16. Pasivos financieros corrientes y no corrientes
16.2 Emisión de obligaciones
Precision Measuring Instruments are provided to measure dimensions to a greater accuracy than can be obtained by the use of a simple engineer’s rule. Where the smallest graduation on a rule is usually either 1 mm or, perhaps, 1/64",
precision instruments are available which measure to 0.01 mm or to 0.0001”. The precision instruments mentioned here would normally be found either in a workshop environment or in a ‘clean room’, which may be part of a company’s Quality Department.
It should also be noted that, whilst very basic forms of the different instruments are described, in order that the principles of operation be understood, the actual precision instruments, found in workshops and ‘clean rooms’ may appear quite different and, in all probability, will possess digital readout facilities.
4.3.1 External Micrometers
An External Micrometer (refer to Fig. 33), as the name implies, is used for measuring (or testing the level of accuracy of) the external sizes of objects.
The standard (or common) external micrometer consists of an appropriately shaped frame, to one end of which is attached an internally threaded barrel (or sleeve).
Ratchet Stop
Graduated Thimble Graduated Barrel with Fiducial Line
Locking Ring Frame Spindle Anvil External Micrometer Fig. 33
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A partially, externally threaded spindle, located in a hollow, tubular thimble, is able to be screwed into (or out of) the sleeve by means of rotating the thimble. The working tip of the spindle has an accurately machined face, to match the equally accurately machined face of the anvil. The anvil is located at the opposite end of the frame and, with the spindle moved sufficiently, the object to be measured is placed between the faces of the anvil and the spindle.
The ratchet stop is used to rotate the thimble so that the spindle moves until the object is held between the faces of the spindle and the anvil. To prevent distortion of the frame and to ensure that the reading is constant when taken by different users of the instrument, the ratchet stop ‘slips’ (3 clicks!) when sufficient pressure is applied to the object being measured.
The principle of the micrometer is based on the lead of the screw thread. This is the distance the thread moves, either forwards or backwards, during one complete revolution of the thimble. If the lead is known, together with the number of revolutions, then the total distance the screw moves can be calculated.
The circumference of the thimble and the length of the barrel are graduated to indicate the measurement of the object that is in contact with the faces of the anvil and the spindle. The barrel also has a datum (fiducial) line, against which the measurements are made, from the bevelled end of the thimble as it uncovers the markings on the fiducial line.
The thimble is bevelled so that its graduations are brought close to those on the fiducial line. The bevelling eliminates shadows and also lessens parallax error when reading the measurement. The body of the micrometer usually has a matt finish, which serves to reduce glare and, thus, aids accurate readings.
The locking ring (some micrometers have a locking lever) is used to lock the spindle, when the instrument is employed as a fixed (or snap) gauge.
The mechanism of the external micrometer is arranged so that the spindle face can only move between 0 - 25 mm (or 0 – 1in) from the anvil face and, thus, the standard micrometer has the capability to measure items which are in this range. For larger items, the size of the frame is simply increased in successive increments of 25 mm (or 1in). For example, the next size of micrometer would be able to measure between 25 mm – 50 mm (1 in – 2 in), the next 50 mm – 75 mm (2 in – 3 in) and so on. While the frames increase in size to accommodate the larger items, the spindle movement (of external micrometers) remains in the range of 0 – 25 mm (0 – 1 in).
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Both Metric and Imperial micrometers (while their units of measurement are different), are operated in a similar manner.
The Metric micrometer uses a thread pitch and, thus, a lead, of 0.5 mm (two threads per millimetre), so that the thimble moves over the barrel a distance of 0.5 mm per revolution. The fiducial line, on the barrel, is marked in increments of 0.5 mm and 1 mm, with numerals at intervals of 5 mm (5, 10, 15, etc.) to 25 mm. The thimble has a total of 50 markings, so that one thimble division represents
1
/50 of 0.5 mm, or 0.01 mm.
