THE ROLE OF fMRI IN CONSUMER NEUROSCIENCE: WHAT HAS IT BEEN DONE AND WHAT IS LEFT TO DO?
3.1.4. Themes and subthemes of consumer neuroscience
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ut and stud inserts can provide wide design flexibility for fastening stampings and sheet metal fabrications together (Fi g u r e 1 ) . All of these fasteners can be installed eco-nomically—except concealed head studs, which require expensive machining operations.Inserts are special nut or stud fasteners that
are designed to be pressed into prepared holes in sheet metal parts. Inserts provide captive male or female threads in materials too thin to tap, when higher strength fastening is required, or when repeated access after assembly is antic-i p a t e d . They are routantic-inely less expensantic-ive than the conventional nut and bolt alternative.
15
Figure 1. Examples of “clinch” nuts and studs.
Inserted Fasteners
A great number of inserts are available in various thread sizes, lengths, classes of fit, mate-rials and finishes to fit most design require-m e n t s. When quantities justify, c u s t o require-m-require-m a d e inserts may prove economically advantageous over standard types.
An important initial consideration in insert selection is access to the prepared hole. If both sides of the sheet are accessible, a wider range of manufacturers’ products is applicable. H o w -e v -e r, wh-en acc-ess is availabl-e to only on-e sid-e of the sheet, rivet nut inserts (Figure 2) should be considered. These inserts can be installed, usually with special tools, like blind rivets.
Alternatively, lack of access to the reverse side may also be solved by inserting fasteners designed for two-side installation, prior to bending or other forming operations. Wh e n
such considerations arise, metalforming suppli-ers can offer practical advice.
Insert Installation
Nut or stud inserts are inserted into punched or drilled holes, then clinched or crimped into the sheet metal. Depending on the type of insert selected, it is clinched against the metal surrounding the prepared hole, t y p i c a l l y deforming and flowing the substrate metal to lock the fastener into position. Knurled flanges and similar features are often incorporated in the insert design to aid in anchoring the fasten-er to the sheet.
Inserts can be installed by standard presses with a punch and anvil set-up (Figure 3) and by hydraulic and pneumatic tools. With automatic feeding capabilities, specially designed presses
Figure 2. Typical of rivet nut inserts.
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can install inserts rapidly.
Some types of inserts, like rivet nut inserts, undergo most of the deformation via clinching, while the workpiece undergoes minimal defor-m a t i o n . H e r e, keyed or ribbed fastener heads are sometimes used to prevent rotation of the insert in the workpiece, and to resist vibration in service.
These types of inserts are usually installed via special pneumatic or hydraulic tools. Fo r h i g h-volume production, fully automated sys-tems including auto feeders and robotics can be utilized.
Cost Considerations
G e n e r a l l y, the more functions an insert per-forms and the more exotic the material and fin-ish, the higher the cost. Beyond that, the cost of nut inserts vs. e x t r u d e d-and tapped holes is always a controversial issue. Often the choice depends on the designer’s preference and expe-rience. The economy of one system vs. the other should be discussed with the supplier.
Extruded holes are ideal in stampings when the part will be tumbled and finished, t h e n assembled with self-tapping screws. B e c a u s e extruded holes are created in a punching opera-t i o n , many sopera-tamping companies advise using this option when additional holes and other features will also be punched and formed to create the final stamping. For higher production
volumes this approach can be the most eco-nomical choice.
Other means of installing threads—weld nuts and studs, s e l f-locating projection weld nuts, e x t r u d e d-a n d-tapped holes—should also be evaluated on a cost/performance basis. M e t a l -forming companies can advise which alterna-tive is most suited for a particular design.
Types of Inserts
To facilitate a particular part design, i n s e r t s can be installed flush (Figure 4) into one side of the sheet, or nonflush, with the head protrud-ing. Flush installations usually require a special head on the insert and/or a countersunk or counterbored hole.
As depicted in Figure 5, additional fastening
Figure 3. Inserts can be installed by standard presses with a punch and anvil set-up.
Figure 4. Example of nut inserts installed flush with the sheet.
installation
before clinching after clinching
flush clinch nut
insert screw from this side
Inserted Fasteners
functions are achieved by nut inserts with s e l f-locking features (accomplished by inter-rupted threads, c o a t i n g s, and special nut d e s i g n s, e t c. ) . “ F l o a t i n g ” fasteners provide for mismatch (e.g., ±0.015 in. (±0.38 mm)) between mating fasteners or holes. Blind-end types form a seal against liquids and foreign contaminants;
a n d , special spring-loaded panel fasteners can be flush-mounted as a single unit.
Depending on the type selected (and the
manufacturer), inserts can be used to join more than two components together. For example, rivet nut inserts can join or “ r i v e t ” two sheets and also provide threads to mount a third com-ponent (Figure 6).
Key Design Parameters
The insert type is chosen based on functional and aesthetic requirements such as flush mount, self-locking nut or concealed head stud.
Figure 5. Nut inserts with self-locking features are another method of fastening.
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Other important factors for consideration in selection are strength, workpiece hardness and t h i c k n e s s, material compatibility, f i n i s h , d i s t o r-tion, clearance and tolerances.
• Retention strength of inserts, as measured by push-out and torque-out values (Figure 7), is a direct result of the metal flow and interlocking that occurs durinterlocking installation. C o n s e -quently, the design of the insert and the materi-al into which it is being instmateri-alled can have a sig-nificant effect on strength. As may be expected, strengths increase with larger diameter inserts and thicker sheets.
