Fig. 1276Fig. 1276.
Fig. 1276.
Fig. 1277Fig. 1277.
Fig. 1277.
As the knurling tool requires to be forced against the work with considerable pressure, there is induced a strain tending to force the tool directly away from the work, as denoted by the arrow inFig. 1276, and this, in a weighted lathe, acts to raise the lathe carriage and weight. This is avoided by setting the tool at an angle, as inFig. 1277, so that the direction of strain is below and not above the pivot on which the cross slide rests. This is accomplished by pivoting the piece carrying the wheels to the main body of the stem, as shown inFig. 1277.
Fig. 1278Fig. 1278.
Fig. 1278.
For use by hand the knurling or milling tool is fitted to a holder and handle, as inFig. 1278, and the hand tool rest is placed somelittle distance from the work so that the knurl can pass over it, and below the centre of the work.
Knurls for screw heads are made convex, concave, or parallel, to fit the heads of the screws, and may be indented with various patterns.
Winding Spiral Springs in the Lathe.—Spiral springs whose coils are close, and which therefore act on distension only, may be wound by simply starting the first coil true, and keeping the wire as it winds on the mandrel close to that already wound thereon.
Fig. 1279Fig. 1279.
Fig. 1279.
Spiral springs with open coils may be best wound as shown inFig. 1279, in which is shown a mandrel held between the lathe centres and driven by a dog that also grips one end of the wirew, of which the spring is to be made. The wire is passed through two blocksb, which, by means of the set-screw in the lathe tool post, place a friction on it sufficient to place it under a slight tension which keeps it straight. The change gears of the lathe are arranged as they would be to cut a screw of a pitch equal to the thickness of the wire added to the space there is to be between the coils of the spring. The first turn of the lathe should wind a coil straight round the mandrel when the self-acting feed motion is put in operation and the winding proceeds, and when the spring is sufficiently long, the feed motion is disconnected, and the last coil is allowed to wind straight round the mandrel, thus giving each end of the spring a flat or level end.
If the wire is of brass it will be necessary to close it upon the mandrel with blows from a lead mallet to prevent it from uncoiling on the mandrel when the end is released, which it will do to some extent in any event.
Fig. 1280Fig. 1280.
Fig. 1280.
If it is of steel it may be necessary to heat the coil red-hot to prevent its uncoiling, and in the coiling it will, if of stout wire, require to be bent against the mandrel during winding with a piece of steel placed in the tool post, as inFig. 1280, in whicharepresents the mandrel,bthe spring wire, anddthe lathe tool post.
Fig. 1281Fig. 1281.
Fig. 1281.
Fig. 1282Fig. 1282.
Fig. 1282.
In the absence of a lathe with a self-acting feed motion, the mandrel may have a spiral groove in it and the piece of steel or other hard metal shown in figure must be used, the feed screw of the slide rest being removed so that the wire can feed itself along as the mandrel rotates. Near one end of the mandrel a small hole is drilled through, there being sufficient space between the hole and the end of the mandrel to admit of a loose washer being placed thereon; the bore of this washer requires to be rather larger in diameter than the outside diameter of the spring, when wound upon the mandrel, and also requires to be provided with a keyway and key. The washerd(Fig. 1281), is slipped over the mandrel, the end of the wirecis inserted in the holeband the spring being wound, the washer is passed up to the end, and the key driven home as inFig. 1282; when the wire is cut off and the mandrel may be taken from the lathe with the spring closely wound round it to be hammered if of brass, and heated if of steel. The hammering should be done over the whole circumference, not promiscuously, but beginning at one end and following along the wire with the blows delivered not more than1⁄4of an inch apart; for unless we do this we cannot maintain any definiterelation between the size of the mandrel and the size of the spring.
When a grooved mandrel is used, its diameter should be slightly less than the required diameter of spring, as when released the coils expand in diameter.
Fig. 1283Fig. 1283.
Fig. 1283.
If it is not essential that the coils be exactly true, take a plain mandrel, such as shown inFig. 1283, and a hook, such as shown ata, fasten the end of the wire either round the lathe dog, or in a hole in the mandrel as before, and wind one full coil of the spring upon the mandrel, then force this coil open until the hook end ofacan be inserted between it and over the mandrel, the other end hanging down between the lathe shears, which will prevent it from rotating, starting the lathe while holding the unwound end of the wire against the hook with a slight pressure, and the winding will proceed as shown in the figure, the thickness ofaregulating the width apart of the coils. It is obvious that if the coil is to be a right-handed one and is started at the carrier end, the lathe must revolve backwards.
