Gaging tools and methods

FIXED AND ADJUSTABLE GAGES

Strictly speaking, any tool or instrument used forsaking measurements might properly be called a gage, but this term, as used by machinists and toolmakers, is generally understood to mean that class of tools which conform to a fixed dimension and are used for testing sizes but are not provided with graduated adjustable members for measuring various lengths or angles. There are exceptions, however, to this general classification.

Measuring instruments, such as the micrometer and vernier caliper, are indispensable because they can be used for determining actual dimensions, and, being adjustable, cover quite a range of sizes. Any form of adjustable measuring tool, however, has certain disadvantages for such work as testing the sizes of duplicate parts, especially when such tests must be made repeatedly, and solid or fixed gages are commonly used. There is less chance of inaccuracy with a fixed gage and it is more convenient to use than a tool which must be adjusted, but owing to the necessity of having one gage for each variation in size, and because of the cost of a set covering a wide range of sizes, solid gages are used more particularly for testing large numbers of duplicate parts in connection with interchangeable manufacture.

Two different types of fixed gages are shown in Fig. 13. The form shown at A is commonly known as a "snap gage." The distance between the measuring surfaces is fixed and represents the size stamped upon the gage, within very close limits. This type of gage can be obtained in various sizes and is used for measuring duplicate parts in connection with general shop work. As a gage of this kind is repeatedly passed over the work, it becomes worn, and, therefore, should be compared or tested occasionally with a standard reference plug or disk. In case of excessive wear, the gage can be closed in slightly smaller than the required size and then be reground or lapped to the original size, as shown by a reference gage.

Sketch B illustrates another form of snap or caliper gage. This is double ended and is intended for both external and internal measurements, the width of the internal end being the same as the distance between the measuring surfaces of the external end.


Limit Gages

With the modern system of interchangeable manufacture, machine parts are made to a definite size within certain limits which are varied according to the accuracy required, which, in turn,, depends upon the nature of the work. In order to insure having all parts of a given size or class, within the prescribed limit so that they can readily be assembled without extra and unnecessary fitting, what are known as "limit gages" are used. One form of limit gage for external measurement is shown at A, Fig. 14. It is double-ended and has a "go" end and a "not go" end; that is, when the work is reduced to the correct size, one end of the gage will pass over it but not the other end: When a single-ended snap gage A, Fig. 13, is used, the diameter of the work may be slightly less than it should be, but by having a gage for the minimum as well as for the maximum size, every part must come within the limits of the gage. This allowance or limit is made to conform to whatever amount experience has shown to be correct for the particular class of fit required.


Adjustable Limit Snap Gage

The snap gage shown at A, Fig. 15, differs from the ordinary single ended type in two particulars: In the first place, it has two sets of measuring plugs and is a limit gage. The lower set forms the "go" end and the upper set the "not go" end. These plugs are also adjustable so that when the gage becomes inaccurate, as the result of wear, the plugs can easily be reset, a standard reference gage being used to determine the distance between them.

The plugs are plain cylinders of hardened steel and are lapped to a snug sliding fit in the hole of the gage body. The ends are square and bear against adjusting screws, the forward ends of which are also lapped square. The clamping screws at the side not only clamp the plugs but tend to force them against the adjusting screws. The handle has an insulated grip.


Plug and Ring Gages

A standard external or ring gage and internal or plug gage is shown at A, Fig. 16. These gages are very accurately made and are used either as reference gages or for setting calipers, etc., or as working gages. One gage manufacturer makes solid gages of this type in diameters varying from 1/16 inch to 3 inches. For larger sizes, up to 6 inches in diameter, the plug gages are made hollow. U. S. standard thread gages are shown at B, Fig. 16. These gages are intended as a practical working standard. The internal gage or plug is a standard to which the external templet is adjusted. The plain unthreaded end of the plug gage is ground and lapped to the exact diameter at the root or bottom of the thread.

Gages for testing the accuracy of tapers are shown in Fig. 17. The ring gage A is used for external tapers and the plug B for holes. The plug accurately fits the ring and when they are assembled, a line on the plug coincides with the end of the ring. This line is used for gaging the depth of holes which must conform to the standard size of the ring gage. When the plug gage is used as a working gage in the shop, the ring is usually kept as a reference gage. On the other hand, if a ring is used for testing external tapers, the plug is often preserved as the reference gage.


Gage for Originating and Accurately Measuring Tapers

When a certain taper or angle must be originated or accurately measured, the disk type of gage shown in Fig. 18 may be employed. The principle of the disk method of taper measurement is that if two disks of unequal diameters are placed either in contact or a certain distance apart, lines tangent to their peripheries will represent an angle or taper, the degree of which depends upon the diameters of the two disks and the distance between them.


Reference Gages

Reference gages are intended for testing the accuracy of working gages such as are used in the shop and toolroom, and for setting other forms of measuring instruments. Reference gages are made in different forms varying from plain blocks or disks to special shapes designed for some particular class of work. The standard set of refer- ence disks made by Brown & Sharpe contains 45 disks varying by sixteenths of an inch, from 4 to 3 inches in diameter. Handles are provided so that these disks can be used in place of standard cylindrical gages, but they are generally used without the handles for setting calipers, testing measuring instruments and for reference purposes.

Plug and ring gages similar to the type illustrated at A, Fig. 16, are also used to some extent for reference purposes, as well as for working gages. In some shops it is the practice to use the plug as a working gage and the ring for testing it, or, in case the ring is required as a working gage, the plug is kept as a standard or reference gage, as previously mentioned.

End-measuring rods and blocks are often used for testing snap gages, etc. Ordinarily, the solid measuring rods are cylindrical in form and may be obtained in sets covering a considerable range of lengths. These rods are used for testing the parallelism and width of two finished surfaces, as well as for setting calipers and testing gages. The ends of some rods are made flat and parallel, whereas others have ends which are sections of spheres, the diameters of which equal the lengths of the rods. The spherical-ended form is very convenient for testing the diameters of rings, cylinders, etc. Some end measuring rods are provided with an insulating handle in the center to prevent expansion from the heat of the hand.


Johansson Gages

The Johansson combination standard gages consist of a series of rectangular steel blocks which are finished on all sides with wonderful accuracy. The opposite sides of each block are parallel and the distance between them is equal to the dimension stamped upon the block, within a limit so small as to be inconceivable. The eighty-one blocks in what is known as Set No. 1 (see Pig. 19) are arranged in four series. The first series contains 9 blocks which vary in thickness from 0.1001 inch to 0.1009 inch, increasing by 0.0001 inch increments. The second series contains 49 blocks varying in thickness from 0.101 inch to 0.149 inch, increasing by 0.001 inch. In the third series there are 19 blocks varying in thickness from 0.050 inch to 0.950 inch, increasing by 0.050 inch. The last series of four blocks has 1, 2, 3 and 4 inch sizes, respectively. The gages for the English system of measurement are adjusted to their sizes at 66 degrees F.


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Gaging tools and methods