Part 9 (2/2)
[Ill.u.s.tration: Fig. 10. End View of Lathe Headstock]
=Cutting a Left-hand Thread.=--The only difference between cutting left-hand and right-hand threads in the lathe is in the movement of the tool with relation to the work. When cutting a right-hand thread, the tool moves from right to left, but this movement is reversed for left-hand threads because the thread winds around in the opposite direction. To make the carriage travel from left to right, the lead-screw is rotated backwards by means of reversing gears _a_ and _b_ (Fig. 10) located in the headstock. Either of these gears can be engaged with the spindle gear by changing the position of lever _R_.
When gear _a_ is in engagement, as shown, the drive from the spindle to gear _c_ is through gears _a_ and _b_, but when lever _R_ is raised thus s.h.i.+fting _b_ into mesh, the drive is direct and the direction of rotation is reversed. The thread is cut by starting the tool at _a_, Fig. 8, instead of at the end.
[Ill.u.s.tration: Fig. 11. End of Square Thread Tool, and Graphic Method of Determining Helix Angle of Thread]
=Cutting a Square Thread.=--The form of tool used for cutting a square thread is shown in Fig. 11. The width _w_ is made equal to one-half the pitch of the thread to be cut and the end _E_ is at an angle with the shank, which corresponds to the inclination _x--y_ of the threads. This angle _A_ depends upon the diameter of the screw and the lead of the thread; it can be determined graphically by drawing a line _a--b_ equal in length to the circ.u.mference of the screw to be cut, and a line _b--c_, at right angles, equal in length to the lead of the thread. The angle [alpha] between lines _a--b_ and _a--c_ will be the required angle _A_. (See end view of thread tool). It is not necessary to have this angle accurate, ordinarily, as it is simply to prevent the tool from binding against the sides of the thread. The end of a square thread tool is shown in section to the right, to ill.u.s.trate its position with relation to the threads. The sides _e_ and _e_{1}_ are ground to slope inward, as shown, to provide additional clearance.
When cutting multiple threads, which, owing to their increased lead, incline considerably with the axis of the screw, the angles for each side of the tool can be determined independently as follows: Draw line _a--b_ equal in length to the circ.u.mference of the thread, as before, to obtain the required angle _f_ of the rear or following side _e_{1}_; the angle _l_ of the opposite or leading side is found by making _a--b_ equal to the circ.u.mference at the root of the thread. The tool ill.u.s.trated is for cutting right-hand threads; if it were intended for a left-hand thread, the end, of course, would incline in the opposite direction. The square thread is cut so that the depth _d_ is equal to the width. When threading a nut for a square thread screw, it is the usual practice to use a tool having a width slightly greater than one-half the pitch, to provide clearance for the screw, and the width of a tool for threading square-thread taps to be used for tapping nuts is made slightly less than one-half the pitch.
=Cutting Multiple Threads.=--When a multiple thread is to be cut, such as a double or triple thread, the lathe is geared with reference to the number of single threads to the inch. For example, the lead of the double thread, shown at _B_, Fig. 12, is one-half inch, or twice the pitch, and the number of single threads to the inch equals 1 1/2 = 2.
Therefore, the lathe is geared for cutting two threads per inch. The first cut is taken just as though a single thread were being cut, leaving the work as shown at _A_. When this cut is finished the work is turned one-half a revolution (for a double thread) without disturbing the position of the lead-screw or carriage, which brings the tool midway between the grooves of the single thread as indicated by dotted lines.
The second groove is then cut, producing a double thread as shown at _B_. In the case of a triple thread, the work would be indexed one-third of a revolution after turning the first groove, and then another third revolution to locate the tool for cutting the last groove. Similarly, for a quadruple thread, it would be turned one-quarter revolution after cutting each successive groove or thread.
