The measuring instruments are required to get the accuracy and complete our jobs as per drawing. These are divided into two groups as follows.
(i) Ordinary Measuring Tools and Instruments
It can measure a minimum of 0.5 mm in the metric system and 1/64 inch in the British system. e.g., the Steel rule.
(ii) Precision Measuring Tools and Instruments
It can measure the smallest and most accurate quantity up to 0.001″ in the British system and 0.01 mm in the metric system. e.g., Micrometer and vernier caliper.
General Measuring Tools
The production of a machine part in a workshop always requires some measuring instruments, according to specific measurement i.e., its accuracy depends upon the quality of measuring tools and the skill of the worker. General measuring tools can be classified into different forms, which are given below.
Length Measuring Tools
These instruments are used to measure the length of an object.
1. Rule
It is a measuring tool, which is made up of wood, carbon steel, stainless steel, or spring steel. It has marking lines on both sides in inches and millimeters. With the help of these lines, the jobs can be measured. It is the most useful tool for direct and indirect measurement.
2. Steel Rule
Steel rules are made up of spring steel or stainless steel. A steel rule has measuring marks on the surface of both sides. These are available in different sizes such as 150, 300, and 600 mm. The accuracy of the reading of a steel rule is 0.5 mm.
When dimensions are given, in a drawing without any indication about the tolerance, it has to be assumed that measurements are to be made with a steel rule.
Measuring with a Steel Rule
The method of using a try square and a steel rule for accurate measurement. For an accurate reading, it is necessary to read vertically and avoid errors arising out of parallax. The steel rule graduations are accurately engraved with the thickness of the line ranging from 0.12 to 0.18 mm.
For maintaining the accuracy of a steel rule, it is important that its edges and surfaces are protected from damage and rusting.
It should not be placed with any cutting tool, because rusting will be carried out. So, it should be polished with oil when it is not in use.
Types of Steel Rule
The following types of steel rules are given below.
(i) Plain Steel Rule
It is a thin and flat linear measuring instrument and its length is 6 inches or 12 inches. One side is marked in inches and another side in centimeters.
Its minimum measuring length is 1/64 inch or 1/4 mm. This is made up of stainless steel, spring steel or high carbon steel.
(ii) Standard Steel Rule
It is also a measuring tool, which is made up of stainless steel, carbon steel, or spring steel. One side of the steel rule is marked in inches and its fractions are marked as one inch into 8, 16, 32, and 64 parts similarly, another side of the rule is marked in the metric system of centimeter and millimeter and each millimeter is divided into two equal parts. Normally, its thickness is 3/64 inch and width 3/4 inch. It is hard and temper.
(ii) Flexible Steel Rule
This type of rule has a generally thin and narrow shape as compared to a plain steel rule.
Since it is flexible, it can be conveniently used to measure the curved surface. It is made up of thin spring steel. So, easily spread on the curved surface and its width is 1/2 inch and length is 6 inches.
(iv) Narrow Steel Rule
It is also similar to a flexible rule. It has less width as compared to the steel standard rule. So, it is called the narrow steel rule. Its width is only 5 mm and its length is equal to 15 cm or 6 inches. The narrow steel rule is used to measure grooves or holes and marked with centimeters or inches.
(v) Hook Rule
This rule has a hook on its one side. So, it is called the hook rule. The hook is always attached to the internal point (that means zero side). So, it is easy to hold it from a hole or point. It is used to get the measurement on the inside caliper.
(vi) Key Seat Rule
This has a length of 15 to 30 cm (6 to 12 inches) and is used to draw parallel lines. It is also called the box-square rule
(vii) Caliper Rule
It is similar to a narrow rule and is marked on its one side. But its width is more as compared to a narrow rule. The caliper rule is used to measure a stand job and its least count is 1/64 inch or 0.5 cm. It is used to measure accurate outer or inner length.
(viii) Shrink Rule
It is used by pattern makers but at the time of marking, shrink allowance would increase from real measuring.
Shrinkage allowances of some metals are as follows.
