Cross-slide equipped with Stop for Regulating Depth of Cut when ThreadingFig. 16. Cross-slide equipped with Stop for Regulating Depth of Cut when Threading
Cross-slide equipped with Stop for Regulating Depth of Cut when Threading
Fig. 16. Cross-slide equipped with Stop for Regulating Depth of Cut when Threading
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 tothe work. A stop is sometimes used for threading which overcomes this difficulty. This stop consists of a screwS,Fig. 16, which enters the tool slide and passes through a blockBclamped in front of the slide. The hole in the block through which the stop-screw passes 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 passing through the fixed block, is placed in the slide so that the end or head will come against the stopB. This change is made because the tool is fed outward when cutting an internal thread.
Gage for grinding and setting Acme Thread ToolsFig. 17. Gage for grinding and setting Acme Thread Tools
Gage for grinding and setting Acme Thread Tools
Fig. 17. Gage for grinding and setting Acme Thread Tools
The Acme Standard Thread.—The Acme thread is often used, at the present time, in place of a square thread. The angle between the sides of the Acme thread is 29 degrees (seeFig. 21) and the depth is made equal to one-half the pitch plus0.010 inch to provide clearance and insure a bearing upon the sides. The thread tool is ordinarily ground to fit a gage having notches representing different pitches. An improved form of Acme thread gage is shown inFig. 17. The tool point is first ground to the correct angle by fitting it to the 29-degree notch in the end of the gage, as atA. The end is then ground to the proper width for the pitch to be cut, by testing it, as atB. The numbers opposite the shallow notches for gaging the width represent the number of threads per inch. With this particular gage, the tool can be set square by placing edgeDagainst the turned surface to be threaded, and adjusting the tool until the end is in line with the gage, as atC. By placing the tool in this position, the angle between the side and the end can also be tested.
Measuring Width of Acme Thread Tool with Vernier Gear-tooth CaliperFig. 18. Measuring Width of Acme Thread Tool with Vernier Gear-tooth Caliper
Measuring Width of Acme Thread Tool with Vernier Gear-tooth Caliper
Fig. 18. Measuring Width of Acme Thread Tool with Vernier Gear-tooth Caliper
In case it should be necessary to measure the end width of an Acme thread tool, for a pitch not on the regular gage, this can be done by using a vernier gear-tooth caliper, as indicated inFig. 18. If we assume that the caliper jaws bear on the sides of the tool at a distanceAfrom the top, equal to1/4inch, then the width of the tool point equals the caliper reading (as shown by the horizontal scale) minus 0.1293 inch. For example, if the caliper reading was 0.315 inch, the width at the point would equal 0.315 - 0.1293 = 0.1857 inch, assuming that the sides were ground to the standard angle of 29 degrees. The constantto be subtracted from the caliper reading equals 2Atan 14° 30' or, in this case, 2 × 0.25 × 0.2586 = 0.1293.
The Whitworth Thread.—The Whitworth (or British Standard Whitworth) thread, which is used principally in Great Britain, has an included angle of 55 degrees, and the threads are rounded at the top and at the root, as shown inFig. 23. The shape of the tool used for cutting this thread is also shown in this illustration. The end is rounded to form the fillet at the root of the thread, and the round corners on the sides give the top of the thread the required curvature. Every pitch requires a different tool, and the cutting end is given the curved form by milling or hobbing. The hob used for this purpose is accurately threaded to correspond with the pitch for which the tool is required, and then it is fluted to form cutting edges, and is hardened. The hob is then used like a milling cutter for forming the end of the thread tool. The tool is sharpened by grinding on the top. The method of cutting a Whitworth thread is, of course, similar to that followed for a U. S. standard or V-thread, in that the tool is set square with the unthreaded blank and at the same height as the lathe centers, in order to secure a thread of the proper form. Care should be taken to turn the blank to the right diameter so that the top of the thread will be fully rounded when the screw is the required size.
