Quality Crimp Handbook
SECTION 8
CRIMP PROCESS CONTROL
The crimp process is the interaction of a terminal, wire,
tooling, personnel, methods and procedures, and
environmental attributes. When this process is
controlled, it will produce a quality termination. Quality
control is an important part of quality crimping. It
should not take excessive setup or inspection time to
do, and it can save a harness manufacturer thousands
of dollars in potential rework or re-manufacturing.
Variability is the slight change that occurs from crimp to
crimp. There are two types of variability, common or
special. Common causes of variation affect the
process uniformly and are the result of many small
sources. Common variability is inherent tolerances
within a reel of wire or terminals. Common variability
also is created by the natural tolerances of the stripping
and crimping machines.
Reducing variability at the common level typically has
* PPM - Parts per million potential defects.
A 25 piece minimum sample needs to be taken from
the crimping process. Calculate the average and
standard deviation for the sample. A capability index is
defined by the formula below. C p may range in value
from zero to infinity, with a larger value indicating a
more capable process. A value greater than 1.33 is
considered acceptable for most applications. C p is
calculated with the following formula.
___Tolerance___
6*Standard Deviation
The C pk index indicates whether the process will
produce units within the tolerance limits. C pk has a
value equal to C p if the process is centered on the
mean of specification; if C pk is negative, the process
mean is outside the specification limits; if C pk is
between 0 and 1 then some of the 6 sigma spread falls
outside the tolerance limits. If C pk is larger than one,
the 6-sigma spread is completely within the tolerance
limits. C pk is calculated with the lesser of the following
formulas:
to come from changes to the wire, terminal, and tooling
manufacturer.
__(USL - Mean)__
3*Standard Deviation
__ (Mean - LSL)__
3*Standard Deviation
Special causes of variation occur irregularly and
unpredictably. Without checks throughout a run, having
a tool become loose after the first hundred crimps or a
jam resulting from a damaged tool may be undetected
until thousands of crimps are made.
Process Capability
Before putting a new crimping tool in production, Molex
recommends that each customer do a capability study,
using the specific wire that will be used in its process.
A capability study, which is based on the assumption of
a normal distribution (bell-type curve), estimates the
probability of a measurement being outside of
specification.
USL = Upper Specification Limit, LSL = Lower
Specification Limit
Six sigma is a goal of many companies because it
represents virtually zero defects. The ability of a
company to achieve a six-sigma level depends on the
amount of common variability in its process. For
example, hand stripping the wire produces more
variability than a stripping machine; crimping hand tools
produce more variability than a press and die set, and
bench terminations produce more variability than a
wire-processing machine.
A part of the variability in crimping will result from the
type of instruments that are used to measure the parts
C pk
0.67
1
1.33
1.67
2
Capability
+/- Sigma % Yield
2 95.45
3 99.73
4 99.99
5 99.99+
6 99.99++
PPM*
45,500
2,699
63
0.57
0
and the operator's ability to repeat the measurement. A
crimp micrometer will measure more accurately than a
dial caliper. An automatic pull force system will
measure better than a hook type scale. It is important
that the measurement gauge has enough resolution.
Order No: TM-638000029
Revision: D
Release Date: 09-04-03
Revision Date: 12-23-09
UNCONTROLLED COPY
Page 15 of 24
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