24-09-2014, 02:35 PM
ABSTRACT NAME OF THE PROJECT- PROCESS CAPABILITY STUDY (GRINDERS) To analyze a process in order to monitor, control, and improve it. The objective is to have a stable, consistent process that produces the fewest defects possible. To control variation so as to avoid product defects. Improve the quality because quality is the degree to which products or services meet the needs of customers. Common objectives for quality professionals include reducing defect rates, Manufacturing products within specifications, and standardizing delivery time. This process capability study helps to achieve such targets. The output of a process is expected to meet customer requirements, specifications, or product tolerances. We conduct a process capability study to determine the extent to which the process can meet these expectations. Our aim is to maintain the quality of the ring which should be within the specification. Its results less scrap, better quality, productivity enhancement and cost reduction. Some Key Points All work is a process. A process turns an input into an output for a
1. PROJECT REPORT
1.1 NAME OF THE PROJECT- PROCESS CAPABILITY STUDY (GRINDERS)
1.1.1 PROJECT AIM
To analyze a process in order to monitor, control, and improve it. The objective is to have a stable, consistent process that produces the fewest defects possible.
To control variation so as to avoid product defects.
1.1.2 SCOPE OF PROJECT
Improve the quality because quality is the degree to which products or services meet the needs of customers.
Common objectives for quality professionals include reducing defect rates,
Manufacturing products within specifications, and standardizing delivery time.
This process capability study helps to achieve such targets.
1.1.3 PROBLEM IDENTIFICATION
The output of a process is expected to meet customer requirements, specifications, or product tolerances. We conduct a process capability study to determine the extent to which the process can meet these expectations.
Our aim is to maintain the quality of the ring which should be within the specification. Its results less scrap, better quality, productivity enhancement and cost reduction.
1.2 INTRODUCTION
ROUGH GRINDING/MEDIUM GRINDING
Rings obtained from foundry have poor surface finish and less dimensional accuracy. Ring blanks are grinded axially to achieve the dimensions required. To grind rings axially center-less grinding machines are used. These are very robust, rigid, heavy machines. In this process ring blanks are fed through a magnetic feeder to the two revolving grinding wheels, separated by a distance which is equal to the thickness of the ring required.
Axial width grinding is carried out in two or three stages as per requirement and different special purpose, dedicated centre-less grinders are used for grinding the ring blanks to achieve required size as per process sheet.
Besly (Rough/Medium grinding Machines)
Some machines have computer controlled settings and some are manually controlled
Used for rough/medium grinding of axial width of rings
Working principle is centre less grinding with horizontal spindles
Parameter controlled is axial width of ring
Manpower – 2 persons per shift
Angle between grinding wheels – 3˚ to 5˚
Coolant used – CIM cool (6-10%) mixed with water
Difference between HMT and Besley is only of the production targets and the shape of machine.
Other parameters:
· Distance between the grinding wheels
· Grinding marks on face of rings
· Variation of axial thickness of ring at different places on the ring
· The difference between the guide plates is o.k.
FINISH GRINDING
To give the finish touch we use finish grinding machines. It grind ring axially. There cut is small but they are more accurate. We measure accuracy in microns they have much less tolerance then rough grinding machines. There speed varies according to the ring size and material. We check the ring with the help of dial gauge.
Dus Grinding Machine
Used for finish grinding of axial width of rings
Working principle is centre less grinding
Parameter to be controlled is axial width of rings
Manpower – 2 persons per shift
Angle between grinding wheels - 3˚ to 5˚
Comparator gauge with dial gauge having least count of .001 mm calibrated with master gauge is used for inspecting the axial width of rings with a sample size of 2 and frequency is at setup, 6-8 times in shift and end of production
Computer controlled settings.
Other parameters:
· Adjustment of guide plates
· Wheel dressing
· Speed of rotation of wheels
· Feed rate
· Cut to be applied
· Marks on the ring
· Angle of the wheels
*For axial width, measuring tool used: dial or digital gauge.
1.3 METHOD OF STUDY
Study is taken on 3 machines are:
· Dus
· Besley 3
· Besley 4
The study was done first by collecting the data for each machine. This data was collected with measuring tool dial gauge of least count 0.001mm.
Total 50 rings are measured at each study by considering the following points:
· Data is collected during machine running.
· All the settings or parameters are ok like wheel settings or cut settings etc.
· Check Dial gauge setting.
· No major change in machine settings during that time.
· Data is collected after regular intervals.
· Check process sheet and specification of rings in sheet.
· Proper note down the data.
After collecting the data study is done by analyzing the data.