When reading a Metric micrometer (refer to Fig. 34) it is, first of all, necessary to decide on the number of divisions, on the fiducial line, which are exposed by the thimble and to note the division on the thimble which also coincides with the fiducial line.
The subsequent actions, to arrive at the dimension being measured, are to:
Note the number of main divisions exposed (as shown at A = 8.00 mm)
Note the additional number of sub-divisions (as shown at B = 0.50 mm)
Note the number of divisions on the thimble (as shown at C = 0.28 mm)
Add all the numbers together to provide the total dimension (8.78 mm).
B 30
25 Fiducial Line (0.5 mm divisions)
0 5
Thimble (0.01 mm divisions) Barrel
C A Metric Micrometer Reading
Fig. 34
A = 8.00 mm B = 0.50 mm C = 0.28 mm Total = 8.78 mm
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Imperial Micrometers measure in decimals of an inch. Their screw threads have forty threads per inch, giving a ‘lead’ of 1
/40" (0.025"), which is the length of each
sub-division on the fiducial line and represents one revolution of the thimble. The thimble circumference is, now, divided into only 25 equal divisions, making one division read 1/25 of 1/40”, which equals 1/1000” (or 0.001") movement of the
spindle.
Barrel markings are made at each tenth of an inch (1, 2, 3, 4, etc) with four sub- divisions between each main mark.
Again, in a similar manner to the Metric micrometer, when taking a dimension, it is necessary to deduce the number of division, on the fiducial line, exposed by the thimble. Next note the mark on the thimble which aligns with the fiducial line and follow similar actions to those employed with the Metric micrometer.
If, for example, nineteen divisions, on the barrel of an Imperial micrometer, were exposed, while the eighth mark on the thimble aligned with the fiducial line, then the total dimension would consist of:
Four 1/10” divisions (sixteen 1/40” divisions) on the barrel
Three further 1/40” divisions on the barrel (making nineteen in all)
Eight 1/1000” divisions on the thimble
In this example the total dimension would be 0.400” + 0.075” + 0.008” = 0.483”. To ensure the integrity of any dimensions it is imperative that the faces of the spindles and anvils of micrometers are kept scrupulously clean.
Micrometers should be stored in a protective case, preferably with a sachet of desiccant (or VPI paper) and not used in extremes of temperature (the temperature of a standards room is usually maintained at 20°C).
Never store a micrometer with its spindle and anvil in contact. Changes in temperature will cause distortion of the frame, with the obvious consequences. Prior to use, the accuracy of a micrometer should be confirmed by doing a check on the zero setting (with the spindle and anvil faces in contact) and a sample check (using slip gauges or similar, accurate standard test pieces), of measurements within the range of the micrometer.
It is possible to do adjustments with special tools, which are provided with micrometers, but any adjustments should normally, only be done by qualified personnel, who will then certify that the micrometer is accurate enough, to be used for aerospace work.
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4.3.2 Internal MicrometersInternal micrometers are used for the precision measuring of internal dimensions, using much the same principles as those used with the external types.
While there are many designs for internal micrometers, to suit particular tasks, space (and time) dictates that consideration be given here only to the type more commonly referred to as the ‘Stick’ micrometer (refer to Fig. 35), which is found in both Imperial and Metric versions.
An Imperial, ‘Stick’ micrometer, consists of a micrometer head, with an overall closed length of only 1½”, a ‘spacing’ collar which has a length of ½" and ten extension rods. The lengths of the rods increase in increments of one inch, with the shortest length being ½” and the longest 9½” (e.g. ½”, 1½”, 2½” etc.).
The internal micrometer differs from the external type in that the thimble travel is only half an inch and so, from closed, the micrometer is capable of measuring internal dimensions from 1½” up to 2”. For dimensions greater than 2” it is then necessary to close the micrometer and attach the smallest extension rod (½”), enabling dimensions up to 2½” to be measured.