United States Standard Thread and Standard Sharp V-ThreadFig. 19. United States Standard ThreadFig. 20. Standard Sharp V-thread
United States Standard Thread and Standard Sharp V-Thread
Fig. 19. United States Standard ThreadFig. 20. Standard Sharp V-thread
Acme Standard and Square ThreadFig. 21. Acme Standard ThreadFig. 22. Square Thread
Acme Standard and Square Thread
Fig. 21. Acme Standard ThreadFig. 22. Square Thread
Whitworth Standard and Standard Worm ThreadFig. 23. Whitworth Standard ThreadFig. 24. Standard Worm Thread
Whitworth Standard and Standard Worm Thread
Fig. 23. Whitworth Standard ThreadFig. 24. Standard Worm Thread
Worm Threads.—The standard worm thread has an angle of 29 degrees between the sides, the same as an Acme thread, but the depth of a worm thread and the width of the flat at the top and bottom differ from the Acme standard, as will be seen by comparingFigs. 21and24. The whole depth of the thread equals the linear pitch multiplied by 0.6866, and the width of the thread tool at the end equals the linear pitch multiplied by 0.31. Gages notched for threads of different pitch are ordinarily used when grinding worm thread tools.
When it is necessary to cut multiple-threaded worms of large lead in an ordinary lathe, difficulty is sometimes experienced because the lead-screw must be geared to run much faster than the spindle, thus imposing excessive strains on the gearing. This difficulty is sometimes overcome by mounting a belt pulley on the lead-screw, beside the change gear, and connecting it to the countershaft by a belt; the spindle is then driven through the change gearing from the lead-screw, instead ofvice versa.
Coarse Threading Attachment.—To avoid the difficulties connected with cutting threads of large lead, some lathes are equipped with a coarse screw-cutting attachment. The arrangement of this attachment, as made by the Bradford Machine Tool Co., is as follows: On the usual reversing shaft, and inside of the headstock, there is a sliding double gear, so arranged as to be engaged with either the usual gear on the spindle, or with a small pinion at the end of the cone. The gears are so proportioned that the ratio of the two engagements is as 10 to 1; that is, when engaged with the cone gear (the back-gears being thrown in) the mating gear will make ten revolutions to one of the spindle, so that when the lathe is ordinarily geared to cut one thread per inch, it will, when driven by the cone pinion, cut one thread in ten inches. This construction dispenses with the extra strain on the reverse gears due to moving the carriage at the rapid rate that would be necessary for such a large lead, when not using an attachment. These attachments are not only extensively used for the cutting of coarse screws but for cutting oil grooves on cylindrical parts.
When cutting a thread of large lead or “steep pitch,” the top of the thread tool should be ground so that it is at right angles to the thread; then the thread groove will be cut to the same width as the tool.
Testing the Size of a Thread.—When the thread tool has been fed in far enough to form a complete thread, the screw is then tested for size. If we assume that a bolt is being threaded for a standard nut, it would be removed from the lathe and the test made by screwing a nut on the end. If the thread were too large, the nut might screw on very tightly or not at all; in either case, the work would again be placed in the lathe and a light cut taken over it to reduce the thread to the proper size. When replacing a threaded part between the centers, it should be put back in the original position, that is, with the “tail” of the driving dog in the same slot of the faceplate it previously occupied.
Testing Diameter of Thread with Calipers and MicrometerFig. 25. Testing Diameter of Thread with Calipers and Micrometer
Testing Diameter of Thread with Calipers and Micrometer
Fig. 25. Testing Diameter of Thread with Calipers and Micrometer
As it is difficult to tell just when a thread is cut to the exact size, special thread calipers having wedge-shaped ends are sometimes used for measuring the diameter of a V-thread or a U. S. standard thread, at the bottom of the grooves or the root diameter, as shown atAinFig. 25. These calipers can be set from a tap corresponding to the size of the thread being cut, or from a previously threaded piece of the right size.
