METHODOLOGY FOR LOCKING FEATURE SELECTION

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METHODOLOGY FOR LOCKING FEATURE SELECTION IN
INTEGRAL SNAP-FIT ASSEMBLY (DETC97/DAC-4003)  
2003.04.11 ? ? ?
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• 1. ABSTRACT
• 2. INTRODUCTION
• 3. LOCKING FEATURES
• Latches
• Cantilever Hooks
• Cantilever Holes
• Traps
• Compressive Beams (or Hooks)
• Leaf Springs
• Annular Snaps
• Finger Grips
• Spring Posts
• Flexible Walls
• - Catches
• Edges

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• 4. LOCKING PAIR SELECTION METHODOLOGY
• - Locking Feature Selection Methodology
• Step 1 Identification of Engaging Surfaces.
• Step 2 Installation (Engaging) Direction and
  Engaging Motion.
• Step 3 Identification of Applicable Locking
  Features.
• Step 4 Selection Based on Ease of Manufacture.
• Step 5 Selection Based on Constrained DOFs and
  Load-carrying Capabilities.
• Step 6 Selection Based on Robustness against
  Dimensional Variations.
• 5. A CASE STUDY
• - Description of the Problem
• The Design Approach (6 steps)
• Constraining Remaining Degrees of Freedoms
• 6. CONCLUSION

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1. ABSTRACT - Design of an assembly for integral
attachment using snap-fit features is important,
none is more important than the selection of
locking features. - Comprised of a latch and
catch component, locking pairs must themselves be
selected using a systematic approach. - This
paper presents that approach as a six-step
methodology, after defining and describing latch
and catch components. - It then demonstrates
the methodology using a case study taken from
real life.
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• 2. INTRODUCTION
• - Growing competition in the market forces
  designers to develop effective ways of reducing
  product cost including better designs and a more
  efficient design process.
• -Considering all stages of a product's
  realization, assembly constitutes a considerable
  share of the entire cost, often exceeding 50.
• - Assembly cost, which is largely due and
  directly proportional to assembly time, can be
  subdivided into two broad categories part
  handling and part insertion
• - Snap-fits are a special category of integral
  features.
• As a group, they undergo elastic deflection to
  allow engagement during the insertion process,
  and elastic recovery to accomplish complete
  attachment (or locking) to provide retention.
• - Complete engagement and successive recovery to
  cause locking is accompanied
• by either an audible or tactile "snap", hence the
  name.
• In order to facilitate the assembly process and
  enhance attachment strength, other features are
  commonly used with snap-fits.
• - In a generic classification these are (1)
  locking, (2) locating, and (3) enhancement
  features

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• - Locking features are used to accomplish final
  attachment of parts brought into proximity and
  caused to engage.
• Locating features are used to guide and align the
  parts during the insertion and engagement
  process, and to provide constraint against
  applied forces after assembly.
• Enhancement features are not unique attachment
  features, but rather are used to enhance the
  strength, ease of assembly or disassembly, or
  ease of manufacturability of locking and locating
  features.
• Locking, locating and enhancement features all
  participate in the attachment process in one way
  or another, these features are collectively
  called integral attachment features
• First, the characteristic properties of generic
  locking feature forms should be known, then the
  specific design problem should be studied, and,
  finally the most suitable locking feature
  alternative should be identified.
• To date, the only significant and systematic
  study of locking feature selection is that was
  performed by Genc et al. (1997a).
• - Fundamental issues of locking features are
  presented, then a methodology for locking feature
  selection follows.
• - A case study is presented to show how the
  methodology works.

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3. LOCKING FEATURES

• They provide mechanical locking by elastic
  deflection and subsequent recovery during
  engagement
• Because of needed flexibility, they are weak
  compared to locating features, which are rigid in
  order to align parts and keep them aligned under
  the action of service
• loads.
• The number of locking features should be kept to
  the minimum needed to hold parts together in an
  assembly.
• Locking features are preferably used to constrain
  parts in the direction opposite to the
  installation direction (retention direction),
  with other directions constrained by either
  natural locating features (e.g., walls, edges of
  parts) or locating features (e.g., ribs, bosses,
  etc.).
• Generically, they consist of two components
  latches and catches.

