Precision Measuring Tools

TABLE OF CONTENTS

INTRODUCTION THE NEED FOR ACCURACY IN MEASURING CARE OF MEASURING TOOLS TYPES OF MEASUREMENTS Linear Measurement Round Work Measurement Polar Measurement LECTURE PROPER Straight Edges/ Hook Rulers/ Meter Stick How to Read Ruler Graduations Metric English Conversions Metric To English English to Metric Practical measurement Work shop CALIPERS Classification Of Calipers As To Use Inside Calipers Outside Calipers Types Of Calipers As To Construction Measuring With A Caliper Setting And Reading With A Caliper Practical Measurement Work Shop MICROMETERS Classification of Micrometers Inside Micrometers Outside Micrometers Depth Micrometers Principle of the Micrometer How To Hold Micrometers How To Read The Graduations Of The Micrometers Practical Measurement Work shop VERNIER CALIPER Uses Of The Vernier Caliper How To Read Graduations Of The Vernier Caliper Practical Measurement Work Shop

Universal Bevel Protractor With A Vernier Uses Of Universal Bevel Protractor Parts of the Universal Bevel Protractor How To Read Graduations Of The Universal Bevel Protractor Practical Measurement Work Shop Gage Blocks/ Filler Gages Uses Of Gage Blocks/ Filler Gages How To Measure With Gage Blocks/ Filler Gages Practical Measurement Work Shop DIAL INDICATORS Uses Of Dial Indicators How To Read Graduations Of The Dial Indicator Practical Measurement Work Shop

Chapter 1

Introduction

The Need for Accuracy in Measuring. Personnel in machine works should begin at once to understand

accuracy in its relative terms. He should appreciate from the start the value of the various measuring tools in obtaining the degree of accuracy the given operations demands. There is no more reason why we should not use a micrometer the moment we return to our work places.

Care of Measuring Tools. It goes without saying that precision measuring tools should be handled

with the greatest care. Good tools are made of hardened steel and will stand a lifetime of one without breakage, but the accuracy of even the finest tool can be quickly impaired by careless or abusive treatment. In working with measuring tolls, be careful to avoid accidental scratches or nicks that will obscure graduations or distort surfaces. Rust is the enemy of all finely finished surfaces. Tools should be wiped clean of fingerprints after using and kept in separate boxes or cases. A light dressing of oil applied with a soft, lint-free cloth will protect tools in storage.

Types of Measurement

Linear Measurement Linear measurements on flat surfaces are perhaps the most common

measurements made in general practice in the machine shop. The tools varies with the sizes of the dimension, the nature of the work, and the degree of accuracy required. It may range from a steel tape, rule, divider or trammel to a micrometer or vernier caliper. The measurement may be made direct as with steel rule or slide caliper. Many related tools such as straight edges, steel squares, and protractors are used in conjunction with linear measuring tools to determine flatness, squareness and angularity.

Round Work Measurement

For round work, measurements are usually made by contact, using tolls with contact point or surfaces such as spring calipers, micrometers, and vernier calipers. Contact measurements are made in two ways: (1) by pre-setting the tool to the required dimensions, using a steel rule, micrometer, or other tool as a gage, and then comparing the set dimensions with the actual size of the work: and (2) the reverse of this method, first setting the contact points to the surfaces of the work and then using a steel rule, vernier caliper to read the size. The first method is generally preferred where repeated test must be made, such as in machining a piece of a given size, or when checking the same dimension on a number of identical parts. The second

method is preferred for determining the actual size of the piece of an accurate measure of variation from a required standard.

Polar Measurement

Measurement in degrees is widely used in engineering and the physical sciences, particularly astronomy, navigation, and surveying. The usual means of locating a star or a point on the surface of the earth is by its angular distance in degrees, minutes, and seconds from certain fixed points or lines. Positions on the surface of the earth are measured in degrees of latitude north and south of the equator and degrees of longitude east and west of the prime meridian, which is usually taken arbitrarily as the meridian that passes through Greenwich, England.

Degree, in trigonometry, arc equalling 1/360 of the circumference of a circle, or the central angle subtended by such an arc. The degree is the common unit of measurement for angles and for arcs of a circle. It is divided into 60 minutes, each equal to 1/21,600 of the circumference of a circle; each minute is divided into 60 seconds, each equal to 1/1,296,000 of the

circumference of a circle. Degrees are usually indicated by the symbol ,

minutes by ‘, and seconds by “, as in 4118’09”, 41 degrees 18 minutes 9 seconds.