The Thread Micrometer.—Another form of caliper for testing threads is shown atB. This is one of the micrometer type and is intended for very accurate work. The spindle of this micrometer has a conical end and the “anvil” is V-shaped, and these ends bear on the sides of the thread or the surfaces which form the bearing when the screw is inserted in a nut or threaded hole. The cone-shaped point is slightly rounded so that it will not bear in the bottom of the thread. There is also sufficient clearance at the bottom of the V-shaped anvil to prevent it from bearing on top of the thread. The diameter as indicated by this micrometer is the “pitch diameter” of the thread and is equal to the outside diameter minus the depth of one thread. This depth may be determined as follows:
Depth of a V-thread = 0.866 ÷ No. of threads per inch;
Depth of a U. S. standard thread = 0.6495 ÷ No. of threads per inch;
Depth of Whitworth thread = 0.6403 ÷ No. of threads per inch.
The movable point measures all pitches, but the fixed anvil is limited in its capacity, for if made large enough to measure a thread of, say,1/4-inch pitch, it would be too wide at the top to measure a thread of1/24-inch pitch, hence each caliper is limited in the range of threads that the anvil can measure. When measuring the “angle diameter” of a thread, the micrometer should be passed back and forth across the thread, in order to make sure that the largest dimension or the actual diameter is being measured. If the micrometer is placed over what seems to be the center of the screw and the reading is taken by simply adjusting in the anvil or point against the thread, without moving the micrometer back and forth across it, an incorrect reading may be obtained.
Testing ThreadFig. 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
Testing Thread
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
If standard threaded reference gages are available, the size of the thread being cut can be tested by comparing it with the gage. Micrometers having small spherical measuring ends (see sketchA,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.
Three-wire System of Measuring Threads.—A method of measuring threads by using an ordinary micrometer and three wires of equal diameter is illustrated atBandC,Fig. 26. Two wires are placed between the threads on one side and one on the opposite side of the screw. The dimensionMover the wires is then measured with an ordinary micrometer. When the thread is cut to a standard size, the dimensionMfor different threads is as follows:
For a U. S. standard thread:
m=d- 1.5155p+ 3w
For a sharp V-thread:
m=d- 1.732p+ 3w
For a Whitworth standard thread:
m=d- 1.6008p+ 3.1657w
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 illustrate the use of the formula for the U. S. standard thread, let us assume that a screw having 6 threads per inch (1/6-inch pitch) is to be cut to a diameter of 11/2inch, and that wires0.140 inch diameter are to be used in conjunction with a micrometer for measurement. Then the micrometer readingmshould be
11/2- 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.
Rivett-Dock Circular Threading Tool in Working PositionFig. 27. Rivett-Dock Circular Threading Tool in Working Position
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 inFig. 27. This tool has a circular-shaped cutterC, having ten teeth around its circumference, 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 slideS, that is fitted to the frameF, and can be moved in or out by leverL. The hub of this lever has an eccentric stud which moves slideSand locks it when in the forward or cutting position. The action of the lever in moving the slide engages the cutter with pawlP, 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 ismoved 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 illustration. 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 frameF. At first a light cut is taken with leverLmoved 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 finishing cut, but with a circular tool this is not necessary, for by using the different teeth successively, the last tooth, which only takes finishing 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.
Diagram Illustrating Method of Cutting a Thread to Compensate for the Error in Pitch due to Shrinkage in HardeningFig. 28. Diagram Illustrating Method of Cutting a Thread toCompensate for the Error in Pitch due to Shrinkage in Hardening
Diagram Illustrating Method of Cutting a Thread to Compensate for the Error in Pitch due to Shrinkage in Hardening
Fig. 28. Diagram Illustrating Method of Cutting a Thread toCompensate for the Error in Pitch due to Shrinkage in Hardening
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 anglea,Fig. 28, the cosine of which equals 0.125÷0.12502; that is, the cosine of angleaequalsthe pitch required after hardening, divided by thepitch necessary to compensate for shrinkage. The angle is then found by referringto 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 distancex, the tool point will have moved a greater distanceyalong the work, the difference between x and y depending upon anglea; hence the tool will cut a thread of slightly greater pitch than the lathe is geared to cut.
