# Gauges in Metrology

• In mass production, where a large number of similar components are manufactured on an interchangeable basis, measuring the dimensions of each part will be a time-consuming and expensive exercise.
• Gauges are one of the most commonly used inspection tools in production shops for quick checking and validation of the dimensions of manufactured parts. Gauging has become an integral part of any machining process as they help in ensuring the required degree of interchangeability among the millions of parts manufactured all over the world.
• Therefore, in mass production, gauges can be used to check for the compliance of the limits of the part with the permissive tolerance limits, instead of measuring the actual dimensions.
• Limit gauges ensure that the components lie within the permissible limits, but they do not determine the actual size or dimensions.
• The gauges required to check the dimensions of the components correspond to two sizes conforming to the maximum and minimum limits of the components. They are called GO gauges and NO GO (or NOT GO) gauges, which correspond, respectively, to the MML and LML of the component.
• MML is the lower limit of a hole and higher limit of the shaft and LML corresponds to the higher limit of a hole and lower limit of the shaft.

## Purpose of using gauges:

• Components are manufactured as per the specified tolerance limits, upper limit and lower limit. The dimension of each component should be within this upper and lower limit.
• If the dimensions are outside these limits, the components will be rejected.
• If we use any measuring instruments to check these dimensions, the process will consume more time. Still, we are not interested in knowing the amount of error in dimensions.
• It is just enough whether the size of the component is within the prescribed limits or not. For this purpose, we can make use of gauges known as limit gauges.

## Types Of Gauges :

Gauges are classified as follows:

1. Plain gauges

(i) According to type

(a) Standard gauges (An exact copy mating part)
(b) Limit gauges (Made to the limits of the dimensions)

(ii) According to purpose

(a) Workshop gauges – Workshop gauges for checking dimensions after manufacture.
(b) Inspection gauges – Inspection gauges for checking parts before final acceptance
(c) Reference, or master or control gauges
(d) Purchase inspection gauges for checking parts of other factories

(iii) According to the geometry of the tested surface

(a) Plug gauges for checking holes
(b) Snap and ring gauges for checking shafts

(iv) According to design

(a) Single circuit and double limit gauges
(b) Single-ended and double-ended gauges

3. Miscellaneous Gauges

(a) Combined – limit gauges
(b) Position gauges
(c) Contour gauges
(e) Taper gauges
(f) Feeler gauges

Standard Gauges

Standard gauges are made to the nominal size of the part to be tested and have the measuring member equal in size to the mean permissible dimension of the part to be checked. A standard-gauge should mate with some snugness.

## Limit Gauges :

• Limit gauges are inspection tools of rigid design, without a scale, which serve to check the dimensions of manufactured parts.
• Limit gauges do not determine the actual value of the inspected dimensions of a part but indicate whether the part dimensions are manufactured within the limits specified.
• These gauges are used in the inspection of interchangeable parts. Usually, two gauges of hardened steel are provided for each dimension to be tested.
• A ‘Go’ gauge should pass over or through a correct feature. It checks the maximum material condition of the part.
• A ‘No Go’ gauge should not pass over or through a correct feature. It checks the minimum material condition of the part.

The general types of limit gauges are

• Plug Gauges – For checking internal dimensions.
• Snap Gauges and Ring Gauges – For checking external dimensions.
1. Solid Gauges – The gauges in which the handle is an integral part of the gauging portion or if the whole gauge is machined from a single piece of metal, they are called as solid gauges.
2. Renewable end gauges – If the handle is manufactured separately and engaged with the gauging portion by means of a suitable locking device, then such gauges are called renewable end gauges.
3. Single-ended gauges – If only a single gauge (either Go or ‘No Go’ gauge) is on one handle, then it is called single-ended gauge.
4. Double-ended Gauge: If both ‘Go’ and ‘No Go’ are combined on one handle on either side, it is called a double-ended gauge.
5. Progressive Guage – A single-ended plain plug gauge with one gauging member having two diameters of the ‘Go’ and ‘No Go’ respectively is called the progressive gauge.

## Plug Gauge :

Plug gauges are of the cylindrical form used in the manner of a plug for inspecting internal dimensions.
A plain plug gauge is used to check the size of a hole. The gauging portion is made of suitable wear-resisting steel and hardened, ground and lapped. For heavy plug gauges, the handle may be made up of light metal alloys and for smaller plug gauges, suitable nonmetallic handles may be provided.

Various types of plain plug gauges like solid type, taper inserted type, fastened type, flat type, and segmental type are available for holes ranging from smaller to bigger sizes.

## Ring Gauge :

Plain ring gauge is a couple of gauges (‘Go’ gauge and ‘No Go’ gauge) having a measurement side with the maximum and minimum limit deviation of the shaft to assure the interchangeability of the shaft.

The gauges are made up of suitable wear-resisting steel and the gauging surfaces are first stabilized, then ground and lapped.