By adding the spacing collar (½”) with the smallest extension rod, measurements up to 3” can be made, then, by removing both collar and rod and using the next rod (length 1½”), it is possible to measure dimensions up to 3½”.
With alternate use of extension rod and rod/collar combinations, the Imperial internal micrometer has a measuring range from 1½” to 12”.
Internal ‘Stick’ Micrometer Fig. 35
1 0 2
Micrometer Head Handle
(replaced by a Grub Screw when the Handle is not required)
Collar
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With Metric internal micrometers, similar principles are used, but the dimensions are, obviously, changed and are not directly related to the measurements used with the Imperial type. The closed body length is 50 mm, thimble range is 10 mm, the collar length is also 10 mm and the seven extension rods are provided in a selection of lengths, which allow measurements (in increments of 20 mm), from 50 mm to 210 mm to be made.
4.3.3 Micrometer Depth Gauge
Whilst only used in specialist applications, a micrometer depth gauge is useful when the depth of a groove or recess needs to be measured with precision. The device (refer to Fig. 36) has a standard micrometer head (but the scale, on the barrel, is reversed) mounted onto a precisely ground base. When the spindle of the micrometer is flush with the face of the base, then the depth gauge reads zero and the thimble is at its maximum distance from the base.
To measure the depth of a recess, the base is placed over the groove and the spindle screwed down until it contacts the bottom of the groove. The reading on the micrometer head indicates the groove depth.
Micrometer Depth Gauge Fig. 36
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4.3.4 Vernier MicrometersSome micrometers (and other precision measuring instruments), have a ‘Vernier’ scale, which enables the instrument to measure to a greater accuracy.
The ‘Vernier’ principle (inventor Pierre Vernier 1580 – 1637)) utilises two accurately graduated scales. The main scale may be fixed, whilst the other (the Vernier scale), moves parallel to the main scale (refer to Fig. 37), or, depending on the instrument (such as with micrometers), it could be the other way round, where the main scale moves while the Vernier scale is stationary.
In the very basic example (refer to Fig. 37) ten divisions on the Vernier scale are made to equal nine divisions on the main scale, so that one Vernier scale division equals one tenth of nine millimetres (0.9 mm). The difference between one main- scale and one Vernier division is, therefore, 0.1 mm.
When the Vernier scale is moved (to the right in this instance), so that the first of the smaller Vernier divisions is aligned with the first main-scale division, the zeros will be displaced by exactly one tenth of one millimetre. If this principle is continued until the second division of each scale is coincident, then the zeros will have moved exactly two tenths of a millimetre apart.
From this it can be seen that, whichever lines on the main and Vernier scales align, then the zero (or datum) marks will be displaced by the small amount shown on the Vernier scale.
Main Scale Vernier Scale Vernier Principle Fig. 37 3 0 10 mm 0 1 2 4 5 6 7 8 9 10
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When the Vernier principle is applied to a micrometer (refer to Fig. 38), the Vernier scale is engraved on the barrel and is, therefore, stationary. The Vernier graduations are scaled, usually, to represent one tenth of those on the thimble, which enables a Vernier micrometer to read dimensions to an accuracy of one tenth of that of a standard micrometer. Consequently the graduations on the Vernier of a Metric micrometer represent 0.001 mm, while those on an Imperial micrometer represent 0.0001”.
The example shows a Metric micrometer reading, where the graduation on the thimble scale does not exactly coincide with the datum line on the barrel. The procedure for reading the dimension is to:
Note the main and sub divisions visible on the barrel (8.5) = 8.500 mm
Note the nearest thimble reading below the datum line (27) = 0.270 mm
Note the Vernier line which aligns with a thimble line (6) = 0.006 mm
Add the readings to provide the total dimension = 8.776 mm. A similar procedure would be followed with an Imperial micrometer.
Care must be taken that it is the Vernier number, which is added, and not the value of the main scale (thimble) reading which aligns with the Vernier line. This is a common fault when reading Verniers.
