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