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.