Chapter 2

Lecture Proper:

Straight edges/ Hook Rulers/ Meter Stick/ Steel Squares Among the most useful tools in the shop are the steel rulers. Although

these are actually rules or rulers, most machinists call them scales. They are made in a variety of kinds such as springs tempered, flexible, narrow, and hooked, and in lengths from 1 to 48 in. The most popular the spring tempered 6 in. rule.

How to Read Ruler Graduations Metric Most steel rules are graduated. That is marked by fine lines upon each

edge of both sides, and often at the ends, in different subdivisions of a meter. The different graduations are classified by number. For the graduation many shopsmen prefer meter, centimeter, and millimeter.

English Most steel rules are graduated. That is marked by fine lines upon each

edge of both sides, and often at the ends, in different subdivisions of an inch. The different graduations are classified by number. For the graduation many shopsmen prefer, have 64th, 32nd , 16th, and 8th.

Conversions

English Metric

1 MILE 5280 FEET 1 KILOMETER 1000 METER

1 YARD 3 FEET 1 METER 100 CENTIMETER

1 FOOT 12 INCHES 1000 MILLIMETER

1 CENTIMETER 10 MILLIMETER

Metric to English

Metric- English

1 KILOMETER 0.6214 MILE

1093 YARDS

3280.8 FEET

1 METER 1.0936 YARDS

3.281 FEET

39.37 INCHES

1 CENT-METER 0.3937 INCHES

1 MILLI-METER 0.03937 INCHES

English to Metric

English- Metric

1 MILE 1.6093 KILO-METER

1609 METER

160900 CENTI-METER

1609000 MILLI-METER

1 YARD .09146 METER

9.146 CENTIMETER

91.46 MILLI-METER

1 FOOT 30.48 CENTI-METER

304.8 MILLI-METER

1 INCH 2.54 CENTIMETER 25.4 MILLI-METER

Practical Measurement Work Shop

Calipers

A caliper is a tool used measuring diameters. It is always used with a steel scale and at times with a micrometer. The caliper itself can not be directly as a steel scale and therefore, when a measurement is taken with a caliper, the opening is measured on the steel scale or micrometer.

Classification of Calipers as to Use:

Inside Calipers Calipers that have measuring end of the legs curving outwardly and are

used to measure inside dimension such as the diameter of a hole. Outside Calipers Calipers that have measuring ends of the legs curving inwardly so that

no part of the legs except the ends ever contact the part being measured.

Types of Calipers as to Construction Spring Joint

Firm Joint calipers

Are similar in construction to the spring type except that the legs are held in a set position by means of a friction joint. Firm joint calipers are particularly convenient in making large measurements.

Transfer Calipers

Are calipers used for making measurement of recesses or diameters, which are larger than the opening through which the caliper is inserted. They are provided with an auxiliary arm against which the one leg bears, thereby allowing collapsing of the leg for removal from the hole while preserving the measurement.

Hermaphrodite Calipers

Caliper that have one inside or outside leg and one leg terminating in a divider point. They are used for such layout work as scribing a line parallel to a shoulder edge, and may be either of the spring or form joint type.

Measuring with a Caliper Accurate use of calipers requires considerable practice and the

development of a sense of touch. To get a delicate sense of touch of a caliper on the work, it should be held lightly and not with a “grab-grip”. If the caliper will just barely hang on the work without falling off, the pressure is about right. It is possible to force calipers over or into work by using too much pressure, and thus introduce an error in reading.

When measuring with outside calipers, the axis of the calipers should be held perpendicular to the axis of the work. With inside calipers, the axes of work and calipers should coincide. It is particularly important with inside calipers that the tips of both feet bear on the work, not one foot and the side of the opposite leg.

Never caliper the work while it is moving or revolving; it is not

accurate and the caliper may get caught and be broken. Calipers are not efficient for accurate measurements, but they are

efficient for measuring stock, roughing cuts, lengths, and any dimensions that

need not be extremely accurate. The caliper may be used if necessary for every close measurement, but it is easier, quicker, and surer to use a micrometer.

