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 brass 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 assembling various machine details. A cylindrical part which is to be held in position by a shrinkage fit is first turned a few thousandthsof 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 classes 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 Association are as follows:
Whether parts are to be assembled 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 assembling, 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.
When threads are cut in the lathe a tooltis used (seeFig. 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-screwSwhich is rotated by gearsa,bandc, 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 gearsaandc(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”Iwhich shows what the size of gearsaandcshould be, or the number of teeth each should have, for cutting any given number of threads per inch.
Measuring Number of Threads per Inch—Setting Thread ToolFig. 1. Measuring Number of Threads per Inch—Setting Thread Tool
Measuring Number of Threads per Inch—Setting Thread Tool
Fig. 1. Measuring Number of Threads per Inch—Setting Thread Tool
Plan and Elevations of Engine LatheFig. 2. Plan and Elevations of Engine Lathe
Plan and Elevations of Engine Lathe
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 boltB, Fig. 2, having a diameter of 11/4inch and seven threads per inch of length, asshown atAinFig. 1, which is the standard number of threads per inch for that diameter. First the change gears to use are found on plateIwhich is shown enlarged inFig. 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” ata(Fig. 2) for different threads, and the third the size of gearcfor the lead-screw. As the thread selected as an example has 7 threads per inch, gearashould 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 assortment provided with the lathe and they are placed on the spindle and lead-screw, respectively.
Intermediate gearbdoes not need to be changed as it is simply an “idler” for connecting gearsaandc. Gearbis mounted on a swinging yokeYso 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 shifting a lever. A lathe having this quick-change gear mechanism is described in the latter part of this chapter.) The workBis placed between the centers just as it would be for turning, with the end to be threaded turned to a diameter of 11/4inch, which is the outside diameter of the thread.
Index Plate showing Gear Changes for ThreadingFig. 3. Index Plate showing Gear Changes for Threading
Fig. 3. Index Plate showing Gear Changes for Threading
The Thread Tool.—The form of tool used for cutting a V-thread is shown atA,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,fof 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 gageGis 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 illustrated atB,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.
Thread Tools and Gage for testing Angle of EndFig. 4. Thread Tools and Gage for testing Angle of End
Thread Tools and Gage for testing Angle of End
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 atA,B,CandDinFig. 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-screwS,Fig. 2. The carriage is engaged with the lead-screw by turning leveruwhich 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 thelathe 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 about1/16inch deep. The carriage is then engaged with the lead-screw, by operating leveru, and the tool moves to the left (in this case1/7inch for each revolution of the work) and cuts a winding groove as atA,Fig. 5. When the tool has traveled as far as the thread is wanted, it is withdrawn by a quick turn of cross-slide handlee, 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 atB,Fig. 5, and this operation is repeated as atCandDuntil 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.
Thread is formed by taking a Number of Successive CutsFig. 5. Thread is formed by taking a Number of Successive Cuts
Thread is formed by taking a Number of Successive Cuts
Fig. 5. Thread is formed by taking a Number of Successive Cuts
When the tool is withdrawn at the end of the first cut, if the carriage is disengaged from the lead-screw and returned by hand, the tool may or may not follow the first cut when the carriage is again engaged with the lead-screw. If the number of threads to the inch being cut is a multiple of the number on the lead-screwS, then the carriage can be returned by hand andengaged with the lead-screw at random and the tool will follow the first cut. For example, if the lead-screw has six threads per inch, and 6, 12, 18 or any number of threads is being cut that is a multiple of six, the carriage can be engaged at any time and the tool will always follow the original cut. This is not the case, however, when the number of threads being cut is not a multiple of the number on the lead-screw.
One method of bringing the carriage back to the starting point, when cutting threads which are not multiples, is to reverse the lathe (by shifting the overhead driving belts) in order to bring the tool back to the starting point without disengaging the carriage; in this way the tool is kept in the same relation to the work, and the carriage is not disengaged from the lead-screw until the thread is finished. This is a good method when cutting short threads having a length of say two or three inches; but when they are longer, and especially when the diameter is comparatively large (which means a slower speed), it is rather slow as considerable time is wasted while the tool is moving back to its starting point. This is due to the fact that the carriage is moved slowly by the lead-screw, but when disengaged, it can be traversed quickly by turning handled,Fig. 2.
A method of returning the carriage by hand when the number of threads being cut is not a multiple of the number on the lead-screw is as follows: The tool is moved a little beyond the right end of the work and the carriage or split nut is engaged with the lead-screw. The lathe is then turned forward by hand to take up any lost motion, and a line is made on the lathe bed showing the position of the carriage. The positions of the spindle and lead-screw are also marked by chalking a tooth on both the spindle and lead-screw gears, which happens to be opposite a corner or other point on the bed. After a cut is taken, the carriage is returned by hand to the original starting point as shown by the line on the bed, and is again engaged when the chalk marks show that the spindle and lead-screw are in their original position; the tool will then follow the first cut. If the body of the tailstock is moved against the bridge of the carriage before starting the first cut, the carriage can be locatedfor each following cut by moving it back against the tailstock, and it will not be necessary to have a line on the bed.
