Fig. 809Fig. 809.
Fig. 809.
To steady work that is unturned and of so great a length that it springs too much to permit of its being turned true, the sleeve or cat head shown inFig. 809is employed; it may contain three or four screwsc, to true it upon the work. The bodybis turned true.
The set-screws are so adjusted upon the work, that the outside runs quite true from end to end. The jaws of the steady rest are then set to just touch the circumference of the sleeve, care being taken that their pressure does not spring the axial line of the work out of its normal straight line. If the shaft is to be turned from end to end, the cat head should be placed sufficiently to one side of the centre of the length of the work and nearer the live centre, that the lathe tool may turn up the work for a distance of at least half its length, or slightly more than half. One half of the work being turned, the shaft is reversed end for end in the lathe, when the cat head may be moved to envelop the turned part, and again set true, or the jaws of the steady rest may be set direct upon the work; in this latter case, however, the friction between the jaws and the work will be apt to leave rings or marks upon the latter.
If the cat head is not set to run quite true upon the work, the latter will not run true when the steady rest is removed, and if the jaws of the steady rest spring the axial line of the work out of its normal straightness, the work will be turned either larger or smaller in diameter in the middle of its length, according to the direction in which the work is sprung.
Suppose, for example, that the work is sprung laterally towards the tool point, then the work will be turned smaller in the middle, or if the work were sprung laterally in the opposite direction, it would be turned larger in the middle than at the ends. If the work is sprung vertically so as to approach or recede from the lathe bed, the amount of the error will be less than if it were sprung laterally, and the nature of the error will depend upon the height of the cutting tool with relation to the work. If, for example, the point is above the centre of the work, and the latter is sprung towards the lathe bed, the work will turn of largest diameter in the middle of its length; or with the tool point placed at the centre of the work, the same result will follow, whether the work be sprung up or down; but if the work be sprung up or away from the lathe bed, and the tool point be placed above the centre, the diameter of the work will be turned smaller than that at the ends.
Fig. 810Fig. 810.
Fig. 810.
When the work is to be turned from end to end or for a considerable distance, a follower rest such as shown inFig. 810should be employed, being similar to the steady rest shown inFig. 802, except that it is open in front, and being fastened to the slide rest carriage, of course travels with the tool; hence the platespmay be either directly in front of the tool or following it, but if the workwhas been turned true and parallel, the platespmay be in front of the tool, or rather may lead it.
The follower rest should always be set to the work when as near as practicable to the dead centre, in which case it will be easier to set it without springing the work.
Fig. 811Fig. 811.
Fig. 811.
Fig. 812Fig. 812.
Fig. 812.
For work of small diameter for which the platespwould be too large, and therefore in the way, the platep,Fig. 811, may be used, being bolted to the follower rest. For work of larger diameter the device shown inFig. 812is sometimes used. It consists of aplatepwith a capc, and bolts for holding the bearingsb,b. These bearings are bored slightly larger in diameter than the finished diameter of the work.
The advantage of the use of this device is that bearings of the requisite bore having been selected they may be inserted and adjusted a proper fit to the work beforepis fastened to the follower rest, thus avoiding the liability of being either too tight or too loose as may happen when the plates cannot be moved or rotated to test the fit. Another and great advantage is that if after the adjustment of the bearingsb,bto the work, the platepis carefully bolted to the follower rest, the liability of springing the work is eliminated, hence truer work will be produced.
Fig. 813Fig. 813.
Fig. 813.
A representative of another class of follower rest is shown inFig. 813, the hubhis accurately bored to receive collars or rings of various diameters of bore to suit the work. The bore ofhmay be made to stand axially true with the lathe centres, and thus avoid the trouble of setting, by employing the steady pins, which, being a close fit in the follower rest and in the lathe carriage will bring the rest to its proper distance from the lathe centres, where it may be secured by the boltb, which may screw into the metal of the carriage or operate to lift a wedge or guide slip so as to grip theV-slide of the carriage and take up any lost motion between the slide in the rest and that in the lathe carriage.
Fig. 814Fig. 814.
Fig. 814.
Fig. 814shows a follower rest in position on the cross slide of a lathe.
There is a large class of small work that could be held between the lathe centres, but that can be more conveniently held in chucks. Chucks are devices for holding work to the live spindle, and may be divided into classes as follows:
1st. Those in which the work is secured by a simple set-screw.