Engagement process of integral attachment locking
features.
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Latches

• Latches
• Latches are that component in a locking pair that
  elastically deflect to allow insertion and then
  recover to complete engagement and accomplish
  locking.
• Because of the requirement for elastic
  deflection, latches are flexible and constitute
  the weak portion of attachments.
• Depending on the specific nature of the
  deflection mechanism, latches appear in different
  forms.
• These can be classified as cantilever hooks
  cantilever holes traps compressive beams (or
  hooks) leaf springs annular snaps finger
  grips spring posts and flexible walls.
• Each of these differs from the others in terms
  of certain characteristics pertaining to the
  deflection and gripping or locking mechanisms.
• A particular form can have any of several
  different shapes (e.g.,straight or 0-degree,
  L-shaped or 90-degree, and U-shaped or
  180-degree).

A compressive beam latch feature with different
shapes.
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Latches

• Cantilever Hooks
• - Cantilever hooks have a beam integral with
  either the base or mating part, and a hook at the
  free end of the beam.
• The beam permits deflection about the catch, and
  the hook functions to lock with the catch.
• The beam portion of the feature may have an L- or
  U- shape to achieve engagement for a particular
  part geometry.
• - In such cases, the retention strength of the
  feature decreases, while its flexibility
  increases.
• - Detailed information on cantilever hooks is
  provided in a study by Luscher (1996).

Examples of cantilever hook and cantilever hole
latch features.
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Latches

• Cantilever Holes.
• Cantilever holes are very similar to cantilever
  hooks, but employ a different locking mechanism.
• Locking is accomplished with a feature in the
  form of a hole.
• Cantilever holes always require a protruding
  catch on the mating half of the assembly pair for
  engagement.
• - Cantilever holes are loaded on-axis.

Examples of cantilever hook and cantilever hole
latch features.
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Latches

• Traps.
• Trap latch features are characterized by a rigid
  beam with an integral flexible hook at the free
  end.
• The rigid portion might be a cantilever beam
  with relatively large cross-sectional area to
  prevent deflection or it might be the wall of a
  part with a flexible protrusion to serve as a
  hook.
• The flexible hook portion interacts with some
  portion of a mating part in an assembly that acts
  like a catch.
• It is molded as part of the beam at an angle, and
  carries the locking loads in compression.

Examples of trap and compressive beam latch
features.
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Latches

• Compressive Beams (or Hooks).
• - Compressive beams are quite similar in geometry
  to cantilever hooks. The only difference is that
  cantilever hooks carry locking loads in tension,
  whereas compressive beams carry locking loads in
  compression.
• - Hook locking mechanism is provided by a
  protrusion at the free end.
• By their design, compressive beam latches are
  loaded eccentrically (as opposed to along their
  center-line) in compression, and in order to
  disengage must have the beam fail by either
  buckling or the hook fail by shear.

Examples of trap and compressive beam latch
features.
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Latches

• Leaf Springs.
• Leaf springs provide deflection in a slightly
  different manner.
• Deflection occurs in a beam in which one or both
  ends is/are fixed.
• Unlike for other latches, the retention direction
  is normal to the beams axis.
• Annular Snaps.
• Annular snaps as the name implies have a
  donut-shaped or annular cross-sectional area.
  Ball-and-socket joints are a good example of an
  annular snap.
• These latch types are very strong in in-plane
  directions, and may or may not be strong in the
  retention direction.

Examples of leaf spring and annular snap latch
features.
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Latches

• Finger Grips.
• Finger grips have grabbing features (fingers) for
  accomplishing locking.
• These fingers usually have symmetric shape
• During insertion, the fingers elastically deflect
  outwards, and then recover to grab a catch on the
  mating part in the locking pair.
• - Fingers can have different shapes depending on
  the shape of the catch.