There are two basic reasons for using - First it allows us to determine when to take action to adjust a process that has fallen out of control. Second, it tells us when to leave the process alone. Knowing when to take action on a process is an important step in defect prevention and eliminates inspection and sorting of product after a large batch has been made. Knowing when to leave a process alone is equally important in keeping variation at minimum.
*In this study process capability sixpack graph is used.
1.4 PROCESS CAPABILITY
The Process Capability is a measurable property of a process to the specification, expressed as a process capability index (e.g., Cpkor Cpm) or as a process performance index (e.g., Ppkor Ppm). The output of this measurement is usually illustrated by a histogram and calculations that predict how many parts will be produced out of specification.
Process capability is also defined as the capability of a process to meet its purpose as managed by an organization\'s management and process definition structures.
The capability of a process should be constantly measured and analyzed. Capability analysis can help you answer following questions:
· Is the process meeting customer specifications?
· How will the process perform in the future?
· Are improvements needed in the process?
· Have you sustained these improvements, or has the process regressed to its previous unimproved state?
Two parts of process capability are –
1) Measure the variability of a process, and
2) Compare that variability with a proposed specification or product tolerance.
1.5 Process Capability Ratio
A process is capable if it has a process distribution whose extreme values fall within the upper and lower specification for a product or service. As general rule, most values of a process distribution fall within ±3 standard deviation of the mean. In other words, the range of values of the quality measure generated by the process is approximately 6 standard deviation. Hence, if a process is capable, the difference between the upper and lower specification, called the tolerance width, must be greater than 6 standard deviation (Process variability). The process capability ratio, Cp, is defined as
Cp = (UTL – LTL) / 6σ
UTL = Upper tolerance limit
LTL = Lower tolerance limit
σ = Standard deviation
Normal Distribution
If Cp is greater than 1.0, the tolerance range is greater than the range of actual process outputs. If Cp is less than 1.0, the process will produce products or services outside their allowable tolerance. Firms often choose an arbitrary critical value for the process capability ratio, such as 1.33, to establish a target for reducing process variability. The value is greater than 1.0 to allow for some change in the process distribution before bad output is generated.
Process Capability Index
The importance of process capability is in assessing the relationship between the natural variation of the process and the design specifications. This is often quantified by a measure known as the Process Capability Index.
Cpk Index
It is possible to predict a relatively wide tolerance band with a relatively small process variation, but in which a significant proportion of the process output lies outside the tolerance band. This does not invalidate the use of Cp as an index to measure the ‘potential capability’ of a process when centered, but suggests the need for another index which takes account of both the process variation and the centering. Such an index is the Cpk, which is widely accepted as a means of communicating process capability.
The overall process Cpk is the lower value of Cpku and Cpkl. A Cpk of 1or less means that the process variation and its centering is such that at least one of the tolerance limits will be exceeded and the process is incapable. As in the case of Cp, increasing values of Cpk correspond to increasing capability. It may be possible to increase the Cpk value by centering the process so that its mean value and the mid-specification or target, coincide. A comparison of the Cp and the Cpk will show zero difference if the process is centered on the target value. The Cpk can be used when there is only one specification limit, upper or lower – a one-sided specification. This occurs quite frequently and the Cp index cannot be used in this situation.
These indexes indicate that the process would have difficulty meeting its upper tolerance limit but that it would easily meet the lower tolerance limit. Cpk summarizes the upper and lower capability indexes into a single number reflecting the worst case; it is often used in specifying quality requirements in purchasing contracts.
We take the minimum care of the two ratios because it gives the worst-case situation. If Cpk is greater than the a critical value greater than 1.0, and the process capability ratio is greater than its critical value, we can finally say the process is capable. If Cpk is less than one of the tolerence limits and is generating defective output.
The capability index will always be less than or equal to the capability ratio. When Cpk equals the process capability ratio, the process is centered between the upper and lower specifications and hence the mean of the process distribution is centered on the nominal value of the design specification.
Difference Between Cp and Cpk
Cpk measures how near we are to our target and how consistent we are in meeting our average performance. IT may happen that a person may be performing with minimum variation, but still be away from the target towards one side of the mean, which indicates lower Cpk, whereas Cp will be high. On the other hand, a person may be on average exactly at the target , but there may be variation in performance although within the specification limits. Here also Cpk will be lower, but Cp will be high. Cpk will be higher only when we consistently meet the target with minimum possible variation.
Effect of Cpk on Management
Process capability study has effect on management decision as:
1) Make or buy.
2) How to direct capital expenditure for machinery.