Spiral springs for railroad cars are wound while red-hot in special spring-winding lathes and with special appliances.
Tools for Hand Turning.—Many of the tools formerly used in hand turning have become entirely obsolete, because they were suitable for larger work than any to which hand turning is now applied; hence, reference to such tools will be omitted, and only such hand tools will be treated of as are applicable to foot lathes and wood turning, their purposes being those for which hand tools are now used.
To the learner, practice with hand tools is especially advantageous, inasmuch as the strain due to the cut is felt by the operator; hence, the effects of alterations in the shape of the tools, its height or position with relation to the work, and also the resistance of the metal to severance, are more readily understood and appreciated than is the case where the tool is held in a slide rest or other mechanical device. If under certain conditions the hand tool does not operate to advantage, these conditions may be varied by a simple movement of the hands, altering the height of the tool to the work, the angle of the cutting edges to the work, or the rate of feed, as the case may be, and instantly perceiving the effects; whereas with tools held by mechanical means, such alterations would involve the expenditure of considerable time in loosening, packing, and fastening the tool, and adjusting it to position.
Small work that is turned by hand may, under exceptionally expert manipulation, be made as interchangeable and more accurate in dimensions than it could be turned by tools operated in special machines. That is to say, it is possible to turn by hand a number of similar small pieces that will be when finished as true, more nearly corresponding in dimensions, and have a finer finish, than it is practicable to obtain with tools operated or guided by parts of a machine. This occurs because of the wear of the cutting tools, which upon small work may be compensated for in the hand manipulation in cases where it could not be in machine manipulation. But with ordinary skill, and under ordinary conditions, the liability to error in hand work induces greater variation in the work than is due to the wear of the tool cutting edges in special machine work; hence, the practical result is that work made by special machinery is more uniform and true to size and shape than that made by hand, while also the quantity turned out by special machines is very much greater.
Fig. 1284Fig. 1284.
Fig. 1284.
The most desirable form of tool for taking a heavy hand cut is the heel tool shown inFig. 1284, which, it may be remarked, is at present but little used on account of the greater expedition of tools held in slide rests. It consists of a steel bar, about3⁄8or1⁄2inch square, forged with a heel atf, so that it may firmly grip the hand rest, and having a cutting edge ate. This bar is about 8 inches long, and is held in a groove in a wooden stock by a strap passing over it, and having a stem which passes down through the handled, in which is fixed a nut, so that by screwing up or unscrewingdthe bar is gripped or released, as the case may be, in a groove in the stock. In use, the endhof the stock is held firmly against the operator’s shoulder, the left hand grasps the stock and presses the tool firmly down upon the face of the hand rest, while with the right the handledis moved laterally, causing the tool to move to its cut. The depth of the cut is put on and regulated by elevating the endhof the stock. The heelfis placed close enough to the work to keepe fnearly vertical, for if it inclines too much in any direction the tool gets beyond the operator’s control. The position of the heelfis moved from time to time along the hand rest to carry the cut along.
A cut of1⁄8inch deep, that is, reducing the work diameter1⁄4inch, may readily be taken with this tool, which, however, requires skilful handling to prevent it from digging into the work.
The shorter the distance from the faceeto the heelfthe more easily the tool can be controlled; hence, asfserves simply as a sharp and gripping fulcrum it need not project much from the body of the steel; indeed, in many cases it is omitted altogether, the bottom of the steel bar being slightly hollowed out instead. No oil or water is required with the heel tool.
The hand rest should be so adjusted for height that the cutting edge of the tool stands slightly above the horizontal level of the work, a rule which obtains with all hand tools used upon wrought iron and steel.
Fig. 1285Fig. 1285.
Fig. 1285.
The graver is the most useful of all hand turning tools, since it is applicable to all metals, and for finishing as well as roughing out the work. It is formed by a square piece of steel whose end is ground at an angle, as shown in the top and the bottom view,Fig. 1285,a abeing the cutting edges,c cthe points, andd dthe heels.
Fig. 1286Fig. 1286.
Fig. 1286.
