Metal screws (self-tapping screws)
Often there is a need for a fastening which will fix
two or more parts together securely for indefinite
periods, yet can be dismantled if necessary. Since
the Second World War, the self-tapping screw has
in many cases taken the place of more traditional
fastenings and has justified itself completely on all
types of motor vehicles. It is a very hard steel
screw designed for joining metal to metal and is
used for joining metal pressings and exterior fittings.
The screw is inserted into a predrilled hole
and it cuts its own thread in the metal, thus making
a very secure joint. When joining thin sheets,
improved characteristics can be obtained if the
hole is plunged or extruded (Figure 7.35). This in
effect thickens the material and gives greater
Figure 7.32Monobolt and placing sequence (Avdel Ltd )
Figure 7.31(a) Standard Nutsert placement (b) Thin sheet Nutsert with bolt in place (Avdel Ltd )
212Repair of Vehicle Bodies
Figure 7.33Avtainer and placing sequence (Avdel Ltd )
1 Drilled hole 3 Shell placed over pin tail
2 Avtainer in position 4 Tool applied: fastener completed. Seal protects against ingress of moisture
Figure 7.34Unhardened wood screws
(European Industrial Services Ltd )
Figure 7.35Fixing self-tapping screws
(European Industrial Services Ltd )
Methods of joining 213
thread engagement; thus the best conditions are
obtained if both sheets are extruded together. If this
device is used, the total sheet thickness should be
used in assessing the required hole size. Pilot holes
may be punched, drilled, rim extruded or moulded
when using selftapping screws.
Screws are available in stainless steel and in
steel with various finishes, and gauges range from
no. 4 to no. 14 and lengths from 6.35 mm, in, to
63.5 mm, 2 in. The types of heads available are
counter-sunk, round head, raised countersunk, pan
head, mushroom head and hexagon head. Most
heads have a single slot for insertion of a screwdriver;
some, known as Supadriv heads, have a
star-type slot and must be placed with a special
type of screwdriver (Figure 7.36).
Self-tapping screws are particularly useful to the
body engineer and trimmer since no access is
needed to the rear side of the work, it being necessary
simply to drill the requisite size of hole and
drive the screw home. The screw will stay secure
until removal is necessary, and can be removed and
replaced many times without becoming slack if
reasonable care is taken. Typical applications are
the mounting of door-pillar trim pads, carpeting or
rubber matting, parcel shelves, glove boxes, window
cappings, ashtrays and kick plates.
A more recent development is the Taptite high
performance thread forming screw which has a
thread like that of a conventional machine screw
(Figures 7.37, and 7.38). The thread on the Taptite
screw has a trilobal cross-section instead of the customary
circular form. As the screw is rotated the
lobe profile rolls the material out of its path to form
the thread. Not only does this eliminate the need for
an additional tapping operation, but it also produces
inherent resistance to vibrational loosening, thereby
obviating the need for separate locking components.
This is achieved by an increase in prevailing
torque which comes from the radial pressure interference
produced at and around the major diameter
and not present on conventional fastenings because
of the need for a clearance fit. These factors mean
that in the assembly shop the operators (and drivers)
can insert more screws in less time and with
less fatigue. In maintenance applications, a standard
machine screw can be substituted if necessary.
The AB screw combines the benefits of Taptite in
a heavy-duty thread forming fastener and is available
in sizes M6 and above (see Figure 7.39).
Another version of the Taptite screw is CA Taptite
(see Figure 7.40), designed in response to the
increasing use of thinner-gauge materials. Fastening
thin sheet poses problems with conventional selftapping
screws, and Taptite offers two solutions.
Either a standard Taptite screw can be driven into
holes which have been rim extruded to provide more
(a)
(e) (f) (g) (h)
(b) (c) (d)
Figure 7.36Self-tapping screw head types: slotted
and (a) countersunk (b) raised countersunk (c) pan;
(d) hexagon; Supadriv and (e) countersunk (f) raised
countersunk (g) pan (h) flange (European Industrial
Services Ltd )
Figure 7.37Supadriv Taptite thread forming screw
(European Industrial Services Ltd )
Figure 7.38Plastite screw (European Industrial
Services Ltd )
214Repair of Vehicle Bodies
metal. Or, where this is not acceptable, the CA
Taptite can be used; this has a gimlet point capable
of self-extruding in thin sheet sections.
