Aircraft Maintenance Measuring Tools PDF

Torque Mechanics Definition of Torque Torque is the turning effect or turning force and is calculated by multiplying the force by the distance from the force to the centre of the nut. It is measured in pound-inches, pound-feet, kilogram-metres, kilogram-centimetres or newton-metres (lb-in., lb-ft, k...

Torque Mechanics Definition of Torque Torque is the turning effect or turning force and is calculated by multiplying the force by the distance from the force to the centre of the nut. It is measured in pound-inches, pound-feet, kilogram-metres, kilogram-centimetres or newton-metres (lb-in., lb-ft, kg-m, kg-cm or N-m respectively). Torque is the force applied multiplied by the lever length. © Aviation Australia Application of torque loading Torque formula: T orque = F orce × Lever Length In aircraft maintenance, you will need awareness of both metric and imperial units and how to convert them. For instance, Airbus is a European company, and typically operates using metric units (kg-m, kg-cm or N-m). Whereas Boeing is an American company and operates using imperial units (lbin, lb-ft). 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 231 of 498 © Aviation Australia Torque units Torque Loading There is a direct relationship between the value of torque applied to a nut and the tensile stress induced in the shaft of a bolt or stud. The term used to refer to this type of tensile stress is preload. The purpose of applying a measured torque loading to a bolted joint is to induce an acceptable value of preload in the bolt. For any bolted joint to develop maximum strength, the fastener must have sufficient torque applied to it to apply a preload to the bolt of a value greater than it would be subjected to by dynamic forces during operation. For example, if an in-service tensile load of 4000 lb will be applied to a bolt, it should be torqued to preload the bolt to a value greater than 4000 lb. This prevents relative movement (fretting) between the mating surfaces that would lead to fatigue and cause the bolt to eventually come loose when the service load is removed. However, it is also important to avoid applying such a high value of torque to a threaded fastener that it is over-stressed and fails. The majority of bolts, nuts and unions on aircraft are subject to a standard torque loading specified in tables in the Aircraft Maintenance Manual (AMM) ATA Chapter 20 Standard Practices – Airframe. Special bolts are subjected to torque loadings which are specified in the applicable chapter of the AMM. Torque load values are normally determined by friction, type of thread, material, and friction between male and female threads. Factors affecting friction include class of fit, lubrication and finish. Under-tightening screw fasteners may result in lack of rigidity between the separate parts of the assembly, causing fretting corrosion – and early failure of a component may occur through fatigue or mechanical breakdown. Conversely, over-tightening is likely to cause immediate failure of the bolts or distortion of one or more parts of the assembly, leading to eventual failure, stress corrosion or cracking. Standard torque-loading values are those generally applied to steel fasteners used in tension applications on aircraft. Lower values are generally quoted for shear nuts, or nuts in shear applications. Lower torque values are also necessary for pipe union nuts. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 232 of 498 A torque wrench is normally used to torque load a fastener. PLI (preload indicating) washers may be specified in some critical bolted joints. Aviation Australia A bolt is subject to it's metallic stress strain which is why correct torquing is important © Aviation Australia Over-torqued bolts 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 233 of 498 Torque Wrenches The Torque Wrench A torque wrench is a precision measuring tool that measures the amount of torque required to turn a fastener. Under controlled conditions, the amount of force required to turn a fastener is directly related to the internal tensile stress in the fastener. The amount of torque is the product of the applied force and the lever arm. The lever arm is the distance between the centre of the applied force and the centre of the driving adapter. All torque wrenches (except the dial type torque wrench) must be held at the knurled handle in order to operate accurately. Digital torque wrench measuring device Deflecting Beam Type Wrench The flexible beam type uses a beam which deflects under load. The amount of deflection is indicated on a scale which is graduated in units of torque. Aviation Australia Torque Wrench – Deflecting Beam Basic Type 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 234 of 498 Deflecting-Beam Dual Indication Type Wrench The torque wrench is pre-set to the desired torque before it is put on a fastener. When the preset torque value is reached, a spring loaded pin (trigger) is released. On release, the pin provides an audible “click” as well as a visual indication. This provides a dual signal. No direct visual access to a scale is needed during torqueing. Once the pin releases - all force should be removed. Deflecting-beam Dual Indication type torque wrench Break-Over Type Wrench The break-over type contains a spring-loaded ratchet device which may be preset before use. When this preset torque is used, the wrench ‘breaks’ to prevent further tightening of the fastener. Alternative terms for this torque wrench are the toggle type and the click-type. Aviation Australia Torque wrench – break-over type For smaller sizes of nuts and bolts, screwdriver type torque wrenches are available which break when the set torque is reached. It is important not to continue tightening the nut or bolt beyond the break point; otherwise, the fastener will be over-tightened. