Crimp Tooling —
Where Form Meets Function
Quality, cost, and throughput are key attributes for any production process. The
crimp termination process is no exception. Many variables contribute to the results.
Crimp tooling, defined here as crimpers and anvils, is one of those variables.
This paper will focus on defining key characteristics of crimp tooling and the effects
those characteristics may have on the production process.
Introduction
Quality, cost, and throughput are associated with specific measurements and linked to process variables. Crimp
height, pull test values, leads per hour, and crimp symmetry are some of the measures used to monitor production
termination processes.
Many variables affect the process such as wire and terminal quality, machine repeatability, setup parameters, and
operator skill.
Crimp tooling is a significant contributor to the overall crimp termination process. The condition of crimp tooling is
constantly monitored in production by various means. These means are often indirect measures. Crimp Quality
Monitors and crimp cross sections are methodologies that infer the condition of the crimp tooling. Visual inspec-
tion of the crimp tooling can be used
to check for gross failures such as tool breakage or tooling deformation which occurred as a result of a machine
crash. Continuous monitoring of production will help determine when
the process needs to be adjusted and the replacement of crimp tooling can be one of the adjustments that is
made.
Crimp tooling can a have positive effect on the quality, cost, and throughput of the termination process. High qual-
ity crimp tooling can produce high quality crimps with less in-process variation over a greater number of termina-
tions.
It is difficult to distinguish critical tooling attributes with visual inspection only. Some attributes cannot be in-
spected even by running crimp samples. This paper will present the reader with information that identifies key
crimp tooling attributes and the effect of those attributes on the crimping process.
Key Crimp Tooling Characteristics
There are four major categories of key characteristics for crimp tooling. These are:
• Geometry and associated tolerances
• Materials
• Surface condition
• Surface treatment
Each of these categories contributes to the overall performance of the production termination process.
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Geometry and Associated Tolerances
Terminals are designed to perform to specification only when the final crimp form is within a narrow range of
dimensions. Controlling critical crimp dimensions is influenced by many factors including:
• Wire size and material variation
• Terminal size and material variation
• Equipment condition
The final quality and consistency of a crimp can never be any better than the
quality and consistency of the tooling that is used. If other variations could
be eliminated, tooling can and should be able to produce crimp forms that
are well within specified tolerances. In addition, variation from one tooling
set to another should be held to a minimum. Crimp tooling features that are
well controlled and exhibit excellent consistency from tooling set to tooling
set can result in shorter setup time as well as more consistent production re-
sults.
Some critical crimp characteristics are directly defined by the tooling form
and are obvious. These include:
• Crimp width
• Crimp length
Other critical crimp characteristics can be related to several tooling form fea-
tures and/or other system factors. These may
be less obvious and include:
• Flash
• Roll, twist, and side-to-side bend
• Up/down bend
• Crimp symmetry
• Bellmouth
The following discussion focuses on two characteristics, crimp width and flash, as examples of how tooling can
affect crimp form. Similar arguments can be applied to the others.
Cross Section Defining Crimp Width,
Crimp Height, and Flash
Crimp Width
Crimp width is a good example of a feature that should be consistent and in control between different
crimpers of the same part number. The reason for this is quite straightforward. For a given terminal and wire
combination, it is necessary to achieve an area index, AI, which is determined by the terminal designer for op-
timal mechanical and electrical performance. Crimp height, CH, and crimp width, CW, directly affect achieving
proper AI. Area index, AI(as a percentage), is defined as:
where At is the total area of the wire and barrel after crimping. A
W
and A
B
are, respectively, the initial cross-
sectional areas of the wire and barrel before crimping.
A typical design point for AI is 80%. In order to maintain
the same AI, the crimp height, CH, needs to change in-
versely to the change of crimp width, CW, in approxi-
mately the same proportion. Thus, if the CW increases
+2%, the CH needs to change approximately -2% in order
to achieve the same AI design point. At first glance that
may not seem significant, but in reality it can be very
significant. Using another general industry design rule of
the ratio of CH to CW of approximately 65%, a typical
set of dimensions used as an example may be: CW = 0.110 in, CH = 0.068 in
Therefore, varying the CW by 2% would result in a CH variation of 2%, or 0.0014 in.
At a CH tolerance of ± 0.002 in, 35% of the total CH tolerance would be used by a
2% variation in CW. Thus, the importance of crimp width control is obvious when
tooling is changed during a production run.
(a)
Cross Sections Showing Min-
imum (a) and Maximum (b)
Area Index per Terminal
Specification—a Variation of
± 3.5%
(b)
Flash
Most crimp terminations have a requirement to limit flash. Flash is defined as the material which protrudes to the
sides of the terminal down and along the anvil. Flash is normal in the crimping process but excessive flash is very
undesirable. Controlling flash requires a balance of several geometric factors. Other factors influencing flash are
related to surface finish and friction, which will be discussed later in this paper.
