User Manual

Quality Crimp Handbook
Order No: ATS-638000029 Release Date: 09-04-03 UNCONTROLLED COPY Page 14 of 23
Revision: C Revision Date: 09-12-06
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 to come
from changes to the wire, terminal, and tooling manufacturer.
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.
Capability
CpK +/- Sigma % Yield PPM*
0.67 2 95.45 45,500
1 3 99.73 2,699
1.33 4 99.99 63
1.67 5 99.99+ 0.57
2 6 99.99++ 0
* 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 each
specification. A capability index is defined by the formula below.
Cp 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. Cp is calculated with
the following formula.
___Tolerance___
6*Standard Deviation
The CpK index indicates whether the process will produce units
within the tolerance limits. CpK has a value equal to Cp if the
process is centered on the mean of specification; if CpK is
negative, the process mean is outside the specification limits; if
CpK is between 0 and 1 then some of the 6 sigma spread falls
outside the tolerance limits. If CpK is larger than one, the 6-sigma
spread is completely within the tolerance limits. CpK is calculated
with the lesser of the following formulas:
__(USL - Mean)__
__ (Mean - LSL)__
3*Standard Deviation 3*Standard Deviation
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 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.
Two operators may measure the same part differently, or the
same operator may measure the part differently when using two
types of gauges. Molex recommends a gauge capability study to
identify what part of the variability is coming from measurement
error. Micro-terminals crimped to small wire sizes need a tight
crimp height range to maintain pull force. The variability from
measurement error can keep CpKs low.
The capability of the crimping tools needs to be re-confirmed if the
production data is significantly different from the capability study.