It is held in a wooden handle, which should be long enough to grasp in both hands, so that the tool may be held firmly. For cutting off a maximum of metal in roughing out the work the graver is held as inFig. 1286, the heel being pressed down firmlyupon the tool rest. The cut is carried along the work by revolving the handle upon its axis, and from the right towards the left, at the same time that the handle is moved bodily from the left towards the right. By this combination of the two movements, if properly performed, the point of the graver will move in a line parallel to the centres of the lathe, because, while the twisting of the graver handle causes the graver point to move away from the centre of the diameter of the work, the moving of the handle bodily from left to right causes the point of the graver to approach the centre of that diameter; hence the one movement counteracts the other, producing a parallel movement, and at the same time enables the graver point to follow up the cut, using the heel as a pivotal fulcrum, and hence obviating the necessity of an inconveniently frequent moving of the heel of the tool along the rest. The most desirable range of these two movements will be very readily observed by the operator, because an excess in either of them destroys the efficacy of the heel of the graver as a fulcrum, and gives it less power to cut, and the operator has less control over the tool.
Fig. 1287Fig. 1287.
Fig. 1287.
Fig. 1288Fig. 1288.
Fig. 1288.
For finishing or smoothing the work the graver is held as inFig. 1287, the edge being brought parallel to the work surface. For brass work the top faces of the graver should be slightly bevelled in the direction shown inFig. 1288.
The graver cuts most efficiently with the work revolving at a fast speed, or, say, at about 60 feet per minute, and for finishing wrought iron or steel requires an application of water.
Fig. 1289Fig. 1289.
Fig. 1289.
To finish work that has been operated upon by a heel tool or by a graver, the finishing tool shown inFig. 1289may be employed. It is usually made about5⁄8or3⁄4inch wide, as the graver is employed for shorter work. It is ground so as not to let the extreme corners cut, and is used at a slow speed with water. The edge of this tool is sometimes oilstoned, causing it to cut with a clean polish. The tool is held level, brought up to the work, and a cut put on by elevating the handle end. To carry the cut forward, the tool is moved along the hand rest to nearly the amount of its width, and is brought to its cut by elevating the handle as before. When the work has been finished as near as may be with this tool, it may be finished by fine filing, the lathe running at its quickest speed; or the file may be used to show the high spots while using the finishing tool.
Fig. 1290Fig. 1290.
Fig. 1290.
Fig. 1291Fig. 1291.
Fig. 1291.
For facing the ends of work the tool shown inFig. 1290, or that shown inFig. 1291, may be used, either of them being made from an old three-cornered file. The cutting edge ata,Fig. 1290, should be slightly curved, as shown. The point of the tool is usually brought to cut at the smallest diameter of the work, with the handle end of the tool somewhat elevated. As the cut is carried outwards the handle end of the tool is depressed, and the point correspondingly elevated. It may be used dry or with water, but the latter is necessary for finishing purposes.
Another form of this tool is shown inFig. 1291. It has two cutting edgesa a, one of which rests on the hand rest while the other is cutting, the tool being shown in position for cutting a right-and a left-hand face, the face nearest to the work being shown in the lower view. This face should be placed against the radial face of the work, and the cut put on by turning the upper edge over towards the work while pressing the tool firmly to the lathe rest.
Fig. 1292Fig. 1292.
Fig. 1292.
For cutting out a round corner the tool shown inFig. 1292, employed either for roughing or smoothing purposes (water being used with it for the latter), the heel causes it to grip the hand rest firmly, and acts as a pivotal fulcrum from which the tool may be swept right and left round the curve, or a portion of it.
This tool, as in the case of all tools used upon wrought iron or steel, should not cut all round its edge simultaneously, as in that case, unless indeed it is a very narrow tool, the force placed upon it by the cut will be too great to enable the operator to hold and control it; hence the cut should be carried first on one side and then on the other, and then at the point, or else the handle end should be moved laterally, so that the point sweeps round the work. It should be brought to its cut by placing its heel close to the work, and elevating the handle end until the cutting edge meets the work.
The point or nose of the tool may obviously be made straight or square, as it is termed, to suit the work, the top rake being omitted for brass work.
Fig. 1293Fig. 1293.
Fig. 1293.
In using this tool for cutting a groove it is better (if it be a deep groove, and imperative if it be a broad one, especially if the work be slight and apt to spring) to use a grooving tool narrower in width than the groove it is to cut, the process being shown inFig. 1293, in whichwrepresents a piece of work requiring the two grooves ataandbcut in it. For a narrow groove asathe tool is made about half as wide as the groove, and a cut is taken first on one side as atc, and then on the other as atd. For a wider groove three or more cuts may be made, as ate,f,g. In all cases the tool while sinking the groove is allowed to cut on the end face only; but when the groove is cut to depth, the side edges of the tool may be used to finish the sides of the groove, but the side and end edge must not cut simultaneously, or the tool will be liable to rip into the work.