To illustrate by using the preceding example, cosine of anglea= 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 angleainFig. 28has been exaggerated in order to more clearly illustrate the principle.)
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 requiredpitch. 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, to83/84of 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 inMachinery, 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.
Lathe with Simple Gearing for Thread Cutting, Compound Geared LatheFig. 29. (A) Lathe with Simple Gearing for Thread Cutting.(B) Compound Geared Lathe
Lathe with Simple Gearing for Thread Cutting, Compound Geared Lathe
Fig. 29. (A) Lathe with Simple Gearing for Thread Cutting.(B) Compound Geared Lathe
As the carriageCand the tool are moved by the lead-screwS(seeFig. 2), which is geared to the spindle, the number of threads to the inch that are cut depends, in every case, upon the number of turns the work makes while the lead-screw is moving the carriage one inch. If the lead-screw has six threads per inch, it will make six revolutions while the carriage and the thread tool travel one inch along the piece to be threaded. Now if the change gearsaandc(see also sketchA,Fig. 29) are so proportioned that the spindle makes the same number of revolutions as the lead-screw, in a given time, it is evident that the tool will cut six threads per inch. If the spindle revolved twice as fast as the lead-screw, it would make twelve turns while the tool moved one inch, and, consequently, twelve threads per inch would be cut; but to get this difference in speeds it is necessary to use a combination of gearing that will cause the lead-screw to revolve once while the lathe spindle and work make two revolutions.
Suppose that nine threads to the inch are to be cut and the lead-screw has six threads per inch. In this case the work must make nine revolutions while the lead-screw makes six and causes the carriage and thread tool to move one inch, or in other words, one revolution of the lead-screw corresponds to one and one-half revolution of the spindle; therefore, if the lead-screw gearchas 36 teeth, the gearaon the spindle stud should have 24 teeth. The spindle will then revolve one and one-half times faster than the lead-screw, provided the stud rotates at the same rate of speed as the main lathe spindle. The number of teeth in the change gears that is required for a certain pitch can be found by multiplying the number of threads per inch of the lead-screw, and the number of threads per inch to be cut, by the same trial multiplier. The formula which expresses the relation between threads per inch of lead-screw, threads perinch to be cut, and the number of teeth in the change gears, is as follows:
Applying this to the example given, we have 6÷9 = 24÷36. The values of 36 and 24 are obtained by multiplying 6 and 9, respectively, by 4, which, of course, does not change the proportion. Any other number could be used as a multiplier, and if gears having 24 and 36 teeth were not available, this might be necessary. For example, if there were no gears of this size, some other multiplier as 5 or 6 might be used.
Suppose the number of teeth in the change gears supplied with the lathe are 24, 28, 32, 36, etc., increasing by four teeth up to 100, and assume that the lead-screw has 6 threads per inch and that 10 threads per inch are to be cut. Then,
By multiplying both numerator and denominator by 4, we obtain two available gears having 24 and 40 teeth, respectively. The 24-tooth gear goes on the spindle stud and, the 40-tooth gear on the lead-screw. The number of teeth in the intermediate or “idler” gearb, which connects the stud and lead-screw gears, is not considered as it does not affect the ratios between gearsaandc, but is used simply to transmit motion from one gear to the other.
We have assumed in the foregoing that the spindle stud (on which gearais mounted) and the main spindle of the lathe are geared in the ratio of one to one and make the same number of revolutions. In some lathes, however, these two members do not rotate at the same speed, so that if equal gears were placed on the lead-screw and spindle stud, the spindle would not make the same number of revolutions as the lead-screw. In that case if the actual number of threads per inch in the lead-screw were used when calculating the change gears, the result would be incorrect; hence, to avoid mistakes, the following general ruleshould be used as it gives the correct result, regardless of the ratios of the gears which connect the spindle and spindle stud:
Rule.—First find the number of threads per inch that is cut when gears of the same size are placed on the lead-screw and spindle, either by actual trial or by referring to the index plate. Then place this number as the numerator of a fraction and the number of threads per inch to be cut, as the denominator; multiply both numerator and denominator by some trial number, until numbers are obtained which correspond to numbers of teeth in gears that are available.The product of the trial number and the numerator (or “lathe screw constant”) represents the gearafor the spindle stud, and the product of the trial number and the denominator, the gear for the lead-screw.