Setting and Reading with a Caliper To set an outside caliper hold the rule in the left hand with the end

against the little finger in such a position that the light falls directly on the scale. Hold the caliper in the right hand, in such a position that it may be adjusted by the adjusting nut between the thumb and finger. Place the end of one leg of the caliper against the end of the scale and against the finger so that it will not slip around, and then adjust the other leg to the desired graduation on the rule. Hold the caliper true and looking squarely at the end to be set to the line, adjust the caliper until the end seems to split the line.

A firm joint caliper is held in about the same way but must be adjusted by tapping lightly against some solid object.

To read an outside caliper it is held substantially as above except that as it is not to be adjusted, the adjusting screw should not be touched.

Calipers are not efficient for accurate measurements, but they are efficient for measuring stock, roughing cuts, lengths and any other dimensions that need not be extremely accurate. The caliper may be used if necessary for very close measurements, but it is easier and quicker and surer to use a micrometer and a gage.

Practical Measurement Work Shop

Micrometers

Micrometer is probably the most commonly used precision device in the industry for obtaining measurements of the order 0.001 to 0.0001 inch its measuring accuracy is based on an accurate screw and a fixed nut. The micrometer is composed of a U- shaped frame, the ends of one leg of which carries a fixed anvil, the other leg carrying a tubular member or barrel. The micrometer nut is fixed in the barrel and is slotted and provided with a tapered thread and nut at its outer end to allow adjustment for wear. The measuring spindle and screw are integral, being supported on the nut at the threaded end and in an accurately fitted hole in the frame of the spindle end. To the outer end of the spindle is fastened a shell or thimble, which extends over the turns around the barrel. The measurement is made between the faces of the anvil and spindle.

Micrometers are made in a variety of sizes ranging from ½ in. to about 48 in., however, the one most commonly used is the 1 in. size for measurements from 0.001 to 1.000 in. they are made also in a variety of styles such as outside micrometers, thread micrometers, and depth micrometers.

Classification of Micrometers They are made also in a variety of styles such as outside micrometers,

inside micrometers, thread micrometers, and depth micrometers. Inside Micrometers Inside micrometers are used for measuring the diameters of holes and

other inside dimensions larger than 1 inch and up 0 several feet, inside micrometers are provided that measure from anvils placed at the end of the thimble and barrel. By means of interchangeable measuring rods a wide range of measurement multiple of 1 inch so that measurement are possible from minimum to maximum size by 0.001 inch intervals.

Outside Micrometers Outside micrometers are used for measuring the diameters and other

outside dimensions. For measuring diameters outside dimensions larger than 1 inch and up 0 several feet, inside micrometers are provided that measure from anvils placed at the end of the thimble and barrel. By means of interchangeable measuring rods a wide range of measurement multiple of 1 inch so that measurement are possible from minimum to maximum size by 0.001 inch intervals.

Depth Micrometers It is used for measuring depth of holes and slots or the distance from

surface to the other. It is similar in general construction to the rule depth gage and vernier depth gage except that a micrometer head is used for measuring. A base whose lower surface is at right angles to the micrometer axis is fastened to the micrometer barrel and is used as reference point. A measuring rod is encased in the hollow screw and is clamped by means of a chuck at the end of a thimble at one of several scribed graduations of the rod. The distance between the graduations is equal to the micrometer screw movement so that a range of measurements from zero to several inches by intervals of 1.001 inch is possible by clamping the rod progressively at the separate scribed graduations. Each graduation on the rod is a V groove into which fingers in the chuck fit for accurately locating the rod.

Other types of micrometers depth gages are made with several interchangeable measuring rods of different lengths in contrast to a single rod with spaced grooves.

Principle of the Micrometer The basis of the tool is an accurate screw, which can be revolved, in a

fixed nut to vary the opening between the two measuring faces, at the end of the screw spindle and the other on the anvil. The graduations on the barrel and thimble indicate precisely the position of the screw spindle and the amount of opening between the measuring faces. The thimble rotates with the screw spindle and travels along the barrel. The graduations on the beveled edge of the thimble accurately subdivide each revolution of the screw so that readings may be taken in units, usually of 0.001 in. or 0.01 in.

How to Hold Micrometers Figure x-xx indicates clearly the proper way to hold the micrometer in

order to accurately measure a piece held in the hand. Note carefully the position of the fingers; the micrometer is held by the little finger or the third finger, whichever is less awkward, against the palm of the hand, which allows the spindle to be operated in either direction with the thumb and index finger. The correct way to hold a micrometer when measuring work not held in the hand, is shown in figure 3-38.