It may also be found advantageous, to use a magnifying glass, to assist in the reading of the smaller Vernier scale and in deciding which lines are actually in alignment. 30 35 25 8 6 2 0 4 0 5
Ten Vernier Scale Marks on Barrel.
Barrel Markings with Fiducial Line.
Thimble Markings
Vernier Micrometer Fig. 38
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4.3.5 Vernier CallipersA Vernier Calliper (refer to Fig. 39), is a versatile precision instrument, used to measure both inside and outside dimensions. In many situations, a Vernier calliper is faster to use than a micrometer but, possibly, needs greater skill in manipulation in order to obtain the correct ‘feel’ and to, thus, ensure accurate readings. Callipers, furthermore, have a working range of up to 150 mm (6 in) as opposed to the micrometer’s more limited movement.
The Vernier scales on Imperial instruments are accurate to 0.001 inch, while Metric Verniers have an accuracy of 0.02 mm.
With some types of calliper, ‘nibs’ are located at the end of both jaws. The nib size, which is etched on the jaw, must be added to any internal dimensions that have been measured.
Two ‘target’ points may also be found on some callipers, one on the beam and one on the sliding jaw. These are used to set spring dividers accurately, when they are being used in a comparator mode. The target points are exactly the same distance apart as the reading on the Vernier and main scale.
Jaws for Internal Measurement.
Jaws for External Measurement Vernier Scale Main Scale Position Lock Vernier Calliper Fig. 39
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4.3.6 Vernier Height Gauge
The Vernier Height Gauge (refer to Fig. 40) is similar in construction to the Vernier calliper, except that an accurately machined, solid base replaces the fixed jaw and the beam is mounted perpendicular to the base, which enables the instrument to be used on a surface plate or table.
The measurements, on the beam, are read in the same manner as those on the Vernier Calliper and they, usually, have both metric and Imperial markings on the same face of the beam.
This instrument can be used for various purposes, when used in conjunction with other suitable attachments. These can include measuring height, comparing and transferring height dimensions (for marking-off), and also as a depth gauge.
Vernier Height Gauge Fig. 40
Precision-Ground Beam and Base
Vernier Scale Scriber Fine Adjustment Control Initial Locking Screw Final Locking Screw
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4.3.7 Vernier ProtractorThe Vernier Protractor (refer to Fig. 41) provides greater precision than is possible when using a standard bevel protractor (or the protractor head of a combination set), and enables angles to be measured to an accuracy of five minutes of arc.
It consists of a grooved blade, a graduated protractor head and a stock with true edges. The protractor head can be slid along the length of the blade to any required position and locked.
The stock rotates about the centre of the protractor and can also be locked in any position. The angles formed by the edges of the stock, relative to the blade, are indicated on the protractor by an index mark ‘0’ on the Vernier scale that is attached to the rim of the stock disc.
The protractor scale is graduated in 180 from each end, meeting at 90 at the middle. This enables both acute and obtuse angles to be measured.
Vernier Protractor Fig. 41
Main Scale on Head Grooved Blade
Stock
Scale Locking Device
Vernier Scale underMagnifier
Fine Adjustment Blade Locking Device
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The Vernier scale is formed into twelve equal parts, which are compared to twenty-three protractor main scale divisions (23°), so that one Vernier scale division represents 23/12 = 1° 55’. The difference between two protractor scale divisions (2°), and one Vernier scale division, (1° 55’) is, therefore, 5’ of arc.
The Vernier scale has each third division numbered 15, 30, 45 and 60, indicating the number of minutes (which make up one degree). There are two separate scales, reading from the centre ‘0’ to left and right, to match the two protractor scales.
The protractor is read from the zero on the protractor scale to the zero on the Vernier scale. This provides the number of whole degrees. The Vernier scale is read in the same direction until the coinciding line is met. The number of the coinciding line, (indicating minutes) must be added to the degrees, read from the protractor scale, to obtain the total value of the angle.