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 235 of 498 Torsion Bar Torque Wrench The torsion bar (dial) type torque wrench uses the ‘torsion beam’ principle. The dial mechanism is sensitive to the twist of the square drive as torque is applied. The display is very accurate and, unlike other torque wrench types, hand position is not critical to accuracy. Aviation Australia Torsion bar torque wrench 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 236 of 498 Torque Wrench Use Calibration of Torque Wrenches A torque wrench is precision tooling used for measuring purposes in aircraft maintenance. As such, it is subject to calibration. A torque wrench must be regularly inspected, tested and calibrated by a facility equipped to do so. The repeat frequency for recalibration is determined by a number of factors. However, 6 months’ validity is typical. The equipment must carry evidence of the facility that certified the equipment and the date it is due for recalibration/inspection. A torque wrench that reaches its calibration due date (as marked on the calibration label) must be considered unserviceable. It must be immediately segregated and then returned to the standards facility responsible for inspection and recalibration. Before using a torque wrench, you should always check it for: Damage Calibration date label attached Calibration date current. Aviation Australia Calibration tag 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 237 of 498 Torque Loading Techniques Torque-loading instructions vary slightly between aircraft types. Manufacturers typically require torque loading to be carried out using ‘dry torque values’, i.e. parts clean and dry or pre-lubricated during manufacture, but no lubricant or anti-seize compounds applied to the threads by maintenance engineers. If additional lubrication is required, it is specified by the AMM. It is important that torque is applied in accordance with AMM instructions. The preload applied to a fastener with inappropriately applied additional lubricant is considerably greater than when the same torque is applied dry. Wherever possible, the torque should be applied to the nut rather than the bolt head as this is a direct measurement of the torque applied to the threads and not a measurement of hole tightness. When it is necessary to tighten a fastener from the bolt or screw side, the torque value must approach the high side of the specified range. Turn the torque wrench with a slow and steady movement during torquing procedures. To remove the roughness from threads and mating surfaces when assembling new components which require high torque loadings: 1. Clean and, where specified, lubricate the threads and mating surfaces of nut, bolt and washer. 2. Tighten the nut to half the specified torque value. 3. Slacken the nut, then finally re-tighten to the specified torque value. When the torque-loaded fastener is secured by a split pin or lockwire, tighten first to the low side of the torque range. If necessary, tighten the fastener so that the next slot aligns with the hole, ensuring that the maximum torque is not exceeded. If the maximum torque is reached and the slot in the nut does not line up with the hole in the bolt, the nut and washer must be changed. It may be necessary to check the frictional torque of stiffnuts before reuse, using a torque wrench. Note that the torque values specified in the AMM are above running torque for self-locking fasteners. Over-Torqued Fasteners Over-torqued fasteners must not be backed off and re-torqued to the correct value. Fasteners which have been tightened beyond the maximum specified torque value must be removed, rendered unserviceable and scrapped. In the case of over-torqued bolt−nut or screw−nut fasteners, both bolt or screw and nut must be discarded. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 238 of 498 Re-Torquing Where it is necessary to re-torque a fastener assembly, the nut must be backed off one full turn and re-tightened to the specified value. The bolt or screw must not be allowed to turn throughout the operation unless the installation requires the torque to be applied to the bolt or screw in the first instance. Torque Load Tables Tables of standard torque values for different thread types and sizes, and for special applications, are normally found in the AMM. Separate tables are often included for ordinary nuts, stiffnuts, union nuts and studs. The AMM may require the frictional torque (roll-on or running torque) of a stiffnut to be added to the specified torque value for the type and size of thread (torque added roll-on, or TAR). The partial table on the right shows typical values for various thread sizes and was taken from an AMM. Note that torque loadings are specified in pound-inches. Aviation Australia Nut tightening torque 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 239 of 498 Progressive or Step Torquing When installation of a part involves tightening a group of more than three fasteners, the AMM often requires that torque tightening be applied in steps. For example, a final torque of 500 lb-in. (57 N-m) may be specified. However, a first step of 300 lb-in. (34 N-m) may be followed by an intermediate step of 400 lb-in. (45 N-m), and the final torque of 500 lb-in. (57 N-m) is then applied. This prevents the component from being unevenly loaded. Pattern Torquing Pattern torquing is required for complex shapes or profiles to prevent distortion or uneven loading. A typical example of where this technique is required is in an automotive cylinder head. A combination of progressive and pattern torquing may be required to prevent component distortion and allow for proper sealing of gaskets. Aviation Australia Crankcase tightening sequence 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 240 of 498 Torque Multipliers As there is a limit to the physical size of a torque wrench for handling purposes, a device known as a torque multiplier is used. These can multiply the applied torque in ratios of 3:1 up to 11:1. They are used for special applications such as engine thrust nuts, propeller retaining bolts and helicopter rotor hub nuts. Torque multiplier Aviation Australia Torque multiplier - sectional view 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 241 of 498 Torque Wrench Extensions If a torque wrench is used in conjunction with a socket type spanner, the nut and torque wrench tang will coincide and the torque applied to the nut may be read directly from the wrench scale. However, in some cases, an extension spanner may have to be used in conjunction with a torque wrench, and the torque applied to the nut will be different from the torque shown on the torque wrench scale. Aviation Australia Torque wrench extensions various adapters The illustration below shows a typical beam type torque wrench which has an extension spanner attached. If this combination is used to torque load a fastener, then the following formula should be used to calculate the wrench scale reading which corresponds to the specified torque value: L Scale reading = Specif ied torque × L + X Where L = distance between the driving tang and the centre of the handle. X = length of extension spanner between centres. Aviation Australia Torque wrench with extension adapter 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 