (customer requirements, company regulations, sampling costs, etc.), but in principle it should be greater than 30, because the sampling space is actually not a normal distribution but t Distribution, when the number of samples is greater than 30 (usually required to be more than 50), it is close to the normal distribution. The formula we use is based on the normal distribution.
The equipment capability index Cmk indicates the capability determined only by the variation of common causes of the equipment. It is different from Cpk Ppk in that the sampling method is different. It is the data of at least 50 pieces when the machine is working stably. Cmk=T/6sigma, sigma can be used for at least 50 consecutive pieces According to the experience of the American industry, 75% of process variation comes from equipment variation, if at least 50 consecutive pieces of data are used to estimate If the sigma estimated by s or the sigma estimated by Rbar/d2 after grouping at least 50 consecutive pieces of data are used to calculate Cpk, the total common cause variation of human, machine, material and method is 8sigma, Cpk=T/8sigma, (for convenience, The above formulas are when the distribution center and the tolerance coincide)
Machine Capability: "Machine Capability" is derived from the ratio of the tolerance to the process variance of the production equipment. Usually, the method of mathematical statistics is used for measurement and proof. At this time, only the short-term discrete is considered, and the factors that affect the process rather than the machine are excluded as much as possible. (compare VDA Volume 4 Part 1)
Process Capability: In contrast to "Machine Capability", looking at the long-term dispersion with consideration of the parameters influencing the process, people call it "Process Capability". The process is capable if the discrete and location statistical characteristic parameters of nominal values and tolerances meet the requirements (at least Cpk = 1.33). If not, process capability must be achieved through process analysis and optimization.
6. CMK calculation formula:
Seven, Cmk actual operation:
- Try to adjust the equipment to the median value required by the specification, and fix the fixtures to prevent the median drift/standard deviation from increasing due to clamping. 2. Prepare >=50
sets of semi-finished products and auxiliary materials of the same batch. Cmk calculation only needs 50pcs, and the extra parts It is the scrapping caused by the equipment adjustment in the first step and the equipment abnormality that is not a process problem. 3. Continuously measure the product characteristics of the 50 sets passing through the station according to the label (note: if it is different acupoints of the same equipment, you can measure and record them separately according to the acupoints. , Of course, it is better to pass Cmk when mixed together) 4. Verify the normality of the data and calculate
Cmk ->Normal)/Q-Das and other tools
such as Minitab operation:
Step 1: Check the normality of the data:
The second step is to perform normal calculation analysis:
Note that if the data is not normally distributed, it can be converted and then normal analysis can be performed;
If it is not normal, it is recommended to follow the following steps (many companies or teaching materials recommend normal transformation, convert non-normal data to normal, and then calculate, this is not the optimal method, and there will always be situations that do not match the actual results)
Re-check the measurement system to see if the measurement system is not capable or abnormal
Review the historical data, and if possible, review the data of other factories to see if the past data is normal
If the final confirmation is that the data is not normal, then verify Which type of distribution the data is closest to, and then use minitab to calculate according to the recommended distribution (this part will be opened separately)
Remarks: Various data distributions exist, but the common data in the factory is normal, and the general non-normal It is caused by an abnormality of the equipment or measurement system. This must be monitored and checked.