The third show whatever error there may be, but do not define its amount; and the same is true of the fifth and sixth.
[Fig. 1389] represents a micrometer caliper for taking minute end measurements. This instrument is capable of being set to a standard measurement or of giving the actual size of a piece, and is therefore strictly speaking a combined measuring tool and a gauge. The U-shaped body of the instrument is provided with a hub a, which is threaded to receive a screw c, the latter being in one piece with the stem d, which envelops for a certain distance the hub a. The thread of c has a pitch of 40 per inch; hence one revolution of d causes the screw to move endways 1⁄40 of an inch.
The vertical lines of division shown on the hub a are also 1⁄40 of an inch apart, hence the bevelled edge of the sleeve advances one of the divisions on a at each rotation.
This bevelled edge is divided into 25 equal divisions round its circumference, as denoted by the lines marked 5, 10, &c. If, then, d be rotated to an amount equal to one of its points of division, the screw will advance 1⁄25 of 1⁄40 of an inch. In the cut, for example, the line 5 on the sleeve coincides with the zero line which runs parallel to the axial line of the hub. Now suppose sleeve d to be rotated so that the next line of division on the bevelled edge of d comes opposite to the zero line, then 1⁄25 part of a revolution of d will have been made, and as a full revolution of d would advance the screw 1⁄40 of an inch, then 1⁄25 of a revolution will advance it 1⁄25 of 1⁄40 inch, which is 1⁄1000 inch.
The zero line being divided by lines of equal division into 40ths of an inch, then, as shown in the cut, the instrument is set to measure 3⁄40ths and 5⁄25ths of a fortieth.
It is to be observed that to obtain correct measurements the work must be held true with the face of the foot b, and the contact between the end of screw c and the work must be just barely perceptible, otherwise the pressure of the screw will cause the U-piece to bend and vitiate the accuracy of the measurement. Furthermore, if the screw be rotated under pressure upon the work, its end will wear and in time impair the accuracy of the instrument. To take up any wear that may occur, the foot-piece b is screwed through the hub, holding it so that it may be screwed through the hub to the amount of the wear.
To avoid wear as much as possible, the screws of instruments of this kind are sometimes hardened, and to correct the error of pitch induced in the hardening, each screw is carefully tested to find in what direction the pitch of the hardened thread has varied, and provision is made for the correction as follows:—
The zero line on the hub a stands, if the thread is true to pitch, parallel to the axis of the screw c, but if the pitch of the thread has become coarser from hardening, this zero line is marked at an angle, as shown in [Fig. 1390], in which a a represents the axial line of the screw and b the zero line.
If the screw pitch becomes finer from hardening, the zero line is made at an angle in the opposite direction, as shown in [Fig. 1391], the amount of the angle being that necessary to correct the error in the screw pitch. The philosophy of this is, that if the pitch has become coarser a less amount of movement of the screw is necessary, while if it has become finer an increased movement is necessary. It is obvious, also, that if the pitch of the thread should become coarser at one end and finer at the other the zero line may be curved to suit.