How to do Pre-control Limits, or
spelled precontrol), is a technique that can be used with X-bar
and R, (Range), control
charts for example, for revealing process
situations and variations
that could produce defects (opposed
to variations which are statistically noteworthy), and establish
control limits without the using the normal calculations that are used
with Upper control
limits and Lower
When using pre-controls, start with a process
centered between specification limits and attempt to identify movement
that could produce some of the process parts being outside a print limit
specification. precontrol does not involve plotting and/or calculations
and requires only three-piece parts to provide control information. The
technique assumes a normal distribution
curve to identify important variations within the spread of a production
process that may produce an increase in defects in the production's
The theory of precontrol can be demonstrated by
making the assumption of a worst case condition that could be tolerated
from a process's capablity.
In other words, when the normal tolerance matches what the print would
allow, and when the process is exactly centered, any shift in the
process would result in some defective
To set the control limits you should draw in two
precontrol (PC) lines, each one quarter of the way inside of the
specification limits. (Figure 1) As you can see, 86% of the piece
parts should be within the PC limit lines, with a delta of 14%, 7% in
from the specification limits. In other words, roughly 1 part in 14
could fall outside of the established PC lines in any typical
The chance that two parts in a row will fall
outside a PC line is 1/14 x 1/14. This should mean that only about one
in every 200 pieces or so could be expected to have 2 piece parts
consecutively in a row within the given outer bands. If 2 consecutive
piece parts are in a row, there may be a much larger probability,
195/196, that the process may have been shifted. It is therefore
recommended that one should reset the process's center at that point. It
is also just as unlikely that any given consecutive piece part could go
beyond one of the established PC lines. In such a case, it could
indicate that the process may have been shifted but not due to natural
variation and that some other factors may have been introduced which
resulted in a wider pattern to the point that defective pieces could be
expected. If the latter is the case, then an immediate fix of the
problem should be made prior to the process continuing to operate
The above theory should lead us to the following
set of rules, (see note at end of page), that sums up the
technique of precontrol:
1. Divide the specification
tolerance range with PC lines at 1/4 of the acceptable tolerance as
shown in the illustrations.
2. Start the process.
3. If any piece part falls
outside the specification limits, then reset the control limits as
4. If only 1 piece part is
within the specification limits but outside a PC lines, check the next
consecutive piece part.
5. If second piece part also
falls outside same PC line, then reset the control limits as prescribed
6. If the second consecutive
piece part is within the PC line, you should continue the process and
reset the control limits as prescribed above, only when two consecutive
pieces in a row fall outside the established PC lines.
7. If two consecutive pieces
show one to be outside the high PC line and one below the low PC line,
you should take action immediately to decrease the variation.
8. When five consecutive pieces
fall between the PC lines, then frequency gauging may start. While
waiting for the five pieces, if one piece goes over the established PC
lines, you should start the count all over again.
9. While frequency gauging, you
should let process alone until a piece goes beyond the established PC
lines. Then inspect
the very next piece part and proceed as in 6 above.
10. When the machine has been
reset, five consecutive piece parts within the established PC lines must
again be produced prior to returning to frequency gauging.
11. If an operator inspects 25
or more times without the need to reset, the gauging frequency could be
reduced so that more piece parts are produced between inspections.
However, if the operator must reset before the 25 inspections are made,
you should raise the gauging frequency. On average, 25 inspections are
required to indicate that the gauging frequency is accurate.
This pre-control theory should point to variations
within any process's aim. It is easy to employ, can use go/no go gages,
and can assure a specific percentage of defective piece parts when
tweaks are made to the process as necessary.
pre-control is an
illustration of the model also known as narrow-limit gauging. The
broader theory offers sampling
procedures to link a predefined possibility of accepting bad piece
parts. Many quality experts have argued about the theory. You should
also investigate an acceptance
control chart that would consider both consumer and producer chances.
The set of rules applies only when 1% to 3%
defective is allowable and the six sigma process spread is no more than
88% of any given tolerance range. You should also check for other rules
that cover special quality
levels and process spreads.