2. Machine capacity index: when a certain equipment processes a certain product of a certain size, the consistency of processing is its equipment capacity (cm); If certain tolerance conditions are applied, the equipment capability index CMK can be calculated.
3. Calculation formula CMK= tolerance zone (upper difference-lower difference)? Excluding six sigma× (1-offset), sigma = (actual value minus median) absolute value plus /N number, and sigma:offset simple = mean minus median/tolerance zone of SPC.
4. The difference between 4.CMK and CPK
CPK: It emphasizes the internal change of the process and the actual internal ability.
CMK: consider short-term dispersion and emphasize the influence of equipment itself on quality;
CPK: The premise of analysis is that the data obeys 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 least1.67;
CMK generally takes 50 samples from 10 group about one hour after the stable production of the machine.
Under the condition of stable and controllable process, CPK takes 25 groups of at least 100 samples at an appropriate frequency.
Extended data:
Ability investigation must determine the characteristics and methods.
Capacity of machines: The capacity of machines and equipment, including molds, shall be verified by the manufacturers or acceptors of machines and molds before new purchase and use.
Under the following specific circumstances, it must be agreed with the customer: new parts order, new mold/equipment, tightening of tolerance, change of processing flow/input state, after maintenance (which has an impact on the product), after machine relocation and after long-term production suspension.
The proof of machine capability should be used to evaluate process capability. In the capacity investigation, the machine should be regarded as an entity together with the mold and, if necessary, with the integrated inspection fixture and adjustment device. Short-term dispersion, the ability index should be at least Cmk= 1.67. If there are deviations, corrective measures must be specified and a new capacity survey must be implemented after the measures are completed.
Process capability: product characteristics and process parameters that have a decisive influence on product quality must be proved. In principle, all characteristics must be within the tolerance range. Important characteristics of product quality must be specified and consistent with customers. If the process capability of important characteristics cannot be proved, measures must be specified.
Baidu encyclopedia -CMK