Fig. 1294Fig. 1294.
Fig. 1294.
Hand Tools for Brass Work.—In addition to the graver as a roughing-out tool for brass work, we have the tool shown inFig. 1294, the cutting edge being at the rounded enda. It is held firmly to the rest, which is not placed close to the work (as in the case of other tools), so as to give the tool a wide range of movement, and hence permit of the cut being carried farther along without moving its position on the rest. It may be used upon either internal or external work.
For finishing brass work, tools termed scrapers are employed.
Fig. 1295Fig. 1295.
Fig. 1295.
Fig. 1295represents a flat scraper, the two end edgesaand the side edges along the bevel forming the cutting edges.
Fig. 1296Fig. 1296.
Fig. 1296.
Fig. 1297Fig. 1297.
Fig. 1297.
Fig. 1298Fig. 1298.
Fig. 1298.
Fig. 1299Fig. 1299.
Fig. 1299.
In this tool the thickness of the endais of importance, since if it be too thin it will jar or chatter. This is especially liable to occur when a broad scraper is used, having a great length of cutting edge in operation. This may be obviated to some extent by inclining the scraper as inFig. 1296, which has the same effect as giving the top face negative rake, causing the tool to scrape rather than cut. The dividing line between the cutting and scraping action of a tool is found in the depth of the cut, and the presentation of the tool to the work, as well as in the shape of the tool. Suppose, for example, that we have inFig. 1297, a piece of workwand a tools, and the cut being light will be a scraping one. Now suppose that the relative positions of the size of the work and of the tool remain the same, but that the cut be deepened as inFig. 1298, and the scraping action is converted into that class of severing known as shearing, or we may reduce the depth of cut as inFig. 1299, and the action will become a cutting one.
Fig. 1300Fig. 1300.
Fig. 1300.
But let the depth of cut be what it may, the tool will cut and not scrape whenever the angle of its front face is more than 90° to the line of tool motion if the tool moves, or of work motion if the work moves to the cut. InFig. 1300, for example, the tool is in position to cut the angle of the front face, being 110° to the direction of tool motion.
Fig. 1301Fig. 1301.
Fig. 1301.
We may consider this question from another stand-point, however, inasmuch as that the tool action is a cutting one whenever the pressure of the cut is in a direction to force the tool deeper into the work, and a scraping one whenever this pressure tends to force the tool away from the work, assuming of course that the tool has no front rake, and that the cut is light or a “mere scrape,” as workmen say. This is illustrated inFig. 1301, the tool ataacting to cut, and atbto scrape, and the pressure of the cut uponaacting to force the tool into the work as denoted by the arrowd, while that uponbacts to force it in the direction of arrowc, or away from the work.
In addition to these distinctions between a cutting and a scraping action we have another, inasmuch as that if a tool is pulled or dragged to its cut its action partakes of a scraping one, no matter at what angle its front face may stand with relation to the work.
Fig. 1302Fig. 1302.
Fig. 1302.
The end face of a flat scraper should be at a right angle to the body of the tool, so that both edges may be equally keen, for if otherwise, as inFig. 1302, one edge asawill be keener than the other and will be liable to jar or chatter.
The flat scraper can be applied to all surfaces having a straight outline, whether the work is parallel or taper, providing that there is no obstruction to prevent its application to the work.
Fig. 1303Fig. 1303.
Fig. 1303.
Fig. 1304Fig. 1304.
Fig. 1304.
Fig. 1305Fig. 1305.
Fig. 1305.
Fig. 1306Fig. 1306.
Fig. 1306.
Thus, inFig. 1303we have a piece of work taper ataandc, parallel ate, and with a collar atd, the scrapersbeing shown applied to each of these sections, and it is obvious that it cannot be applied to sectionabecause the collardis in the way. This is remedied by grinding the scraper as inFig. 1304, enabling it to be applied to the work as inFig. 1305. Another example of the use of a bevelled scraper is shown inFig. 1306, the scrapershaving its cutting edge parallel to the work and well clear of the armh.
Fig. 1307Fig. 1307.
Fig. 1307.
The round-nosed scraper is used for rounding out hollow corners, or may be made to conform to any required curve or shape. It is limited in capacity, however, by an element that affects all scraping tools, that if too great a length of cutting edge is brought into action at one time, chattering will ensue, and to prevent this the scraper is only made of the exact curvature of the work when it is very narrow, as atsinFig. 1307.
