b.Adjustable Gages.

Adjustable gages can be adjusted by moving the scale or by moving the gaging surface to the dimensions of the object being measured or gaged.For example, on the dial indicator, the face is adjusted to align the indicating hand with the zero point on the dial. On verniers, the measuring surface would be moved to the dimensions of the object being measured.

(1) Dial Indicators.

(a)   Dial indicators are used by the machinist in setting up workpieces in machines and in checking the alignment of machinery. Proficiency in the use of the dial indicator requires a lot of practice; the more one uses it, the more it will aid in doing more accurate work.

(b)   Dial indicator sets (figure 2 ) usually have several components that permit a wide variation of uses.The contact points allow the indicator to be used on different types of surfaces. The universal sleeve permits flexibility of setup. The clamp and the holding rods permit setting the indicator to the work. The hole attachment is used to indicate the variation, or run out, of the inside surfaces of holes and the tool post holder can be used to clamp the indicator in various lathe setups. Figure 3  shows some of the practical applications of the dial indicator.

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(c) When preparing to use the dial indicator, there are several things that should be checked. Dial indicators come in different degrees of accuracy. Some will give readings to one ten thousandths (0.0001) of an inch, while others will indicate to only five thousandths (0.005) of an inch. Dial indicators also differ in the total range or amount that they will indicate. If a dial indicator has a total of one hundred thousandths (0.100) of an inch in graduations on its face, and has a total range of two hundred thousandths (0.200) of an inch, the needle will only make two revolutions before it begins to exceed its limit and jams up. The degree of accuracy and the range of a dial indicator is usually shown on its face. Before using a dial indicator, carefully depress the contact point and release it slowly; rotate the movable dial face so that the dial needle is on zero. Depress and release the contact point again and check to ensure that the dial pointer returns to zero; if it does not, have the dial indicator checked for accuracy.

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(d) Care.Dial indicators and other instruments that have a mechanically operated dial as part of their design are easily damaged by misuse and lack of proper maintenance. The following instructions apply to dial indicators in general:

  1. Make sure the dial indicator that has been selected for use has the range capability required. When a dial indicator is extended beyond its design limit, some lever, small gear, or rack in the housing must give way to the exerted pressure applied on it. The dial indicator will be rendered useless if this happens.
  2. Never leave a dial indicator on any surface that will be subjected to a shock (such as hammering on a part when dialing in on the workpiece); an erratic and uncontrolled movement of a surface could cause the dial to be over traveled.
  3. Protect the dial when it is not being used. Provide a storage area where the dial will not receive accidental blows, and where dust, oil, and chips will not come in contact with it.
  4. When a dial indicator becomes sluggish or sticky in operating, it may be either damaged or dirty. Also, one may find that the pointer is rubbing the dial crystal or that the pointer is bent or rubbing the dial face. A sluggish dial should never be oiled. Oil will compound the problem. A suitable cleaning solvent should be used to remove all dirt and residue.

(2)   Vernier Caliper. A vernier caliper can be used to measure both inside and outside dimensions. To take a measurement, position the appropriate sides of the jaws to the surface to be measured and read the side marked inside or outside as required. There is a difference in the zero marks on the two sides that is equal to the thickness of the tips of the two jaws, so be sure to read the correct side. Vernier calipers are available in sizes ranging from 6 inches to 6 feet and are graduated in increments of thousandths (0.001) of an inch. The scales on the vernier calipers made by different manufacturers may vary slightly in length or number of divisions; however, they are all read basically the same way. Detailed instructions for reading and using the vernier calipers are covered in a later section of this course.

(3)   Vernier Height Gage.A vernier height gage (figure 4 ) is used to lay out work for machining operations or to check the dimensions on the surfaces of work which has been machined. The offset scriber allows one to measure from the surface plate with readings taken directly from the scale without having to make any calculations. If a straight scriber were used, the actual height would have to be calculated by taking into account the distance between the surface plate and the zero mark. Some models have a slot in the base for the scriber to move down to the surface and a scale that permits direct reading. Another attachment is a rod that permits depth readings. Small dial indicators can be connected to the scriber to permit extremely close work in checking or laying out work. A vernier height gage is read the same way as the vernier caliper.