There are different methods of indexing the work when cutting multiple threads, in order to locate the tool in the proper position for cutting another thread groove. Some machinists, when cutting a double thread, simply remove the work from the lathe and turn it one-half a revolution by placing the tail of the driving dog in the opposite slot of the faceplate. This is a very simple method, but if the slots are not directly opposite or 180 degrees apart, the last thread will not be central with the first. Another and better method is to disengage the idler gear from the gear on the stud, turn the spindle and work one-half, or one-third, of a revolution, as the case might be, and then connect the gears. For example, if the stud gear had 96 teeth, the tooth mes.h.i.+ng with the idler gear would be marked with chalk, the gears disengaged, and the spindle turned until the chalked tooth had made the required part of a revolution, which could be determined by counting the teeth. When this method is used, the number of teeth in the stud gear must be evenly divisible by two if a double thread is being cut, or by three for a triple thread, etc. If the stud is not geared to the spindle so that each makes the same number of revolutions, the ratio of the gearing must be considered.
[Ill.u.s.tration: Fig. 12. Views ill.u.s.trating how a Double Square Thread is Cut]
=Setting Tool When Cutting Multiple Threads.=--Another method, which can sometimes be used for setting the tool after cutting the first groove of a multiple thread, is to disengage the lock-nuts from the lead-screw (while the spindle is stationary) and move the carriage back whatever distance is required to locate the tool in the proper position for taking the second cut. Evidently this distance must not only locate the tool in the right place, but be such that the lock-nuts can be re-engaged with the lead-screw. Beginning with a simple ill.u.s.tration, suppose a double thread is being cut having a lead of 1 inch. After the first thread groove is cut, the tool can be set in a central position for taking the second cut, by simply moving the carriage back 1/2 inch (one-half the lead), or 1/2 inch plus the lead or any multiple of the lead. If the length of the threaded part were 5 inches, the tool would be moved back far enough to clear the end of the work, or say 1/2 + 5 = 5-1/2 inches. In order to disengage the lock-nuts and re-engage them after moving the carriage 5-1/2 inches (or any distance equal, in this case, to one-half plus a whole number), the lead-screw must have an even number of threads per inch.
a.s.sume that a double thread is being cut having 1-1/4 single threads per inch. The lead then would equal 1 1-1/4 = 0.8 inch, and if the carriage is moved back 0.8 2 = 0.4 inch, the tool will be properly located for the second cut; but the lock-nuts could not be re-engaged unless the lead-screw had ten threads per inch, which is finer than the pitch found on the lead-screws of ordinary engine lathes. However, if the movement were 0.4 + 0.8 2 = 2 inches, the lock-nuts could be re-engaged regardless of the number of threads per inch on the lead-screw. The rule then, is as follows:
_Divide the lead of the thread by 2 for a double thread, 3 for a triple thread, 4 for a quadruple thread, etc., thus obtaining the pitch; then add the pitch to any multiple of the lead, which will give a movement, in inches, that will enable the lock-nuts to be re-engaged with the lead-screw._
Whenever the number obtained by this rule is a whole number, obviously, the movement can be obtained with a lead-screw of any pitch. If the number is fractional, the number of threads per inch on the lead-screw must be divisible by the denominator of the fraction.
To ill.u.s.trate the application of the foregoing rule, suppose a quadruple thread is to be cut having 1-1/2 single threads per inch (which would be the number the lathe would be geared to cut). Then the lead of the thread = 1 1-1/2 = 0.6666 inch and the pitch = 0.6666 4 = 0.1666 inch; adding the pitch to twice the lead we have 0.1666 + 2 0.6666 = 1.499 inch. Hence, if the carriage is moved 1-1/2 inch (which will require a lead-screw having an even number of threads per inch), the tool will be located accurately enough for practical purposes. When the tool is set in this way, if it does not clear the end of the part being threaded, the lathe can be turned backward to place the tool in the proper position.
[Ill.u.s.tration: Fig. 13. Indexing Faceplate used for Multiple Thread Cutting]
The foregoing rule, as applied to triple threads or those of a higher number, does not always give the only distance that the carriage can be moved. To ill.u.s.trate, in the preceding example the carriage movement could be equal to 0.499, or what is practically one-half inch, instead of 1-1/2 inch, and the tool would be properly located. The rule, however, has the merit of simplicity and can be used in most cases.