Lathes with Compound Gearing.—When gearing is arranged as shown atA,Fig. 29, it is referred to as simple gearing, but sometimes it is necessary to introduce two gears between the stud and screw as atB, which is termed compound gearing. The method of figuring compound gearing is practically the same as that for simple gearing. To find the change gears used in compound gearing, place the “screw constant” obtained by the foregoing rule, as the numerator, and the number of threads per inch to be cut as the denominator of a fraction; resolve both numerator and denominator into two factors each, and multiply each “pair” of factors by the same number, until values are obtained representing numbers of teeth in available change gears. (One factor in the numerator and one in the denominator make a “pair” of factors.)
Suppose the lathe cuts 6 threads per inch when gears of equal size are used, and that the number of teeth in the gears available are 30, 35, 40 and so on, increasing by 5 up to 100. If 24 threads per inch are to be cut, the screw constant 6 is placed in the numerator and 24 in the denominator. The numerator and denominator are then divided into factors and each pair of factors is multiplied by the same number to find the gears, thus:
The last four numbers indicate the gears which should be used. The upper two having 40 and 30 teeth are thedrivinggears and the lower two having 80 and 60 teeth are thedrivengears. The driving gears are gearaon the spindle stud and gearcon the intermediate stud, meshing with the lead-screw gear, and the driven gears are gearsbandd. It makes no difference which of the driving gears is placed on the spindle stud, or which of the driven is placed on the lead-screw.
Fractional Threads.—Sometimes the lead of a thread is given as a fraction of an inch instead of stating the number of threads per inch. For example, a thread may be required to be cut, having3/8-inch lead. The expression “3/8-inch lead” should first be transformed to “number of threads per inch.” The number of threads per inch (the thread being single) equals:
To find the change gears to cut 22/3threads per inch in a lathe having a screw constant of 8 and change gears varying from 24 to 100 teeth, increasing by 4, proceed as follows:
As another illustration, suppose we are to cut 13/4thread per inch on a lathe having a screw constant of 8, and that the gears have 24, 28, 32, 36, 40 teeth, etc., increasing by four up to one hundred. Following the rule:
The gears having 72 and 64 teeth are thedrivinggears, and those with 36 and 28 teeth are thedrivengears.
Change Gears for Metric Pitches.—When screws are cut in accordance with the metric system, it is the usual practice to give the lead of the thread in millimeters, instead of the number of threads per unit of measurement. To find the change gears for cutting metric threads, when using a lathe having an English lead-screw, first determine the number of threads perinch corresponding to the given lead in millimeters. Suppose a thread of 3 millimeters lead is to be cut in a lathe having an English lead-screw and a screw constant of 6. As there are 25.4 millimeters per inch, the number of threads per inch will equal 25.4 ÷ 3. Place the screw constant as the numerator, and the number of threads per inch to be cut as the denominator:
The numerator and denominator of this fractional expression of the change-gear ratio are next multiplied by some trial number to determine the size of the gears. The first whole number by which 25.4 can be multiplied so as to get a whole number as the result is 5. Thus, 25.4 × 5 = 127; hence, one gear having 127 teeth is always used when cutting metric threads with an English lead-screw. The other gear required in this case has 90 teeth. Thus:
Therefore, the following rule can be used to find the change gears for cutting metric pitches with an English lead-screw:
Rule.—Place the lathe screw constant multiplied by the lead of the required thread in millimeters multiplied by 5, as the numerator of the fraction, and 127 as the denominator. The product of the numbers in the numerator equals the number of teeth for the spindle-stud gear, and 127 is the number of teeth for the lead-screw gear.