A dominant factor in controlling flash is controlling the clearance between the crimper and anvil during the crimp
process. Defining the ideal clearance could in itself be a simple matter were it not for two facts:
• In order to minimize terminals’ sticking in the crimper, the sides
of the crimper are tapered. Thus the clearance between the
anvil and crimper varies throughout the stroke.
• Crimper and anvil sets are typically designed to terminate two
to four wire sizes. This creates multiple crimp heights. Since the
sides of the crimper are tapered to minimize terminal sticking,
the maximum clearance permitted without creating flash must
be assigned to the maximum crimp height specified for the
tooling set. In addition, a minimal clearance must be maintained
for the smallest crimp height specified by the tooling set to
prohibit contact between the anvil and crimper.
Crimper to anvil clearance is thus a combination of crimp width, crimper leg taper,
anvil width, and crimp height. The critical design point is at the largest crimp
height. This contribution to the gap is directly dependent on dimensional control.
The following is offered as an example:
Nominal condition: CH = 0.073 in, CW = 0.110 in
Crimper leg taper = 3.0 degree
Anvil Width = 0.109 in
Nominal anvil to crimper total clearance = 0.005 in
The clearance can grow rapidly with small changes to
the nominal dimensions:
CH remains unchanged = 0.073 in
Increase in crimp width, CW, = 0.0008 in
Increase in crimper leg taper = 0.8 degree
Decrease in anvil width = 0.0008 in
The total increase in total clearance is this case =
0.0026 in
This more than a 50% increase in the nominal design
clearance, which can result in unacceptable flash (see right).
Dimensional control is clearly critical.
Crimper-to-Anvil Clearance = X + Y
at the Final Crimp Height
(a)
Significant flash can be generated
with excessive anvil to crimper
clearance, as shown by nominal
design condition (a) and +0.003 in
over nominal condition (b)
(b)
Materials
The material selection for tooling is critical. The material must
be able to meet the in-service demands placed on the tooling components. The two critical tooling components
to be reviewed are the wire crimper and the anvil.
The wire crimper and the anvil have different functional demands. Both have the need to withstand high loads and
moderate shock. However, the wire crimper is in fact an aggressive forming tool. It must withstand high shear
loading that is a result of frictional loads generated as the terminal barrel slides along the crimper surfaces in the
forming process, and then as the terminal barrel is plastically deformed and extruded to complete the termination.
The anvil experiences some of the same conditions but to a much lower level of severity.
The wire crimper and the anvil can be likened to a punch and die in the world of metalworking. The materials used
in punch and die applications have been well documented, along with the material selection process. The added
severity of the aggressive forming and the terminal and wire extrusion during crimping add complexity
to the material selection. The material selection process involves:
• Strength of materials with emphasis on toughness needed to
withstand the moderate shocks generated during crimping
• Wear resistance to maintain form
In addition to the above design considerations, there exists another phenomenon that occurs during crimping that
can significantly shorten the useable life of a wire crimper. Material can be transferred from the terminal barrel to
the wire crimper. This material buildup can result in unacceptable terminations. The crimped terminal surfaces can
actually be deformed by the indentations of the deposited material. Crimp deformation may result due to in-
creased friction. Tooling wear can be accelerated due to higher crimp forces. Surface treatments that minimize
this material transfer are critical to extended tooling life.
Strength of Materials
Crimpers and anvils are designed to be able to withstand stresses that are typically encountered during crimping.
The basic design of tooling with reference to size and geometry has been well analyzed and generally stresses
generated during crimping are able to be accommodated. However, there are always demanding applications that
will tax the design to its stress limits. In those cases, geometry and material may depart from the standard design.
These exceptions are dealt with on a one-by-one basis and will not
be discussed here.
It is the unique requirement of stress and shock that needs to be discussed. Peak crimp loads go from zero to
maximum in less than 40 ms. Tooling needs to withstand this load cycle at a rate of greater than once per second.
Several classes of tool steels are suitable and are well described in the material handbooks. It is the processing of
these materials that can make a significant performance difference.
In order to withstand the rapid loading to a high stress on a repeated basis, the surface of the material must mini-
mize cracks and imperfections that may be generated during the machining and/or heat treat operations. It is im-
portant that grain structure be controlled in size and orientation to achieve maximum and consistent service life.
Decarburization of the surface during heat treating must be controlled. Heat treating process controls are critical
to reproducing the optimal surface. Machining processes must also be controlled to avoid surface cracking due to
excessive heat generation during overly aggressive material removal. Likewise, localized tempering may occur,
which can soften material beyond the effective range.