242 of 498 Example A simple way to calculate the scale reading required is set out in the following example. A specified torque loading is required to be 300 lb-in. and the lengths of the wrench and extension spanner are 10 in and 5 in. respectively. Without an extension, the required force to be applied to the handle of a 10 in spanner to torque the nut to 300 lb-in would be: 300 lb ⋅ in = 30 lb 10 in The total torque required on a nut divided by the length of the tool (distance between the nut and the handle) gives the force required on the handle to produce said torque. Adding an extension (as shown above) therefore reduces the force required on the handle. The formula is 300 lb-in. divided by the new distance between nut and wrench handle, which is: 300 lb. in. = 10 in. + 20 lb 5in. However, the scale reading when the force is on the (now 15 inch) handle is still based on the 10 in length of the tool. Therefore, the torque reading on the tool at 20 lb will show 20 lb × 10 in. = 200 lbin, but you are actually applying 300 lb-in. If you add an extension to a scaled tool, you must perform the conversion and torque according to the new length. In the previous example, with a break type wrench, the adjustment would be preset at 200 lb-in. Note: For the purpose of conversion, 1 lb-in. = 115 kg-cm or 0.113 N-m. When using an extension spanner with a torque wrench, the spanner and wrench should be as nearly as possible in line. If it is necessary to diverge by more than 15° from a straight line (due, for example, to intervening structure), then the direct distance (D) between the nut and wrench handle must be substituted for L + X in the formula for calculating wrench scale reading, and the scale reading in this instance is equal to specified torque X. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 243 of 498 Aviation Australia Torque wrench with extension spanner (new length is from handle to tip in a straight line) Whenever a torque wrench is used, confirm that the specified torque and the wrench scale are in the same units; if not, then the specified torque should be converted by calculation to the units shown on the wrench scale and any measurements should be taken in the appropriate units. When applying torque, the wrench handle should be lightly gripped and force applied smoothly at 90° to the axis of the wrench. Torquing Considerations Torque wrench operation within the first quarter of its scale is insufficiently precise for aviation use. Consequently, the torque wrench selected for a particular use should have a range such that the specified torque falls within the upper range of the scale. Fasteners which may have been tightened beyond the maximum specified torque value must be removed and scrapped. Where it is necessary to re-torque a fastener assembly, the nut must be backed off part of a turn and re-tightened to the specified value. Whenever a torque wrench is used, it must be confirmed that the specified torque and the wrench are in the same units. If they are not, then the specified torque should be converted, by calculation, to the units shown on the wrench scale. Any measurements taken must be taken in appropriate units. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 244 of 498 Useful Conversions lb-ft x 1.35 = N-m N-m x 0.74 = lb-ft lb-in. x 0.113 = N-m N-m x 8.849 = lb-in. lb-in. x 1.15 = kg-cm kg-cm x 0.87 = Ib-in. Checking a Torque Wrench Prior to Use Beam type torque wrenches should be checked before use to ensure that the scale reading is 0. Immediately prior to using a calibrated torque wrench for an aircraft maintenance task, it is best practice to use a torque setting rig (illustrated below) to check its function and accuracy at the torque value required by that task. If a torque setting rig is not available, the torque wrench may be checked for accuracy with the aid of a spring balance. The spring balance must be attached at the centre point of the wrench handle and the force applied tangentially to the arc of movement. The wrench scale reading should correspond to the spring balance reading multiplied by the wrench length. Aviation Australia Torque setting rig 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 245 of 498 Other Methods of Obtaining a Desired Preload The preload applied by torque loading is less than the required value if threads are dirty or in poor condition, and greater than the required value if excessive lubrication has been applied. For certain applications, an aircraft manufacturer may choose to specify other methods to apply a required value of preload to a threaded fastener. These methods can be more consistently accurate than torque loading because they are unaffected by variations in thread friction between the nut and the bolt. These methods include: Nut rotation method Bolt stretch method Preload indicating washers. Nut Rotation Method Some manufacturers specify that the nut should be tightened until it is snug and then turned a further 120o to 240o (1/3 to 2/3 turn). Bolt Stretch Method In the bolt-stretch method, as is used with some connecting rod bolts, the length of the bolt is measured before torque is applied and the nut is tightened until the bolt is stretched a specified amount. In this case, the length of the bolt is measured with a micrometer or a vernier caliper. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 246 of 498 Preload Indicating Washers For certain critical bolted joints, the manufacturer may specify the use of preload indicating (PLI) washers to ensure greater precision in setting preload than torque loading can provide. PLI washers consist of concentric inner and outer rings and two high-strength steel washers as shown. The outer ring is thinner than the inner ring and has a series of radial holes drilled through it. A stiff wire tool is inserted in holes in the outer ring and used to check whether the ring is free to rotate. As the nut is tightened, the inner ring is compressed until, at a predetermined preload, the outer ring is nipped between the washers. At this point, the outer ring can no longer be rotated and tightening is complete. PLI washers are unaffected by thread or nut friction or by lubrication and provide a means of preloading a bolt which is more consistent than torque loading. The preload applied to the particular size of bolt can be varied to suit its application by changes in the material or dimensions of the inner ring. However, since the inner ring is compressed during tightening, it can only be used once, and if slackened, must be replaced. Due to the method