Description: This phase of the procedure is used to replicate and check the supplier's homework. It is used to verify machine warmup and setup, as well short- term stability.
Objectives: After completing this phase, the machine acceptance team should have verified the supplier's process setup and production methods , process warm-up period, and short-term (30 consecutive parts ) stability.
Phase I participants include the machine acceptance team and the supplier of the machine.
Method (Figure 16.8): Fundamentally there are three items of concern in this phase:
Before data collection confirm gage capability by performing a repeatability and reproducibility (R&R) study (deemed capable if ratio < 0.1).
Have vendor set up the process to make parts to your needs.
Start up the process at production cycle times and wait the vendor-prescribed warmup period.
Data collection: After supplier-prescribed warmup period, collect 30 consecutive parts. Number the parts to maintain the time order. Measure all 30 parts. Record the measurements in time order and inspect all parts for witness marks (deformations caused by handling). Have the supplier fix the machine if the witness marks are too frequent or destructive to the parts' performance.
Data analysis:
Plot the 30 measurements on an Xbar and R chart with the data arranged into subgroups of three. Look for any out-of-control conditions. Don't cheat yourself by being loose with the definitions of control. Remember that you are the one buying the machine.
Investigate the distribution of the 30 parts. If they appear to have a normal distribution, then proceed to calculate P pk . If the distribution is not normal, do calculations based on the mirror imaging technique or probability paper with nonnormal data.
If the control chart exhibits out-of-control conditions, then an action plan with the vendor must be developed and executed. After the problem has been identified and fixed, start back at the beginning of Phase I.
Potential capability for this size of data is determined to be four standard deviations within specification. If four standard deviations (99.994%) are in specification, then it is believed that there is a good chance of achieving long-term stability with three standard deviations (99.73%) within specification.
If C p and C pk are < 1.33, instruct the supplier to reduce the variation (reduce Rbar) in the process. If C p > 1.33 but C pk < 1.33, then the process needs to be centered on the middle of the specification. In either case, do not accept this machine or proceed to repeat Phase I study until after the supplier has made appropriate changes.
Proceed to phase II for a C pk > 1.33 or for a nonnormal distribution with 99.994% of its cumulative frequency distribution within specification. A good way to make sure that Phase I has met all the requirements is to use a checklist such as the one in Table 16.2.
If at least 99.994% of the distribution is not within the specification limits, then develop an action plan based on the following questions. | |||
---|---|---|---|
Yes | No | Action Required | |
Is the lack of capability because of the average? | _____ | _____ | ____________ |
Is the lack of capability because of the range? | _____ | _____ | ____________ |
Were there any trends or patterns over the time window of the study? | _____ | _____ | ____________ |
Does the histogram suggest any unusual conditions such as nonnormality? | _____ | _____ | ____________ |
Does the study log show any unusual occurrences that would help explain apparent incapability? | _____ | _____ | ____________ |
Does either of the control charts give signals of unusual variation that would suggest stratification? | _____ | _____ | ____________ |
Should the study be rerun? | _____ | _____ | ____________ |
Thirty parts were collected as part of a Phase I study. A histogram of the 30 parts is given. Note that the histogram is skewed to the low side, indicating process stream effects of warmup. The control chart does show a warmup trend as indicated by the histogram. From this, it is concluded that the supplier did not determine an adequate warmup period. The next Phase I study will wait 12 cycles (four groups of three) more than the supplier-determined warmup period before data are collected. Note that the R chart is in control; hence it makes sense to look at the Xbar chart.
Sample size | 30 | Actual | EST | Limits | |
---|---|---|---|---|---|
99.730% | |||||
Target | 15.0000 | Low | 14.5090 | 14.4339 | 14.5000 |
Average | 14.9635 | High | 15.3036 | 15.4930 | 15.2000 |
Std. Dev. | 0.1765 | Range | 0.7945 | 1.0591 | 0.7000 |
Skewness | -0.5175 | % < Low limit | 0.00 | 0.43 | |
Kurtosis | 0.0538 | % > High limit | 6.67 | 9.01 | |
Normality test made | % Out of range | 6.67 | 9.44 | ||
Normality assumed | |||||
Subgroup size = 3 | |||||
WARNING: C p and C pk values for information only! | C P | 0.6609 | |||
Process not stable due to: Xbar chart | C pk | 0.4467 |
Cum Prob | Midpoint | Frequency |
---|---|---|
0.001 | 14.4000 | 0 + |
0.002 | 14.4500 | 0 + |
0.004 | 14.5000 | 1 + * ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” |
0.010 | 14.5500 | 0 + |
0.020 | 14.6000 | 0 + |
0.038 | 14.6500 | 2+.* |
0.068 | 14.7000 | 0+. |
0.113 | 14.7500 | 0+. |
0.177 | 14.8000 | 1 + *. |
0.260 | 14.8500 | 5 + **.** |
0.360 | 14.9000 | 2 + **. |
0.470 | 14.9500 | 3 + **. |
0.582 | 15.0000 | 3 + **. |
0.688 | 15.0500 | 6 + **. *** |
0.780 | 15.1000 | 4 + **.* |
0.855 | 15.1500 | 0+. |
0.910 | 15.2000 | 1+. ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” ” |
0.948 | 15.2500 | 1 +. |
0.972 | 15.3000 | 1 +. |
0.986 | 15.3500 | 0 + |
0.993 | 15.4000 | 0 + |
0.997 | 15.4500 | 0 + |
0.999 | 15.5000 | 0 + |
1.000 | 15.5500 | 0 + |
1.000 | 15.6000 | 0 + |
Note: Histogram 1 "*" = 1 sample (actual data); "." = estimated distribution. |
LCL = 14.7087 Center = 14.9635 UCL = 15.2182 | ||||||
---|---|---|---|---|---|---|
Subgroup size used: 3 | ||||||
Mean | SN | 14.5000 | 14.7000 | 14.9000 | 15.1000 | 15.3000 |
+ ” ” ” ” + ” ” ” ” + ” ” ” ” + ” ” ” ” + ” ” ” ” | ||||||
14.6564 | I | *: | + | : | ||
14.8760 | I | : | *+ | : | ||
14.9894 | I | : | +* | : | ||
15.1273 | I | : | + | * | : | |
15.0353 | 5 | I | : | + | * | : |
14.9598 | I | : | + | : | ||
14.9273 | I | : | *+ | : | ||
15.0336 | I | : | + | * | : | |
15.0184 | I | : | + | * | : | |
15.0111 | 10 | I | : | + | * | : |
LCL = 0.000 Center = 0.2498 UCL = 0.6419 | ||||||
---|---|---|---|---|---|---|
Subgroup size used: 3 | ||||||
Range | SN | 0.0000 | 0.2000 | 0.4000 | 0.6000 | 0.8000 |
+ ” ” ” ” + ” ” ” ” + ” ” ” ” + ” ” ” ” + ” ” ” ” | ||||||
0.3215 | : | + | * | : | ||
0.0457 | : * | + | : | |||
0.0225 | : * | + | : | |||
0.1617 | : | * + | : | |||
0.4722 | 5 | : | + | * | : | |
0.5187 | : | + | * | : | ||
0.2688 | : | + * | : | |||
0.1730 | : | * + | : | |||
0.2932 | : | + * | : | |||
0.2202 | 10 | : | * + | : |