Chapter 3: Linear Measurement

Production Engineering and Design Dept.

Assoc. Prof. Helmi Abulila

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Chapter 3: Linear Measurement

3.1 Introduction Linear measurement applies to measurement of lengths, diameters, heights, and thickness including external and internal measurements. The dimension to be measured is aligned with the graduations of the scale. Linear measuring instruments are designed either for line instruments, the measurement is taken between two end surfaces as in micrometers, slip gauges etc. The instruments used for linear measurements can be classified as: 3.2 Classification of Instruments

1. Direct measuring instruments 2. Indirect measuring instruments

The direct measuring instruments are of two types: 1. Graduated 2. Non Graduated

3.2.1 Graduated instruments



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3.2.2 Non-graduated instruments

Calipers

Surface Plate A surface plate is a hard, solid, and horizontal flat plate, which is used as the reference plane for precision inspection, marking out, and precision tooling set-up (Fig. 4.1). Since a surface plate is used as the datum for all measurements on a job, it should be finished to a high degree of accuracy.

V-BLOCKS V-blocks are extensively used for inspection of jobs with a circular cross section. The major purpose of a V-block is to hold cylindrical workpieces to enable measurement. The cylindrical surface rests firmly on the sides of the ‘V’, and the axis of the job will be parallel to both the base and the sides of the V-block.

V-block V-block with a stirrup clamp Magnetic V-block



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3.3 Precision The graduated instruments include rules, vernier calipers, vernier height gauges, vernier depth gauges, micrometers, dial indicators etc. The non-graduated instruments include calipers, trammels, telescopic gauges, surface gauges, straight gauges, wire gauges, screw pitch gauges, thickness gauges, slip gauges etc. They can also be classified as:

1. Non-precision instruments such as steel rule, calipers etc. 2. Precision measuring instruments, such as vernier instruments,

micrometers, dial gauges etc.

1) Non-precision type instruments

Steel Rule Calipers

2) Precision type instruments



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3.4 Dimensions A very common measurement is that of dimensions, i.e., length, width, height of an object Dimensions of the measuring instruments are classified as follows: 3.2.1 Low resolution devices

Steel rule Steel rule with assistance of Calipers Dividers & Surface gauges Thickness gauges

3.2.2 Medium resolution devices Micrometer Micrometer with assistance of Telescoping Extendable ball gauges

Vernier calipers Dial indicators Microscope.

3.2.3 High resolution devices Gauge blocks Gauge block with assistance of Mechanical comparator Electronic comparator Pneumatic comparator Optical flats

3.4.1 Steel Rule Types of steel rules (a) Narrow tempered steel rule (b) Flexible fillet rule (c) Short rule with holder (d) Angle rule (e) Steel measuring tape.



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3.4.2 Callipers There are many jobs whose dimensions cannot be accurately measured with a steel rule alone. However, callipers are the original transfer instruments to transfer such measurements on to a rule.

Classification of calipers

Types of callipers (a) Outside caliper Calliper being used to transfer a (b) Inside calliper (c) Divider dimension from a job to a rule (d) Hermaphrodite calliper

Measurement using callipers (a) Outside calliper

(b) Inside calliper (c) Divider



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3.4.3 Vernier principle When two scales (main and auxiliary scales) or division slightly different in size are used, the difference between them can be utilized to enhance the accuracy of measurement.

Construction

Working



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1. Types of Vernier Calipers:

Direct reading of an external length

using digital Vernier caliper

Dial Calliper

2. Vernier Depth Gauge

A vernier depth gauge is a more versatile instrument, which can measure

up to 0.01 mm or even finer accuracy. Designed for use in toolrooms,

workshops, inspection departments to measure depths of holes, slots,

recesses, and so on.



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Standard Depth Gauge

3. Vernier Height Gauges Designed for use in tool rooms, workshops, inspection departments to measure or mark off vertical heights and locating center distances.