The other surfaces are finished smooth and a suitable anti-corrosive coating is applied to protect against climatic conditions. The periphery of the ring gauge is knurled to provide grip while handling.

## Snap (or) Gap Gauges :

A snap gauge is a limit gauge, having a C-shaped frame with hardened, adjustable anvils at opposite ends, used to check diameters, lengths, and thicknesses.

### Types of snap Gauges :

(a) Rib type snap gauge

(i) Double-ended snap gauge:
(ii) Single-ended progressive type:

(b) Plate Snap Gauges

(i) Double-ended type
(ii) Single-ended progressive type:

(a) Rib type snap gauge

### (i) Double-ended snap gauge:

This gauge is also called a solid snap gauge. The solid snap gauge is machined and ground out of a single block of metal in the form of
a ‘C’ on either side. On one side is the ‘Go’ gauge made to the upper limit and the other side in the ‘No Go’ gauge made to the lower limit of the dimension. These gauges are conveniently used for inspecting sizes in the range of 3 mm to 100 mm.

### (ii) Single-ended progressive type:

In this type of snap gauge, both the apertures i.e ‘Go’ and ‘No Go’ are on the same side and are arranged such that the part being measured can pass through them in sequence. The part, if within tolerance, will pass through the ‘Go’ aperture (upper limit) (lower end) but will not pass through the ‘No Go’ aperture (lower or minimum size limit) (upper end). These gauges are suitable for sizes ranging from 100 to 250 mm.

### (b) Plate Snap Gauges

These gauges are made up of suitable quality wear-resistant steel plates. The gauging surfaces are hardened, stabilized, ground, and lapped. The other surfaces are finished smooth and all sharp corners and edges are removed.

### (i) Double-ended type:

In this type both ‘Go’ and ‘No Go’ gauges are on either side of the same plate. These are used for sizes ranging from 2 mm to 100 mm. A double-ended type plate snap gauge is shown in Fig

### (ii) Single-ended progressive type:

Here both ‘Go’ and ‘No Go’ are on the same side of the plate. On the other side is provided a pad of suitable non conducting material for convenient handling. These gauges are used for sizes ranging from 100 mm to 250 mm. A single-ended progressive type plate snap gauge is shown in Fig

The adjustable type gap (or) snap gauges are in the form of a horse-shoe frame with their gauging anvils adjustable. The anvils are adjusted by means of independent and fixedly threaded screws. The aperture gap is set to the required limit by means of slip gauges and the screws are locked in position. Adjustable snap gauges can be reset to compensate for wear or re-tasked for measuring a different dimension. They offer the flexibility of building up our own gauge within any size in the specified range. It is preferred where the number of components to be inspected is less but the variety of sizes to be inspected is more.

## Miscellaneous Gauges :

### Combined Limit Gauge

A single plug gauge used for inspecting both the upper and lower limits of cylindrical holes is called a combined limit gauge.

### Position Gauge

A position gauge is used to check the position of a feature on the part in relation to another reference point or surface. The shape and design of a position gauge depend upon the feature to be checked. Position gauge design can be based either on the feel of the operator or on the principle of the sighting. When in contact with the ‘Go’ side, no light will pass between the reference surface and gauge surface, and when in contact with the ‘No Go’ side, the light will pass.

### Contour Gauges

Contour gauges are also called as profile gauges used for checking the dimensional accuracy and cross-sectional shape of a surface. These gauges are made of similar profile as that of the work.

A contour gauge consists of a set of pins (made of steel or plastic) held together tightly in a frame such that they are parallel to each other and in the same plane. So, when the gauge is pressed against an object, the pins which are free to move independently conform to the shape of the object. Now, the gauge can be used to draw the profile or copy it onto another surface.

Examples of simple contour gauges are radius gauges for gear tooth profiles, form tool profiles, thread pitches, etc.

### Taper Gauge :

• Taper plug gauges are used to check tapered holes. It has two check lines. One is a GO line and another is a NOGO line.
• During the checking of work, NOGO line remains outside the hole and GO line remains inside the hole.
• They are various types of taper plug gauges are available as shown in fig. Such as
1) Taper plug gauge —plain
2) Taper plug gauge —tanged.
3) Taper ring gauge plain
4) Taper ring gauge —tanged.

• For checking internal threads, (nuts, bushes) plug thread gauges are used. Similarly, ring thread gauges are used for checking external threads (bolts, screw s).

### Form Gauges :

• Form gauges may be used to check the contour of a profile of a workpiece.
• Form gauges are nothing but template gauges made of sheet steel.
• A profile gauges may contain two outlines which indicate the limits of a profile

### Screw Pitch Gauge :

• Screw pitch gauges are used to check the pitch of the thread immediately. It is very much the everyday tools used to pick out a required screw.
• The number of flat blades with different pitches is pivoted in a holder. The pitch value is marked on each blade.

