PRECISION MEASURING EQUIPMENT

GENERAL

Precision measurement is a process of accurately deriving the numerical value or size of a physical

dimension in standard units of measurement such as inches, centimetres, degrees, minutes and

seconds, by comparing the dimension with a precision standard of length or angle. This note

describes various commercially available precision measuring equipment and the length or angle

standards with which they are associated. Such tools and standards are used primarily for the

measurement of gages, but they are also used, to a considerable extent, in the inspection of

manufactured parts.

LENGTH STANDARDS

Line-graduated standards

Line graduated standards used in gage inspection commonly have the form of accurately ruled scales

and dials embodied in various types of precision measuring instruments.

Precision gage blocks

Precision gage blocks (see fig.1 ) are square or rectangular pieces of hardened alloy steel or sintered

carbide, having lapped, flat and parallel measuring surfaces on opposite ends. This permits combining

them, by wringing, to produce any desired size. They are usually measured by comparison with

standard blocks, which have been calibrated and certified by the National Bureau of Standards, using

comparator readings in millionths of an inch. Original or absolute measurements of gage blocks are

derived by comparison with standard wave lengths of light by means of an interferometer. Gage

blocks are most commonly used as length standards in gage inspection and are usually supplied in

sets varying from 28 to 103 blocks of different lengths. The most generally used set of the inch system

has 81 blocks with possibly two additional carbide wear blocks. Gage blocks are made in several

degrees of accuracy, of which the most generally used is ± 0.000004 inch and designated as “A”

quality The highest or laboratory grade, designated as “AA,” has a guaranteed accuracy of ±

0.000002 inch for all blocks up to 1 inch in length and ± 0.000002 inch per inch of length for larger

blocks. Other lower grades are also available. Variations in the lengths of blocks have a random

distribution between plus and minus. The total error in a block combination, therefore, seldom

exceeds twice the error in a single block, and often is actually less than that of a single block. If the

error were permitted to be in one direction only, the accumulated error in a combination might be

considerable. Although precision gage blocks are most widely used as masters for checking other

gages, their use in the shop for setting of tools and in layout work is increasing. Chromium plated and

carbide blocks are used where blocks are subject to considerable wear.Gauge blocks are calibrated

to be accurate at 68 °F (20 °C) and should be kept at this temperature when taking measurements.

This mitigates the effects of thermal expansion

Metric Gauge Blok Set (fig 1)

Wringing

Wringing (see fig 2) is the process of sliding two blocks together so that their faces lightly bond.

Because of their ultra flat surfaces, when wrung, gauge blocks adhere to each other tightly as

documented.

Air pressure applies pressure between the blocks because the air is squeezed out of the joint. Surface tension from oil and water vapor that is present between the blocks. Molecular attraction occurs when two very flat surfaces are brought into contact.

Wringing Slip Gages (Fig 2)

SCALE AND VERNIER.

Precision measuring equipment which embody line-graduated standards, scales, or graduated circles, equipped with verniers for subdividing scale divisions, are grouped into the following types:

Vernier caliper. The vernier calliper (see fig. 16) consists of a solid L-shaped frame and a sliding jaw which can be moved along the long arms of the L-shaped frame. The sliding jaw is in two sections joined by a horizontal screw. The larger section carries the measuring jaw and vernier scale; the smaller section or binding clasp has an adjusting nut acting on the horizontal screw. Both sections can be clamped independently to the long arm of the L-shaped frame. Thus the sliding jaw can be moved into an approximate position, the clasp clamped, and the larger section and measuring jaw moved to the exact position by movement of the adjusting nut. The main scale is divided into 1.0-, 0.1-, and 0.025-inch intervals, but only the 1.0 and 0.1 intervals are numbered. The vernier scale has 25 divisions. A point is placed on the sliding head and another one of the frames so that dividers can be set to transfer distances. For some applications the vernier caliper has definite advantages over the micrometer caliper (see 4.4. 1) because of its wide range and because it can be used for measuring both inside and outside dimensions. It does not, however, havethe accuracy of a micrometer caliper. A vernier caliper should show any l-inch interval of length within an accuracy of ±0.001 inch. In any 12 inches its accuracy should be within ±0.002 inch, and the error should not increase by more than about ±0.001 inch for every 12 inches thereafter.