If the lathe has a metric pitch lead-screw, and a screw having a given number of threads per inch is to be cut, first find the “metric screw constant” of the lathe or the lead of thread in millimeters that would be cut with change gears of equal size on the lead-screw and spindle stud; then the method of determining the change gears is simply the reverse of the one already explained for cutting a metric thread with an English lead-screw.
Rule.—To find the change gears for cutting English threads with a metric lead-screw, place 127 in the numerator and the threads per inch to be cut, multiplied by the metric screw constant multipliedby 5, in the denominator; 127 is the number of teeth on the spindle-stud gear and the product of the numbers in the denominator equals the number of teeth in the lead-screw gear.
Lathe having Quick Change-gear MechanismFig. 30. Lathe having Quick Change-gear Mechanism
Fig. 30. Lathe having Quick Change-gear Mechanism
Quick Change-gear Type of Lathe.—A type of lathe that is much used at the present time is shown inFig. 30. This is known as the quick change-gear type, because it has a system of gearing which makes it unnecessary to remove the change gears and replace them with different sizes for cutting threads of various pitches. Changes of feed are also obtained by the same mechanism, but the feeding movement is transmitted to the carriage by the rodR, whereas the screwS1is used for screw cutting. As previously explained, the idea of using the screw exclusively for threading is to prevent it from being worn excessively, as it would be if continually used in place of rodR, for feeding the carriage when turning.
End and Side Views of Quick Change-gear MechanismFig. 31. End and Side Views of Quick Change-gear Mechanism
End and Side Views of Quick Change-gear Mechanism
Fig. 31. End and Side Views of Quick Change-gear Mechanism
The general construction of this quick change gear mechanismand the way the changes are made for cutting threads of different pitch, will be explained in connection withFigs. 30,31and32, which are marked with the same reference letters for corresponding parts. Referring toFig. 30, the movement is transmitted from gearson the spindle stud through idler gearI, which can be moved sidewise to mesh with either of the three gearsa,borc,Fig. 31. This cone of three gears engages gearsd,eandf, any one of which can be locked with shaftT(Fig. 32) by changing the position of knobK. On shaftTthere is a gearSwhich can be moved along the shaft by hand leverLand, owing to the spline or keyt, both the sliding gear and shaft rotate together. ShaftT, carrying gearsd,eandfand the sliding gearS, is mounted in a yokeY, which can be turned about shaftN, thus making it possible to lower sliding gearSinto mesh with any one of a cone of eight gearsC,Fig. 31. The shaft on which the eight gears are mounted has at the end a small gearmmeshing with gearnon the feed-rod, and the latter, in turn, drives the lead-screw, unless gearois shifted to the right out of engagement, which is its position except when cutting threads.
Sectional Views of Quick Change-gear MechanismFig. 32. Sectional Views of Quick Change-gear Mechanism
Sectional Views of Quick Change-gear Mechanism
Fig. 32. Sectional Views of Quick Change-gear Mechanism
With this mechanism, eight changes for different threads or feeds are obtained by simply placing gearSinto mesh with thevarious sized gears in coneC. As the speed of shaftTdepends on which of the three gearsd,eandfare locked to it, the eight changes are tripled by changing the position of knobK, making twenty-four. Now by shifting idler gearI, three speed changes may be obtained for gearsa,bandc, which rotate together, so that the twenty-four changes are also tripled, giving a total of seventy-two variations without removing any gears, and if a different sized gearswere placed on the spindle stud, an entirely different range could be obtained, but such a change would rarely be necessary. As shown inFig. 30, there are eight hardened steel buttonsB, or one for each gear of the coneC, placed at different heights in the casing. When leverLis shifted sidewise to change the position of sliding gearS, it is lowered onto one of these buttons (which enters a pocket on the under side) and in this way gearSis brought into proper mesh with any gear of the coneC. To shift leverL, the handle is pulled outward against the tension of springr(Fig. 32), which disengages latchland enables the lever to be lifted clear of the button; yokeYis then raised or lowered, as the case may be, and leverLwith the sliding gear is shifted laterally to the required position.