c.Fixed Gages.

Fixed gages cannot be adjusted.They can generally be divided into two categories, graduated and nongraduated. The accuracy of a machinist's work, when using fixed gages, will depend on the ability to determine the difference between the work and the gage. For example, a skilled machinist can take a dimension accurately to within 0.005 of an inch or less when using a common rule. Practical experience in the use of these gages will increase ones ability to take accurate measurements.

(1) Rules.

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(a)  Steel Rule. The steel rule with the holder set (figure 18, view A, ) is convenient for measuring recesses. It has a long tubular handle with a split chuck for holding the ruled blade. The chuck can be adjusted by a knurled nut at the top of the holder, allowing the rule to be set at various angles. The set has rules ranging from 1/4 to 1 inch in length.

(b)  The Angle Rule. The angle rule (figure 18, view B) is useful in measuring small work mounted between centers on a lathe. The long side of the rule (ungraduated) is placed even with one shoulder of the work. The graduated angle side of the rule can then be positioned easily over the work.

(c) The Keyseat Rule.Another useful measuring device is the keyseat rule (figure 18, view C). It has a straightedge and a 6 inch machinist's type rule arranged to form a right angle square. This rule and straightedge combination, when applied to the surface of a cylindrical workpiece, makes an excellent guide for drawing or scribing layout lines parallel to the axis of the work. This measuring device is very convenient when making keyseat layouts on shafts.

(d) Care.Rules, like any other measuring tool, must be taken care of if accurate measurements are to be obtained. Do not allow them to become battered, covered with rust, or otherwise damaged in such a way that the markings cannot be read easily. Do not use them for scrapers; once rules lose their sharp edges and square corners, their general usefulness is decreased.

(2) Scales.A scale is similar in appearance to a rule, since its surface is graduated into regular spaces. The graduations on a scale, however, differ from those on a rule because they are either smaller or larger than the measurements indicated. For example, a half­size scale is graduated so that 1 inch on the scale is equivalent to an actual measurement of 2 inches. A 12 inch long scale of this type is equivalent to 24 inches. A scale, therefore, gives proportional measurements instead of the actual measurements obtained with a rule. Like rules, scales are made of wood, plastic, and metal. They generally range from 6 to 24 inches.

(3)   Acme Thread Tool Gage.The Acme thread cutting gages (figure 19) are hardened steel plates with cutouts around the perimeter. Each cutout is marked with a number that represents the number of threads per inch. These gages provide a standard for thread cutting tools that are being ground. The tool is also used to align the Acme thread cutting tool prior to machining them on a lathe. The sides of the Acme thread have an included angle of 29° (14 1/2° on each side) and that is the angle made into the gage. The width of the flat on the point of the tool varies according to the number of threads per inch. The gage provides different slots to use as a guide when grinding the tool. Setting the tool up in the lathe is simple. First, ensure that the tool is centered to the work as far as the height is concerned. Then, with a gage edge laid parallel to the centerline of the work, adjust the side of the tool until it fits the angle on the gage very closely.

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(4)  Center Gage.The center gage (figure 20 ) is used like the Acme thread gage. Each notch and the point of the gage has an included angle of 60°.The sixty­degree angles of the gage are used for checking Unified and American threads, as well as older American National or U.S.Standard threads, and for checking thread cutting tools. The center gage is also used to check the angle of lathe centers. The edges are graduated into 1/4, 1/24, 1/32, and 1/64 of an inch for ease in determining the pitch of threads on screws. The back of the center gage has a table giving the double depth of the threads in thousandths of inch for each pitch. This information is also useful in determining the size of tap drills.

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(5) Thickness (Feeler) Gages.

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(a)   Thickness (feeler) gages (figure 21 ) are used to determine distances between two mating parts. The gages are made in various shapes and sizes; usually 2 to 26 blades are grouped into one tool and graduated in thousandths of an inch.

(b)   Most thickness blades are straight, while others are bent at the end at 45 degree and 90 degree angles. Some thickness gages are grouped so that there are several short and several long blades together. Thickness gages are also available in single blades and in strip form for specific measurements.

(c)   Some gages are fixed in leaf form, like a jackknife. This type allows the checking and measuring of small openings such as contact points, narrow slots, and so forth. They are widely used to check the flatness of parts, in straightening and grinding operations, and in squaring objects with a try square.

