Drill Bits and Sizes PDF

Drill Bits and Sizes Drill Bits Drill bits are cutting tools used to remove material to create holes, almost always of circular cross section. Drill bits come in many sizes and shapes and can create different kinds of holes in many different materials. In order to create holes, drill bits are attach...

Drill Bits and Sizes Drill Bits Drill bits are cutting tools used to remove material to create holes, almost always of circular cross section. Drill bits come in many sizes and shapes and can create different kinds of holes in many different materials. In order to create holes, drill bits are attached to a drill, which powers them to cut through the work piece, typically by rotation. The drill grasps the upper end of a bit called the shank in the chuck. Drill bits come in standard sizes. A comprehensive drill bit and tap size chart lists metric- and imperial-sized drill bits alongside the required screw tap sizes. Certain specialised drill bits can create holes with a non-circular cross-section. In the United States, fractional inch and gauge drill bit sizes are in common use. In nearly all other countries, metric drill bit sizes are most common, and all others are anachronisms or are reserved for dealing with designs from the United States. The British standards for replacing gauge size drill bits with metric sizes in the United Kingdom were first published in 1959. Aviation Australia Various drill bits 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 369 of 498 Drill Sizing Metric Sizing Bits at size increments of 1 mm are most commonly available. Sets of bits in 1-mm increments might be found in a market stall, in 0.5-mm increments at any hardware store and in 0.1-mm increments at any engineers' store. Sets are not commonly available in smaller size increments, except for drill bits below 1-mm diameter. Drill bits of the less routinely used sizes, such as 2.55 mm, must be ordered from a specialist drill bit supplier. This sub-set of standard sizes is in contrast to general practice with number gauge drill bits, for which it is rare to find a set on the market which does not contain every gauge. Metric dimensioning is routinely used for drill bits of all types, although the details of BS 328 apply only to twist drill bits. For example, a set of Forstner bits may contain 10-, 15-, 20-, 25- and 30-mm diameter cutters. Examples of common drill sizes including metric and number and letter gauge sizing 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 370 of 498 Number and Letter Gauge Drill Bit Sizes Number drill bit gauge sizes range from size 80 (the smallest) to size 1 (the largest), followed by letter gauge size A (the smallest) to size Z (the largest). Although the ASME B94.11M twist drill standard, for example, lists sizes as small as size 97, sizes smaller than 80 are rarely encountered in practice. An 80 gauge drill bit Number and letter sizes are commonly used for twist drill bits rather than other drill forms, as the range encompasses the sizes for which twist drill bits are most often used. Number and letter gauge drill bits are still in common use in the United States and, to a lesser extent, the United Kingdom, where they have largely been superseded by metric sizes. Other countries which formerly used the number series have for the most part also abandoned these in favour of metric sizes. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 371 of 498 Fractional Sizes ANSI B94.11M-1979 sets size standards for jobber-length straight shank twist drill bits sized from 1/64 in. to 1 in., in 1/64-in. increments. For Morse taper shank drill bits, the standard continues in 1/64-in. increments up to 1 3/4 in., then 1/32-in. increments up to 2 1/4 in., 1/16-in. increments up to 3 in., 1/8-in. increments up to 3 1/4 in., and a single 1/4-in. increment to 3 1/2 in. One aspect of this method of sizing is that the size increment between drill bits becomes larger as bit sizes become smaller: 100% for the step from 1/64 to 1/32, but a much smaller percentage between 1 47/64 and 1 3/4. Drill bit sizes are written down on paper and etched onto bits as irreducible fractions. So, instead of 78/64 in. or 1 14/64 in., the size is noted as 1 7/32 in. The next page contains a chart providing the decimal-fraction equivalents that are most relevant to fractional-inch drill bit sizes (that is, 0 to 1 by 64ths). Decimal places for 0.25, 0.5 and 0.75 are shown to thousandths (0.250, 0.500, 0.750), which is how machinists usually think about them: ‘two-fifty’, ‘five hundred’, ‘seven-fifty’. Decimal fraction equivalents 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 372 of 498 Fractional drill bit sizing Jobber-Length Drill Jobber-length drills are the most common type of drill. The length of the flutes is between nine and 14 times the diameter of the drill, depending on the drill size. So a 1/2-in. (12.7-mm) diameter drill can drill a hole 4 1/2 in. (114.3 mm) deep since it is nine times the diameter in length. A 1/8-in. (3.2-mm) diameter drill can drill a hole 1 5/8 in. (41.3 mm) deep since it is 13 times the diameter in flute length. Jobber drill bit 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 373 of 498 Aircraft Length Drill Extended reach or long series drills are commonly called aircraft length from their original use in manufacturing riveted aluminium aircraft. For bits thicker than a minimum size such as 1/8 in., they are available in fixed lengths such as 6, 8, 12 or 18 in. rather than the progressive lengths of jobber drills. Three 11/32" diameter drills