Inside Micrometer Calliper:
The inside micrometer calliper is useful for making small measurements from 5 to 25 mm. In this instrument, unlike a regular micrometer, the axis of the instrument does not coincide with the line of measurement. In addition, unlike the outside micrometer where there is a surface contact between the job and the instrument, the contact between the job and the instrument is line contact. The nibs, as the contacts are called, are ground to a small radius. As a necessity, this radius has to be smaller than the smallest radius the instrument can measure. Therefore, all measurements are made with line contacts.
As illustrated in Fig. 1, the movable jaw can be moved in and out by the rotation of the thimble. One complete rotation of the thimble moves it by one division on the barrel scale. A locknut can be operated to hold the position of the movable jaw for ease of noting down a reading. While taking measurements, it needs to be rocked and centralized to assure that the axis of the instrument is parallel to the line of measurement. This makes the instrument prone to rapid wear. It is therefore needless to say that the instrument needs to be checked and calibrated regularly.
Fig.1 Inside micrometer calliper
This instrument perfectly complies with Abbe’s law. The axis of an inside micrometer is also its line of measurement. It is useful for measuring the inside diameter of cylinders, rings, and other machine parts. The inside micrometer set has several accessories, which have to be assembled together for taking the readings. The main unit is the measuring head, which has a thimble that moves over a barrel, same as in the case of an outside micrometer. Graduated scales are
provided on the barrel and thimble, which give readings up to an accuracy of 0.01 mm, but with a limited range. The rear end of the measuring head has a contact surface, whereas extension rods. Figure 2 illustrates the construction details of an inside micrometer.
Fig.2 Inside micrometer
The inside micrometer is more common than the inside micrometer calliper because of the flexibility it affords to measure a large range of dimensions. A range of 25 mm above the length of extension rods is commonly used in shops. A standard set will have five extension rods of ranges 50–75, 75–100, 100–125, 125–150, 150–175, and 175–200 mm.
In addition to extension rods, a spacing collar (usually 12.5 mm in length) is provided for smaller adjustments in the range of measurements. The micrometer is also provided with a detachable handle for easier handling of the instrument.
The best practice for using an inside micrometer is to first measure the dimension approximately by using a steel rule. Then select a suitable extension rod that can adequately cover the range of measurements. Insert the extension rod into the chuck and set the instrument to read zero. Now fix the handle and lower the instrument into the gap where the dimension is to be measured. Operate the thimble until the two contact surfaces establish a firm contact with the surfaces of the job.
While measuring diameters, it is always recommended to lightly move the instrument to and fro so that the actual diameter is sensed by the person taking measurements. Now, operate the locknut and take out the instrument. The micrometer reading has to be added to the length of the extension rod to get the actual reading.