(d)   The leaf­type gage can be used with a combination of blades to obtain a desired gage thickness. Always place the thinner blades between the heavier ones to protect the thinner ones and to prevent them from kinking. Do not force the blades into openings which are too small as the blades may bend or kink. A good way to get the “feel” of using a thickness gage correctly is to practice with the gage on openings of known measurements.

(6) Radius Gage.

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(a)   The radius gage (figure 22) is used to check, in any position and at any angle, both inside and outside radii. This gage is often underrated in its usefulness to the machinist. The blades of the fillet and radius gages are made of hard­rolled steel. The double­ended blades of the gage have a lock which holds the blade in position. The inside and outside radii are on one blade on the gage.Each blade of the gage is marked in 64ths. Each gage has 16 blades.

(b)   Whenever possible, the design of most parts includes a radius located at the shoulder formed when a change is made in the diameter. This radius gives the part an added margin of strength at that particular place. When a square shoulder is machined in a place where a radius should have been, the possibility that the part will fail by bending or cracking is increased. The blades of most radius gages have both concave (inside curve) and convex (outside curve) radii in almost all of the common sizes.

(7) Straightedges.

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(a)   General. Straightedges look very much like rules, except that they are not graduated. They are used primarily for checking surfaces for straightness; however, they can also be used as guides for drawing or scribing straight lines. Two types of straightedges are shown in figure 23. View A shows a straightedge made of steel which is hardened on the edges to prevent wear; it is the one the machinist will probably use the most. The straightedge shown in View B has a knife edge and is used for work requiring extreme accuracy.

(b)   Care.The straightedges should always be kept in a box when they are not in use. Some straightedges are marked with two arrows, one near each end, which indicate the balance points. When a box is not provided, place the resting pads on a flat surface in a storage area where no damage to the straightedge will occur from other tools. Place the straightedge so that the two balance points set on the resting pads.

(8) Machinist's Square. The most common type of machinist's square is a hardened steel blade securely attached to a beam. The steel blade is not graduated. This instrument is very useful in checking right angles and in setting up work on shapers, milling machines, and drilling machines. The size of the machinist's squares range from 1 1/2 to 36 inches in blade length. The same care should be taken with them as with micrometers.

(9) Sine Bar.

(a)   General. A sine bar (figure 24) is a precision tool used to establish angles which require extremely close accuracy. When used in conjunction with a surface plate and gage blocks, angles are accurate to within 1 minute (1/60°) . The sine bar may be used to measure angles on a workpiece and to lay out an angle on the workpiece that is to be machined. Work may be mounted directly to the sine bar for machining. The cylindrical rolls and the parallel bar, which make up the sine bar, are all precision ground and accurately positioned to permit such close measurements. Any scratches, nicks, or other damage should be repaired before the sine bar is used, and care must be exercised in using and storing the sine bar.

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(b)   Use.

  1. A sine bar is a precisely machined tool steel bar used in conjunction with two steel cylinders. In the type shown in figure 25 , the cylinders establish a precise distance of either 5 inches or 10 inches from the center of one to the center of the other, depending upon the model used.The bar itself has accurately machined parallel sides. The axes of the two cylinders are parallel to the adjacent sides of the bar within a close tolerance. Equally close tolerances control the cylinder roundness and freedom from taper. The slots or holes in the bar are for convenience in clamping workpieces to the bar. Although the illustrated bars are typical, there is a wide variety of specialized shapes, widths, and thicknesses.
  2. The sine bar itself is very easy to set up and use. One does not need to have a basic knowledge of trigonometry to understand how it works. When a sine bar is set up, it always forms a triangle. A right triangle has one 90° angle. The base of the triangle formed by the sine bar is the surface plate (figure 25). The side opposite is made up of the gage blocks that raise one end of the sine bar. The hypotenuse is always formed by the sine bar. The height of the gage block setting may be found in two ways. The first method is to multiply the sine of the angle needed by the length of the sine bar. The sine of the angle may be found in any table of trigonometric functions. The second method is to use a table of sine bar constants. These tables give the height setting for any given angle (to the nearest minute) for a 5 inch sine bar. Tables are not normally available for 10 inch bars because it is just as easy to use the sine of the angle and move the decimal point to the right.