### Radius and Fillet Gauges  :

• The radius of curvature can he measure by using these gauges. The radius may be either outer or inner radius.
• According to the type of radius to be measured, the end of the blade is made to either concave or convex profile.

### Feeler Gauge :

• Feeler gauges are used for checking the clearance between mating surfaces.
• They are mainly used in adjusting the valve clearance in automobiles.
• They are made from 0.03to 1.0mm thick of 100mm long. The blades are pivoted in a holder.

### WIRE Gauge :

Wire gauge is using to measure the width of the sheets and the diameter of the wires. It is accessible in the shape of plates. Slots of dissimilar of sizes are available on a plate to measure the preferred thickness or dia. These slots former numbered it is available as of gauge 1 to 30. Sheets and wire which is to be calculated passes during these slots and it is checked that it would not be movable and together of its that must touch the slot. Currently, the number on the slot is its gauge figure.

### Pin Gauge :

This gauge is the diameter of large bores which cannot be simply measured by micrometer or vernier and so on., can be measured by means of a pin by spherical ends, the length of which is slightly less than the micrometer to be measured the pin is located in the bore by its ends in
contact in the bore surface, its axis so being a chord of the circle. Keep one end in contact by the bore surface, the other end is brought crossways to make contact at the other side. The distance among the two points of contact is measured and as of this dimension and the length of the pin gauge, the diameter of the bore is determined.

## Points to be remembered for Gauge Design

•  The form of the ‘Go’ gauges should be a replica of the form of the mating parts.
• ‘Go’ gauges should enable several related dimensions to be checked simultaneously.
• During the inspection, Go gauges must always be put into conditions of maximum impassibility.
• ‘No Go’ gauges are for checking a single element of the feature at a time.
• ‘No Go’ gauges must always be put into conditions of maximum passibility during the inspection.

## Material for Gauges

The material used to manufacture gauges should satisfy the following requirements:
1. The material used in the manufacture of gauges should be hard and wear-resistant for prolonged life.
2. It should be capable of maintaining dimensional stability and form.
3. It should be corrosion resistant.
4. It should be easily machinable, in order to obtain the required degree of accuracy and surface finish.
5. It should have a low coefficient of expansion, in order to avoid temperature effect.

High-carbon steel

• It is the most suitable and inexpensive material used for manufacturing gauges.
• It can be heat treated suitably to provide stability and high hardness.
• It can easily be machined to a high degree of accuracy.

Mild steel

• Mild steel gauges are the most suitable for larger gauges.
• They are suitably heat-treated by carburizing to the required depth and then case hardened on their working surfaces to allow for final grinding and finishing.
• After hardening, internal stresses are relieved to improve stability.

Chromium-plated gauges

• Chromium-plated gauges are very popular and extensively used for gauging.
• Chromium plating makes the surface of the gauge very hard, and resistant to abrasion and corrosion
• It is also very useful in reclaiming worn-out gauges. For gauging aluminum or other materials having an abrasive action, chromium-plated gauges are extensively used.

Glass

• Glass gauges are not very popular although they have good wear and corrosion resistance properties.
• The problem with these gauges is that they either get damaged or are easily broken if dropped.
• They are not affected by changes in temperature and have a very low coefficient of thermal expansion.

Invar

• Although Invar, which contains 36% of nickel, has a low coefficient of expansion, it is not suitable over a long period.

Elinvar

• Elinvar has 42% of nickel, is more stable than Invar, and also has a low coefficient of expansion.

## Gauge Tolerance (Gauge Maker’s Tolerance)

• Gauges, like any other component, cannot be manufactured to their exact size or dimensions.
• In order to accommodate these dimensional variations, which arise due to the limitations of the manufacturing process, the skill of the operator, etc., some tolerance must be allowed in the manufacture of gauges.
• Thus, the tolerance that is allowed in the manufacture of gauges is termed gauge maker’s tolerance or simply gauge tolerance.
• The normal practice is to take gauge tolerance as 10% of the work tolerance.

## Wear allowance

• During the inspection, the NOT GO side should not enter or pass.
• The NOT GO gauge engages fully with the work and therefore does not undergo any wear. Hence, there is no need to provide an allowance for wear in case of NOT GO gauges.
• The GO side of the gauge should enter the hole or just pass over the shaft under the weight of the gauge without using undue force.
• During the inspection, the measuring surfaces of the gauge constantly rub against the mating surfaces of the workpiece. Therefore, the GO gauges suffer wear on the measuring surfaces and thus lose their initial dimension. Hence, wear allowance is provided for GO gauges to extend their service life.
• The wear allowance provided for the GO gauge is added in a direction opposite to wear. This allowance is added in for a plug gauge while subtracted for a ring or gap gauge.
• A wear allowance of 10% of gauge tolerance is widely accepted in industries.