Part 10 (2/2)

Micrometers having small spherical measuring ends (see sketch _A_, Fig.

26) are sometimes used for this purpose. The ball points are small enough to bear against the sides of the thread and the diameter, as compared with the reference gage, can be determined with great accuracy.

[Ill.u.s.tration: Fig. 26. (A) Testing Size of Thread with Ball-point Micrometer. (B) Testing Size of V-thread by the Three-wire System. (C) Testing the Size of a U. S. Standard Thread]

=Three-wire System of Measuring Threads.=--A method of measuring threads by using an ordinary micrometer and three wires of equal diameter is ill.u.s.trated at _B_ and _C_, Fig. 26. Two wires are placed between the threads on one side and one on the opposite side of the screw. The dimension _M_ over the wires is then measured with an ordinary micrometer. When the thread is cut to a standard size, the dimension _M_ for different threads is as follows:

For a U. S. standard thread:

_m_ = _d_ - 1.5155_p_ + 3_w_

For a sharp V-thread:

_m_ = _d_ - 1.732_p_ + 3_w_

For a Whitworth standard thread:

_m_ = _d_ - 1.6008_p_ + 3.1657_w_

In these formulas, _d_ = standard outside diameter of screw; _m_ = measurement over wires; _w_ = diameter of wires; _p_ = pitch of thread = 1 number of threads per inch.

To ill.u.s.trate the use of the formula for the U. S. standard thread, let us a.s.sume that a screw having 6 threads per inch (1/6-inch pitch) is to be cut to a diameter of 1-1/2 inch, and that wires 0.140 inch diameter are to be used in conjunction with a micrometer for measurement. Then the micrometer reading _m_ should be

1-1/2 - 1.5155 1/6 + 3 0.140 = 1.6674 inch

If the micrometer reading were 1.670 inch, it would indicate that the pitch diameter of the screw was too large, the error being equal to difference between 1.667 and the actual reading.

[Ill.u.s.tration: Fig. 27. Rivett-Dock Circular Threading Tool in Working Position]

=Rivett-Dock Threading Tool.=--A special form of thread tool, which overcomes a number of disadvantages common to an ordinary single-point thread tool, is shown in Fig. 27. This tool has a circular-shaped cutter _C_, having ten teeth around its circ.u.mference, which, beginning with tooth No. 1, gradually increase in height, cutter No. 2 being higher than No. 1, etc. This cutter is mounted on a slide _S_, that is fitted to the frame _F_, and can be moved in or out by lever _L_. The hub of this lever has an eccentric stud which moves slide _S_ and locks it when in the forward or cutting position. The action of the lever in moving the slide engages the cutter with pawl _P_, thus rotating the cutter one tooth at a time and presenting a different tooth to the work for each movement of the lever. When the slide is moved forward, the heel or underside of the tooth which is in the working position rests on a stop that takes the thrust of the cut.

When the tool is in use, it is mounted on the tool-block of the lathe as shown in the ill.u.s.tration. The cutter is set for height by placing a tooth in the working position and setting the top level with the lathe center. The cutter is also set square with the work by using an ordinary square, and it is tilted slightly from the vertical to correspond with the angle of the thread to be cut, by adjusting frame _F_. At first a light cut is taken with lever _L_ moved forward and tooth No. 1 on the stop. After this cut is completed, the lever is reversed which rotates the cutter one tooth, and the return movement places tooth No. 2 in the working position. This operation is repeated until the tenth tooth finishes the thread. It is often necessary, when using a single-point thread tool, to re-sharpen it before taking the finis.h.i.+ng cut, but with a circular tool this is not necessary, for by using the different teeth successively, the last tooth, which only takes finis.h.i.+ng cuts, is kept in good condition.

=Cutting Screws to Compensate for Shrinkage.=--Some tool steels are liable to shrink more or less when they are hardened; consequently if a very accurate hardened screw is required, it is sometimes cut so that the pitch is slightly greater than standard, to compensate for the shrinkage due to the hardening operation. As the amount of contraction incident to hardening is very little, it is not practicable to use change gears that will give the exact pitch required. A well-known method of obtaining this increase of pitch is by the use of a taper attachment.

For example, suppose a tap having 8 threads per inch is to be threaded, and, owing to the contraction of the steel, the pitch must be 0.12502 inch instead of 0.125 inch. The lathe is geared to cut 8 threads per inch or 0.125 inch pitch, and then the taper attachment is set to an angle _a_, Fig. 28, the cosine of which equals 0.125/0.12502; that is, the cosine of angle _a_ equals _the pitch required after hardening_, divided by the _pitch necessary to compensate for shrinkage_. The angle is then found by referring to a table of cosines. The tap blank is also set to the same angle a by adjusting the tailstock center, thus locating the axis of the work parallel with the slide of the taper attachment.

When the carriage moves a distance _x_, the tool point will have moved a greater distance _y_ along the work, the difference between x and y depending upon angle _a_; hence the tool will cut a thread of slightly greater pitch than the lathe is geared to cut.

To ill.u.s.trate by using the preceding example, cosine of angle _a_ = 0.125/0.12502 = 0.99984. By referring to a table of cosines, we find that 0.99984 is the cosine of 1 degree, approximately; hence, the taper attachment slide and the work should be set to this angle. (The angle _a_ in Fig. 28 has been exaggerated in order to more clearly ill.u.s.trate the principle.)

[Ill.u.s.tration: Fig. 28. Diagram Ill.u.s.trating Method of Cutting a Thread to Compensate for the Error in Pitch due to Shrinkage in Hardening]

As is well known, it is objectionable to cut a thread with the tailstock center offset, because the work is not rotated at a uniform velocity, owing to the fact that the driving dog is at an angle with the faceplate. For a small angle such as 1 degree, however, the error resulting from this cause would be very small.

If a thread having a pitch slightly less than standard is needed to fit a threaded part which has contracted in hardening, the taper attachment can also be used provided the lathe is equipped with special gears to cut a little less than the required pitch. Suppose a screw having a pitch of 0.198 inch is required to fit the thread of a nut the pitch of which has been reduced from 0.200 inch to 0.198 inch. If gears having 83 and 84 teeth are available, these can be inserted in a compound train, so as to reduce the 0.200 inch pitch that would be obtained with the regular gearing, to 83/84 of 0.200 or 0.19762 inch. This pitch, which is less than the 0.198 inch pitch required, is then increased by using the taper attachment as previously described. (This method was described by Mr. G. H. Gardner in MACHINERY, February, 1914.)

=Calculating Change Gears for Thread Cutting.=--As previously mentioned, the change gears for cutting threads of various pitches are shown by a table or ”index plate” attached to the lathe. The proper gears to be used can be calculated, but the use of the table saves time and tends to avoid mistakes. Every machinist, however, should know how to determine the size of gears used for cutting any number of threads to the inch.

Before referring to any rules, let us first consider why a lathe cuts a certain number of threads to the inch and how this number is changed by the use of different gears.

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