Aluminum inserts in aluminum exhibit lower p u s h-out and torque-out values than steel inserts in steel. When higher strength is needed, both carbon steel and stainless steel inserts can be used in aluminum workpieces. For applica-tions that require optimum strength—such as when replacing weld studs—high-t o r q u e-r e s i s-tant studs with heavy heads can be specified to boost pull-through values.
For rivet nut inserts, tensile strength, t h r e a d strength and shear strength, as well as t o r q u e-out values can be used in determining what insert type will resist design stresses.
Highest thread strength is provided by stainless steel inserts, followed by steel, then aluminum.
However, most steel self-clinching fasteners are heat treated and will then prove the strongest threads.
• H a r d n e s s of the workpiece is also an important criterion. In general, inserts are rec-ommended for use in workpieces up to a specif-ic maximum hardness according to the insert’s material type. For self-clinching fasteners the fastener must always be significantly harder than the workpieces.
• T h i c k n e s s e s from as low as 0.020 in.
(.5 mm) up to about 1⁄2 i n . (12.7 mm) are suit-able for inserts depending on the manufacturer and insert type. For nut inserts in sheet metal, typical workpiece thicknesses range from 0.030 in. (0.75 mm) to 0.125 in. (3 mm), correspond-ing to thread sizes from #2-56 to 5⁄16-18.
For best performance, insert size (diameter and threads/in.) and shank length for a given type of insert should correspond to the mini-mum thickness recommended by the manufac-t u r e r. H o w e v e r, shank lengmanufac-ths recommended for a specific minimum thickness can usually be used in thicker workpieces so that one “ s t a n-dard” size can be used throughout an assembly and for similar parts.
Figure 6. Here a rivet nut insert is used to join or “rivet”
two sheets and also provide threads to mount a third component, an angle bracket.
Figure 7. Strength of inserts is measured by push-out and torque-out values.
self-clinching nut flush mount stud
Inserted Fasteners
• Compatible Materials should be used to avoid galavanic corrosion in the insert and workpiece. Inserts are available in a wide vari-ety of materials, including aluminum alloys, car-bon steel, stainless steel and brass.
In service environments where this is not a problem, steel and stainless steel inserts can be used in aluminum to achieve higher strengths.
Steel and stainless steel inserts are installed in aluminum after anodizing or other finishing o p e r a t i o n s. Stainless hardware is installed in steel after plating.
• F i n i s h . Various corrosion-resistant finishes to meet commercial and military specs may be s p e c i f i e d . Typical for steel are black oxide and zinc plating, to which a clear or other chromate can be applied for additional corrosion protec-tion.
For aluminum, anodizing or color anodizing are the two options. Stainless steel inserts are usually passivated to enhance corrosion resis-t a n c e. Sresis-tandard finishes vary among manufac-turers; custom and unique finishes usually com-mand a premium cost and require extended lead times.
• Edge distortion is hardly ever a problem if the insert manufacturer’s recommendations are
followed. Closer proximity to the edge leads to edge distortion, which may interfere with sub-sequent assembly and/or part function. Wh i l e smaller than minimum edge distances can be u s e d , they usually require special fixtures to restrain the sheet during installation. This extra expense is usually cost-prohibitive and often distortion cannot be prevented.
• C l e a r a n c e must be adequate. While hole locations and their distances from other fea-tures are usually dictated by design analysis and the methods that create them (see punched holes and slots in Stamping Production C h a p t e r ) , these should be tempered by such factors as additional clearance for the insert installation equipment, the subsequent assem-bly tool, and the length of the insert (Figure 8).
• A c c e s s i b i l i t y should be considered in the design stage. Particularly important is the close-ness of inserts to bends and formed features.
Even if the insert is installed prior to bending to accommodate a difficult manufacturing s e q u e n c e, subsequent access to the insert is a must so that the mating fastener can be installed.
Tolerance Considerations
Generally, hole diameters for clinch nuts and studs should not be toleranced by the designer since the supplier implements nut/stud manu-facturing tolerances to maintain insert loca-t i o n s. Usually only loca-the diameloca-ter of loca-the hole should be specified with a reference indicating the insert type, length, etc.
Typically, the supplier can advise whether the holes need deburring. Many manufacturers rec-ommend no deburring, since this extra metal can result in a better clinch. In contrast, manu-facturers of rivet-nut inserts ordinarily recom-mend burr-free holes.
When accuracy of hole location is important, holes should be dimensioned from a datum (not chain dimensioned) to avoid accumulation of tolerances (Figure 9) and resulting
misunder-Figure 8. Illustration of insufficient clearance.
s t a n d i n g s. If tolerances achievable with inserts and punched holes are not acceptable for the d e s i g n , then another method of fastening should be considered.
During insert installation, even with the anvil and punch on center, some movement occurs as the insert is put into the hole and clinched. As a result of this variable, as well as variations in concentricity of hardware, and hole location, t h e accumulative tolerances can range as much as 0.015 in. (0.38 mm) from the design centerline.
If tighter tolerances are required, use of a fix-ture can provide more accurate hole location.
Perpendicularity of inserts to the sheet is usually quite consistent as a result of proper installation.
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Figure 9. “Recommended” and “not recommended”
dimensioning for accurate positioning of holes.