These variations in final material and surface conditions are not readily detectable with a visual inspection. They
can manifest themselves during service and result in unacceptable tooling performance.
Wear Resistance
Wear is generally described as the gradual deterioration of a surface through use. Several types of wear exist and
include adhesive, abrasive, and pitting. By design, the tooling is able to withstand normal surface loads. Thus, pit-
ting is typically not an issue.
The primary wear mode experienced by crimp tooling is adhesive wear. Adhesive wear occurs as two surfaces
slide across each other. Under load, adhesion, sometimes referred to as cold welding, can occur. Wear takes place
at the localized points of adhesion due to shear and deformation. Adhesion is highest at the peaks of surface fin-
ish because that is where the load is greatest. During crimping, the ideal conditions exist for adhesive wear. That is,
• High loading due to crimp force
• Sliding surfaces due to crimp formation, and terminal and
wire extrusion
Wear will generally manifest itself more significantly at edges of a surface. However, adhesive wear is often ob-
served over substantial areas of the tooling. It is important to note here that the wire crimper is the component
most susceptible to adhesive wear. Generally, adhesive wear will be directly related to load and to the amount of
relative movement between the two materials. Although the anvil may have equal loading, the amount of relative
movement between the terminal and tooling is many times more at the crimper than at the anvil. The insulation
crimper typically experiences lower adhesive wear because the load is reduced compared to the wire crimp and
the relative movement is less than that of the wire crimper, since there is no terminal and wire extrusion at the in-
sulation crimp.
Adhesive wear can be controlled in the selection of the material. Different alloys exhibit better or worse wear properties.
These properties can be measured and are well documented. Adhesive wear
is inversely proportional to the hardness of the material. Thus, the harder the material, the less adhesive wear. In crimp
tooling, there is often a tradeoff that is made. In order to achieve higher wear resistance, the material often exhibits
lower toughness by composition, hardness, or both. The final material selection is often based on years of experience.
One material may have high wear characteristics and lower toughness, and be suitable for a small terminal since the
margin of safety on stress is high. Another terminal may be large and the toughness could be of more importance due a
lower stress design margin. The ability to design and manufacture crimpers from several materials will enable optimal
material selection for a specific application.
The final property that affects adhesive wear is surface finish. As stated earlier, adhesion is highest at the peaks of the
surface. Thus, the smoother the finish, the less significant the peaks and the less significant the adhesion. Adhesive wear
can be reduced with a lower surface finish. Surface finish affects other crimping performance parameters. These are dis-
cussed in the next section.
Abrasion can occur depending on terminal surfaces. If a terminal is plated with an abrasive substance, the tooling
could suffer from abrasive wear. This would be an atypical condition
and would be handled by special design.
Other applications where abrasive wear is the primary wear mode involve terminals made of steel and stainless steel.
Extensive testing has shown chromium plating is the best surface treatment that can be used on crimpers designed for
these abrasive terminals. However, in these applications, crimpers will not last as long as those crimpers used to crimp
terminals made of other, less abrasive base materials. Using a lubricant (in those applications where this is acceptable)
has shown to increase the life of the crimper. However, even when lubricated the crimper life can be expected to be
shorter when crimping steel or stainless steel terminals.
Once abrasive wear has taken place to the point where the chromium plating has been removed from the base tool steel
of the crimper, as successive crimp cycles occur, further wear will happen very quickly. Without the protective
chromium plating, the underlying surface will then be subject to either further abrasive wear, or adhesive wear. For this
reason, care should be taken to replace the crimper as soon as wear is visible on the surface of the crimper.
Surface Condition
Surface condition can affect the performance of the crimp tooling as well as the longevity of service. As noted in
the previous section, a hard, smooth surface has improved adhesive wear properties and, thus, longer service life.
The other attribute that needs to be considered is friction.
Friction is a contributing factor in determining the final
crimp form and process characteristics. Low tooling friction
results in lower crimping force and thus can influence crimp
form as well as tooling life. Consistent frictional characteris-
tics between tooling sets will result in reduced process
variation.
Friction of the crimp tooling surfaces is influenced by fac-
tors similar to those that influence adhesive wear—hard-
ness and surface finish. Generally, harder materials exhibit
lower coefficients for sliding friction. Friction coefficients
have also been shown to be related to surface finish. Manu-
facturing processes need to produce consistent results
such that when tooling sets need to be changed in produc-
tion, minimum disruption in crimp quality is achieved. It has
been found that maintaining surface hardness above Rc 55 as
well as keeping surface finishes to 8 micro-inches or less is
desirable to obtain consistent crimp results and minimize
adhesive wear.
Typical Effect of Friction on Crimp Force