Examples of finger grip latch features.
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Latches

• Spring Posts.
• Spring posts are very similar to finger grips.
  They too have symmetric hook or locking
  mechanisms at the end of a post or beam.
• These are very commonly used features in the
  automobile industry for attaching decorative
  trim, for example. In general, the catch for
  spring post latches is a hole.
• Flexible Walls.
• Flexible walls function the same way leaf springs
  function.
• A protrusion or a penetration feature on the wall
  enables engagement and locking with a catch.
• Using flexible wall features where appropriate,
  may help decrease the complexity of parts in
  terms of their manufacturing.

Examples of spring post and flexible wall latch
features.
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Catches

• Catches
• Catches comprise the second component of locking
  pairs.
• They are the rigid part in the pairs.
• Catches can have different forms including edges,
  ledges, notches, holes, cantilever
• catches, grooves, and rigid posts.
• These differ from one another in geometry. Catch
  to use depends on the latch selected.

Examples of edge, ledge, and notch catch features.
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Catches

• Edges.
• Edges require no penetration or special
  protrusion, as they are themselves the protrusion
  of a surface.
• The hook or lock or finger feature of a latch
  grabs and holds the opposing part by its edges,
  which function as a catch.
• Ledges.
• Ledges are protrusion features molded onto the
  surfaces of parts.
• They may have different cross-sectional shape,
  such as triangular or rectangular, based on
  choice.
• Notches.
• - The main purpose of using a notch feature is to
  constrain additional degrees of freedom once it
  is engaged with a latch feature.

Examples of edge, ledge, and notch catch features.
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Catches

• Holes.
• Holes are penetrating features into or through
  part surfaces, and hole-form are assumed to be
  rigid.
• Depending on the latch in the locking pair, hole
  catches can be of circular or rectangular shape.
• Cantilever Catches.
• Cantilever catches are a special variant of holes
  in which the hole is located in the surface of a
  feature that extends out from a wall or surface.
• A latch feature can engage with a cantilever
  catch in two ways (1) the latch can pass through
  the frame for engagement, or (2) the hook or
  locking mechanism of a latch feature can engage
  with the catch without passing through the frame.

Examples of hole, cantilever, and groove catch
features.
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Catches

• Grooves.
• Grooves are a special variant of a notch in which
  the penetration or recess is into the surface
  rather than the perimeter or edge of a part.
• Unlike notches, grooves do not pass trough the
  entire thickness of a part
• Like a notch, a groove provides additional
  constraint.

Examples of hole, cantilever, and groove catch
features.
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Catches

• Rigid Posts.
• Rigid posts are non-deflecting extended
  protrusions or columns on part surfaces.
• They are very similar to spring post latch
  features. They are totally rigid so engagement
  depends on the mating latchs deflection.
• - The mating latches to rigid posts are usually
  finger grips.

Examples of rigid post catch features.
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Matrix of Locking Pairs

• Having defined various forms of latches and
  catches, it is worthwhile to identify possible
  locking pair combinations. Each locking pair has
  a latch and a catch component.
• Possible locking pairs are those latches and
  catches that can be brought together to
  accomplish engagement based on their
  complementary geometry.
• - In practice, this matrix either guides or
  provides validation of selections for designers
  for particular applications.
• - If the designer has the freedom to decide on
  either a latch or a catch feature, a latch
  feature
• should be selected, and then a suitable catch
  chosen.

