M ISCELLANEOUS M EASURING T OOLS

There are a number of specialist measuring tools, which are, usually, only found in selected workshops or in Quality Departments. These tools are normally used in conjunction with surface plates or tables, which are used to give the measuring operation a standard flat surface to base the measuring upon.

4.4.1 Gauge Blocks

Gauge Blocks (also known as Slip Gauges), are, simply, precision-ground blocks of metal that are used either alone, or in combination with other blocks, to give extremely accurate measurements. The blocks are made from high-carbon steel or cemented carbide and are hardened, ground and lapped so that:

• Opposite faces are flat • Opposite faces are parallel

• Opposite faces are , accurately, the stated distance apart.

The opposite faces are of such a high degree of surface finish, that, when two blocks are wrung (pressed, with a simultaneous slight twist, by hand) together, they will remain firmly attached to each other. This characteristic, of gauge blocks, enable them to be built up, into combinations, which give sizes varying in increments of 0.01 mm (0.0004 in), and whose overall accuracy is of the order of 0.00025 mm (0.00001 in) even with workshop grade blocks.

Gauge blocks are supplied in sets of 50, 78 or 105 pieces and protective blocks are provided for use with inspection and workshop grades. The protective blocks should, where possible, be used as the end blocks of all combinations, and the smallest number of gauge blocks should always be used when making up a combination.

JAR 66 CATEGORY B1 MODULE 7 MAINTENANCE PRACTICES (MECHANICAL)

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engineering

4.4.2 Dial Test Indicator (DTI)

The Dial Test Indicator, or DTI, consists of a small dial, to the circumference of which, is connected a spring-loaded plunger. When the plunger is placed on a surface and moved over that surface (or the surface moved beneath the plunger), any variations in the surface condition will be indicated on the dial of the gauge. A DTI (also called a Clock Gauge) is used, not for measuring the actual size of a component, but to indicate small differences in size or for indicating the amount of eccentricity (parallelism, run out) of revolving parts.

Its dial is graduated to indicate thousandths of an inch or, in metric values, in increments of 0.01 mm or 0.001 mm, depending on the sensitivity required. The dial has the zero datum at the top (12 o’clock position), with scales of equal value to either side, which enables plus and minus values to be measured.

An important feature of the gauge dial is that the dial can be rotated by a ring bezel, enabling it to be readily set to zero. The gauge can thus be used as a comparator, or as an instrument for comparative measurements, as well as for direct measurements.

Internally, the plunger has a rack (or straight) gear, which drives a small pinion. The pinion is fixed to a larger gear, which drives a second pinion. This pinion is also fixed to a second, larger gear, which drives a further, third, small pinion to which the pointer of the gauge is attached.

The compound gear train magnifies the plunger movement and outputs its displays, via the pointer, onto the dial. A spring keeps the plunger in contact with the surface being tested.

The flatness of a surface of a workpiece, can be checked, by attaching a DTI to a scribing block that is standing on a surface plate. The surface being checked is set beneath the DTI plunger and the bezel is zeroed. The workpiece is moved beneath the DTI and variations in flatness are displayed and quantified by the dial reading. A bar may also be checked for bowing by using a DTI, attached to a scribing block, whilst the bar is supported by ‘V’ blocks.

4.4.3 Feeler Gauges

Feeler Gauges have a wide application and consist of a series of thin, flexible, steel blades in varying thicknesses (normally from 0.04 mm to 1.00 mm or from 0.0015 in to 0.015 in). The blades are secured in a protective, metal scabbard, by a pin. It is important that those blades not in use should be withdrawn into the scabbard, to prevent accidental distortion, especially of the thinnest blades.

JAR 66 CATEGORY B1 MODULE 7 MAINTENANCE PRACTICES (MECHANICAL)

uk

engineering

Feeler gauges are used to measure very small, fixed gaps between faces. The blades are chosen to comply with the measurement given in the maintenance instructions. Sometimes there will only be a need to use a single blade whilst, at other times, a combination of blades may be required to achieve the given dimension. Feeler gauges are classified by the length of the blade.

4.4.4 Screw Pitch and Radius Gauges

These gauges are used to check the profiles of threads and radii, by comparison with sets of blades held in a case much like feeler gauges. The thread gauges are available in all thread types and the radius gauges have both an internal and external radius.

4.4.5 Go/No-Go Gauges

Where a single dimension has to be repeatedly measured, a comparator-type of gauge is used which checks, simply, whether a component is within a pair of dimensions, usually referred to as maximum and minimum dimensions. These gauges are referred to as ‘GO/NO-GO’ gauges (Fig 42) and, providing the item being checked passes through one jaw of the gauge, (the ‘GO’ dimension), and fails to pass through the other jaw, (the ‘NO-GO’ dimension), it is considered to be satisfactory.

4.4.6 Straight Edges

Straight Edges can be found in a variety of types, from a precision-ground rule, to heavy-duty, cast iron straight edges, (e.g. the ‘camel back’ straight edge). The lighter straight edges are used to either visually check the flatness of a surface (by holding it up to a light), or to use feeler gauges. Heavy-duty straight edges will, probably, be used, to check other items for straightness, by a similar ‘bluing’ method to that used on surface tables.

GO NOGO

Plug Type Gauge

N

O

G

O G

O

Calliper or Snap Gauge

Limit (GO/NO-GO) Gauges Fig. 42

JAR 66 CATEGORY B1 MODULE 7 MAINTENANCE PRACTICES (MECHANICAL)

uk

engineering