Indicator used when Cutting ThreadsFig. 6. Indicator used when Cutting Threads
Indicator used when Cutting Threads
Fig. 6. Indicator used when Cutting Threads
Indicator or Chasing Dial for Catching Threads.—On some lathes there is an indicator for “catching threads,” as this is called in shop language. This is a simple device attached to the carriage and consists of a graduated dialDand a worm-wheelW(seeFigs. 2and6) which meshes with the lead-screw, so that the dial is revolved by the lead-screw when the carriage is stationary, and when the carriage is moved by the screw, the dial remains stationary. The indicator is used by engaging the carriage when one of the graduation lines is opposite the arrow mark; after a cut is taken the carriage is returned by hand and when one of the graduation lines again moves opposite the arrow, the half-nuts are thrown into mesh, as before, and this is repeated for each successive cut, thus causing the tool to always come right with the thread. If the number of threads per inch is even, engagement can be made when any line is opposite the arrow, but for odd numbers such as 3, 7, 9, 11, etc., one of the four long or numbered lines must be used. Of course, if the thread being cut is a multiple of the number on the lead-screw, engagement can be made at any time, as previously mentioned.
Principle of the Thread Indicator.—The principle upon which the thread indicator operates is as follows: The number of teeth in worm-wheelWis some multiple of the number of threads per inch of the lead-screw, and the number of teeth in the worm-wheel, divided by the pitch of the screw, equals the number of graduations on the dial. For example, if the lead-screw has six threads per inch, the worm-wheel could have twenty-four teeth, in which case the dial would have four divisions, each representing an inch of carriage travel, and by sub-dividing the dial into eighths (as shown) each line would correspond to1/2inch of travel. The dial, therefore, would enable the carriage to be engaged with the lead-screw at points equal to a travel of one-half inch. To illustrate the advantage of this suppose ten threads per inch are being cut and (with the lathe stationary) the carriage is disengaged and moved1/6inch or one thread on the lead-screw; the tool point will also have moved1/6inch, but it will not be opposite the next thread groove in the work as the pitch is1/10inch. If the carriage is moved another thread on the lead-screw, or2/6inch, the tool will still be out of line with the thread on the work, but when it has moved three threads, or1/2inch, the tool will then coincide with the original cut because it has passed over exactly five threads. This would be true for any number of threads per inch that is divisible by 2. If the thread being cut had nine threads per inch or any other odd number, the tool would only coincide with the thread at points 1 inch apart. Therefore, the carriage can only be engaged when one of the four graduations representing an inch of travel is opposite the arrow, when cutting odd threads; whereas even numbers can be “caught” by using any one of the eight lines.
This indicator can also be used for “catching” fractional threads. As an illustration, suppose 111/2threads per inch are to be cut, and the carriage is engaged for the first cut when graduation line 1 is opposite the arrow; engagement would then be made for each successive cut, when either line 1 or 3 were opposite the arrow, or in other words at spaces equal to a carriage movement of 2 inches. As the use of the indicator whencutting fractional threads is liable to result in error, it is better to keep the half-nuts in engagement and return the carriage by reversing the lathe.
Replacing Sharpened Thread Tool.—If it is necessary to sharpen the thread tool before the thread is finished, it should be reset square with the work by testing with the thread gage as atB,Fig. 1. The carriage is then engaged with the lead-screw and the lathe is turned forward to bring the tool opposite the partly finished thread and also to take up any backlash or lost motion in the gears or half-nut. If the tool-point is not in line with the thread groove previously cut, it can be shifted sidewise by feeding the compound restEin or out, provided the latter is set in an angular position as shown in the plan view,Fig. 2.
If the thread tool is ground flat on the top as atA,Fig. 4, it is not a good tool for removing metal rapidly as neither of its two cutting edges has any slope. In order to give each cutting edge a backward slope, it would be necessary to grind the top surface hollow or concave, which would be impracticable. When a course thread is to be cut, a tool shaped as atBcan be used to advantage for rough turning the thread groove, which is afterward finished to the correct depth and angle by toolA. This roughing tool is ground with a backward slope from the point and the latter is rounded to make it stronger.
