Process capability index (Cpk) is a statistical tool, to measure the ability of a process to produce output within customer's specification limits. In simple words, it measures producer's capability to produce a product within customer's tolerance range.
When a large amount of data is to be evaluated, one takes the help of statistics. It is known that when the variation in the data is due to common causes only, the frequency distribution is a bell curve and from that various attributes are arrived at for evaluation.
While manufacturing a large number of components for the automotive industry, it is demanded that the evaluation be done on a statistical basis (as against 100% inspection) to make the manufacturing process dependent and not inspection (people) dependent. Hence the produce of a batch or day is to be evaluated based on sampling at various intervals and an index is arrived at. This index is generally mutually agreed upon between the supplier and the buyer. The end-users of the components are increasingly demand this capability index> 1.67 irrespective of the tolerance and the supplier and without knowing the implication accepts it and then struggles to meet the requirement. While both know what formula is to be applied for calculations, they are not aware as to how to the right results. So, many times this task is imposed upon the machine builder. Some builders accept this condition blindly, even without knowing the input conditions of the components to be machined and get into trouble at the last minute, thus creating difficulties for both the parties.
The generally accepted practice in the machine-building industry is that a short term capability is evaluated for a few critical dimensions at the machine builder’s end before the machine is accepted to be shipped. This is carried out by the machine builder with suitable work holding and tooling in a thermally stable condition of the machine. The basis generally is an hour’s production and is carried out without changing the tool offsets in the case of turning machines. The short term capability (Cp) and capability index (Cpk) is then evaluated (sometimes this is also known as machine capability (Cm) and machine capability index (Cmk)) based on measurement data of the critical dimensions. Geometrical tolerances if any are evaluated on a percentile basis or by evaluating on statistical basis subject to prior understanding or agreement.
The work is normally entrusted to one of the few chosen machine tool builders stating one’s requirement. The machine builder then scrutinizes the requirement and sends out a quote for the total turnkey work to the customer. This value-added service is expensive due to the time spent by the experts and the accumulated knowledge base that is required, which consists of:
- Data on similar components
- Machine behavior in production
- Most current Work holdings, Tooling
- Productivity requirement
At ACE we have been making efforts to educate customers while making such offers for turning solutions. Here is how we go about doing it. We make a study of the parameters involved that influence the machining process.
The following are of primary importance in turning:
These are design and manufacturing related issues and are taken care of during the various stages of basic machine design and a systematic assembly with checklists. At ACE all necessary data is recorded and retained for future scrutiny if needed.
All the above are very tricky issues except in a few cases like rigid components held by a standard 3 jaw power chuck. Knowledge built up over years, extensive experimentation, study of up-to-date tooling materials and a thorough background in analysis become necessary for finding out suitable solutions to be offered with the required confidence level. The uniqueness at ACE is to offer the customer the best possible solution under the given circumstances at a sensible price, but not necessarily the cheapest solution.
The work piece
Special shapes demand ingenious work holding solutions; exotic materials demand very special cutting tools, etc. At ACE the knowledge base created over the years is of vital importance.
Prior study of thermal distortion created by changing environment, effect of heat generated during cutting material, effect of heat generated by friction etc. have to be understood before Cp, Cpk commitment.
The operator completes the chain, in many cases, owing to financial or time constraints the user cannot go in for the recommended work holding or process and hence wants to use the operator for certain functions as above. An untrained, semi-trained or over confident operator can be a very weak link in the man-machine-work piece chain. Periodic training programs are held for customers & their operators at ACE.
Evaluating a machine based on Cp/Cpk is a different setting all together. The following preconditions need to be satisfied: a. The consistency of incoming components has to be verified. b. Gauges need to be calibrated and gauge capability is to be established (buyer’s responsibility). c. Machining trials to be conducted to establish scatter, trends (special causes) etc., in the stabilized state of the machine. d. Corrective actions to be taken. The component related problems are very difficult to anticipate during the proposal stage (this requires a substantial amount of accumulated experience & knowledge). This may need to be addressed by the redesign of tooling or work-holding if necessary. e. Stability of the process has to be established. If the process is not stable, then the underlying special causes have to be analysed and the corrective action is to be taken. This might even involve changing the incoming component requirement or changing the cycle time of the process to accommodate additional passes of the tool. It might also need a stable temperature environment for the machine. Here it becomes essential to involve the user and his co-operation. f. The study needs to be re-conducted at least a couple of times and the number of components with the right input conditions are to be supplied by the user after verification. g. The effort requires a substantial amount of man-hours and components. Presence of the user representative is required for each trial on an average, at least 3 days per process. If the number of operations is more than proportionally higher number of days is required for trials. The first attempt generally is never fully successful. h. The process can never be improved at the user’s premises unless it becomes their responsibility. The machine builder’s representative working in isolation cannot improve results at the user’s premises. This is a real-life experience. With the experience gained over a thousand tooled-up turning machines, we know the pitfalls of conducting the process capability study i.e. establishing ‘Cpk’. While this has become the keyword for demanding results from the machine builder, very few companies have established a manual for evaluating the same.
This brings in a lot of difficulties and misunderstanding during trials. Also, many times the machine builder is forced to accept whatever conditions imposed without even studying or understanding them with the attitude let us worry about it during trials and the over confidence puts both the machine builder and the user into difficulties related to avoidable delays.
It has been a part of our belief that we should offer cost-effective solutions to our customers and timely deliveries. But this service being engineering and manpower extensive, we find there is a need to spell out the requirement and responsibilities.
Incoming and outgoing Component drawings
M/c User-before quote
Supply of components checked as per input drawing
Method of measurement/ evaluation (if users gauges are to be used they need to be calibrated)
The critical / major parameters to be evaluated for Cp/Cpk
Has to be specified by the user. Commented upon by the builder before order.
Method of evaluation
Mutually settled before hand.
Only short-term evaluation of Cp/Cpk will be done at builder’s work
M/c builder in the presence of user
Long term test of Cpk with X-R charts is to be done at the user’s premises
By the users engineers only. ( If the user does not do this, there is no scope of improvement or maintenance of standards)
Method of Evaluation
The method of evaluation for Cp/Cpk is a long drawn process and requires a large number of components.
All over the world, the standards of evaluation for machined components are standardized. The following methods are accepted by most of the reputed automotive manufacturers (Ford, GM & Chrysler) as the norm. At the machine builder’s end – Short term evaluation (process with bilateral tolerance) evaluation is carried out.
Cpk > 1.67 is generally a goal and in practice generally very difficult to attain for very small values (say