For broad curves it is made of more curvature, so as to limit the length of cutting edge, as is shown in the same figure ats′, and is swept round the work so as to carry the cut around the curve.
There are, however, other means employed to prevent chattering, and as these affect the flat scraper as well as the round-nosed one, they may as well be explained with reference to the flat one.
Fig. 1308Fig. 1308.
Fig. 1308.
Fig. 1309Fig. 1309.
Fig. 1309.
First, then, a thin scraper is liable to chatter, especially if used upon slight work. But the narrower the face on the end of the scraper, the easier it is to resharpen it on the oilstone, because there is less area to oilstone. A fair thickness is about1⁄20inch; but if the scraper was no thicker than this throughout its whole length, it would chatter violently, and it is for this reason that it is thinned at its cutting end only. Chattering is prevented in small and slight work by holding the scraper as inFig. 1308, applying it to the top of the work; and to reduce the acting length of cutting edge, so as to still further avoid chattering, it is sometimes held at an angle as in the top view inFig. 1309,sbeing the scraper andrthe tool rest.
When the scraper is applied to side faces, or in other cases in which a great length of cutting edge is brought into action, a piece of leather laid beneath the scraper deadens the vibration and avoids chattering.
Fig. 1310Fig. 1310.
Fig. 1310.
It is obvious that the scraper may be given any required shape to meet the work,Fig. 1310representing a scraper of this kind; but it must in this case be fed endways only to its cut, if the work is to be cut to fit the scraper.
Fig. 1311Fig. 1311.
Fig. 1311.
Fig. 1312Fig. 1312.
Fig. 1312.
InFig. 1311is shown a half-round scraper, which is shown inFig. 1312in position to scrape out a bore or hole. This tool is made by grinding the flat face and the two edges of a worn-out half-round smooth file, and is used to ease out bores that fit too tightly. The cutting edges are carefully oilstoned, and the work revolved at a very quick feed.
Fig. 1313Fig. 1313.
Fig. 1313.
Fig. 1314Fig. 1314.
Fig. 1314.
When a number of small pieces of duplicate form are to be turned by hand, a great deal of measuring may be saved and the work very much expedited by means of the device shown inFig. 1313. It consists of a tool stock or holder, the middle of which, denoted bya, is square, and contains three or four square slots, with a set-screw to each slot to hold different turning tools. Each end of the stock is turned parallel, as denoted byb,c. InFigs. 1313and1314,d,e, andfare the tools, andg,h, are the set-screws.
Fig. 1315represents top and side view of a plate, of which there must be two, one to fasten on the headstock and one on the tailstock of the lathe, as shown inFig. 1316. InFig. 1317the manner of using the tool is shown, similar letters of reference denoting similar parts in all the figures.
Fig. 1315Fig. 1315.
Fig. 1315.
Fig. 1316Fig. 1316.
Fig. 1316.
The platesp pare bolted by screws to the headblockhand the tailstocktof the lathe. The tool holder is placed so that the cylindrical endsb,c, rest on the ends of these plates, and in the anglesp′ p′. The cutting tooldis sustained, as shown, upon the lathe restr. In use the operator holds the stockain his hands in the most convenient manner, using the tooleas a handle when there is a tool in the position ofe. The cutting point of the tool is pressed up to the workw, and the feed is carried along by hand. It is obvious, however, that when the perimeters ofa bmeet the shoulderso o,Fig. 1315, of the platesp p, the tool cannot approach any nearer to the diametrical centre of the work; hence the diameter to which the tool will turn is determined by the distance of the shoulderoof the platepfrom the centre of the lathe centres, as shown inFig. 1316by the linel. In carrying the cut along it is also obvious that the lateral travel of the stock or holder must end when the end of the square partacomes against the side face of either of the plates. In the engraving we have shown the tooldcutting a groove in the workw, while the shoulder of the holder is against the plate fastened to the lathe tailstockt; and so long as the operator, in each case, keeps the shoulder against that plate, the grooves upon each piece of work will be cut in the same position, for it will be observed that the position in the length of the work performed by each tool is determined by the distance of the cutting part of each tool from the end of the square partaof the tool holder. All that is necessary, then, is to adjust each tool so that it projects the proper distance to turn the requisite diameter and stands the required distance from the shoulders of the square to cut to the desired length, and when once set error cannot occur.