Special faceplates are sometimes used for multiple thread cutting, that enable work to be easily and accurately indexed. One of these is ill.u.s.trated in Fig. 13; it consists of two parts _A_ and _B_, part _A_ being free to rotate in relation to _B_ when bolts _C_ are loosened. The driving pin for the lathe dog is attached to plate _A_. When one groove of a multiple thread is finished, bolts _C_ are loosened and plate _A_ is turned around an amount corresponding to the type of thread being cut. The periphery of plate _A_ is graduated in degrees, as shown, and for a double thread it would be turned one-half revolution or 180 degrees, for a triple thread, 120 degrees, etc. This is a very good arrangement where multiple thread cutting is done frequently.
[Ill.u.s.tration: Fig. 14. Correct and Incorrect Positions of Tool for Taper Thread Cutting]
=Taper Threading.=--When a taper thread is to be cut, the tool should be set square with axis _a--a_ as at _A_, Fig. 14, and not by the tapering surface as at _B_. If there is a cylindrical part, the tool can be set as indicated by the dotted lines. All taper threads should be cut by the use of taper attachments. If the tailstock is set over to get the required taper, and an ordinary bent-tail dog is used for driving, the curve of the thread will not be true, or in other words the thread will not advance at a uniform rate; this is referred to by machinists as a ”drunken thread.” This error in the thread is due to the angularity between the driving dog and the faceplate, which causes the work to be rotated at a varying velocity. The pitch of a taper thread that is cut with the tailstock set over will also be slightly finer than the pitch for which the lathe is geared. The amount of these errors depends upon the angle of the taper and the distance that the center must be offset.
=Internal Threading.=--Internal threading, or cutting threads in holes, is an operation performed on work held in the chuck or on a faceplate, as for boring. The tool used is similar to a boring tool except that the working end is shaped to conform to the thread to be cut. The method of procedure, when cutting an internal thread, is similar to that for outside work, as far as handling the lathe is concerned. The hole to be threaded is first bored to the root diameter _D_, Fig. 15, of the screw that is to fit into it. The tool-point (of a tool for a U. S. standard or V-thread) is then set square by holding a gage _G_ against the true side of the work and adjusting the point to fit the notch in the gage as shown. The view to the right shows the tool taking the first cut.
[Ill.u.s.tration: Fig. 15. Method of setting and using Inside Thread Tool]
Very often the size of a threaded hole can be tested by using as a gage the threaded part that is to fit into it. When making such a test, the tool is, of course, moved back out of the way. It is rather difficult to cut an accurate thread in a small hole, especially when the hole is quite deep, owing to the flexibility of the tool; for this reason threads are sometimes cut slightly under size with the tool, after which a tap with its shank end held straight by the tailstock center is run through the hole. In such a case, the tap should be calipered and the thread made just small enough with the tool to give the tap a light cut.
Small square-threaded holes are often finished in this way, and if a number of pieces are to be threaded, the use of a tap makes the holes uniform in size.
=Stop for Thread Tools.=--When cutting a thread, it is rather difficult to feed in the tool just the right amount for each successive cut, because the tool is moved in before it feeds up to the work. A stop is sometimes used for threading which overcomes this difficulty. This stop consists of a screw _S_, Fig. 16, which enters the tool slide and pa.s.ses through a block _B_ clamped in front of the slide. The hole in the block through which the stop-screw pa.s.ses is not threaded, but is large enough to permit the screw to move freely. When cutting a thread, the tool is set for the first cut and the screw is adjusted until the head is against the fixed block. After taking the first cut, the stop-screw is backed out, say one-half revolution, which allows the tool to be fed in far enough for a second cut. If this cut is about right for depth, the screw is again turned about one-half revolution for the next cut and this is continued for each successive cut until the thread is finished.
By using a stop of this kind, there is no danger of feeding the tool in too far as is often done when the tool is set by guess. If this form of stop is used for internal threading, the screw, instead of pa.s.sing through the fixed block, is placed in the slide so that the end or head will come against the stop _B_. This change is made because the tool is fed outward when cutting an internal thread.
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