(c) Care.Although sine bars have the appearance of being rugged, they should receive the same care as gage blocks. Because of the nature of their use in relation with other tools or parts that are heavy, they are subject to rough usage. Scratches, nicks, and burrs should be removed or repaired. They should be kept clean of abrasive dirt, sweat, and other corrosive agents. Regular inspection of the sine bar will locate such defects before they are able to affect the accuracy. When sine bars are stored for extended periods, all bare metal surfaces should be cleaned and then covered with a light film of oil. Placing a cover over the sine bar will further prevent accidental damage and discourage corrosion.

(10) Parallel (Bars) Blocks. Parallel blocks (figure 26 ) are hardened, ground steel bars that are used in laying out work or setting up work for machining. The surfaces of the parallel blocks are all either parallel or perpendicular, as appropriate, and can be used to position work in a variety of setups with accuracy. They generally come in matched pairs and standard fractional dimensions. Care should be used in storing and handling them to prevent damage. If it becomes necessary to regrind the parallel blocks, be sure to change the size that is stamped on the ends of the blocks.


(11) Ring and Plug Gages.

(a)   General. Ring, plug, snap, and precision gage blocks are used as standards to determine whether or not one or more dimension is within specified limits. Their measurements are included in the construction of each gage, and they are called fixed gages. However, some snap gages are adjustable. Gages are used for a wide range of work, from rough machining to the finest tool and die making. The accuracy required of the same type of gage will be different, depending on their use.




(b)   Ring Gages.

  1. A ring gage (figure 27 ) is a cylindrical­shaped disk that has a precisely ground bore. Ring gages are used to check machined diameters by sliding the gage over the surface. Straight, tapered, and threaded diameters can he checked by using the appropriate gage. The ring gage is also used to set other measuring instruments to the basic dimension that is required for their particular operation. Normally, ring gages are available with a GO and a NO GO size that represents the tolerance allowed for that particular size or job.
  2. The plain gage is an external gage of the circular form. For sizes between 0.059 and 0.510 inch, ring gages are made with a hardened steel bushing and pressed into a soft metal body. The thickness of the gage will range from 3/16 to 1 5/16 inches. On ring gages, the GO gage (1) is larger than the NO GO gage (2). The GO and the NO GO ring gages are separate units. They can be distinguished from each other by an annular groove (3) cut in the knurled outer surface of the NO GO gage. Ring gages made for diameters of 0.510 to 1.510 inches are the same as those in figure 27, except that there is no bushing; they are made all in one piece. Ring gages, sized from 1.510 to 5.510 inches are made with a flange (4) . This design reduces the weight, making the larger sizes easier to handle.
  3. Ring gages are used more often in the inspection of finished parts than of parts in process. The reason for this is that the finished parts are usually readily accessible; whereas parts in a machine that are supported at both ends would have to be removed to be checked.
  4. The use of the ring gage (figure 28 ) is an important function when checking the accuracy of parts. Proper use of the ring gage requires a sensitive sense of feel by the individual inspecting the finished parts.
  5. To check the shank diameter of a pivot stud (figure 28) line the stud (view A) (1) up with the hole (2) and press it in gently. If the stud will not go in, the shank is too large. With the stud in the hole (view B) , check the piece for taper and out­of­roundness by sensing any wobble.
  6. After checking the part in the GO gage, check it in the NO GO gage. The stud must not enter this gage to establish it as being between the desired limits.


The GO ring gage controls the maximum dimension of a part and the NO GO plug gages control the minimum dimension of a hole. Therefore, GO gages control the tightness of the fit of the mating parts and the NO GO gages control the looseness of the fit of the mating parts.


(c) Plug Gages.A plug gage (figure 29 ) is used for the same type of jobs as a ring gage except that it is a solid shaft­shaped bar that has a precisely ground diameter for checking inside diameters or bores.

(12)  Micrometer Standards.Micrometer standards are either disk or tubular shaped gages that are used to check outside micrometers for accuracy. Standards are made in sizes so that any size micrometer can be checked. They should be used on a micrometer on a regular basis to ensure continued accuracy.

(13)  Gage Blocks.