Matrix of possible locking pair combinations of
latches and catches.
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3. LOCKING PAIR SELECTION METHODOLOGY

• Designing plastic parts with integral attachment
  features is relatively new compared to
  traditional designs employing fasteners such as
  screws, rivets, etc.
• In practice, designers tend to use one of a
  limited number of designs with which they are
  familiar, rarely exploring new possibilities
  because they often do not have a complete
  knowledge of integral attachment possibilities.
• This causes them to stay with an old design and
  use trial-and-error methods to incorporate any
  new materials or dimensions.
• There are several pieces of information that the
  designer knows at the outset.
• 1. The geometry of the parts to be attached.
• 2. The degrees of freedom which must be
  constrained.
• 3. The direction of installation and the
  assembly motion
• Locking features must be used to constrain the
  retention direction (opposite of installation
  direction), and their final type and form are
  dependent on a number of issues.

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Locking Feature Selection Methodology

• The proposed methodology for locking feature
  selection involves six steps.
• The order of the first three steps is always as
  shown in Figure, the order of the last three
  steps is left to the designers choice.
• The six steps are
• (1) identification of engaging mating part
  surfaces
• (2) identification of the installation direction
• (3) identification of applicable locking feature
  pairs
• (4) down-selection among options based upon ease
  of manufacture
• (5) down-selection based upon constrained DOFs
  and load-carrying capability and finally
• (6) down-selection
• -The first three steps help identify
  alternative/candidate locking pairs, while the
  last three help choose among these alternatives
  based upon design objectives.

Methodology for locking feature selection.
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Locking Feature Selection Methodology

• Step 1 Identification of Engaging Surfaces.
• Installation direction and assembly motion are
  selected, interfacing or interacting surfaces of
  mating parts become key to further steps in the
  design process.
• There are three types of generic engaging
  surfaces
• (1) butt surfaces
• (2) T-butt surfaces
• (3) lap surfaces
• This stage of the design no feature has yet been
  placed, identification of engaging surfaces is
  the first task to be accomplished.
• They are designed to constrain parts principally
  in the direction opposite the installation
  direction (i.e.,in the retention direction).

Engaging surface types.
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Example issues (1) Accessibility Locking
features should be placed in such locations that
they can be accessed easily to allow disassembly
if it that is important. (2) Structural
Rigidity Using portions that are too flexible
may cause unintentional disassembly or result in
loose attachment, vibration or rattle. (3)
Appearance In many products, aesthetics is an
important design consideration. (4)
Dimensional Variations -Since injection or
other molding processes are generally used in the
production of plastic parts, variations are very
likely to occur because of shrinkage or warpage.
-Such variations can cause misalignment during
assembly, loose attachment, unintentional
disengagement, or unintended loading after
assembly. -Therefore, locking features should be
located so that they are not or are relatively
less influenced by dimensional variations.
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Locking Feature Selection Methodology

• Step 2 Installation (Engaging) Direction and
  Engaging Motion.
• Installation direction refers to the
  translational direction in which one part is
  moved to be joined with another to create an
  assembly.
• This direction is often dictated by the
  production/assembly environment
• Indeed, overall installation direction and
  engaging direction are the same vector.
• There are at least two parts in an assembly, one
  engaging surface is on the base part toward which
  the mating part is moved.
• - Possible assembly motions are push, slide, tip,
  and spin while possible engaging motions can only
  be push (normal to the engaging surfaces) or
  slide (motion parallel to the engaging surfaces).

Examples of engaging surfaces, engaging direction
and motion.
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Locking Feature Selection Methodology

• Step 3 Identification of Applicable Locking
  Features.
• - Latch and catch pairs should be located on
  engaging surfaces at this stage.
• Since these components are interchangeable,
  location of the latch, for instance, can be
  either on the base part or on the mating part.
• To optimize a design, designers should inspect
  all possible latches and possible corresponding
  catches to see whether they are applicable or not
  with the given engaging surface geometry, and
  engaging direction and motion.
• By using the locking pair matrix, designers move
  directly to the corresponding row of possible
  latches or column of possible catches, and
  identify possible alternative pairs to be listed,
  evaluated, and down-selected later in the process.