This plain description of the device, however, does not convey an adequate idea of its importance. Suppose, for example, that it is required to turn a number of duplicate pieces, each with a certain taper: all that is necessary is to adjust the platespin their distances from the lathe centres. If the large end of the taper on the work is required to stand nearest the lathe headstocka, the platepon the headstock must be moved until its shoulderois farther from the lathe centre. If, however, the work requires to be made parallel, the platespmust be set the same distance for the axial line of the centres. If it be desired to have a parallel and a taper in proximity upon the same piece of work, the tool must have one of its cylindrical ends taper and use it upon the taper part of the work.
Fig. 1317Fig. 1317.
Fig. 1317.
InFig. 1317the tooldis shown cutting a square groove. The tool atfserves to turn the parallel partx, and the toolewould cut theV-shaped groovei.
All kinds of irregular work may be turned by varying the parallelism and form of the cylindrical endsb c; but in this event the shoulderso o,Fig. 1315, should be madeV-shaped and hardened to prevent them from rapid wear.
Fig. 1318Fig. 1318.
Fig. 1318.
Fig. 1319Fig. 1319.
Fig. 1319.
Screw Cutting with Hand Tools.—Screw threads are cut by hand in the lathe with chasers, of which there are two kinds, the outside and the inside chaser. InFig. 1319is shown an outside, or male, and inFig. 1318an inside, or female chaser. The width of a chaser should be sufficient to give at least four teeth, and for the finer thread pitches it is better to have six or eight teeth, the number increasing as the pitch is finer, and the length of the work will permit. The leading tooth should be a full one, or otherwise it will break off, and if in cutting up the chaser a half or less than a full tooth is formed it should be ground off. The tooth points should not be in a plane at a right angle to the chaser length, but slightly diagonal thereto, as inFig. 1319, so that the front edge of the chaser will clear a bolt head or shoulder, and permit the leading tooth to pass clear up to the head without fear of the front edge of the steel meeting the shoulder.
Fig. 1320Fig. 1320.
Fig. 1320.
Fig. 1321Fig. 1321.
Fig. 1321.
Fig. 1322Fig. 1322.
Fig. 1322.
Fig. 1323Fig. 1323.
Fig. 1323.
Fig. 1324Fig. 1324.
Fig. 1324.
The method of producing a chaser from a hob is shown inFig. 1320, in whichhis a hob, which is a piece of steel threaded and serrated, as shown, to give cutting edges to act, as the hob rotates, upon the chaserc. If the chaser is cut while held in a constant horizontal plane, its teeth will have the same curvature as the hob, or, in other words, they will fit its circumference. Suppose that the chaser, being cut up by the hob and then hardened, is applied to a piece of work of the same diameter as the hob and held in the same vertical plane, as inFig. 1320, it is obvious that, there being no clearance, the teeth cannot cut. Or, suppose it be applied to a piece of work of smaller diameter, as inFig. 1324, it cannot cut unless its position be lowered, as inFig. 1322, or else it must be elevated, as inFig. 1323. In either case the angleof the thread cut will be different from the angle of the sides of the chaser teeth, and the thread will be of improper depth. Thus, on referring toFig. 1321, it will be seen that the chaserchas a tooth depth corresponding to that on the workwalong the horizontal dotted lineeonly, because the true depth of thread on the work is its depth measured along a radial line, as lineforg, and the chaser teeth are, at the cutting edge, of a different angle. This becomes more apparent if we suppose the chaser thickness to be extended up to the dotted lineh, and compare that part of its length that lies within the two circlesi j, representing the top and bottom of the thread, with the length of radial lineg, that lies within these circles. If, then, the chaser be lowered, to enable it to act, it will cut a thread whose sides will be of more acute angle than are the sides of the chaser teeth or of the hob from which it was cut. The same effect is caused by using a chaser upon a larger diameter of work than that of the hob from which the chaser was cut, because the increased curvature of the chaser teeth acts to give the teeth less contact with the work, as is shown inFig. 1325, for the teeth cannot cut without either the lower cornersaof the teeth being forced into the metal, or else the chaser being tilted to relieve them of contact. To obviate these difficulties and enable a chaser to be used upon various diameters of work, it is, while being cut up by the hob, moved continuously up and down, as denoted inFig. 1326, byaandb, which represent two positions of the chaser. The amount of this movement is sufficient to make the chaser teeth more straight in their lengths, and to give them a certain amount of clearance, an example of the form of chaser thus produced being shown inFig. 1327, applied to two different diameters of work, as denoted by the circleaand segment of a circleb,crepresenting the chaser.