(a)   Gage blocks (figure 30 ) are available in sets from 5 to 85 blocks of different dimensions. Precision gage blocks are made from a special alloy steel. They are hardened, ground, and then stabilized over a period of time to reduce subsequent waxing. They are rectangular in shape with measuring surfaces on opposite sides. The measuring surfaces are lapped and polished to an optical flat surface and the distance between them is the measuring dimension. The dimension may range from 0.010 of an inch up to 20 inches.

(b)   Gage blocks are used as master gages to set and check other gages and instruments. They are accurate from eight millionths (0.000008) of an inch to two millionths (0.000002) of an inch, depending on the grade of the set.To visualize this minute amount, consider that the thickness of a human hair divided by 1,500 equals 0.000002 of an inch. The degree of accuracy applies to the thickness of the gage block, the parallelism of the sides, and the flatness of the surfaces. The gages are lapped so smooth and flat that when they are “wrung” or placed one on top of the other in the proper manner, one cannot separate them by pulling them straight out; they have to be slipped to the side and then off. A set of gage blocks has enough different size blocks that any measurement can be established within the accuracy and range of the set. As one might expect, anything so accurate requires exceptional care to prevent damage and to ensure continued accuracy. A dust­free, temperature­controlled atmosphere is preferred. After the gage blocks are used, each block should be wiped clean of all fingerprints and coated with a thin layer of white petroleum to prevent them from rusting.

(c) Gage blocks are used for various precision measurements. Before using a set of new gage blocks, remove the coat of rust preventing compound with a chamois or a piece of cleaning tissue, or by cleaning them with an approved solvent. Gage blocks and any other measuring tool used with them must be free of grease, oil, dirt, and any other foreign matter to avoid a lapping action whenever the block is moved, and to ensure accurate measurement. When using gage blocks, take particular care when measuring hardened workpieces to avoid scratching the measuring surfaces.


When building gage blocks (wringing them together) to obtain a desired dimension, care should be exercised to avoid damaging them.

Step 1. To build or stack precision gage blocks (figure 31 ) to take measurements, bring the blocks together (view A) and move them slightly back and forth. This minimizes scratching, as it will detect any foreign particles between the surfaces.

Step 2. Shift the blocks. If the blocks are clean, they will begin to take hold.

Step 3.Slide the two blocks together (view B), using a slight pressure and a rotary motion.

Step 4. Shift the gage blocks so that the sides are in line. Any combination of the gage blocks may be stacked together in this manner. The combination will be as solid as a single block.


The adhesive force that binds the two gage blocks together is a combination of molecular attraction and the suction cup action due to the film of oil or moisture on the surfaces being wrung together.

Separate the gage blocks by sliding them apart, using the same movement as when wringing them together.



Do not leave blocks wrung together for long periods of time since the surfaces in contact will tend to corrode.

(d) Ordinary changes in temperature have a significant effect on the measurements made with precision gage blocks. The standard measuring temperature is 68°F, which is just a little lower than the average temperature in most shops. Since the room temperature affects the work as well as the block, the expansion in the work will be matched in most cases by a similar expansion in the block. The coefficient of expansion of several metals and blocks are listed below:

Material Millionths of an inch
Steel 5.5 to 7.2 per degree F
Iron 5.5 to 6.7
Phosphor bronze 9.3
Aluminum 12.8
Copper 9.4
Gage blocks 6.36 to 7.0

(e)   Handle blocks only when they must be moved and hold them between the tips of your fingers so that the area of contact is small. Hold them for short periods of time only.


Avoid conducting body heat into the block by careless handling. Body heat may raise the temperature of the block, causing serious error in a measurement, particularly if a long stack of blocks is being handled.

(f)   When using gage blocks, consider the source of error resulting from the temperature. Metals other than iron and steel (such as aluminum) have a much different coefficient of linear expansion, which will result in a difference between the room measurement and the standard measuring temperature measurement. Careless handling of gage blocks may produce an error of several millionths of an inch, and this error increases proportionally with the dimension of the block.

(g)   The temperature of the work may be either lower or higher than the room temperature as a result of a machining operation. This difference may be sufficient to cause a sizable error.