As plastic materials are increasingly used to
replace aluminium, zinc die castings and other metal
parts, so new fasteners have been designed capable
of joining plastic components together. A screw can
be driven directly into thermoplastics, making it a
very cost effective way of making positive joints.
The screw can be removed and reinserted into the
same hole many times and will always pick up the
same thread, thus maintaining a strong joint.
Polymate is a round bodied, twin-threaded selftapping
screw with a closely controlled critical helix
angle and thread pitch. The torque required to drive
the screw is low, while the clamping load achieved
is very high (see Figure 7.41).
Steel hammer drive screws
These sheet metal fasteners (Figure 7.42) are used
only where permanent fixing is required, as they
are difficult to extract once placed. They are made
in a very hard steel, and the head shapes available
are round head and countersunk. The screws are
inserted in predetermined holes of the correct size
and hammered so that the spiral thread is forced to
cut into the material. The cutting action of the
thread reduces the size of the hole after the pilot
point has passed through.
Screw nails
These are made of hardened steel and are used for
fastening thin sheet metal on to wood structures
(Figure 7.43). They are hammer driven through
light-gauge sheet metal, taking care not to bend or
break them, so that the hardened spiral thread cuts
into the burr in the sheet metal and then worms its
way into the wood. This makes a secure metalto-
wood joint which is ideal for panelling in the
Figure 7.39AB type self-tapping screw (European
Industrial Services Ltd )
Figure 7.41Polymate screw (European Industrial
Services Ltd )
Figure 7.42Hammer driven screw (European
Industrial Services Ltd )
Figure 7.40CA Taptite screw (European Industrial
Services Ltd )
Methods of joining 215
construction of vehicle bodies. The screws are
obtainable with countersunk, flat and round heads.
Coach screws
These are designed for securing heavy-gauge metal
and fittings to timber, and are obtainable with either
hexagon or square heads suitable for using a spanner
or socket wrench. Sizes range from 4.763 mm,
in diameter up to 12.7 mm, in and in lengths
from 19 mm to 254 mm, in to 10 in (Figure 7.44). 3
friction between the head of the bolt and the parent
metal. A spring washer placed under an ordinary
nut assists in preventing the slackening of the nut
when the joint is subject to vibrations.
The basic differences between bolts, set screws
and carriage bolts are illustrated in Figure 7.45. The
identification of bolts is outlined in Figure 7.46.
Figure 7.44Coach screw, square head (European
Industrial Services Ltd )
Figure 7.45(a) Bolt (b) set screw (c) carriage bolt
(European Industrial Services Ltd )
Square head
Bolting
Bolts are used extensively in the manufacture of
motor vehicles, as components which are built in
sections and then bolted together can easily be dismantled
for repair or replacement. One example of
this type of construction is the car body, which has
certain panels which are made separately and later
bolted in position so that they can be replaced as
individual units if damaged. Another application of
bolting is in cases where fabrications are too large
for workshop assembly; then the article is made in
sections which are assembled on site.
A bolt is a cylindrical rod having a head on one
end and a thread cut along part of its length from
the other end. Bolts are available in mild steel,
alloy steel, stainless steel and brass, and can be
made of other metals and alloys for special applications.
A nut and bolt is the most common means
of joining. The unthreaded portion of the shank of
the bolt is intended for fitting into clearance holes,
and a washer can be used to minimize the effect of
Figure 7.46Bolt identification. An ISO metric bolt or
screw made of steel and larger than 6 mm in diameter
can be identified by either of the symbols ISO M or M
embossed or indented on top of the head. In addition
to marks to identify the manufacturer, the head is also
marked with symbols to indicate the strength grade,
e.g. 8.8; 10.9; 12.9; 14.9. As an alternative, some bolts
and screws have the M and strength grade symbol on
the flats of the hexagon (Rover Group Ltd )
The following types of head are available onbolts:
Hexagon head This is the most frequently used
shaped, and it is suitable for all spanners.
Square head The square headed bolt is used
mainly for heavy engineering and structural work.
Countersunk head A bolt of this type has a head
shaped to fit into conical recesses in the work surface
so that it lies level and flush. A slot is provided in the
head for insertion of a screwdriver.
Figure 7.43Screw nail (European Industrial
Services Ltd )
216Repair of Vehicle Bodies