Examples

Vernier bevel protractor. The vernier bevel protractor (see fig. 21 ) also called a universal bevel

protractor, is an angle measuring instrument designed for the layout and checking of angles to an

accuracy greater than 1 degree. It consists of two members, a base and a blade, which can be set in

any angular relationship to each other. The position of one member relative to the other is indicated

on a circular dial which has four O- to 90-degree scales. By means of a vernier, the scale can be read

to the nearest 5 minutes or degree. The blade is adjustable and, by means of a clamping mechanism

and thumb nut, can be locked to the central connecting arm at any position along its axis. The ends of

the blades are usually beveled to allow the protractor to be used in measuring from square to

undercut shoulders where a square end blade would not enter. The blade assembly (blade and

central arm) is locked to the scale and base by a large central locking nut. Either a separate or

integral means of fine adjustment is usually provided to allow controlled final movement of the blade

through a small arc. The instrument shown in figure 21 has an adjustable arm or acute angle blade for

the purpose of measuring small angles on pieces so shaped that they would be difficult to measure

from the fixed base. The back of the vernier bevel protractor is flat so that the tool may be placed flat

on a surface plate or on the work being checked.

Picture

Directions for reading the vernier protractor. The vernier indicates every 5 minutes or one twelth of a degree. When the zero on the vernier exactly

coincides with a graduation on the scale, the reading is in exact degrees, as in the upper scale shown

on figure 22, in which the reading is 17 degrees O minutes. When the zero graduation of the vernier

does not coincide exactly with a graduation on the scale, the number of one twelth of a degree or 5

minute intervals to be added to the whole degree reading is determined by counting the number of

divisions from the zero of the vernier to the first line on the vernier that coincides with a line on the

scale. As each division on the vernier represents 5 minutes, the number of these divisions multiplied

by 5 will be the number to be added to the whole number of degrees. For example, the lower scale

shown on figure 22 shows the zero on the vernier between 12 and 13 on the scale. The zero on the

vernier has, therefore, moved 12 whole degrees to the right from zero on the scale. In the same

direction, the tenth line of the vernier, representing 50 minutes is the line which exactly coincides with

a line on the scale. The reading, then, is 12 degrees 50 minutes. Since the divisions, both on the

scale and on the vernier, are numbered both to right and left from zero, any size of angle can be

measured. The readings on the scale and on the vernier are taken either to the right or left, according

to the direction in which the zero on the vernier is moved.

examples

DIMENSIONS AND TOLERANCES

Dimension: A dimension is a specified size of a geometrical characteristic such as a diameter, length, angle, circumference, or centre distance.

Nominal size: The nominal size is the designation which is used for the purpose of general identification.

Basic size: The basic size of a dimension is the theoretical size from which the limits of size for that dimension are derived by the application of the allowance and tolerances.

Design size: The design size of a dimension is the size in relation to which the limits of tolerance for that dimension are assigned.

Actual size: The actual size of a dimension is the measured size of that dimension on an individual part.

Limits of size: These limits are the maximum and minimum sizes permissible for a specific dimension.

Fit: The fit between two mating parts is the relationship existing between them with respect to the amount of clearance or interference which is present when they are assembled.

Tolerance: The tolerance on a dimension is the total permissible variation in its size. The tolerance is the difference between the limits of size.

Allowance: An allowance is an intentional difference in correlated dimensions of mating parts. It is the minimum clearance (positive allowance) or maximum interference (negative allowance) between such parts.

Tolerances: Variation in size can be restricted but not avoided. However, a certain amount of variation can be tolerated without impairing the functioning of a part. The purpose of tolerances is to confine such variation within definite limits.