(h)   Theoretically, the measuring pressure should increase proportionally with the area of contact. For practical purposes, it is better to use a standard measuring pressure. The most commonly used pressure is 1/2 to 2 pounds.

(i) Gage blocks are used in the layout and checking of tools, dies, and fixtures. They are also used in machine setups, in checking parts in the process of being manufactured, and finished parts.

(j) Gage blocks are commonly used in setting adjustable instruments and indicating gages and verifying inspection gages. Gage blocks are used to verify the accuracy of ring and snap gages and many other special­purpose gages. The classification of blocks depends largely on the accuracy required. Typical classification is as follows:

(14) Care.The following steps should be followed when caring for precision gage blocks:

Step 1. Observe particular care when using gage blocks to measure hardened work. The danger of scratching is increased when the work is as hard as the block, or harder.

Step 2. Never touch the measuring surfaces of the blocks any more than necessary. The moisture from one's hands contains acid which, if not removed, will eventually stain the blocks.

Step 3.Before using the gage blocks, ensure that there is no grease, oil, dirt, or other foreign substances on the block.

Step 4. Every time a set of blocks is used, all of the blocks which have been cleaned for use must be covered with a light film of acid­free oil, such as boiled petroleum, before they are put away. Wipe them with an oiled chamois as the blocks are returned back to their places in the case. 

(15)   Classes and Standards for all Makes of Gages.

Class X ­ Precision lapped to close tolerances for many types of masters and the highest quality working and inspection gages.
Class Y ­ Good lapped finish to slightly increased tolerances for inspection and working gages.
Class ZZ ­ (Ring gages only). Ground only to meet the demand for an inexpensive gage, where quantities are small and tolerances liberal.

Table 1 lists the tolerances for ring gages in each class:


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(16)   Thread Measuring Wires.

The most accurate method of measuring the fit or pitch diameter of threads, without the use of the expensive and sophisticated optical and comparator equipment, employs thread measuring wires. These wires are accurately sized, depending on the number of threads per inch, so that when they are laid over the threads in a position that allows an outside micrometer to measure the distance between them, the pitch diameter of the threads can be determined. Sets are available that contain all the more common sizes. Detailed information on computing and using the wire method is covered in most machinist and technical manuals.

(17) Surface Plate.


(a)   General. A surface plate (figure 32) provides a true, smooth, plane surface. It is often used as a level base for surface and height gages from which to make accurate measurements. Surface plates are usually made of close grain cast iron, are rectangular in shape, and come in a variety of sizes.

(b)   Uses.The surface plate is used with such tools as parallels, squares, surface gages, angle plates, and sine bar in making layout lines. Angle plates are used to mount work at an angle on the surface plate.

  1. View A of figure 32 shows a surface gage and a V­block combination used in laying out a piece of stock. To set the surface gage for height, first clean the top of the surface plate and the bottom of the surface gage. Then, place the squaring head of a combination square is shown in view B of figure 32. The scale is secured in the square head so that it does not move and is in contact with the surface of the plate. Settings are then made on the surface gage to be transferred to the workpiece.
  2. The surface plate is also used for checking surfaces that are being scraped for flatness (figure 33) . To perform this operation, a surface plate and nondrying prussian blue are used.The plate is covered with a light coat of blue. The workpiece is placed on top of the plate and blue, and moved over the surface. The blue will stick to the high spots on the workpiece, revealing the areas that are to be scraped. Once the blue areas are scraped, the piece is checked again. This process is continued until the blue coloring shows on the entire surface of the workpiece.


(18) Screw Pitch Gage.


(a) General. Screw pitch gages (figure 34 ) are made for checking the pitch of U.S.Standard, Metric, National Form, and Whitworth cut threads. These gages are grouped and retailed in a case or handle, as are the thickness gages.The number of threads per inch is stamped on each blade, in which are cut the exact form of threads of the various pitches. Some types are equipped with blade locks. The triangular shaped gage has 51 blades covering a wide range of pitches, including 11 1/2 and 27 threads­per­inch for V­form threads.

(b) Screw pitch gages are used to determine the pitch of an unknown thread by setting one of the pitch blades (one that matches the threads) against the threads that have already been cut (figure 34, view E). The pitch of a screw thread is the distance between the center of one tooth to the center of the next tooth.