of tightening, PLI washers can only be used with self-locking nuts. Aviation Australia PLI washers 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 247 of 498 Calibration Requirements Aircraft maintenance requires precision measurements to ensure that the fit and relationship of parts are correct. Accurate measurement is essential. For example, the correct functioning and reliability of an engine depends on installation of parts which meet specified dimensional tolerances. In addition, in-service limits need to be observed to ensure that wear limits are not exceeded. In your job, you will be required to confidently and competently take measurements using the various types of precision measuring tools. A technician’s skill and awareness must include the care and checking of precision measuring instruments. Prior to any use of a precision measuring instrument, ensure you check accuracy by zeroing. An instrument that does not zero correctly may need calibrating. Calibration sticker 1. Using these tools and equipment requires that the organisation has a procedure to inspect/service and, where appropriate, calibrate such items on a regular basis and indicate to users that the item is within any inspection, service or calibration time limit. A clear system of labelling all tooling, equipment and test equipment is therefore necessary, giving information on when the next inspection, service or calibration is due and if the item is unserviceable for any other reason that may not be obvious. A register should be maintained for all precision tooling and equipment, together with a record of calibrations and standards used. 2. Inspection, service or calibration on a regular basis should be in accordance with the equipment manufacturer’s instructions, except where organisations can show by results that a different time period is appropriate in a particular case. 3. To re-calibrate, a precisely sized piece of metal called a standard or gauge block is inserted between the anvil and the spindle. The spindle is then closed on the block to create a pressed fit. A special tool is slipped into a hole in the micrometer sleeve and carefully rotated until the longitudinal line aligns exactly with the zero mark on the thimble. Note: Temperature variation can affect the calibration process. In aircraft maintenance, many jobs require precise measuring for close tolerance fits. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 248 of 498 Measuring Tools Common Measuring Tools Some of the more common measuring devices include: Protractors Feeler gauges Vernier calipers Micrometers Dial indicators Go/no-go gauges Bore and depth gauges. Measuring tools are often used in conjunction with mark-out or lay-out tools. Feeler gauge 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 249 of 498 Micrometers 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 250 of 498 Mark-Out Tools Dimension mark-out on metal parts, regardless of the accuracy, is typically accomplished by using mark-out dye and a marking tool. Common mark-out tools include: Scribers Dividers Vernier height gauge. A vernier height gauge combines a measuring tool with a mark-out tool. Common mark-out tools 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 251 of 498 Straight Edge A straight edge is a tool with a precision straight edge used for drawing or cutting straight lines or checking the straightness of work. It is similar to a rule but usually much thicker. Straight edges have precision ground edges, one square edge and the other bevelled. They can be graduated or not graduated, in lengths from 300 to 1200 mm. Straight edge and feeler gauges 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 252 of 498 Engineer’s Square An engineer’s square is a precision instrument with hardened steel blades. It is used for: Marking out lines at right angles to an edge Checking the flatness of the surface Squaring up work Checking the accuracy of surfaces at 90° to each other. It can also be used as a guide for your scriber when marking out lines. Precision engineer’s squares must be handled with care - do not use them for any other purpose. Engineer’s square 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 253 of 498 Bevel Protractor Vernier protractors have a vernier scale to enable them to be set to a much greater accuracy. Generally this is 5 min (1/12°). The main scale is graduated into 360°, reading from 0°–90°–0°–90°–0°. The vernier scale is made to cover an arc of 23° of the main scale. It is then divided into 12 equal parts. The length or angle of each division is therefore 1/12 of the total arc of 23°. The illustrations below show examples of a vernier protractor. Vernier protractor 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 254 of 498 Inclinometer An inclinometer or clinometer is an instrument for measuring angles of slopes (or tilt). It measures the inclination of an object with respect to gravity. Inclinometers are often used to measure flight control surface angles during rigging operations. Inclinometer 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 255 of 498 Feeler Gauge Feeler or thickness gauges are used for measuring small spaces. They measure clearances that are necessary when fitting one part to another or when making adjustments. They are also useful when checking wear between parts. Feeler gauges consist of a set of hardened steel or brass blades or leaves of various thicknesses. Each blade is commonly etched with its size in decimal imperial, metric or both measurements. Feeler gauge Feeler gauges may be used singly or in combination to achieve a desired thickness. The pivot often includes a locking nut to lock the blades in the desired position. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 256 of 498 Micrometers The Micrometer When a part has to be measured to three decimal places in inches, or two decimal places in metric, an accurate method of measurement is required, often a micrometer. The various types of micrometers all work on the same principle and can be classified as either external (outside), internal or depth micrometers, depending on the type of measurement they have been designed for. Micrometers are available to take accurate imperial or metric measurements and to read to 0.0001 in. or 0.002 mm by the addition of a vernier scale on the barrel. The most common imperial outside micrometer measures external dimensions from 0 to 1 in. The equivalent micrometer with a metric scale measures from 0 to 25 mm. A standard range of micrometers is available in steps of 1 in. (25 mm) up to 8 in. (200 mm). For specific applications, specialist micrometers have been made to measure distances up to 6 ft. Outside micrometers (handheld) 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 257 of 498 Micrometer - depth Micrometer - 6 feet 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 258 of 498 Micrometer Construction These are the main parts of the external micrometer: Frame – Roughly semicircular in shape, holding anvil, barrel and lock nut Anvil – Made of hardened steel, forms the fixed datum Spindle – Made of hardened steel with a ground thread of 1/40 in. pitch (imperial) or 0.5 mm pitch (metric); front face of the spindle forms the moving datum Locking Ring – Enables the spindle to be locked in position Barrel or Sleeve – Graduated in major divisions of measurement Thimble – Graduated in minor divisions of measurement Ratchet – Torque limiting device; ensures consistent pressure on the work and protects the mechanism from overload. It should be noted that some micrometers are fitted with an alternative spindle lock as shown here. Micrometer construction 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 259 of 498 Micrometer construction lock 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 260 of 498 Using a Micrometer A micrometer is a very accurately ground screw-thread rotating in a fixed nut, thus opening or closing the distance between the two measuring surfaces on the ends of the anvil and spindle. A piece of work is measured by placing it between the anvil and spindle faces and rotating the spindle using the thimble until both the anvil and spindle faces nearly contact the work. The spindle is then rotated by the ratchet torque-limiting device until the measuring faces make gentle contact with the work. The user must hold the measuring faces square to the work when measuring. A micrometer is not accurate if the thimble is over-torqued or under-torqued into contact with the work. A torque limiting ratchet or spring clutch is fitted to most thimbles to ensure a light and consistent pressure for each reading. This maintains accuracy and consistency (repeatability) between readings and enables different users to obtain identical readings for any given dimension. Micrometer manufacturers typically recommend three clicks of the ratchet before taking the reading. The ratchet also protects the micrometer mechanism from overload, which prolongs its life and maintains its accuracy with repeated use. However, an experienced user may choose to rotate the thimble directly from its knurled portion until the jaws contact the work, as shown below. A skilled user develops a light and consistent touch (feel) which applies a torque value to the thimble that is equivalent to the gentle torque of the ratchet. A 1-in. or 25-mm outside micrometer should be held in the right hand as shown below; the little finger is placed in the U-shaped frame. The forefinger and thumb are placed near the thimble to rotate it while the middle finger and ring finger support the micrometer. This enables the thimble or ratchet to be rotated while you maintain a secure hold on the instrument. The left hand is free to hold the work piece if required. A knurled clamping ring or lock lever is usually fitted on the frame to enable the spindle to be locked at the measured dimension. This allows the instrument to be removed from inaccessible areas for reading. The desired work dimension is then found from the micrometer reading, indicated by the graduations on the sleeve and thimble as described on the following pages. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 261 of 498 Using a micrometer 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 262 of 498 Checking and Adjusting the Micrometer Always check the zero reading of the micrometer before use. For a 0–1 in. or 0–25 mm, screw down the spindle to meet the anvil. When the ratchet slips, the reading should be 0. For larger micrometers, e.g. 1–2 in. or 25–50 mm, test pieces are provided of various sizes to place between the spindle and anvil. For an internal micrometer, check with a gauge or caliper of known accuracy. For a depth micrometer, using the 0–1 in. or 0–25 mm rod, unscrew the thimble until the rod end is flush with the base and check that the zeros align. Adjustment of the Micrometer Adjustment to obtain zero reading can be made: At the barrel by using the C-spanner provided and rotating the barrel until a zero reading is obtained, or sometimes at the anvil For thread wear and slack spindle, by screwing in the split friction nut with the special spanner provided with the micrometer. Note: When used in the aviation industry, adjustment may only be carries out by the standards facility responsible for the calibration and re-certification of the micrometer. Instruments bearing a calibration sticker must not be tampered with and must be returned to the standards facility if found to be out of calibration. Instruments without a calibration sticker must not be used. Precautions for Using and Caring for a Micrometer Following these precautions enables measurements that are accurate and able to be repeated by another user. It also protects the instrument mechanism from corrosion, rapid wear and distortion. A micrometer is a precision instrument. Treat it with care at all times. Keep the micrometer lightly oiled. Do not allow the micrometer to come into contact with harmful materials, e.g. filings. Do not drop the micrometer or allow it to come into contact with tools. The spindle, anvil and work must be wiped clean before measurement. Check the date on the calibration sticker. Reject the micrometer if it is out of calibration or if the sticker is missing. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 263 of 498 Do not tamper with any adjustments on the instrument. Check for a zero reading. Reject the micrometer if the reading is not zero. Continue to check for accuracy frequently during use. Following the zero check, the jaws must be opened up to a dimension larger than that of the work. To achieve this, rotate the thimble anti-clockwise (looking at end of thimble) using its knurled portion. Do not grip the thimble and use the off-centre mass of the frame to twirl the body around. This causes rapid thread wear. Either use the ratchet torque limiter to bring the measuring faces into contact with the work, or use a light but consistent touch (feel) to directly turn the thimble. Do not use force. Hold the measuring faces square to the work when measuring. Do not attempt to measure parts or components that are rotating. Be certain that the work is stationary before applying the micrometer. Do not set the micrometer to a size and then try to push it over the work, i.e. it must not be used as a go/no-go gauge. Store the micrometer safely in its box with the spindle open after use. Note that the jaws should not be touching during storage. Micrometer storage 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 264 of 498 Standard English Micrometer The spindle of the English micrometer has an external thread of 40 TPI, thus the pitch is 1/40 or 0.025 in. Therefore, one complete revolution of the thimble advances the spindle face towards or away from the anvil face precisely 1/40 or 0.025 in. The longitudinal line on the sleeve is divided into 40 equal parts by vertical lines that correspond to the number of threads on the spindle. Therefore, each vertical line designates 1/40 or 0.025 in. and every fourth line, which is longer than the others, designates 1/10 or 0.100 in. Standard English micrometer - scales For example, the line marked 1 represents 0.100" (0.025 × 4), line 2 represents 0.200 in. (0.025 × 8) and line 3 represents 0.300" (0.025 × 12), etc. The bevelled edge of the thimble is divided into 25 equal parts. As one rotation equals 0.025 in., each division represents 0.001 in. Rotating the thimble from one of these lines to the next moves the spindle longitudinally 1/25 of 0.025, or 0.001 in.; rotating two divisions represents 0.002 in.; etc. Twenty-five divisions indicate a complete revolution of 0.025 or 1/40 in. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 265 of 498 Reading the micrometer to one thousandth of an inch (0.001 in.): Read the whole number of major divisions (1/10 or 0.100 in., shown on sleeve): 0.200. Read the number of minor divisions after the whole number (each minor division is equal to 0.025 in.): 0.075. Identify the line on the thimble coinciding with the datum line. This indicates thousandths of an inch: 0.011. Add the measurements to arrive at a total reading: 0.286 in. Excluding micrometers designed for special purposes, the measuring scale is limited to 1 in., and consequently a range of sizes is necessary. The standard range contains sizes 0-1 in. to 7-8 in. in steps of 1 in. When using micrometers other than the 0-1 in., the gap in the frame, e.g. 6 in. in the 6-7 in. micrometer, must be added to the reading on the scale. Reading a standard English micrometer 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 266 of 498 English Vernier Micrometer The following sections describe the process of reading various micrometers. A measurement on a standard English micrometer For additional accuracy, some micrometers are produced with a vernier scale on the sleeve, enabling measurements to be made to an accuracy of tenths of a thousandth of an inch (0.0001 in.). 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 267 of 498 Reading the Vernier Micrometer Carry out the initial readings as with a standard micrometer. When the datum line of the sleeve falls between the graduations on the thimble, the unknown amount is read by using the vernier scale on the sleeve. Note which line on the vernier scale coincides with a line on the thimble, then read the number on the vernier scale and add this to the normal reading. This may be written as a sum: 0.2000 +0.0750 +0.0110 +0.0007 Total 0.2867 in. English vernier micrometer 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 268 of 498 Metric Micrometer The spindle of the metric micrometer has an external screw thread with a pitch of 0.5 mm. Therefore, one complete revolution of the thimble advances the spindle face towards or away from the anvil face by precisely 0.5 mm. The longitudinal line on the sleeve is graduated in millimetres, each subdivided in 0.5 mm. Therefore it requires two revolutions of the thimble to advance the spindle a distance equal to 1 mm. The bevelled edge of the thimble is divided into 50 divisions, every fifth line being numbered. As one rotation of the thimble advances the spindle 0.5 mm, each division on the thimble represents 1/50 of 0.5 mm, or 0.01mm; two graduations equal 0.02 mm; etc. Reading a standard metric micrometer Reading in hundredths of a millimetre (0.01 mm): The whole number of major divisions (1 mm shown on sleeve): 10.00 The number of minor divisions after the whole number (each minor division is equal to 0.5 mm): 0.50 The line on the thimble coinciding with the datum line, which indicates hundredths of a millimetre: 0.33 Total reading: 10.83 mm. Excluding micrometers designed for special purposes, the measuring scale is limited to 25 mm, and consequently a range of sizes is necessary. Metric micrometers range upwards in steps of 25 mm (0–25 mm, 25–50 mm, etc.). 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 269 of 498 When using micrometers other than 0–25 mm, the gap in the frame, e.g. 50 mm on a 50–75 mm micrometer, must be added to the reading on the scale. Metric Vernier Micrometer For additional accuracy, some micrometers are produced with a vernier scale on the sleeve. Using this enables measurements to be made to an accuracy of two thousandths of a millimetre (0.002 mm). Carry out the initial readings as with a standard micrometer. When the datum line of the sleeve falls between the graduations on the thimble, the unknown amount is read by using the vernier scale on the sleeve, which measures two thousandths of a millimetre (0.002 mm). Note which line on the vernier scale coincides with a line on the thimble, then read the number on the vernier scale and add this to the normal reading. This may be written as a sum: 17.000 +0.500 +0.270 +0.004 Total 17.774 mm Reading the metric Vernier micrometer scale 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 270 of 498 Electronic Micrometers Micrometers are now available that give a digital read-out of measurements. They can convert imperial measurements into metric and vice versa at the press of a button. Micrometer digital display 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 271 of 498 Internal Micrometer This micrometer, used for measuring internal dimensions, is similar in principle to the external type. It consists of a measuring head similar to the barrel and thimble of outside micrometers, with extension rods or a spindle fitted at either end. Thus, by using different lengths of rod, the measuring range can be varied. Internal Micrometer (Imperial) The internal micrometer set comprises a 2-in. micrometer head, extension rods of varying length in 1in. increments and a 1/2 in. collar. The sleeve scale has a range of only 1/2 in. but, by