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Locking Feature Selection Methodology

• Step 4 Selection Based on Ease of Manufacture.
• The fourth step, which addresses the ease of
  manufacturing parts with integral features once
  they are selected and placed
• Injection molding is a very popular and growing
  method of plastic part production. These
  processes, undercuts are very important design
  concerns due to the fact that they increase the
  complexity of die design (to allow part release
  and, removal), thereby increasing overall product
  cost.
• The goal is to avoid having undercuts in a
  design. This can be accomplished in several ways
  without using inserts, including
• (1) by judicious selection of the mold closure
  direction
• (2) by judicious selection of parting line
• (3) by changing the geometry of certain
  attributes on parts
• In order to assess the ease of manufacturing
  locking features, the following concepts need to
  be known. Features can be located on part
  surfaces in three different ways
• (1) out-of-plane
• (2) outof-plane at an edge
• (3) in-plane

Examples for mold closure directions for a
cantilever hook feature with different locations
and orientations.
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Locking Feature Selection Methodology
- By thinking about manufacturing, certain
locking pairs can be found that result in
undercuts for some features in a given mold
closure direction. - One way of avoiding
undercuts, is to add a hole to the part as part
of the feature. , it can be seen that an
out-of-plane feature can be manufactured using
either of two mold closure directions, i.e., x-
and y-directions. In order to manufacture this
part in the z direction, a hole can be added to
the locking feature to avoid an undercut.
Examples for mold closure directions for a
cantilever hook feature with different locations
and orientations.
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Locking Feature Selection Methodology

• Step 5 Selection Based on Constrained DOFs and
  Load carrying Capabilities.
• - They constrain some degrees of freedom (DOFs)
  kinematically. By kinematically, it is meant that
  constrained directions may or may not carry
  service loads safety, but motion is constrained.
• The number and direction of DOFs constrained by a
  locking pair depend entirely on the geometry of
  that locking pair.
• In contrast to this case, if significant loads
  are applied to the parts, the load-carrying
  capability of locking pairs in certain directions
  will drive their selection.
• In this latter case, possible locking pairs
  should be studied, and the ones best suited to
  carrying the loads in their preferred direction
  should be selected.
• In a final case, loading only in a direction
  opposite the installation direction to provide a
  high retention force, or, contrarily, to provide
  a low assembly insertion force might be
  important..

Examples of constrained DOFs and load carrying
capabilities for locking pairs for straight butt
(a and b) and T-butt (c and d) type surfaces.
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Locking Feature Selection Methodology

• Step 6 Selection Based on Robustness against
  Dimensional
• Variations.
• Shrinkage and warpage are two major concerns for
  parts manufactured by injection or other molding
  techniques.
• Dimensional variations caused by shrinkage
  and/or warpage affect the product at two stages
  (1) during assembly, (2) after assembly (in
  service).
• After assembly, the result can be a loose
  assembly or unintended loading (preloading) of
  features.
• In addition, loose assemblies may squeak or
  rattle under vibrational environment.
• Too tight results in loading or pre-loading of
  the locking features.
• Therefore, locking features are preferred that
  are robust to dimensional variations.
• - Studies by Lewis et al. (1997a) and Wang et al.
  (1995) provide detailed information on
  bayonet-fingers.

Examples of lap-type surfaces containing locking
pairs with different degrees of robustness
dimensional variations.
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5. A CASE STUDY

• Description of the Problem
• The problem involves the design of a grill that
  is to fit into an opening in the front spoiler of
  a sports car.
• Design objectives are multiple and include, in
  no intended order of priority structural
  performance, ease of assembly, ease of
  manufacture, and aesthetics.
• Since the spoiler, which is the base part here,
  is already in use, there is no opportunity to
  make any modification in terms of geometry.
• In addition, because of limited work space under
  the hood of the car, the grill has to be
  installed into the spoiler from the front end.

