BS ISO 7870-8:2017 pdf download – Control charts Part 8: Charting techniques for short runs and small mixed batches
b) Step 2: The next step is to determine if a difference in certain characteristics causes two or more processes to behave significantly different. This information can be obtained, for example, by
1) expert knowledge/workshops,
3) preliminary experiments, and/or
4) statistical analysis of existing data about the processes.
If there are no significant differences or the differences are systematic and can be compensated by normalization and no other practical reasons stand against it, the characteristics can be grouped and joint control charts can be applied.
c) Step 3: In the course of the application of control charts, more data are collected and more knowledge is gained about the processes. Therefore, it is wise to periodically recheck that the grouping conditions are still valid. This is especially true if alarms are frequently raised where no assignable cause can be found. To be able to flexibly group and re-group processes, it is important to record the characteristics as metadata along with the measured data so that each measurement value is associable with a group of processes.
EXAMPLE In Table 2, the grouping is done for the characteristics given in Figure 4. Without grouping, 360 combinations have to be monitored. With grouping, the number of combinations to be monitored is reduced to 4. For this example, it is assumed that tools, tolerances and measurement process have no significant effect on the process, the difference in nominal value can be compensated by normalization and the material and production machine have a significant influence on the process which cannot be compensated.
5.3 Typical applications The short run chart can be applied to product characteristics or process parameters regardless of the type of feature or differences in units of measure and process spread; for instance, a single short run chart can be set up to monitor multiple characteristics such as taper, parallelism, ovality and hardness on a cylinder. Another application is the small batch identification ticket that travels through the various processing stages of a job. A single short run chart is used to monitor quality performance at the various process stages of a job; for instance, at machining operations, heat treatment, anodizing and painting. At a milling machine, one short run chart can be set up to monitor different operations performed, such as drill, bore and ream in the X, Y and Z axes. The principles embodied in short run charts can also be applied to situations other than short run, small batch situations; for instance, the gap between a motor vehicle door and body panels where the gap is measured at different locations, A, B, C and D on a single door/body combination. A single chart then obviates the use of four different standard SPC charts, one for each location measured.
5.4 Preliminary process diagnosis As with any SPC implementation, it is important to pre-establish the types of major potential sources of variation (dominance) in a process. This will determine not only the SPC approach to be adopted but whether SPC is the correct tool to be applied. As a once-only exercise, certain questions need to be posed and considered about the short run and small batch process. In a manufacturing situation, the most likely principal sources of process variation should be assessed, for instance:
a) source material dominance: where the incoming material or previous operation has a major influence;
b) set-up dominance: where the characteristic is highly reproducible once properly set up;
c) operator dominance: where the process is highly dependent on the skill, care and attention of operational personnel;
d) time dominance: where the process can drift (for instance, with tool wear and lack of replenishment of solution mix ratios);
e) fixture or pallet dominance: where the fixtures or pallets holding the parts are a large source of inconsistency;
f) process parameter dominance: where the output is dependent on process parameters (for instance, depth and speed of cut, temperature of oil);
g) environmental dominance: where the temperature and humidity of the manufacturing area change;
h) information dominance: where variation and nonconformities are caused by frequent job or specification changes or poor measurement information.BS ISO 7870-8 pdf download.