The micrometer type instruments also have many useful applications for inside measurementsl. These comprise the measurement of an object feature bounded by walls having parallel elements in the gaging plane, such as the diameter of a cylindrical bore or the width of a parallel-sided groove.
Inside measurements , particularly of bore diameters, are affected by more variable than the measurement of length between outside surfaces . Small bore sizes prevent the introduction of inside micrometers . The depth of the bore , when its diameter must be measured at a greater distance from the open end, can cause difficulties for micrometer applications. Finally, the requirement of measuring a cylindrical feature across its axis and in a plane normal to the axils , calls for conditions that may be accomplished only partially when relying solely on the skill of the operator , this being the case when using a plain inside micrometer.
The particular aspects of inside measurements created the need for a variety of inside measuring instruments operating on the micrometer principle, or used in conjunction with a regular outside micrometer.
The particular aspects of inside measurements created the need for a variety of inside measuring instruments operating on the micrometer principle , or used in conjunction with a regular outside micrometer.
The inside micrometer consists of a head part comprising the micrometer screw , and of independent measuring rods. The rods are made in different lengths , in steps of one inch , can be assembled with the head part by means of a threaded connection and are accurately positioned on a shoulder. The micrometers screw of the smaller size heads has a 1-inch movement, and the rods are attached either directly , or by using a 1-inch spacing collar, which is supplied with the instrument . Inside micrometer heads for measurement of bore diameters larger than 8 inches have a full inch of screw travel.
The smallest bore that can be measured with this type of micrometer is 2 inches in diameter (exceptionally 1 inches), and the maximum diameter depends on the available rods , a practical upper limit being about 32 inches . A handle can be attached to the head to permit measurement at greater depths inside a bore.
Bore diameter measurements by transfer. Small bores cannot be measured directly because of space limitations . It is possible , however, to transfer the inside dimension to be measured by using an appropriate means that will represent the reverse replica of the inside length. The resulting physical outside length can then be measured with a standard outside micrometer. Examples of such transfer devices ate the following:
The small hole gage, consisting of a split ball that can be expanded to the size of the diameter of the bore to be measured. Rotating the knurled knob of the handle advances the inside screw whose conical end causes the ball halves to separate to the required extent. The actual spread of the balls is small , on the order of 1 inch , and therefore these gages are supplied in sets encompassing a wider range of dimensions.
The telescoping gage has a tubular member to which a handle is attached at right angle position. The tubular member has either one or two plungers , which are under spring pressure and telescope into the fixed tube. The free ends of the plungers or , in the case of a single plunger, the closed end of the fixed member also has spherical forms and are hardened to serve as contact elements. When introduced into the hole to be measured , the previously retracted plungers are released to extend to a length equal to the diameter of the object. In this position the plungers can be locked again by turning a knurled screw in the end of the handle . Subsequently , the thus fixed length of the telescoping rod can be measured with a regular outside micrometer.
Three-point contact internal micrometer. Several of the difficulties connected with bore diameter measurements by a micrometer can be reduced when using the three-point internal micrometer. The self-aligning property of this instrument is particular useful when measuring deep bores , for which purpose an extension part can be attached to the basic tool.
A precisely ground spiral on a cone surface functions as a seat for the shafts of the three self-aligning measuring points . These are contained in the measuring head of the instrument and are spaced 120 degrees apart. Advancing the cone along its axis spreads the engaged measuring points radially, resulting in a larger envelope circle, and the cone movement in the inverse sense causes the spring loaded contact members to retract . The cone is attached to a spindle whose axial position is shown on the micrometer sleeve and thimble . The smallest graduation of the micrometer varies from 0.0001 inch to 0.0005 inch, depending on the size and range of the instrument.
Three-point internal micrometers are available in sizes from 0.275-inch to 8-inch bore diameters . The individual tools have measuring ranges varying from 0.075 inch for the smallest size, to a full inch for larger sizes . Usually these micrometers are procured in sets to cover a more extended range of measurable bore diameters.
It is advisable to recalibrate these instruments periodically , using a standard ring gage and applying the regular micrometer adjustment procedures.