2nd. Drill chucks, which are applied mainly to drive drills, but which may also be used to drive very small work to be operated upon by cutting tools, the mechanism causing the jaws to move simultaneously to grip or release the work.
3rd. Independent chucks, in which the jaws are operated separately.
4th. Universal chucks, which are larger than drill chucks, and in which the jaws operate simultaneously.
5th. Combination chucks, in which the jaws may be operated either separately or simultaneously as may be required.
Fig. 815Fig. 815.
Fig. 815.
Referring to the first,Fig. 815represents a simple form of set-screw chuck, the stemsfitting into the live centre hole, and the outer end being pierced to receive a drill shank, and the iron from which a piece of work may require to be turned, which is secured in the chuck by the set-screwb. In the case of drill or other cutting tools, however, it is better that they be provided with a flat placea, to receive the set-screw pressure, and enable it to hold them more securely. The objections to this class of chuck are threefold: First, each chuck is suitable for one diameter of work only; secondly the screw headbis in the way; and thirdly, the set-screw pressure is in a direction to set the work out of true, which it will do unless the work is a tight fit to the bore of the chuck. In this case, however, it is troublesome to insert and remove the drill, unless the bore of the socket is relieved on the half circumference nearest to the set-screw, as shown atcin the end view, in which case the efficiency of the chuck is greatly enhanced.
Referring to the second class they are made to contain either two or three jaws.
Fig. 816Fig. 816.
Fig. 816.
When two jaws are employed they are made to slide in one slideway, and are operated therein by a right and left-handed screw, causing them to simultaneously advance or recede from the chuck axis.Fig. 816represents a chuck of this class, the jaws fitting one into the other to maintain each other in line, and prevent their tilting over from the pressure.
Fig. 817Fig. 817.
Fig. 817.
In scroll chucks the mechanism for operating the jaws is constructed upon two general principles. The first may be understood fromFig. 817, in which the body of the chuck is provided upon its end face with a scrollc, with which the ends of the jawsaengage. These jaws fit into radial slots in the shelle, which iscapable of rotation uponband is held thereto by the capd; hence rotatingecarries around the jawsa, and the threadccauses them to approach or recede from the chuck axis, according to their direction of rotation.
Fig. 818Fig. 818.
Fig. 818.
The second general principle upon which small drill chucks are constructed may be understood fromFig. 818, in whichcmay be taken to represent the end of a lathe spindle or a stem fitting into the live centre hole in the same. At the other end it is to receive the shelldwhich screws upon it.dis coned at the outer end of its bore, and the jawseare made to fit the cone, and it is obvious that ifdbe rotated to screw farther uponc, the coned bore ofdwill act to force the jawsenearer to the chuck axis and cause them to close upon and grip the work. To operatedit is knurled or milled atg, or it may have pin spanner holes as ath. In this class of chuck it is essential that the direction of rotation ofdto close the jaws must be opposite to that in which the drill rotates, otherwise the resistance of the work against the jaws would causedto rotate uponc, and the work to become released from the jaw grip. Furthermore, as the larger the work the more severe the duty in driving it, it is usually provided by the construction of such chucks that the jaws shall be opened to their maximum when at their nearest approach to the body (asc) of the chuck, and shall close as they move outward or away from the same. This principle of moving the jaws radially by means of a cone sliding upon a cone is applied in numerous ways, thus sometimes the jaws are provided with wings that slide upon a cone or in slide ways that are at an angle to the chuck axis.
Fig. 819Fig. 819.
Fig. 819.
Fig. 820Fig. 820.
Fig. 820.
Fig. 821Fig. 821.
Fig. 821.
Figs. 819,820, and821represent Gage’s patent chuck, in which the gripping surfaces of the jaws are serrated to increase the grip, and to further secure the same object the jaws move at an angle instead of in a radial line, so that the body of the jaws is more directly in the line of strain, and therefore resists it better. The serrations are left-handed, so that the tendency is to force the drill forward and toward the cut, supposing them to act as a nut and screw upon the drill shank. The jaws are supported by the central cylindrical piece that contains them out to the extreme end, and have in addition a lug which slides in radial grooves.Fig. 819is a side elevation, with a piece of the shell removed to show the jaw and its slide way, and an end view showing the arrangement of the jaws.Fig. 820is a sectional side elevation, andFig. 821, two views of the jaws removed from the chuck;arepresents the jaws with the lugeto slide in the radial slots provided inb. The wingsa′of the jaws slide in the ways inb, the ways passing through the openingfinFig. 821;cis the cone for causing the jaws to open and close radially. The driving piecehhasaleft-hand thread operating inb. It also has a collar abutting over one side against the end ofb, and secured on the other by the capi, which threads into the shellg. A pin incsecures it to the capi, so that if rotated both move together. On the other hand, ifhbe rotated andgis held stationary, the thread onhoperates onbas a nut, causing it to slide, carrying the jaws with it, and the jaws are simultaneously opened or closed according to the direction of rotation ofh.Fig. 819shows the jaws screwed partly out, and therefore partially closed, while inFig. 820the jaws are shown within the chuck, and therefore opened to their fullest extent.
