by Prof. Eduardo J. Stefanelli Reading and interpretation of micrometer in thousandth of inch
This page assumes that you have already interacted with the page: micrometer in millimeter hundredth resolution – use, reading and interpretation and with the page: reading of ruler or line gauge, in tenth inch.
The operation of the micrometer in thousandth inch
figure 1 – thimble in partial view to reveal the shealth marks
A complete turn in the thimble (and of the fuse) produces the advance equal to the thread pitch. The screw of this fuse, usually, has the lenght of the one inch and the pass of 25 thousandths of inch.
In the sleeve there is a graduation that controls the advance of the thimble/fuse – figure 1. The inch (10 tenths) is divided by nine main marks, the distance between them is the tenth of inch, these marks have numbers to help the reading. The tenths of inch are divided by three marks that control the advance of twenty and five thousandth of inch (0.1/4 in = 0.025 in). This means that each time that a new mark is revealed by the thimble, the fuse moved close to twenty and five thousandth of inch(0.025in).
The resolution of the thousandth inch micrometer
The configuration most commonly found in the metal / mechanical workshops is the micrometer spindle having a pitch of 0.025 in and the barrel 25 divisions-Figure 2. Thus, by turning the micrometer spindle each marking of the barrel passing through the ‘Reference line’ will indicate that The movable stopper moved away (or approached) 0.001 in (one thousandth of an inch).
figure 2 – image of the graduation of the thimble corrected to reveal the 25 marks/divisions
Reading of the micrometer in thousandth inch by the measurement formation
- to determine the value of the measure, it is necessary to add:
- the value obtained in the graved scale in the sleeve, in tenths of inch (0.1in)
- represented by the ( o ) ‘in green color’ in the animation below
- with 0.025in x number of counters of 0.025in exposed (0.1in /4 in)
- represented by ( o ) ‘in blue color’
- with the value of the thimble aligned with the ‘reference line’, in thousandths of inch
- represented by ( o ) ‘in the orange color’
- click in the icon in the inferior corner of the interactive screen animation to alternate the value of the measure
- use the icon ‘eye’ to hide the values total and partial
- with the ‘eye closed’, click in the colored circles to show the value related to them
Commented examples of reading and interpretation of micrometer in thousandth inch
The comprehension of the reading of this instrument is very intuitive for those who know the vernier scale: simulator of reading and interpretation in thousandth inch resolution 0.001″ . Evidently, it is necessary to transfer the manipulated data given by the vernier to the one given by the thimble. See also the figure 2 of the page: Micrometer in inch with resolution tenth of thousandth – use, reading and interpretation
Imagine that the scales have come to the positions as shown in the figure below, after the jaws are kept around the object. The lock can be used to assure that readings don’t change due to the movement of the thimble. In the figure, it can be seen that the marking on the main scale which is just to the left of the thimble is 7 mm. However, the half scale division that is visible below the main scale shouldn’t be neglected and thus the reading on the main scale will be 7.5mm. For the auxiliary scale reading, it is noted that the 22nd division on the thimble scale matches with the main scale. Thus the thimble scale reading would be 0.22 mm. The final reading will be the addition of these two readings i.e. 7.5 + 0.22 = 7.72 mm.
Let’s take one more example to understand it properly. Suppose the scales came to the positions shown in the figure. The main scale reading would be the marking that is fully visible immediately to the left of the thimble, i.e. 5.5 mm. For the auxiliary reading, the 30th division of the thimble matches with the main scale and thus its reading will be 0.30 mm. The final reading will be the addition of the readings of both the scale i.e. 5.5 + 0.30 = 5.80 mm.