Cutting Thread by using Compound RestFig. 7. Cutting Thread by using Compound Rest
Cutting Thread by using Compound Rest
Fig. 7. Cutting Thread by using Compound Rest
Use of Compound Rest for Thread Cutting.—Another form of thread tool is shown atA,Fig. 7, which is very good for cutting V-threads especially of coarse pitch. When this tool is used, the compound restEis set to an angle of 30 degrees, as shown, and it is fed in for the successive cuts by handlewin the direction indicated by the arrow. It will be seen that the point a of the tool moves at an angle of 60 degrees with the axis of the work, thus forming one side of the thread, and the cutting edgea—b, which can be set as shown atB, forms the opposite side and does all the cutting. As this edge is given a backward slope, as shown, it cuts easily and enables threading operations to be performed quickly. Threads cut in this way are often finished by taking a light cut with a regular thread tool. Thecutting edgea—bis ground to an angle of 60 degrees (or slightly less, if anything) with the side, as shown by sketchA.
When cutting threads in steel or wrought iron, some sort of lubricant is usually applied to the tool to preserve the cutting end and give a smooth finish to the thread. Lard oil or a mixture of equal parts of lard oil and paraffin oil are often used for this purpose. If the thread is small, the lubricant may be applied from an ordinary oil can, but when cutting comparatively large threads, it is better to have a stream of oil constantly playing upon the tool-point. This constant flow may be obtained by mounting a can having a spout leading to the tool, on a bracket at the rear of the carriage.
ThreadsFig. 8. (A) V-thread.(B) U. S. Standard Thread.(C) Square Thread.(D) Left-hand Thread.(E) Double Square Thread.(F) Triple Square Thread
Threads
Fig. 8. (A) V-thread.(B) U. S. Standard Thread.(C) Square Thread.(D) Left-hand Thread.(E) Double Square Thread.(F) Triple Square Thread
Threads Commonly Used.—Three forms of threads or screws which are in common use are shown inFig. 8; these are the V-thread (A), the U. S. standard (B), and the square thread (C). The shapes of these threads are shown by the sectioned parts. The V-thread has straight sides which incline at anangle of 60 degrees with each other and at the same angle with the axis of the screw. The U. S. standard thread is similar to the V-thread except that the top of the thread and bottom of the groove is left flat, as shown, and the width of these flats is made equal to1/8of the pitch. The square thread is square in section, the widtha, depthband spacecbeing all equal. All of these threads are right-hand, which means that the grooves wind around to the right so that a nut will have to be turnedtoward the right to enter it on the thread. A left-hand thread winds in the other direction, as shown atD, and a nut is screwed on by turning it to the left.
Multiple Threads.—Threads, in addition to being right-and left-handed, are single, as atA,B,CandD, double, as atE, and triple, as atF, and for certain purposes quadruple threads or those of a higher multiple are employed. A double thread is different from a single thread in that it has two grooves, starting diametrically opposite, whereas a triple thread has three grooves cut as shown atF. The object of these multiple threads is to obtain an increase in lead without weakening the screw. For example, the threads shown atCandEhave the same pitchpbut the leadlof the double-threaded screw is twice that of the one with a single thread so that a nut would advance twice as far in one revolution, which is often a very desirable feature. To obtain the same lead with a single thread, the pitch would have to be double, thus giving a much coarser thread, which would weaken the screw, unless its diameter were increased. (The lead is the distancelthat one thread advances in a single turn, or the distance that a nut would advance in one turn, and it should not be confused with the pitchp, which is the distance between the centers of adjacent threads. Obviously the lead and pitch of a single thread are the same.)
U. S. Standard Thread, Thread Tool, and GageFig. 9. U. S. Standard Thread, Thread Tool, and Gage
U. S. Standard Thread, Thread Tool, and Gage
Fig. 9. U. S. Standard Thread, Thread Tool, and Gage
Cutting a U. S. Standard Thread.—The method of cutting a U. S. standard thread is the same as described for a V-thread, so far as handling the lathe is concerned. The thread tool must correspond, of course, to the shape of a U. S. standard thread. This tool is first ground to an angle of 60 degrees, as it would be for cutting a V-thread, and then the point is made flat as shown inFig. 9. As will be recalled, the width of this flat should be equal to1/8of the pitch. By using a gage like the one shown atG, the tool can easily be ground for any pitch, as the notches around the periphery of the gage are marked for different pitches and the tool-point is fitted into the notch corresponding to the pitch wanted. If such a gage is not available, the width of the flat at the point can be tested by using, as a gage, a U. S. standard tap of the same pitch as the thread to be cut.
When cutting the thread, the tool is set square with the blank, and a number of successive cuts are taken, the tool being fed in until the width w of the flat at the top of the thread is equal to the width at the bottom. The thread will then be the right size provided the outside diameterDis correct and the tool is of the correct form. As it would be difficult to measure the width of this flat accurately, the thread can be tested by screwing a standard nut over it if a standard thread is being cut. If it is being fitted to a tapped hole, the tap itself is a very convenient gage to use, the method being to caliper the tap and then compare its size with the work.