fitting the 1/2 in. collar to the extension rods, readings up to 1 in. can be made, e.g. with a 4–5 in. rod, without the collar a range of 4–4.5 in. can be measured; with the collar, 4.5–5 in. can be measured. Internal micrometer - component parts 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 272 of 498 Operation The micrometer is assembled with the appropriate extension rod and, if necessary, the collar is seated against the micrometer head and locked in place by the locking screw. A handle may be attached if required. The instrument is then inserted between the faces to be measured and the thimble unscrewed until each end of the instrument is contacting a face; the instrument must be perpendicular to each face to obtain an accurate measurement. The instrument is removed without disturbing the setting, and a reading is obtained in the same way as with an external micrometer, with the length of the collar and extension rod, if fitted, added. Note: The marked length on the extension rod includes the minimum length (2 in.) of the micrometer head. To check for accuracy, set to a given measurement and measure the internal micrometer with an external micrometer or vernier caliper of known accuracy. Internal micrometer in use Caliper type internal micrometers 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 273 of 498 Depth Micrometer The micrometer depth gauge, as the name implies, was designed to measure the depth of holes, slots, recesses, keyways, etc. The principle is the same as for other micrometers. The essential differences with this instrument are that the scale is read from right to left on the barrel and the thimble divisions are graduated in the opposite direction to an ordinary micrometer. In practice, the reading is taken from the graduations on the barrel, which are covered. Micrometer - depth extension rods The instrument consists of a hardened, ground and lapped base combined with a micrometer head. Measuring rods of differing length are inserted through a hole in the micrometer screw and brought to a positive seat by a knurled nut. The screw is precision ground and has a 1-in. movement. The rods are furnished to measure in increments of 1 in. Each rod protrudes through the base and moves as the thimble is rotated. When micrometers have extension pieces or rods, they are made as a matched set and cannot be interchanged with any other micrometer set. Before using the instrument, ensure that the base, end of rod and work are wiped clean. Hold the base firmly against the work, turn the thimble until the rod contacts the bottom of the slot or recess, and read the measurement. Note: Scales may be in either the English or the metric system. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 274 of 498 Depth micrometer scales - example is English scale Depth micrometer in use 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 275 of 498 Specialist Micrometers Specialist micrometers, such as bore micrometers, may use other scaling systems. One of these is shown below. It is capable of reading to 0.0002 in. On some bore micrometers, graduations of the main scale may be found on both sides of the datum line to enable more accurate readings to be taken. On the thimble, each thousandth of an inch is sub-divided into five divisions, giving the ability to measure to 0.0002 in. Bore micrometers can also be found with vernier scales that give them the ability to measure to 0.0001 in. Versions are available which offer two-point measurement, and others have interchangeable anvils or contacts so that a wider range of measurements can be taken. Bore micrometer Bore micrometer scale 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 276 of 498 Specialised micrometers Thread minor diameter-measuring micrometer 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 277 of 498 Thread minor diameter-measuring micrometer - second example Adaptors - thread minor diameter-measuring micrometer 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 278 of 498 Pipe wall thickness micrometer Long-reach micrometer 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 279 of 498 4-in. range micrometer Using a 7-8 in. range micrometer 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 280 of 498 Vernier Slide Caliper The vernier slide caliper is a precision instrument for making accurate linear measurements. It consists of two main parts: A beam incorporating a fixed jaw with a fixed scale engraved on the beam A sliding jaw with a vernier scale which can be moved along the beam. The sliding jaw is fitted with clamping screws to fix it to the beam when the approximate position of reading is reached. An adjusting screw permits the sliding jaw to be finely adjusted to achieve the final measurement. In general, the precautions to prevent damage to micrometers are also applicable to vernier calipers. The principle of the vernier scale is as follows. A line of known length (the main scale) contains a certain number of divisions. A second line of equal length (the vernier scale) is divided into a number of divisions which is one less than those on the first line. When the two lines are compared, each division of the vernier scale represents a proportion of its corresponding division on the main scale. Vernier slide caliper 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 281 of 498 Reading Scales Reading an Imperial Vernier Scale This example of the imperial vernier caliper has 25 spaces on the vernier scale that occupy exactly the same distance as 24 spaces on the bar scale. Each graduation on the bar represents 25 thousandths of an inch (0.025 in.). The line on the vernier scale that is aligned with one of the bar scale marks indicates the number of thousandths of an inch to add to the measurement indicated on the bar before the 0 on the vernier scale. Using the vernier scale of a slide caliper Reading an imperial vernier scale graduated for 25 thousandths of an inch (0.025 in.) 