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(a) Care.Vernier gages also require careful handling and proper maintenance if they are to remain accurate. The following instructions apply to the vernier gages in general:

  1. Always loosen the binding screws before attempting to move the sliding arms.
  2. Never force a gage into position. Forcing, besides causing an inaccurate reading, is likely to force the arms out of alignment.
  3. When taking a measurement, use only gentle pressure on the fine adjustment screw. Heavy pressure will force the two scales out of parallel. 
  4. Prior to putting a vernier gage away, wipe it clean and give it a light coat of oil. (Perspiration from the hands will cause the instrument to corrode rapidly.)

(b) Use.The most accurate means of using the height gage is to place the workpiece on the top of the surface plate. After the correct setting has been made, place the base of the vernier height gage on the surface plate and scribe the desired height onto the workpiece.

(4) Depth Gages.A depth gage is an instrument for measuring the depth of holes, slots, counter bores, recesses, and the distance from the surface to some recessed part. The most commonly used depth gages are the vernier depth gage, the rule depth gage, and the micrometer depth gage.

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(a)   Vernier Depth Gage.The vernier depth gage (figure 5 ) consists of a graduated scale (1) either 6 or 12 inches long. It also has a sliding head (2) similar to the one on the vernier caliper. The sliding head is designed to bridge holes and slots. The vernier depth gage has the range of the rule depth gage. It does not have quite the accuracy of a micrometer depth gage.It cannot enter holes less than 1/4 inch in diameter. However, it will enter a 1/32 inch slot. The vernier scale is adjustable and may be adjusted to compensate for wear.

(b)   The Rule Depth Gage.The rule depth gage is a graduated rule with a sliding head designed to bridge a hole or slot, and to hold the rule perpendicular to the surface on which the measurement is taken. This gage has a measuring range of 0 to 5 inches. The sliding head has a clamping screw so that it may be clamped in any position. The sliding head has a flat base which is perpendicular to the axis of the rule and ranges in size from 2 to 2 5/8 inches in width and from 1/8 to 1/4 inch in thickness.

(c) Micrometer Depth Gage.The micrometer depth gage consists of a flat base attached to the barrel (sleeve) of a micrometer head. These gages have a range of 0 to 9 inches, depending on the length of the extension rod used.The hollow micrometer screw (the threads on which the thimble rotates) has a range of either 1/2 or 1 inch. Some are provided with a ratchet stop. The flat base ranges in size from 2 to 6 inches. Several extension rods are normally supplied with this type of gage.

(5)    Dial Vernier Caliper.A dial vernier caliper looks much like a standard vernier caliper and is also graduated in one thousandths (0.001) of an inch. The main difference is that instead of a double scale, as on the vernier caliper, the dial vernier caliper has the inches marked only along the main body of the caliper and a dial with two hands to indicate hundredths (0.010) and thousandths (0.001) of an inch. The range of the dial vernier caliper is usually 6 inches.

(6)  Dial Bore Gage.One of the most accurate tools for measuring a cylindrical bore, or for checking a bore for out­of­roundness or taper, is the dial bore gage.The dial bore gage (figure 6) does not give a direct measurement; it gives the amount of deviation from a preset size, or the amount of deviation from one part of the bore to another. A master ring gage, an outside micrometer, or a vernier caliper can be used to preset the gage.A dial bore gage has two stationary spring­loaded points and an adjustable point to permit a variation in range. These three points are evenly spaced to allow accurate centering of the tool in the bore. A fourth point, the tip of the dial indicator, is located between the two stationary points. By simply rocking the tool in the bore, the amount of variation on the dial can be observed. Accuracy to one ten thousandth (0.0001) of an inch is possible with some models of the dial bore gage.

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(7) Internal Groove Gage.The internal groove gage is very useful for measuring the depth of an O­ring groove or of other recesses inside a bore. This tool allows one to measure a deeper recess, or one that is located farther back into the bore, than would be possible with an inside caliper. As with the dial bore gage, this tool must be set with gage blocks, a vernier caliper, or an outside micrometer. The reading taken from the dial indicator on the groove gage represents the difference between the desired recess or the groove depth and the measured depth.

