Part 8 (2/2)
=Allowance for Given Pressure.=--By transposing the preceding formula, the approximate allowance for a required ultimate tonnage can be determined. Thus, _a_ = 2_P_ _AF_. The average ultimate pressure in tons commonly used ranges from 7 to 10 times the diameter in inches.
a.s.suming that the diameter of a machine steel shaft is 4 inches and an ultimate pressure of about 30 tons is desired for forcing it into a cast-iron hub having a length of 5-1/2 inches, what should be the allowance?
_A_ = 4 3.1416 5-1/2 = 69 square inches,
_F_, for a diameter of 4 inches, = 115. Then,
2 30 _a_ = -------- = 0.0075 inch.
69 115
=Shrinkage Fits.=--When heat is applied to a piece of metal, such as iron or steel, as is commonly known, a certain amount of expansion takes place which increases as the temperature is increased, and also varies somewhat with different kinds of metal, copper and bra.s.s expanding more for a given increase in temperature than iron and steel. When any part which has been expanded by the application of heat is cooled, it contracts and resumes its original size. This expansive property of metals has been taken advantage of by mechanics in a.s.sembling various machine details. A cylindrical part which is to be held in position by a shrinkage fit is first turned a few thousandths of an inch larger than the hole; the diameter of the latter is then increased by heating, and after the part is inserted, the heated outer member is cooled, causing it to grip the pin or shaft with tremendous pressure.
General practice seems to favor a smaller allowance for shrinkage fits than for forced fits, although in many shops the allowances are practically the same in each case, and for some cla.s.ses of work, shrinkage allowances exceed those for forced fits. In any case, the shrinkage allowance varies to a great extent with the form and construction of the part which has to be shrunk into place. The thickness or amount of metal around the hole is the most important factor. The way in which the metal is distributed also has an influence on the results. Shrinkage allowances for locomotive driving wheel tires adopted by the American Railway Master Mechanics a.s.sociation are as follows:
Center diameter, inches 38 44 50 56 62 66 Allowance, inches 0.040 0.047 0.053 0.060 0.066 0.070
Whether parts are to be a.s.sembled by forced or shrinkage fits depends upon conditions. For example, to press a driving wheel tire over its wheel center, without heating, would ordinarily be a rather awkward and difficult job. On the other hand, pins, etc., are easily and quickly forced into place with a hydraulic press and there is the additional advantage of knowing the exact pressure required in a.s.sembling, whereas there is more or less uncertainty connected with a shrinkage fit, unless the stresses are calculated. Tests to determine the difference in the quality of shrinkage and forced fits showed that the resistance of a shrinkage fit to slippage was, for an axial pull, 3.66 times greater than that of a forced fit, and in rotation or torsion, 3.2 times greater. In each comparative test, the dimensions and allowances were the same.
The most important point to consider when calculating shrinkage fits is the stress in the hub at the bore, which depends chiefly upon the shrinkage allowance. If the allowance is excessive, the elastic limit of the material will be exceeded and permanent set will occur, or, in extreme cases, the ultimate strength of the metal will be exceeded and the hub will burst.
CHAPTER IV
THREAD CUTTING IN THE LATHE
When threads are cut in the lathe a tool _t_ is used (see Fig. 2), having a point corresponding to the shape of the thread, and the carriage is moved along the bed a certain distance for each revolution of the work (the distance depending on the number of threads to the inch being cut) by the lead-screw _S_ which is rotated by gears _a_, _b_ and _c_, which receive their motion from the spindle. As the amount that the carriage travels per revolution of the work, and, consequently, the number of threads per inch that is cut, depends on the size of the gears _a_ and _c_ (called change gears) the latter have to be changed for cutting different threads. The proper change gears to use for cutting a given number of threads to the inch is ordinarily determined by referring to a table or ”index plate” _I_ which shows what the size of gears _a_ and _c_ should be, or the number of teeth each should have, for cutting any given number of threads per inch.