A good method of cutting a U. S. standard thread to a given size is as follows: First turn the outside of the blank accurately to diameterD, and then turn a small part of the end to diameterrof the thread at the root. The finishing cut for the thread is then taken with the tool point set to just graze diameterr. If ordinary calipers were set to diameterrand measurements taken in the thread groove, the size would be incorrect owing tothe angularity of the groove, which makes it necessary to hold the calipers at an angle when measuring. To determine the root diameter divide 1.299 by the number of threads per inch and subtract the quotient from the outside diameter. Expressing this rule as a formula,
in whichDequals outside diameter;N, the number of threads per inch; andr, the root diameter. The number 1.299 is a constant that is always used.
End View of Lathe HeadstockFig. 10. End View of Lathe Headstock
End View of Lathe Headstock
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 gearsaandb(Fig. 10) located in theheadstock. Either of these gears can be engaged with the spindle gear by changing the position of leverR. When gearais in engagement, as shown, the drive from the spindle to gearcis through gearsaandb, but when leverRis raised thus shiftingbinto mesh, the drive is direct and the direction of rotation is reversed. The thread is cut by starting the tool ata,Fig. 8, instead of at the end.
End of Square Thread Tool, and Graphic Method of Determining Helix Angle of ThreadFig. 11. End of Square Thread Tool, and Graphic Method of Determining Helix Angle of Thread
End of Square Thread Tool, and Graphic Method of Determining Helix Angle of Thread
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 inFig. 11. The widthwis made equal to one-half the pitch of the thread to be cut and the endEis at an angle with the shank, which corresponds to the inclinationx—yof the threads. This angleAdepends upon the diameter of the screw and the lead of the thread; it can be determined graphically by drawing a linea—bequal in length to the circumference of the screw to be cut, and a lineb—c, at right angles, equal in length to the lead of the thread. The angle α between linesa—banda—cwill be the required angleA. (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 illustrate its position with relation to the threads. The sideseande1are 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 linea—bequal in length to the circumference of the thread, as before, to obtain the required anglefof the rear or following sidee1; the anglelof the opposite or leading side is found by makinga—bequal to the circumference at the root of the thread. The tool illustrated 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 depthdis 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.
Views illustrating how a Double Square Thread is CutFig. 12. Views illustrating how a Double Square Thread is Cut
Views illustrating how a Double Square Thread is Cut
Fig. 12. Views illustrating how a Double Square Thread is Cut
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 atB,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 atA. 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 atB. 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 theproper 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 meshing 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.
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 illustration, 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 back1/2inch (one-half the lead), or1/2inch 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 say1/2+ 5 = 51/2inches. In order to disengage the lock-nuts and re-engage them after moving the carriage 51/2inches (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.
Assume that a double thread is being cut having 11/4single threads per inch. The lead then would equal 1 ÷ 11/4= 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-screwof 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 illustrate the application of the foregoing rule, suppose a quadruple thread is to be cut having 11/2single threads per inch (which would be the number the lathe would be geared to cut). Then the lead of the thread = 1 ÷ 11/2= 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 11/2inch (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.
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 illustrate, in the preceding example the carriage movement could be equal to 0.499, or what is practically one-half inch, instead of 11/2inch, and the tool would be properly located. The rule, however, has the merit of simplicity and can be used in most cases.
Indexing Faceplate used for Multiple Thread CuttingFig. 13. Indexing Faceplate used for Multiple Thread Cutting
Indexing Faceplate used for Multiple Thread Cutting
Fig. 13. Indexing Faceplate used for Multiple Thread Cutting
Special faceplates are sometimes used for multiple thread cutting, that enable work to be easily and accurately indexed. One of these is illustrated inFig. 13; it consists of two partsAandB, partAbeing free to rotate in relation toBwhen boltsCare loosened. The driving pin for the lathe dog is attached to plateA. When one groove of a multiple thread is finished, boltsCare loosened and plateAis turned around an amount corresponding to the type of thread being cut. The periphery of plateAis 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.
Correct and Incorrect Positions of Tool for Taper Thread CuttingFig. 14. Correct and Incorrect Positions of Tool for Taper Thread Cutting
Correct and Incorrect Positions of Tool for Taper Thread Cutting
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 axisa—aas atA,Fig. 14, and not by the tapering surface as atB. 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.
Method of setting and using Inside Thread ToolFig. 15. Method of setting and using Inside Thread Tool
Method of setting and using Inside Thread Tool
Fig. 15. Method of setting and using Inside Thread Tool
Internal Threading.—Internal threading, or cutting threads in holes, is an operation performed on work held in the chuck oron 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 diameterD,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 gageGagainst 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.
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.