Using of The Micrometer Caliper and Applications
introduction
Most engineering work has to be measured to much greater accuracy than it is possible to achieve with a rule, even when aided by the use of calipers. To achieve this greater precision, measuring equipment of greater accuracy and sensitivity has to be used. One of the most familiar measuring instruments used in engineering workshops is the micrometer caliper.
The constructional details
The constructional details of a typical micrometer caliper are shown in figure 1
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| FIG 1 |
B Spindle –Thread ground and made from alloy steel, hardened throughout, and stabilised
C Locknut –Effective at any position. Spindle retained in perfect alignment
D Barrel –Adjustable for zero setting. Accurately divided and clearly marked, pearl chrome plated
E Main nut –Length of thread ensures long working life
F Screw adjusting nut –For effective adjustment of main nut
G Thimble adjusting nut –Controls position of thimble
H Ratchet –Ensures a constant measuring pressure
I Thimble –Accurately divided and every graduation clearly numbered
J Steel frame –Drop forged
K Anvil end –Cutaway frame facilitates usage in narow slots
the principle of operation
The operation of this instrument depends upon the principle that the distance a nut moves along a screw is proportional to the number of revolutions made by the nut and the lead of the screw thread. Therefore by controlling the number of complete revolutions made by the nut and the fractions of a revolution made by a nut, the distance it moves along the screw can be accurately controlled. It does not matter whether the nut rotates on the screw or the screw rotates in the nut, the principle of
operation still holds good. In a micrometer caliper, the screw thread is rotated by the thimble
which has a scale that indicates the partial revolutions. The barrel of the instrument has a scale which indicates the ‘whole’ revolutions. In a standard metric micrometer caliper the screw has a lead of 0.5 millimetre and the thimble and barrel are graduated as in figure 2
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| FIG 2 |
Since the lead of the screw of a standard metric micrometer is 0.5 millimetre and the barrel divisions are 0.5 millimetre apart, one revolution of the thimble moves the thimble along the barrel a distance of one barrel division (0.5 mm). The barrel divisions are placed on alternate sides of the datum line for clarity. Further, since the thimble has 50 divisions and one revolution of the thimble equals 0.5 millimetre, then a movement of one thimble division equals: 0.5 millimetre/50 divisions = 0.01 millimetre.
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| FIG 3 |
Figure 3 shows the scales for a micrometer graduated in ‘inch’ units. The micrometer screw has 40 TPI (threads per inch), therefore the lead of the screw is 1/40 inch (0.025 inch). The barrel graduations are 1/10 inch subdivided into 4. Therefore each subdivision is 1/40 inch (0.025 inch)
and represents one revolution of the thimble. The thimble carries 25 graduations. Therefore one thimble graduation equals a movement of 0.025 inch/25 = 0.001 inch. This is one-thousandth part of an inch and is often referred to by engineers as a ‘thou’. Thus 0.015 inch could be referred to as 15 ‘thou’.
micrometer caliper reading
As in figure 2 .A metric micrometer caliper reading is given by:
• The largest visible ‘whole’ millimetre graduation visible on the barrel, plus
• The next ‘half’ millimetre graduation, if this is visible, plus
• The thimble division coincident with the datum line.
Therefore the micrometer scales shown in Fig.2 read as follows:
9 ‘whole’ millimetres = 9.00
1 ‘half’ millimetre = 0.50
48 hundredths of a millimetre = 0.48
= 9.98 mm
As in figure 3 . An inch micrometer reading is given by:
Therefore the micrometer scales shown in Fig. 3 read as follows:
3 tenths of an inch = 0.300
1 fortieth of an inch = 0.025
23 thousandths of an inch = 0.023
= 0.348 inch
care of Micrometer caliper
Unless a micrometer caliper is properly looked after it will soon lose its initial accuracy. To maintain this accuracy you should observe the following precautions:
• Wipe the work and the anvils of the micrometer clean before making a measurement.
• Do not use excessive measuring pressure, two ‘clicks’ of the ratchet are sufficient.
• Do not leave the anvil faces in contact when not in use.
• When machining, stop the machine before making a measurement. Attempting to make a measurement with the machine working can ruin the instrument and also lead to a serious accident. This rule applies to all measuring instruments and all machines.
Although easy to read and convenient to use, micrometer calipers have two disadvantages:
• A limited range of only 25 millimetres. Thus a range of micrometers is required, for example: 0–25 millimetres, 25–50 millimetres, 50–75 millimetres, and so on.
• Separate micrometers are required for internal and external measurements. The micrometer caliper so far described can be used only for external measurements.
further application
Internal micrometer
An internal micrometer is shown in Fig. 4. It is used for measuring bore diameters and slot widths from 50 millimetres to 210 millimetres.
For any one extension rod its measuring range is 20 millimetres. A range of extension rods in stepped lengths is provided in the case with the measuring head. It suffers from two important limitations.
• It cannot be used to measure small holes less than 50 millimetres diameter.
• It cannot be easily adjusted once it is in the hole and this affects the accuracy of contact ‘feel’ that can be obtained.
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| FIG 4 |
Figure 5 shows the principle of the micrometer cylinder gauge. It is used for measuring the diameters of holes to a high degree of accuracy. It uses a micrometer-controlled wedge to expand three equi-spaced anvils until they touch the walls of the bore. Unfortunately it has only a limited measuring range and the range cannot be extended by the use of extension rods . A separate instrument has to be used for each range of hole sizes.
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| FIG 5 |
This is used for measuring the depth of holes and slots. You must take care when using a depth micrometer because its scales are reversed when compared with the familiar micrometer caliper. Also the measuring pressure tends to lift the micrometer off its seating. A depth micrometer is shown
in Fig. 6. The measuring range is 25 millimetres for any given rods. Typical rods give a range of 0 to 25 mm, 25 to 50 mm, 50 to 75 mm.
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| FIG 6 |
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| FIG 7 |
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