(8)   Universal Bevel. The universal bevel (figure 7) , because of the offset in the blade, is very useful for bevel gear work and for checking angles on lathe workpieces which cannot be reached with an ordinary bevel. The universal bevel must be set and checked with a protractor, or another suitable angle­measuring device, to obtain the desired angle.

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(9)  Cutter Clearance Gage.The cutter clearance gage (figure 8 ) is one of the simplest gages to use, yet it is suitable for gaging clearance on all styles of plain milling cutters which have more than 8 teeth and a diameter range from 1/2 inch to 8 inches. To gage a tooth with the instrument, bring the surfaces of the “V” into contact with the cutter and lower the gage blade upon the tooth to be gaged.Rotate the cutter sufficiently to bring the tooth face into contact with the gage blade. If the angle of clearance on the tooth is correct, it will correspond with the angle of the gage blade. Cutter clearance gages that have an adjustable gage blade for checking clearance angles of 0°­30° on most common cutter styles are also available.

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(10)    Adjustable Parallel. The adjustable parallel (figure 9) consists of two wedges connected on their inclined surfaces by a sliding dovetail. The distance between the two outside parallel surfaces is varied by moving the mating parts together or apart. The distance is then measured with a micrometer. An adjustable parallel can be locked at any height between the maximum and the minimum limits. This instrument, constructed to about the same accuracy of dimensions as parallel blocks, is very useful in leveling and positioning setups in a milling machine or in a shaper vise. Adjustable parallels are available in various sizes depending on the nature of the work.

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(11)    Surface Gage.A surface gage (figure 10) is used to measure or gage an object and to indicate the parallelism of surfaces. It is used primarily in layout and alignment of the work. The surface gage is commonly used with a surface plate and a scriber to transfer dimensions and layout lines to the work. In some cases, a dial indicator is used with the surface gage to check the trueness or alignment of an object or workpiece. The surface gage consists of a base with an adjustable spindle (1) to which may be clamped a scriber or an indicator (2) . Surface gages are made in several sizes and are classified by the length of the spindle. The smallest spindle is 4 inches long, the average 9 to 12 inches, and the largest 18 inches. The scriber is fastened to the spindle with a clamp. The bottom and the front end of the base of the surface gage have deep V­grooves. The grooves allow the gage to measure from a cylindrical surface. The base has two gage pins (3) . They are used against the edge of a surface plate or a slot to prevent movement or slippage.

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(12)   Toolmaker's Buttons. Toolmaker's buttons (figure 11) are hardened and ground cylindrical pieces of steel, used to locate the centers of holes with extreme accuracy. As many buttons may be used as necessary on the same layout by spacing them the proper distance from each other with gage blocks.

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(13)   Telescoping Gages.

(a) General. Telescoping gages (figure 12 ) are used to gage large holes and to measure inside distances. These gages are equipped with a plunger (1) that can be locked in the measuring position by a knurled screw or locking nut (2) in the end of the handle (3).Maximum measuring capacity is 6 inches. Measurements must be calipered on the gage by a micrometer, as in the case of the small hole gages.They are also used when measurements cannot be taken with a standard micrometer. Telescoping gages are particularly adaptable for roughly bored work and odd sizes and shapes of holes.

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(b) Uses. To use the telescoping gage loosen the knurled locking nut (2) at the end of the handle (3). Compress the plungers, place them into the hole to be measured, release the turning handle screw (2), slightly tilt the telescoping gage, and rock it back and forth slightly, while at the sametime gradually tightening the turning handle screw (2). Remove the gage from the hole. Take measurements only once. Repeated attempts will produce an inaccurate reading. Measure the gage setting with an outside micrometer.

(14) Small Hole Gages.

(a)   General. Small hole gages (figure 13 ) are similar to telescoping gages. They are smaller in size, adjustable, having a rounded measuring member. A knurled screw in the end of the handle is turned to expand the ball­shaped end in small holes and recesses. A micrometer is used to measure the ball end. Maximum measuring capacity is 1/2 inch. The set of four or more gages is used to check the dimensions of small holes, slots, grooves and so forth from approximately 1/8 to 1/2 inch in diameter.