including an aircraft length drill The image shows a long series drill compared to its diametric equivalents. All are 11/32 in. (8.7 mm) in diameter. The equivalent Morse taper drill, shown in the middle, is the usual length for a taper shank drill. The lower drill bit is the jobber or parallel shank equivalent. Centre Drill Bit Sizes Centre drills are available with two different included angles: 60° is the standard for drilling centre holes (for example, for subsequent centre support in the lathe), but 90° is also common and used when locating holes prior to drilling with twist drills. Centre drills are made specifically for drilling lathe centres, but are often incorrectly used as spotting drills. Centre drills-numbers 1 to 6 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 374 of 498 Centre drill bit sizing chart Bolt Tolerance To eliminate friction during fitting, close-tolerance bolts can be placed in a freezer prior to fitment. Since the specs for a close-tolerance bolt are so tight, cad plating is not possible. Standard-tolerance bolts are manufactured to +0.000 to -0.0055 in. of the diameter’s nominal size. Close-tolerance bolts are manufactured to +0.000 to -0.0005 in. of the diameter’s nominal size. Bolt design 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 375 of 498 Bolt holes of primary connecting elements have close tolerances. For general application, it is generally permissible to use the first lettered drill size larger than the nominal bolt diameter except where an AN hex bolt is a light drive fit (reamed). A light drive fit is defined as an interference fit and is approximately 0.0006 in. for a 5/8-in. diameter bolt. Therefore, for a 3/8-in. bolt (0.375 in.), it would be acceptable to have a V letter drill for the hole (0.377 in.) or 0.002 in. clearance. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 376 of 498 Tolerance and Allowance Fits and clearances are manufacturing standards (British Standards Institute) that allow massproduced parts to fit within limits. This is a cost-effective method of production compared to the old system of individual manufacture and ensures parts are interchangeable. If a hole is acceptable between the limits of 1.000 in. (low limit) and 1.0015 in. (high limit), we could say that the allowable manufacturing error of 0.0015 in. could be tolerated. This allowable error is called the tolerance. Nominal Dimension Hole 1.000 in. -0.000 in. +0.0015 in. If a shaft was to fit this hole, it would need to be determined what class of fit would suit the application – clearance fit, for example. The shaft would need to be slightly smaller than the hole, so if the smallest hole is 1.0000 in., then the largest shaft would have to be 0.9985 in. (This would give a clearance of 0.0015 in. at the hole low limit and 0.003 in. at the hole high limit.) It could then be determined how much more clearance would be acceptable (between the two parts) to give a tolerance to the manufacture of the shaft. This shaft could be 0.9975 in. to 0.9985 in. (tolerance of 0.001 in.), which would give a maximum clearance of 0.004 in. to a minimum clearance of 0.0015 in. Nominal Dimension Shaft 0.9980 in. +0.0005 in. -0.0005 in. The allowance represents the tightest permissible fit for proper construction. In this case, the allowance is 0.0015 in. Allowance can be negative (shrink fit). Tolerance and allowances (British standard) 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 377 of 498 Classes of Fit Fits In engineering, the term fit refers to the degree of tightness or looseness between mating parts. The mating of mechanical components is classified into three broad classes: Clearance fits; Interference fits; and Transition fits. Each main class is further sub-divided into several types of fit, and each type is classified by a letter symbolising the shaft that produces the fit using a standard hole. Fit types 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 378 of 498 Clearance Fits Where it is necessary to have movement between parts, the hole must be larger than the male component (shaft) to provide a suitable clearance between them. A clearance fit allows for a calculated amount of clearance between the parts on assembly, as shown below. This provides freedom of movement and space for lubrication. Clearance fit Running Fit Running fits have a clearance between mating surfaces sufficient to allow one item to rotate freely within the other without danger of seizing. Sliding Fit Sliding fits have a smaller clearance than running fits and are designed for use when the mating surfaces have to slide but not rotate. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 379 of 498 Interference Fits In an interference fit, no movement is possible after assembly, the inner part being larger than the part into which it fits. When parts are required to fit tightly together, such as when a shaft is required to fit into a hole in a boss, the outside of the shaft must be slightly larger than the hole. When the shaft is pushed in, the force between the components tends to expand the boss and contract the shaft. If the difference in size, which is called the allowance for the fit, is suitable, the expansion of the boss and the contraction of the shaft are slight. The elasticity of the components maintains the fit. However, if the allowance is too great, one or both of the parts may fracture or the smooth finish of the mating surfaces may be damaged. Interference fit Driving Fit A driving fit is when a shaft is slightly larger than the hole and the allowance is such that the parts can be assembled by driving. Force Fit In a force fit, the parts have to be forced together, usually by presses. The interference is larger than a driving fit. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 380 of 498 Shrink Fit A shrink fit is when the parts being joined are to be held in place permanently. The outer member can be heated or the shaft can be cooled. Interference is greater than force fit. Transition Fits Between the two major classifications of interference fit and clearance fit is an intermediate type, usually called a transition fit. This occurs when the mating parts are almost the same size. Very small differences in size can be obtained between parts that have finely finished and accurate surfaces. Very small changes in tolerance can produce considerable differences in the nature of the fit. This may result in either a clearance fit or an interference fit. The type of fit (clearance/interference) can be determined by selective assembly of pieces on either side of the allowance. The diagram below shows transition fit complete with: Basic dimensions tolerances An interference fit combination A clearance fit combination. © Aviation Australia Transition fit 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 381 of 498 List of Assembly Fits The broad categories of fits can be broken down further to specific allowances for engineering applications. Running Fit The shaft is designed to rotate in the hub. Sliding Fit Parts can be assembled by hand with slight pressure. Push Fit Parts can be assembled with a little more pressure than a sliding fit. Driving Fit Parts can be assembled by cushioned blows with a hammer. Force Fit There is heavy interference between the parts. Use of a hydraulic press is usually necessary for assembly. Shrink Fit To avoid damage to the parts, the outer part is expanded by heating in order to permit easy assembly. When the fit cools, the tightness of fit returns. In extreme cases, the inner part may also be shrunk by severe cooling. Aircraft Engine Assembly The following section describes some of the fits in a turbine engine assembly. Heat/Shrink Fits (Interference Fit) Turbine wheel to turbine shaft assembly Bearings to shafts and gears Sleeves to cases. Heat/shrink fits between the turbine shaft (shrunk with dry ice) and the turbine wheels (heated with heating element) allow assembly of the two components. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 382 of 498 Press Fits (Transition Fit) bushings bearings sleeves. The turbine shaft bearing is normally pressed onto the turbine shaft assembly. The turbine assembly tie bolts are a press type fit which are normally assembled by applying a predetermined torque to the tie bolt nuts to draw the bolt home into the turbine wheel bolt holes. The spacers between the turbine wheels are normally a light press type fit. Clearance Fit Turbine/compressor blades to discs Case split line bolts Piston to cylinder. The turbine blades are a clearance type fit in which the blades are inserted into the turbine wheels by hand and just slide into the fur tree. Fits in a turbine assembly 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 383 of 498 Fit Tolerances and Allowances Tolerances of Size As it is not practicable to produce a large number of similar items of identical size, parts are manufactured to an acceptable size with a specified tolerance range. As long as the tolerance remains within permissible variations, all of the parts will fit a single mating part with a similar type of fit. When this is the case, the parts are said to be interchangeable. Interchangeability of parts is important in mass production because it enables: Reduction in costs Simplicity in fitting and assembly Quick replacement of worn assemblies or single parts of an assembly. These techniques are widely used throughout the automotive, engineering and aerospace industries. To provide for unavoidable imperfections in workmanship or machine accuracy, an acceptable amount of tolerance is usually applied to the dimensions of an object when it is drawn. This is done by indicating an upper and lower limit of size for each dimension. Every dimension on a drawing has a tolerance and may be expressed: By specific limits of size applied directly to the dimension In a general tolerance note referring to those dimensions on a drawing for which tolerances are not otherwise specified. Three types of tolerances can be applied: unilateral, bilateral and limits of size. Their definitions are as follows. Unilateral Tolerances A unilateral tolerance means variation is permitted only in one direction from the specified dimension, e.g. 20.5 + 0.02 or 20.5 - 0.02. Bilateral Tolerances A bilateral tolerance means variation is permitted in both directions from the specified dimension, e.g. 20.50 ± 0.25. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 384 of 498 Limits of Size Limits of size are the extremes of size allowed for a dimension. Two limits are possible: the maximum allowable size and the minimum allowable size. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 385 of 498 Fit Definitions Basic Size Basic size is the size about which the limits of a particular fit are fixed. It is the same for both ‘shaft’ and ‘hole’. It may also be referred to as the nominal size. Upper Limit of Size Upper limit of size (ULS) is the largest size permitted by a toleranced dimension. 20.50 ± 0.25 ULS = 20.75 Lower Limit of Size Lower limit of size (LLS) is the smallest size permitted by a toleranced dimension. 