Shapes of spoiler and grill (sketched by Gene
Hopkins of Saratoga Technologies, Inc.).
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5. A CASE STUDY

• The Design Approach
• - The first task is to inspect the parts and
  identify DOFs to be constrained, applicable
  installation directions, and assembly motions.
• The output of this task, which is the initial
  phase of the design process, provides the input
  information for the locking feature selection
  process that will be described in the following
  step.

Design inputs before the feature selection
process.
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5. A CASE STUDY

• Step 1 Identification of engaging surfaces.
• - The first step is to inspect the spoiler with
  integral back grill in terms of accessibility for
  disassembly, rigidity of attachment, sensitivity
  to dimensional variations from manufacture, and,
  most importantly,
• appearance from the front side (i.e.,
  aesthetics).
• Step 2 Identification of engaging direction and
  motion.
• In this step, the engaging direction and motion
  of engaging surfaces were identified.
• Step 3 Identification of possible locking pairs.
• It should be remembered that the geometry of the
  spoiler cannot be changed . Using the matrix of
  locking pairs latch options can be seen for an
  edge catch.
• These latches are (1) cantilever hooks (2)
  compressive beams (3) traps (4) leaf springs
  and, finally, (5) flexible wall features.

Alternative latches for an edge catch pair.
Identification process of engaging surfaces.
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5. A CASE STUDY

• Step 4 Selection based upon ease of manufacture.
• - The grill has five blade or vanes. Due to the
  orientation of these vanes, mold closure
  direction is parallel to the vanes
• Step 5 Selection based upon DOFs and
  load-carrying capability.
• Once the grill and spoiler are brought together,
  the walls of these parts and the back grill
  constrain most of the translational and
  rotational DOFs naturally. Rotational DOFs must
  be dealt with using multiple locking pairs to
  overcome moments.
• Possible loadings are impact and wind loading.
  Both of these loads would be applied from the
  front side in the installation direction. There
  are no critical loads applied to the locking
  features in the retention direction.

Alternative features that do not create undercuts.
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• Constraining Remaining Degrees of Freedoms
• The designer needs to constrain the remaining
  DOFs after considering the design of the mating
  parts.
• The following provides only a brief summary.It
  should be recalled that retention direction is
  primarily constrained by locking features.
  Therefore, by using the locking feature form just
  selected, the designer should constrain the
  retention direction.
• - In order to find the minimum number of locking
  features required to constrain pure translational
  motion in the retention direction and related
  rotational DOFs, a central load should be applied
  in the retention direction, and additional
  locking feature should be located and oriented in
  such a way that they balance the force couple
  created by the applied central load, thereby,
  constraining translational and rotational DOFs.
• - The rest of the DOFs should be constrained by
  locating features. After the retention direction
  is taken care of, the installation direction
  should be constrained in the same way, and
  remaining directions should follow.
• - After constraining the retention direction,
  there are no DOFs to be constrained for this
  particular case study. Thus, it is said that the
  design is fully constrained, or kinematically
  constrained. A kinematically constrained design
  is called basis design.
• basis design can be further enhanced, by
  considering each DOF independently, depending on
  loading situation, and by adding more features ,
  or changing the sizes of features.
• Finally, enhancement features can be added to the
  design for further improvements in performance,
  ease of assembly/disassembly, and ease of
  manufacture.

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• 6. CONCLUSION
• Designing plastic parts with integral snap-fit
  attachment features is attractive in terms of
  reducing part count, reducing tool requirements,
  and reducing assembly time,
• In integral attachment design, one of the most
  important steps is locking feature selection
• for a particular application.
• This paper focused on locking feature selection.
  First, fundamental issues concerning locking or
  locking pairs were addressed, then a
  classification of locking feature forms including
  latches and catches was presented.
• - Matrix of possible latch-catch locking pairs
  was presented. Finally, a methodology for locking
  feature selection was proposed.
• This locking feature selection methodology also
  consists of six steps to help designers select a
  particular form of locking feature pair based on
  design objectives and constraints.
• - Last but not least, a case study was presented
  to illustrate use of the methodology.

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