When making a measurement be sure the micrometer is held square across the diameter. Turn the spindle down to the work, but not down too hard. It is easy to spring a micrometer 0.001 or 0.002 inch and this not only gives a false measurement but also injures the micrometer.

It seems easy for some people occasionally to read a micrometer 0.025 over or under; such a mistake is inexcusable. It is even more careless to add 25 and 5 and call it 35, or 75 and 5 and read it 85. Be careful when using a micrometer to hold it properly, to adjust it carefully, and to read it accurately.

The ratchet stop. In the use of the micrometer, the measuring pressure is dependent on the operator’s sense of “feel”. Errors of 0.0001 to 0.0003 inch may be introduced in a measurement as a result of this factor. In order to overcome this source of error, a device known as a ratchet stop is provided for use with the micrometer. Attached to the end of the thimble, it serves as a constant-torque turning means. The outer knurled member, when turned, exerts a turning force on the thimble through ratchet teeth under spring pressure. When the spindle and anvil contact the part to be measured and the correct pressure has been reached, the ratchet teeth will slip, preventing further pressure from being applied. Ratchet stops are set to give a measuring pressure of approximately 25 lb per square in. of spindle face area.

How to Read the Graduations of the Micrometers The instructions that follow on hoe to read a micrometer caliper apply

to a micrometer that reads thousandths of an inch only. How to read a

micrometer graduated in ten- thousandths of an inch. There are ten longitudinal spaces laid out on the sleeve. This set of spaces is the vernier scale by means of which a reading as fine as 0.0001 inch can be made. Figure 3-46 is an enlarged view of the barrel showing the vernier scale.

The vernier scale is used in reading a micrometer whenever the longitudinal line on the spindle does not coincide, that is, is not in alignment with the line on the thimble.

A vernier scale, consisting of 10 spaces running parallel to the longitudinal lines on the sleeve, covers exactly nine of the spaces on the beveled edge of the thimble. These lines are numbered from 0 to 0 (meaning from 0 to 10).

Since the 10 spaces or divisions of the vernier equal to the overall space of nine divisions of the thimble, then one division on the vernier equals 1/10 of 9/10,000 inch (0.0009 inch).

Graduations on the thimble, then one division on the vernier equals 1/1000 or 10/10,000 inch. The difference between one division on the thimble and one division on the barrel is therefore (10/10,000) - (9/10,000) or 1/10,000 or 0.0001 in.

The zero lines of the vernier and the thimble coincide when the reading is exact in thousandths and the difference between lines on the thimble and the lines on the vernier at 1, 2,3 etc. equals 0.0001 in. 0.0002 in. etc. thus as the 1, 2, 3, etc., vernier lines coincide with any thimble line, the thimble has moved past the exact setting 1, 2, 3, etc., ten thousandths of an inch.

Practical Measurement Work Shop

Vernier Caliper

The vernier caliper is a measuring tool much used in machine shops, especially in tool and die making departments where fine exact work is done. The vernier caliper is wrench like and has a scale that allows direct reading of the adjusted width between the jaws of the wrench. The vernier caliper is similar in principle to the caliper rule described before except that the vernier scale is added for making precision measurements. IN addition, much greater refinements are made in the quality of the device with respect to the accuracy of the graduations, the squareness and parallelism of the jaw, and the fit of the movable jaw on the bar. The width of graduations on vernier devices are smaller than those on rules, being of the order of .003 or .004 in. it will be evident that in order to use the vernier caliper to the observed accuracy some degree of skill and judgment is necessary.

Uses of the Vernier Caliper The vernier caliper is used for making inside as well as outside

measurements. The ends of the jaws are provided with nibs that are ground on the outside to a radius less than the smallest inside diameter which may be measured.

How to Read Graduations of the Vernier Caliper The vernier consists of a fixed scale and a vernier scale, which is

movable in relation to the former. The vernier scale makes possible the

accurate reading of a measurement to a fractional part of the smallest division on the fixed scale and therefore converts a scale into a precision device. Although a vernier scale can theoretically be designed to indicate to very small fractions of length, it is limited practically to about 0.001 in. when applied to simple scales.

