SETTING THE THREADING TOOL

The threading tool should be set so that its top face is level with the center of the piece being threaded. Fig. 1 shows two methods of setting a tool to cut sharp V threads, or U. S. standard threads. The tool must be ground to fit perfectly the angle a cb of the center gauge d, which equals 60°.

In a square thread the space theoretically is equal to the thread width, Fig. 2 (a). Practically, the space equals 1.0015

60

width for clearance.

FIG. 1

The cutting part a of the tool is set at. an angle with the vertical side of the shank of the tool, so that it will follow the space being cut between the threads without.

bearing against either side.

Fig. 2 (c) is a right-hand tool.

and (d) a corresponding left-hand tool. The angle of side rake:

is found by drawing a straight line a b, Fig. 2 (e), equal in length to the circumference of the bolt at the root of the thread, and at one end a perpendicular b c equal to the lead of the thread.

(a)

Then the points a and c are joined by a straight line

FIG. 3

(b)

a c. The angle cab is the angle of side rake. The lead equals the width of the thread plus the width of the space. The distance c d is made equal to the width of the space, and the dotted lines d e and bf are drawn parallel to the slanting line a c.

The distance g h between the lines a c and d e is the shortest distance between the threads and represents the width of the tool at the cutting edge. The tool is shown in section in its correct angular position at i. In addition to its angle of side rake, it should have clearance on each side, as at j, so that it will not rub against the

sides of the groove, which are represented by the lines a c and ed. The angle of front rake should be 15°, as shown in view (b). The point of the tool, for a distance

of in. back from

the cutting edge,

FIG. 4

should have the correct width of the thread space, and the back of this point should be narrower, so as to cut a clean thread. It is a good plan to drill a hole b in (a) where the thread is to stop.

In Fig. 3 are shown two methods of setting an inside thread

ing tool. The face a should be turned up before the thread is cut, but if this is not done, the center gauge b can be set against the face of the chuck or

face plate.

In

cutting tapered

threads, the tool is set square to the axis, as shown in Fig. 4, when a taper attachment is used, which is the same as for cutting straight V threads. When the taper is threaded by setting over the tail

FIG. 5

stock the piece is first set over, and the tool is then set square to the axis as shown in Fig. 5.

CALCULATING CHANGE GEARS

Simple-Geared Lathe. To calculate the change gears for cutting a thread on the simple-geared lathe shown in Fig. 1. use the following formula:

EXAMPLE.-Eight threads per inch are to be cut on a lathe having 4 threads per inch on the lead screw. What change gears are required?

SOLUTION.-By the formula, X. A 32-tooth gear is placed at e on the stud and a gear g of 64 teeth is placed on the lead screw i. If these two gears are not found in the gear stack, use any other multipliers giving 2 gears in the same proportion. Thus, a 28-tooth gear could be used on the spindle and a 56-tooth gear on the lead screw.

If the fixed gear d'in Fig. 1 has twice as many teeth as fixed gear b on the spindle, the lathe is then said to be geared 1 to 2, and the number of teeth of change gear e obtained by the rule will have to be multiplied by 2. If the gear d has half as

many teeth as gear b, the lathe is geared 2 to 1 and the number of teeth on gear e by rule must be divided by 2.

The idler ƒ is used to fill the space between gears e and g, the gears h and c being reversing gears on the lever j.

Compound gearing makes use of two pairs of change gears, de and g h, Fig. 2. To find the two pairs of change gears to cut a given thread, use the following rule:

FIG. 1

Rule.-Make a fraction having the number of threads per inch on the lead screw above the line and the number of threads to be cut below the line. Separate the fraction thus formed into two factors and then multiply the part of each factor above the line and the part of each factor below the line by some number that will give results corresponding to number of teeth in gears in the gear stack. The gears corresponding to the numbers above the line in these factors are the drivers d and e, placed on the studs