Cast iron : 1.0 mm/100 mm
Brass: 1.6 mm/100 mm
Aluminium: 1.6
Steel: 2.1 mm/100 mm
Copper: 1.6 mm/100 mm
(ix) Folding Rule
It is normally 300 mm long with 2 folds. It can be conveniently carried out. It is used to measure accurate outer or inner length.
3. Steel or Canvas Tape
It is used to measure a long distance and is available in 1/2 inch or 3/4 inch width and length of 5 to 100 ft. In steel tape, an inch is divided up into 8 positions, and a centimeter is divided up into 10 positions. The steel tape is made up of a thin spring steel layer and measures to any type of profile due to its flexibility.
4. Scale
It looks like a rule and has markings on both sides at equal distances, similar to a rule but its marking is different from the steel rule. It may be smaller or bigger than its original size. It is used to measure as per ratio either as in inches and centimeters or millimeters.
Measurement by Using Scale
When we measure the dimensions of an object, many factors should be considered as follows.
(i) Always measure any job by steel rule in a standing position, not in a sleeping position.
(ii) Do not measure from its head because it can be rubbing due to regular use.
(iii) The job to be measured is always nabbed with parallel to its side.
Angle Measuring Instruments
These precision instruments are used to measure the angle of an object as follows.
1. Try Square
The try square is a measuring instrument, which is used to check the squareness (angle of 90°) of a surface.
The accuracy of a measurement by a try square is about 0.002 mm per 10 mm length, which is accurate enough for most workshop purposes. The try square has a blade with parallel surfaces. The blade is fixed to the stock at 90°.
Types of Try Square
Try squares are mainly of four types, which are given below
(i) Fixed Try Square
This type of try square is used for general work, where extreme accuracy is required. Its blade is fixed with the stock by riveting.
(ii) Adjustable Try Square
This is adjustable on the stock with the help of a pin and knurled nut. It is more useful due to the squareness of the job and can be checked on either side of the stock. When we tighten the nut of this square, then it makes 90⁰ angle between the stock and blade. It should not be used, where higher accuracy is required.
(iii) Die Maker’s Try Square
It is used for making die or punch and for that, two screws are fitted in the stock. The blade is fixed with stock at some angle with the help of a small screw. To measure different types of shapes, it has four blades.
(a) Standard Blade
The standard blade has a width of 12 mm and a length of 65 mm.
(b) Bevel Blade
The bevel blade has a width of 12 mm. It is one head made up of 45⁰ and the other head at 30⁰.
(C) Narrow Blade
The narrow blade has which of 4 mm and a length of 60 mm. Its length of one head is 16 mm and its width is 2.4 mm.
(D) Upset Blade
The upset blade has a width of 3 mm.
(iv) L-Square
Normally, its base is made of mild steel and a plate of spring steel or mild steel. Its size is 1.5 mm in thickness, 30 to 50 mm in width, and 150 to 200 mm in length. It’s both arms same working signs in each and a millimeter with their functions mark.
It is used to measure alignments and paralleness.l It is generally used by carpenters, Taylor masters, mason sheet metal, and in drawing sections.
Uses of Try Square
(i) The try square is used to check the squareness of the machine job or filed surfaces.
(ii) Check the flatness of machined surfaces.
(iii) Mark Lines at 90⁰ to the edges of the workpiece.
(iv) Set the workpiece at 90⁰ on work-holding devices such as vices.
2. Bevel Square
The bevel square is a marking tool used to transfer a known angle to the material from 0 to 180°. Mostly, blades are made up of iron and stock is of any other material. It is generally used in Beldon to check the bevel, which is made at corners.
Combination Bevel Square
It is similar to the bevel square and is used to measure an angle with the help of a protractor. In comparison to a bevel square, it is more convenient to measure the angle between rough surfaces.
In the construction of this type of bevel square, it is stuck with a blade that is attached by hooks, which is called the main blade. This main blade consists of a slot in the middle to hinge auxiliary blade.
3. Combination Set
The size of the combination set is given by the length of the steel rule. The combination set can be used for different types of work, such as layout work, measurement, and checking angles.