20.50 ± 0.25 LLS = 20.25 Tolerance Tolerance is the total amount of variation permitted by the limits of the dimension. It is expressed as the mathematical difference between the upper and lower limits of size. 20.50 ± 0.25 Tolerance = 0.50 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 386 of 498 Allowance Allowance is the intentional difference between the sizes of mating parts to provide the required class of fit. See the diagram below. Allowance diagram Effects of Limits on Allowance The diagram below shows a method of displaying tolerance information on a drawing. This diagram depicts mating parts (a hole and shaft). Acceptable component limits - showing tolerance information on a drawing 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 387 of 498 Applying the definitions of the upper and lower limits of size to the dimensions shown in the diagram above, the following are obtained: Upper limit Lower limit Hole Shaft 1.500 + 0.001 1.500 - 0.001 = 1.501 = 1.499 1.500 + 0.000 1.500 - 0.002 = 1.500 = 1.498 Variations in Allowance These variations in shaft size vary the amount of allowance between the following extremes: M aximum Allowance = U pper Limit of H ole − Lower Limit of Shaf t = 1.501 − 1.498 = 0.003 Clearance M inimum Allowance = Lower Limit of Shaf t − U pper Limit of H ole = 1.500 − 1.499 = 0.001 Clearance 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 388 of 498 Calculating Tolerances Worked Example The diagram below shows another set of mating parts. Mating parts with upper and lower limits drawn From this diagram, the following data can be determined: Basic size, upper and lower limits of size and tolerance 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 389 of 498 Methods of Expressing Tolerance The table below shows different methods of expressing tolerances on dimensions. Methods of expressing tolerance Calculating Clearances The amount of clearance is found by calculating the upper and lower limits of both the male and female components. The smallest shaft is then subtracted from the largest hole to calculate the slackest fit. The largest shaft is subtracted from the smallest hole to calculate the tightest fit. Worked Example The diagram below shows the same set of mating parts used in the tolerance example. Mating parts with upper and lower limits drawn 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 390 of 498 From this diagram, the clearances can be calculated as follows. Clearances calculation tables The tolerances of the individual items will produce a minimum clearance of 0.040 in. and a maximum clearance of 0.144 in. Therefore, the limits of size for these mating parts will result in a clearance fit. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 391 of 498 Acceptable Component Limits During maintenance, it is often necessary to measure mating parts and determine if they are suitable for further service. Common areas of wear are: Shafts and bolts acting as hinge pins Elongation of holes in moving controls. Once the measurement of both parts has been carried out, the dimensions are checked against applicable maintenance manuals. Example The diagram below provides limits of size. Acceptable component limits A shaft size of 1.497 is unserviceable and a replacement shaft is required. A hole size of 1.502 is also unserviceable. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 392 of 498 Press / Interference Fits A press fit (also known as an interference fit or sometimes a friction fit) exists when the male member is larger than the female member. When press fits are assembled, one of the following processes is involved: force shrink freeze. Press fits have many applications. Some of the more common are: A bush assembled in a casing or body A train wheel tyre assembled to the hub Sections on a built-up crankshaft assembled to one another. Pins, journals and webs are often assembled without keying. The grip due to the interference fit is used to transmit the entire torque. The interference in most cases is not allowed to vary much; otherwise, excessive interference could give rise to distortion in the assembly, while too little interference could achieve insufficient grip. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 393 of 498 Force Fits The most accurate method of assembling a force fit is by hydraulic press, with the operator exercising control of the operation. Where light work is concerned, a small lever or screw press gives the operator a sense of feel, which enables them to mate the parts without damage. The proper allowance for a forced fit depends on several factors, such as: The mass of the metal The kind and quality of the material of which the parts are composed The size of the work. Because of these factors, the tables of suitable allowances for force fits, which are found in standard engineering handbooks, should be used as a guide only and modified if necessary to suit particular circumstances. Assembling Force Fits The following points should be considered when assembling/dismantling components requiring the application of force. The contacting surfaces of both members must be free of grit. It is vital that the axis of the male and female stay aligned. If either part is canted, one or both may be damaged. An assembly arbour is often used to maintain the correct alignment. Tests have demonstrated that smooth mating surfaces perfectly free from surface film give the best grip. However, a heavy lubricant, such as white lead and lard oil mixed to the consistency of paint, is usually applied before assembly to reduce the possibly of seizure and abrasion of the surfaces. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 394 of 498 Shrink / Freeze Fitting Shrink fitting generally refers to techniques in which thermal expansion and contraction effects are used to achieve an interference fit. The following sections will outline some of the processes involved to assemble or dismantle components requiring the application of heat. In shrink fitting of parts, the outer or female member is heated, causing it to expand sufficiently to enable the two parts to fit together with little or no effort. Shrinkage allowance varies to a great extent, and the following factors of the part to be shrunk must be considered: Construction Type of material Form or shape. The thickness of the metal around the hole is the most important factor, but the way the metal is distributed also influences the results. The diagram below shows two examples of a crankshaft web marked A and B. For example, the rectangle crank web (A) is a bad shape for a shrink fit as the web with uneven thickness is liable to fracture by concentrating the stretching of the metal at the weakest point. A better shape and the one generally used is shown at B. Shrink fits – bad and good shapes 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 395 of 498 Temperatures for Shrink Fits The desired amount of expansion (or contraction) can be given to a piece by heating (or cooling) it to a defined temperature, where the means are available to measure this temperature. This method is often used instead of measuring the actual bore. The required temperature depends on the total expansion required and the coefficient of linear expansion of the metal. The coefficient of linear expansion is the increase in unit length of any section of metal in any direction for an increase in temperature of 1 °C. The total required expansion in diameter consists of the allowance for shrinkage plus an amount for clearance. The value of the coefficient of linear expansion for various materials is listed below: Aluminium 0.00002/C Cast iron 0.00001/C Steel 0.000011/C Nickel steel 0.000012/C. Various heating and cooling methods can be employed, such as: Heating: Immersion in hot water, oil or metal baths Steam Air and gas, oxygen and gas hand torches Oil, gas or electric furnaces Electrical resistance heating Electric induction heating. Cooling: Immersion in liquids cooled by ice, dry ice or refrigeration Cooling in a refrigerator Packing in dry ice Immersion in liquid air or liquid nitrogen. Freeze fitting is when the temperature of a part is reduced to eliminate friction during installation/fitting. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 396 of 498 Shrink Fit Considerations Working with heat and cold in press fitting involves additional risks to personal safety. The following points should be considered during the assembly/dismantling of components requiring the application of heat/cold. The contacting surfaces of both members must be free of grit. It is crucial that the axis of the male and female stay aligned. If either part is canted, one or both may be damaged. An assembly arbour is often used to maintain the correct alignment. Use appropriate PPE, including shields and protective clothing, where heat or severe cold is involved. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 397 of 498 Inspecting for Maintenance Limits Part Limits Typically, for aircraft inspection, limits are the bounds of any of the following: travel wear play clearance bow twist. Aviation Australia Maintenance inspection limits Travel is the distance between the extreme positions reached by a mechanism executing a reciprocating motion. For example, in a linear actuator is would be the distance travelled between fully extended and fully retracted. Play is the relative movement between two aircraft parts. Bow is the amount that a side of a surface deviates from being straight. The limit for the amount of bow, when inspecting aircraft parts, is expressed in the relevant maintenance or overhaul manual. The usual method used to inspect a shaft or other part for bow is to use a Dial Test Indicator. The compressor blade bow is usually checked with feeler gauges and a straight edge to measure any discrepancies. Twist is the rotation of a shaft or component about its longitudinal axis. When inspecting parts for twist, refer to the manufacturer’s manual for the proper inspection technique or method. Normally a shaft has a reference line or datum points that can be compared to maintenance manual dimensions or limits. Visual inspection can also reveal twist. Wear is the amount of material loss due to friction, rotation or any other form of movement on a part or component. When checking for wear, always refer to the manufacturer’s manuals for wear limits and inspection methods. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 398 of 498 Some of the methods of checking for wear: Micrometer Vernier gauge Dial test indicator Go/no-go gauge Feeler gauges. Micrometer 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 399 of 498 Inspection Methods Many inspection methods are available for checking shafts, bearings and other aircraft parts. The methods listed in the manufacturer’s manuals should be followed. Some common methods are: Visual Micrometers, verniers, etc. Dial test indicators Dye penetrant inspection Eddy current Inspection Magnetic particle inspection Radiographic (X-ray) inspection Ultrasonic inspection. At all times, follow the manufacturer's inspection requirements. Visual inspection 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 400 of 498