Depth gauges with reversible base
Or protractor
A depth gauge is designed to measure the depth of
holes or recessed surfaces. It consists of a
reversible head or a protractor base which sits on
the top surface of the workpiece in use, and a graduated
steel rule which can move vertically through
the centre of the head. The rule can be locked in
any desired position by a thumbscrew. When the
head is placed on the workpiece the rule is lowered
to the bottom of the recess, then locked in this
position when the size or depth of the recess is
indicated. Figure 6.11a and b illustrate the two
types.
Figure 6.9Calipers (Neill Tools Ltd (Moore and
Wright))
Figure 6.10Oddleg calipers (Neill Tools Ltd (Moore
and Wright))
Measuring and marking-out instruments 191
Feeler gauges
A feeler gauge (Figure 6.12) is used to measure
small clearances between two objects, and in some
cases to establish the amount of wear or distortion
of a component part. Each tempered steel blade has
a number indicating its thickness in millimetres.
The thicknesses are carefully graded in order
that various clearances ranging from 0.01 mm to
1.0 mm can be measured. The gauge illustrated in
Figure 6.12 is imperial but this tool is now mostly
made in metric sizes.
Screw pitch gauge
A screw pitch gauge (Figure 6.13) consists of a
metal case with two sets of flat pivoting blades, each
having teeth cut on its edge to correspond to a screw
thread form. In use, the blades are placed in turn on
to the profile of the thread to be checked until an
(a) (b)
Figure 6.11(a) Reversible depth gauge (b) protractor depth gauge (Neill Tools Ltd (Moore and Wright))
Figure 6.12Feeler gauges (Neill Tools Ltd (Moore
and Wright))
Figure 6.13Screw pitch gauge (Neill Tools Ltd
(Moore and Wright))
192Repair of Vehicle Bodies
exact match is found in the gauge. This matching
thread form will give the correct name, size and
pitch of the thread in question. The gauge illustrated
in Figure 6.13 is imperial, but screw threads in
common use are: Système International (SI) metric,
International Organization for Standardization (ISO)
metric, American National, Whitworth, and Unified.
Radius gauges
These are used to measure small internal and
external radii. They are supplied as a set having a
number of pivoting flat blades each of hardened
and tempered steel, cut to a specific radius size.
They are used as a template to check radii sizes on
the workpiece (Figure 6.14).
The size of a drill is found by inserting the drill in
the holes until a hole is found which fits the drill
exactly. These gauges are made in either fractional
or decimal sizes, number sizes, or lettered sizes.
Imperial standard wire gauges
A wire gauge (Figure 6.16) can either be oblong or
circular in shape. Slots cut into its outside edge
give accurate wire gauge sizes, usually numbers
1 to 36 SWG or 0.2 to 10.0 mm. It is still used by
the body worker and sheet metal worker, as metal
sheet thickness may be given in SWG size. The
gauge is used by slipping it over the edge of a
metal sheet until the corresponding slot is found,
and the gauge size is then read off.
6.3 Precision marking-out and measuring
instruments
The average body worker will not normally come
into contact with precision marking and measuring
instruments. However, measurement is the basis
of engineering, and an understanding of the more
common precision instruments is essential if he is
to realize that greater accuracy is possible than that
which is achieved by the rule and scriber.
Marking-off table
This is made of close-grained cast iron, rigidly
constructed and supported on very substantial
legs. The top surface is machined level and finally
scraped to a degree of flatness which is determined
by the use to which the table is to be put.
Figure 6.14Radius gauges (external and internal
radii) (Neill Tools Ltd (Moore and Wright))
Figure 6.15Drill gauge (Neill Tools Ltd (Moore and Wright))
Drill gauge
The drill gauge (Figure 6.15) is a flat piece of metal
having a selection of holes of accurate diameters.
Measuring and marking-out instruments 193
The poorest grade is used for marking out material
appertaining to body work; it also supports the
work during marking out, and is often used as a
datum from which all measurements can be taken.
Marking-off tables must be used for no other
purpose except marking out and measuring. When
not in use they should be lightly oiled and covered
against accidental damage and corrosion.
Surface plate
A flat surface is one of the fundamentals of engineering,
and the flatness of a surface can be verified by
testing it against the flatness of a standard surface,
i.e. the surface plate. Surface plates are similar in
construction and grading to the standard of finish of
a marking table, but are smaller in size and have two
carrying handles. The back is strongly ribbed to give
rigidity to the surface. In use, the surface plate is
covered with a special marking compound and then
lightly rubbed on the workpiece to be tested. The
marking dye is transferred to the parts of the workpiece
which actually come into contact with the surface
of the plate, and if the surface of the workpiece
is uneven, only the high spots will be marked. Any
high spots must be scraped and the workpiece must
be retested until the whole surface comes in contact
with the surface plate. Surface plates should be kept
covered at all times when not in use.