[Ill.u.s.tration: Fig. 1. Measuring Number of Threads per Inch--Setting Thread Tool]
[Ill.u.s.tration: Fig. 2. Plan and Elevations of Engine Lathe]
=Selecting the Change Gears for Thread Cutting.=--Suppose a V-thread is to be cut on the end of the bolt _B_, Fig. 2, having a diameter of 1-1/4 inch and seven threads per inch of length, as shown at _A_ in Fig. 1, which is the standard number of threads per inch for that diameter.
First the change gears to use are found on plate _I_ which is shown enlarged in Fig. 3. This plate has three columns: The first contains different numbers of threads to the inch, the second the size gear to place on the ”spindle” or ”stud” at _a_ (Fig. 2) for different threads, and the third the size of gear _c_ for the lead-screw. As the thread selected as an example has 7 threads per inch, gear _a_ should have 48 teeth, this being the number given in the second column opposite figure 7 in the first. By referring to the last column, we find that the lead-screw gear should have 84 teeth. These gears are selected from an a.s.sortment provided with the lathe and they are placed on the spindle and lead-screw, respectively.
[Ill.u.s.tration: Fig. 3. Index Plate showing Gear Changes for Threading]
Intermediate gear _b_ does not need to be changed as it is simply an ”idler” for connecting gears _a_ and _c_. Gear _b_ is mounted on a swinging yoke _Y_ so that it can be adjusted to mesh properly with different gear combinations; after this adjustment is made, the lathe is geared for cutting 7 threads to the inch. (The change gears of many modern lathes are so arranged that different combinations are obtained by simply s.h.i.+fting a lever. A lathe having this quick-change gear mechanism is described in the latter part of this chapter.) The work _B_ is placed between the centers just as it would be for turning, with the end to be threaded turned to a diameter of 1-1/4 inch, which is the outside diameter of the thread.
=The Thread Tool.=--The form of tool used for cutting a V-thread is shown at _A_, Fig. 4. The end is ground V-shaped and to an angle of 60 degrees, which corresponds to the angle of a standard V-thread. The front or flank, _f_ of the tool is ground back at an angle to provide clearance, but the top is left flat or without slope. As it is very important to grind the end to exactly 60 degrees, a gage _G_ is used, having 60-degree notches to which the tool-point is fitted. The tool is clamped in the toolpost as shown in the plan view, Fig. 2, square with the work, so that both sides of the thread will be cut to the same angle with the axis of the work. A very convenient way to set a thread tool square is ill.u.s.trated at _B_, Fig. 1. The thread gage is placed against the part to be threaded, as shown, and the tool is adjusted until the angular sides of the point bear evenly in the 60-degree notch of the gage. The top of the tool point should be at the same height as the lathe centers, as otherwise the angle of the thread will not be correct.
[Ill.u.s.tration: Fig. 4. Thread Tools and Gage for testing Angle of End]
=Cutting the Thread.=--The lathe is now ready for cutting the thread.
This is done by taking several cuts, as indicated at _A_, _B_, _C_ and _D_ in Fig. 5, the tool being fed in a little farther for each successive cut until the thread is finished. When these cuts are being taken, the carriage is moved along the bed, as previously explained, by the lead-screw _S_, Fig. 2. The carriage is engaged with the lead-screw by turning lever _u_ which causes the halves of a split nut to close around the screw. The way a lathe is handled when cutting a thread is as follows: After the lathe is started, the carriage is moved until the tool-point is slightly beyond the right end of the work, and the tool is fed in far enough to take the first cut which, ordinarily, would be about 1/16 inch deep. The carriage is then engaged with the lead-screw, by operating lever _u_, and the tool moves to the left (in this case 1/7 inch for each revolution of the work) and cuts a winding groove as at _A_, Fig. 5. When the tool has traveled as far as the thread is wanted, it is withdrawn by a quick turn of cross-slide handle _e_, and the carriage is returned to the starting point for another cut. The tool is then fed in a little farther and a second cut is taken as at _B_, Fig.
5, and this operation is repeated as at _C_ and _D_ until a ”full”
thread is cut or until the top of the thread is sharp. The thread is then tested for size but before referring to this part of the work, the way the carriage is returned to the starting point after each cut should be explained.
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