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3.4.4 Micrometers The micrometer screw gauge essentially consists of an accurate screw having about 10 or 20 threads per cm and revolves in a fixed nut. The end of the screw forms one measuring tip and the other measuring tip is constituted by a stationary anvil in the base of the frame. The spindle is a slide fit over the barrel and barrel is the fixed part attached with the frame. The barrel is graduated in unit of 0.05 cm. i.e. 20 divisions per cm, which is the lead of the screw for one complete revolution. The thimble has got 25 divisions around its periphery on circular portion. Thus it sub-divides each revolution of the screw in 25 equal parts, i.e. each division corresponds to 0.002 cm. A lock nut is provided for locking a dimension by preventing motion of the spindle.



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A = 7.00 mm B = 0.5 mm C = 22 x 0.01 = 0.22 mm

A = 7.00 mm B = 0 mm C = 38 x 0.01 = 0.38 mm

Micrometer thimble reading 5.78mm

Micrometer reading 5.783millimeters



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4. Types of Micrometer The following paragraphs briefly highlight the use of each type of micrometer in metrology applications: 1. Outside Micrometers

Dial-indicating Micrometer V-anvil Micrometer

Screw Thread Micrometer (measuring pitch Diameter of screw threads)



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2. Inside Micrometers

Standard inside Micrometers

3. Depth Micrometer



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4. Floating Carriage

Micrometer a floating carriage micrometer, sometimes referred to as an

effective diameter measuring micrometer, is an instrument that is used

for accurate measurement of ‘thread plug gauges.

3.4.5 Special Purpose Gauges Tap and Drill Gauges (consisting of a flat rectangular steel plate with holes accurately drilled and identified according to their size).

3.4.6 Rectangular Gauge Blocks Slip gauges, also called gauge blocks, can counter some of these limitations

and provide a high degree of accuracy as end standards. In fact, slip gauges

are a direct link between the measurer and the international length standards.

It is made of hardened alloy steel having a 30 mm × 10 mm cross section. The

length between the measuring surfaces, flatness, and surface conditions of

measuring faces are the most important requirements of slip gauges.



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Slip gauges are available in three basic shapes: rectangular, square with a central hole, and square without a central hole. Accordingly, slip gauges are designated into five grades, namely grade 2, grade 1, grade 0, grade 00, and inspection grade. Grade 2: This is the workshop-grade slip gauge. Typical uses include setting up machine tools, milling cutters, etc., on the shop floor. Grade 1: This grade is used for tool room applications for setting up sine bars, dial indicators, calibration of vernier, micrometer instruments, and so on. Grade 0: This is an inspection-grade slip gauge. Limited people will have access to this slip gauge and extreme care is taken to guard it against rough usage. Grade 00: This set is kept in the standards room and is used for inspection/calibration of high precision only. It is also used to check the accuracy of the workshop and grade 1 slip gauges. Calibration grade: This is a special grade, with the actual sizes of slip gauges stated on a special chart supplied with the set of slip gauges. For instance, the set of 103 pieces consists of the following:

1. One piece of 1.005 mm 2. 49 pieces ranging from 1.01 to 1.49 mm in steps of 0.01 mm 3. 49 pieces ranging from 0.5 to 24.5 mm in steps of 0.5 mm 4. Four pieces ranging from 25 to 100 mm in steps of 25 mm

A set of 56 slip gauges consists of the following: 1. One piece of 1.0005 mm 2. Nine pieces ranging from 1.001 to 1.009 mm in steps of 0.001 mm 3. Nine pieces ranging from 1.01 to 1.09 mm in steps of 0.01 mm 4. Nine pieces ranging from 1.0 to 1.9 mm in steps of 0.1 mm 5. 25 pieces ranging from 1 to 25 mm in steps of 1.0 mm 6. Three pieces ranging from 25 to 75 mm in steps of 25 m

Functional features of a slip gauge

Chapter 3: Linear Measurement

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