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(b)   Uses.The small hole gages perform the same function as the telescoping gages, except that they are used to transfer measurements in smaller work. To use the small hole gages (figure 13, view B) fit the ball­shaped point (1) into the hole or slot (2). Expand the ball­shaped end by turning the screw (3) at the end of the handle. Use the same procedures in taking measurements of the hole as explained in (13) (b) above for the telescoping gages.After the measurements have been made, use an outside micrometer to gage the measurement.

(15) Snap Gages.

(a)  General. The plain snap gage is made in two general types, the nonadjustable and the adjustable.

(b)  Nonadjustable Snap Gage.The nonadjustable type (figure 14) is of a solid construction, having two gaging members, GO (1) and NO GO (2) as shown in figure 14. The part to be inspected is first tried on the GO side and then the gage is reversed and the part is tried on the NO GO side. Some solid snap gages (3) have combined gaging members in the same set of jaws, known as a progressive snap gage. The outer member (4) gages the GO dimension and the inner member (5) the NO GO dimension.

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(c) Adjustable Snap Gages.

  1. Three standard designs of the adjustable type of snap gage are available (figure 14, view B, ), consisting of a light, rigid frame with adjustable gaging pins, buttons, or anvils. These pins or buttons may be securely locked in place after adjustment. The locking screws are tightened to hold the gaging dimensions.
  2. One type of adjustable snap gage is made in sizes that range from 1/2 to 12 inches (1). This gage is equipped with four gaging pins and is suitable for checking the dimensions between surfaces. Another type is made in sizes that range from 1/2 to 11 1/4 inches (2) . This gage is equipped with four gaging buttons and is suitable for checking flat or cylindrical work.
  3. The third type is made in sizes from 1/2 to 11 5/8 inches (3).This type is equipped with two gaging buttons and a single block anvil, and is especially suitable for checking the diameters of shafts, pins, studs, and hubs.

(d) Using an Adjustable Snap Gage.Before the snap gage is used to check parts, the GO and NO GO buttons, pins, or anvils must first be set to the proper dimensions (figure 15, views A through D,  indicate the steps used for making the proper settings).

  1. To make the proper settings, the snap gage should be clamped in a vise (soft jaws) or a holder (figure 15, view A). Adjust the GO dimension first or, if desired, reverse the procedure and adjust the NO GO dimension first.
  2. After determining the correct dimension, the gage should be set.Select a master disk, a precision gage block, or a master plug of the correct size. Loosen the locking screw (2) (figure 15, view B) , and turn the adjusting screw (3) until the dimension (4) is set.
  3. Take the gage block selected for the NO GO dimension and check it against the setting (5) (figure 15, view C).If the NO GO dimensions are incorrect, place the gage block in place and turn the other adjusting screw (3) until the NO GO dimension (5) is set.
  4. After adjusting the gage for proper dimensions with the master precision piece (6) in place (view C), tighten the locking screws (2) (view D) . Recheck to make sure that the dimensions have not changed before the gage is used to check the workpiece.

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(e) Gaging Flat Parts. Gaging flat parts with the snap gage is illustrated in figure 16, views A through D. Inspection of machined components or parts is vital when they are being matched or assembled with other parts to form a completed unit. Therefore, the inspector must be proficient in the use of gages to be able to accept or reject parts being tested by the GO or NO GO standards.

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  1. To gage flat parts, position the gage so that the pins or buttons (1) (view A) are square with the flat surfaces on the part (2).
  2. Take the work to be measured and place it at the front of the first pin or button. Using a slight hand pressure, push the gage (3) (view B) over the part.
  3. If the part is within limits, the NO GO pins will stop the part (view C).However, if the part is undersize, it will be possible to push the part past the NO GO pins (view D).

(f) Gaging Cylindrical Parts. Figure 17, views A through D, will be used in illustrating gaging cylindrical parts.

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  1. To gage cylindrical parts, locate the gage on the part with the solid anvil (1) on top (view A). Rock the gage (2) as indicated by the shaded segment in figure 17, view A, where the GO dimension is checked.
  2. If the shaft is not oversize, the first button (3) (view B) on the gage will pass over it easily.
  3. Move the gage to the position shown in view C.If the NO GO button (4) stops the gage, the shaft is within limits. However, if the gage can be rocked further, as shown in view D, then the part diameter is too small, since it has passed over the NO GO button. This is known as a reject.