Practical Measurement Work Shop

Universal Bevel Protractor with a Vernier

Uses of Universal Bevel Protractor A Universal Bevel Protractor is a measuring tool used to measure

angles. With the help of a vernier, it will measure angles accurately.

Parts of the Universal Bevel Protractor

How to Read Graduations of the Universal Bevel Protractor Figure x-xx show part of a vernier protractor. The upper scale is the

one showing degrees, each tenth degree being numbered. The lower scale is the vernier, each line being equal to ½ degree or 5 minutes. Each space on the vernier is 5 minutes shorter than two spaces on the scale.

When the zero on the vernier exactly coincides with the graduation on the scale, the reading is in the exact degrees. When the graduation marked 0 on the vernier does not exactly coincide with any graduation on the scale, the graduation on the vernier that does coincide with any graduation on the scale indicates the number of twelfths of a degree to be added to the whole degree reading.

Practical Measurement Workshop

Gage Blocks/ Filler Gages

Uses of Gage Blocks/ Filler Gages A set of gage blocks usually used in machine shops where extreme

accuracy is desired. They are rectangular blocks of steel very carefully hardened and finished. Their surfaces are so fine and parallel that when rubbed together in the proper manner they will stick together.

Each block is very accurate to within a few millionth of an inch. They are very often referred to as precision or gage blocks. Whenever accuracy is desired to four decimal places in any length or height dimension, these blocks are used. For example, if a machinist has a dimension of 2.467 in. that he has to use in setting up a job and that dimensions has to be very accurate, he makes a combination of various size blocks whose total will equal 2.467 in.

Precision blocks have other uses like checking the accuracy of measuring tools such as micrometers and vernier calipers. It is good practice on the part of machinists to have their measuring instruments checked occasionally for accuracy.

How to Measure with Gage Blocks/ Filler Gages To build up any dimension using gage blocks, the first consideration is

to use a few blocks as possible. The second point to be remembered is always to work from right to left of the decimal point when making the combination. In other words, eliminate the last figure in the dimension sought firs. A typical set of 81 blocks divided into the following:

1 block - 0. 050 in 10 blocks - 0.100 in. to 0.1009 in. by 0.0001 in. steps

50 blocks - 0.101 in. to 0.150 in. by 0. 001 in steps 16 blocks - 1. 200 in. to 0. 950 in. steps 4 blocks - 1. 000 in. to 4. 000 in. by 1 in. steps

Example: Set up the 1. 3247 in. dimension

Step 1 Write the dimension on paper 1.3247 check

Step 2 Eliminate the last figure to the right of the decimal point by selecting a block with a 7 in. the fourth place. In this case, use the 0.1007 block. This amount still to be accounted for. The size of the block is written in the last column on the right for addition to prove our problem

0.1007 0.1007

Result 1. 2240

Step 3 Eliminate the last figure to the right other than zero

0. 112 0. 112

Result 1.1300

Step 4 Eliminate the last figure again 0.13 0.130

Result 1.000

Step 5 Eliminate the 1.000 1.000 1.000

Result 0.000 1.3427

Practical Measurement Work Shop

Dial Indicators Dial indicator is an instrument in which the movement of a spring

loaded measuring spindle is magnified by means of a rack and train of gears which actuate a pointer on a graduated dial. Typically scale graduations are 0.001, 0.0005, and 0.0001 in. for various types and sizes while the magnification varies from about 50 to one to 1250 to one. The rim of the dial or bezel is rotatable so that for a certain setting the zero on the scale may be brought opposite the position of the pointer for convenient reading. Although the dial indicator is a direct measuring type of device, it is commonly used as a measurement from a standard figure.

Uses of Dial Indicators dial indicator has many common uses. It may be used as a means of

checking run out or out of roundness of a piece rotated between centers or to check the trueness of the rotating part of a machine either by a clamp or by means of a heavy base to which it is attached. In a similar manner it may be used to set or check the parallelism of one surface with respect to another. When mounted to a base and upright commonly known as test set, it may be used on a surface plate to test the size straightness, and parallelism of parts. Built- in dial indicators are frequently used in such devices as calipers, snap gages, bore gages, thickness gages, height gages, and bench comparators for determining variations in size from a master or standard.

How to Read Graduations of the Dial Indicators

Practical Measurement work Shop