The combination set has three parts i.e., protractor head, square head, and center head with a rule. According to the requirement of the job, any head is used to fit with a steel rule that has a groove at the backside surface. This groove consists of the locking pins of all heads. The different parts of the combination set are discussed below.
(i) Protractor Head
The protractor head can be rotated and set to any required angle. The protractor head is used for marking and measuring O angles with an accuracy of 1°. The spirit level is attached to setting jobs in a horizontal plane.
(ii) Square Head
It is made up of cast steel and used to fix at steel rule. It has one measuring face at 90° and another at 45° to the rule. It is used to mark and check 90° and 45° angles. It can also be used to set workpieces on the machines and measure the depth of slots.
(iii) Centre Head
It is V-shaped and made of cast iron. Its internal angle is 90⁰ with the slot on its hypotenuse. This is along with the rule. It is used for locating the center of cylindrical jobs.
To ensure accurate results, the combination set should be cleaned well after use and should not be mixed with cutting tools, either while using or storing.
Comparing Instruments
These instruments are used to compare the two objects of different sizes. Some different types of comparing instruments are given below.
1. Caliper
Calipers are simple measuring instruments, these are used to transfer the measurements from a steel rule to objects and vice-versa.
Types of Calipers
Calipers are of different types, depending upon the type of joint and shape of the legs.
On the basis of joint, calipers are classified as.
(ii) Firm Joint Caliper
In the case of firm joint caliper, both legs are pivoted at one end. To take measurements of a workpiece, the caliper is opened roughly to the required size. The setting accuracy is done by tapping the caliper lightly on a wooden surface.
(ii) Spring Joint Caliper
For this type of caliper, the legs are assembled by means of a pivot loaded with a spring. For opening or closing the caliper legs, a screw and nut are provided.
Spring joint caliper has the advantage of quick setting. The setting will not change unless the nut is turned. The size of a caliper is specified by its length, which is the distance between the pivot center and the tip of the leg.
The accuracy of the measurement depends very much on the sense of feel or touch while measuring the job. You should get the feel when the legs are just touching the surface.
On the basis of leg, calipers are classified as.
There are three types of calipers according to the leg.
(i) Outside Caliper
The caliper is used for measuring the outer length, width, and diameter, and is known as the outside caliper.
(ii) Inside Caliper
The caliper used for measuring inner length, slot or gap width, and inner diameter is known as the inside caliper.
(iii) Transfer Caliper
Transfer calipers are used for measuring chamfered grooves or frames. A screw is attached with a small auxiliary leaf to one of the caliper legs. The leaf is locked to the leg. The legs may be opened or closed as needed to clear the obstruction. The legs are then brought back and locked to the leaf, restoring them to the original setting.
Transfer caliper is available in two shapes i.e., inner transfer caliper and outer transfer caliper
(iv) Hermaphrodite Caliper
In this type of caliper, one leg is bent inside and the other pointed. A hermaphrodite caliper is used to find out the center of parallel lines or circular shafts. According to need, its legs can be turned outside. It is also called the Jenny caliper or the old legs caliper.
2. Straight Edge
Straight edges are made up of hardened steel and used for checking the straightness of an edge of a workpiece. It is also used for checking surfaces uniformly. Both edges have a finished level and are parallel with one edge usually bevelled. e.g., A guide of the machine. There are three types of straight edges as discussed below.
(i) Cast Iron Straight Edge
It is made up of cast iron and its length is about 3 m. To protect it from curving, its upper part is attached with a rib so that it is strong. It is used to measure the straightness of the surface and the long guideways of the machine.
(ii) Steel Straight Edge
It is made up of high carbon steel and its length is 2 m. Its one side is thin and tapered. So, it is easily used to check the straightness of the workpiece.
(iii) Tool Maker’s Straight Edge
Its size is small in length. It is used by die makers and machine fitters. Its head is made up of a pointed taper so that easily checks the straightness of the die and the surface of slides.
So far, we have discussed the measuring tools and measuring instruments. However, it is essential to have accuracy in the measurement. In this way, specific precision measuring instruments are to be studied.