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 282 of 498 Example 1. First note how many whole inch, tenth and fortieth (0.025) lines there are on the fixed scale to the left of the zero line on the vernier scale: 1.325 in. 2. Note on the vernier scale the number of divisions from 0 to where a graduation mark on the vernier scale coincides with one on the main fixed scale. The line of coincidence indicates the number of thousandths to be added to the fixed scale reading: 0.012 in. 3. Add the figures to determine the reading. Therefore, the reading is 1.337 in. Use of a vernier slide caliper - fixed scale 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 283 of 498 Reading a Metric Vernier Scale Fixed Scale In this example, each division on the fixed scale is 1 mm wide and every 10th division is numbered. There are ten 1-mm divisions between each numbered division; therefore, each numbered division equals 10 mm. Vernier Scale In this example, the vernier scale is 49 mm long and is divided into 50 divisions. Each small division of the vernier scale represents 1/50 mm, which is equal to 0.02 mm. There are 10 numbered divisions, with five smaller divisions between each of these. Therefore, one numbered division represents 0.02 × 5 = 0.1 mm. If the vernier scale is moved to the right until the first graduation to the right of 0 on the vernier scale coincides with the first graduation on the fixed scale, the vernier scale has been moved 0.02 mm. If the 12th graduation is made to coincide, the vernier scale has been moved 12 × 0.02 mm = 0.24 mm. This consists of two numbered vernier divisions 0.2 mm Plus two small divisions, 2 × 0.02 0.04 mm Total 0.24 mm Use of a vernier slide caliper scale 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 284 of 498 Reading a metric vernier graduated for fiftieths of a millimetre (0.02 mm). Example 1. First note how many whole centimetres and fifth (0.2) lines there are on the fixed scale to the left of the zero line on the vernier scale: 3.2 mm. 2. Note on the vernier scale the number of divisions from 0 to where a graduation mark on the vernier scale coincides with one on the main fixed scale. The line of coincidence indicates the number of hundredths to be added to the fixed scale reading: 0.16 mm. 3. Add the figures to determine the reading. Therefore, the reading is 3.36 mm. Reading the scale of a vernier slide caliper 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 285 of 498 Vernier Height Gauge The vernier height gauge consists of a graduated frame held in a vertical position to an accurately ground base. The vernier scale is read in the same manner as the vernier tools already discussed. A scribing point is attached to the frame and used in conjunction with a surface plate to accurately scribe lines parallel to an edge. The diagram below shows an example of a vernier height gauge. Vernier height gauge 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 286 of 498 Other Measuring Devices Introduction to Other Measuring Devices While outside micrometers and vernier calipers are probably the most commonly used precision measuring tools, many variations of them are available to suit special measuring tasks. The more specialised types of measuring tools are usually used in component overhaul shops. Dial Indicators Dial indicators are precision instruments used to measure movement between certain engine and airframe parts. These indicators are also used to determine an out-of-round condition on a shaft as well as the plane of rotation of a disk. Dial indicators can be used for checking: Out of round Bow Warp End float Backlash Flatness Run-out. Dial indicator 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 287 of 498 A dial indicator is used to set work true in lathe chucks, parallel between centres, or square or parallel to machine tables. They are also used to register variations in stipulated sizes of machine parts. The dial may be graduated in a clockwise and anti‑clockwise direction from zero to give plus or minus readings, or it may give a continuous reading from zero in a clockwise direction only. The indicator is operated by a spring-loaded plunger which, contacting the work, registers variations in distance, movement and concentricity. Dial indicator with direct access A dial indicator is also used for measuring surfaces that can be accessed directly. Use of a dial indicator with direct access - example 1 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 288 of 498 Use of a dial indicator with direct access - example 2 Dial Test Indicators A Dial Test Indicator (DTI) is a special type of dial indicator. It is primarily used in machine set-ups. The DTI measures displacement at an angle of a lever or plunger perpendicular to the axis of the indicator. A regular dial indicator measures linear displacement along that axis. A DTI is used for measuring surfaces that cannot be accessed directly. Dial Test Indicator - DTI 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 289 of 498 Dial Test Indicator (DTI) in use with indirect access Bore Gauges A bore gauge is used to quickly and accurately check dimensions of holes, such as engine cylinder bores. A bore gauge incorporates a dial indicator with a telescoping gauge. The nominal dimension of the hole is set with the dial indicator zeroed. The bore gauge is placed in the hole and the dial indicator read to show the amount of oversize or undersize. Bore gauges 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 290 of 498 Bore gauge in use 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 291 of 498 Digital Instruments Precision measuring instruments are available with electronic digital read-outs. These are some advantages of digital read-outs: Quick, no reading calculations Eliminates reading errors Can be zeroed at any setting Display can be set to imperial and metric. However, the disadvantage of digital instruments is the readout is battery powered and the batteries will need to be replaced. Digital instruments 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 292 of 498 Go/No-Go Gauges Go/no-go gauges are used to easily and quickly determine whether parts are within specified limits, without the need to adjust the gauge for each reading. These gauges have two sets of jaws or gaps. One set pair is slightly over the required size, and the other pair is slightly under the required size. The gauges can be adjustable or non-adjustable. The high size set is the go size, and the low size set is the no-go size. The difference in dimensions between the high and low sizes represents the acceptable tolerance for a part. As shown in the illustration below, the part within the specified limits will enter the first gap, but will not pass through the smaller gap. A part that will not enter the first gap is oversized. A part that will pass through both gaps is undersized. A typical application for these gauges is checking control cable terminal fittings after swaging. Go/nogo gauges are also particularly useful for repetitive work as they provide a quick and easy way to check large numbers of similar parts for accuracy. Go/no-go gauges 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 293 of 498