Fig. 822Fig. 822.
Fig. 822.
Fig. 823Fig. 823.
Fig. 823.
Figs. 822and823represent a chuck employed by the Hancock Inspirator Co., of Boston, for very true work. This chuck will not get out of true by wear, and holds brass work against a good lathe-cut without indenting it.
Fig. 824Fig. 824.
Fig. 824.
Fig. 822shows the chuck complete.Fig. 823is a mid-section of chuck complete.Fig. 824is a side and an end of the work-gripping piece. The chuck is composed of three pieces,a,bandc. Pieceascrews upon the lathe spindle and is bored to receivec; piecebscrews uponaand receives the outer end ofc, which is provided with a double coned e, and is split nearly its full length at three places, one of which is shown atf, so that whenbis screwed uponathe two cones upona,bcompressc, and cause the diameter of its bore to decrease and grip the work. The splitsfare made long, so thatcshall not close at its outer end only, but on both sides of the cones, and thus grip the work parallel.
There are several advantages in this form of construction; thusthe parallel bore ofa, in whichcfits, is not subject to strain or wear, and therefore remains true and holdsctrue. Furthermore,bhas no tendency to wear out of true, because it fits uponaat the partg, as well as at its threaded end, while the coneeofcalso acts to keep it true. Asbis screwed up with a wrench fitting its hexagon exterior, the work can be held against any amount of cut that the lathe will drive.
It is obvious that the capacity of the chuck, so far as taking in range of different diameters, is quite limited, but the excellence of its execution far more than compensates for this when work is to be turned out true and correct to standard gauge.
To increase the range of capacity of the chuck, the split piece only needs to be changed. Before hardening the split piece the jaws should be sprung well apart, so that they will spring open when released by unscrewing the outside shell to release the work and insert another piece.
In proportion as the diameter of the work is increased it requires to be more firmly held, and the chucks are made with jaws moved by screws operated by wrench power. These chucks are made with two, three, or four jaws, and the bite of the jaw is shaped to suit the nature of the work, the gripping area being reduced for very small work, and serrated parallel to the chuck axis so as to form gripping teeth for firmly gripping rough work, as shown in some of the followingexamples:—
Fig. 825Fig. 825.
Fig. 825.
Fig. 826Fig. 826.
Fig. 826.
Figs. 825and826represent the Horton two-jawed chucks with false or slip jaws, which are removable so that jaws of various shapes in the bore may be fitted to the same chuck, thus enabling the jaws to be varied to suit the shape of the work to be held. The jaws are secured in place by the pins shown.
Fig. 827Fig. 827.
Fig. 827.
Fig. 827shows a two-jawed solid jaw chuck, the bite of the jaws being made hollow, so as not to mark the surface of the work, while they will hold it very firmly.
Fig. 828Fig. 828.
Fig. 828.
InFig. 828is shown what is termed a box-body two-jawed chuck, which is mainly used by brass turners. The object of this form of body is to permit the flanges, &c., of castings escaping the face of the chuck.
Fig. 829Fig. 829.
Fig. 829.
Fig. 829also represents a two-jawed chuck, the body being cylindrical, and having aV-groove atato receive the work. The screwsc,dmay act independently of each other, or a continuous screw may be used, having, as in the figure, a left-hand thread atc, and a right-hand one atd, so that the jaws move simultaneously when the screw is operated. The difference between these two methods being asfollows:—
When one screw is used the jaws will hold the work so that the centre of rotation will be midway between the points of contact of the jaws of the chuck and the work, hence work cannot be set eccentrically, unless pieces of iron are inserted between it and one of the jaws. When two screws are used the jaws may be operated separately, and one jaw may be set to such distance from the centre of rotation as the necessities of the work may require; but in this case more adjustment is required to set either square or cylindrical work to rotate on its axis than when the jaws operate simultaneously as with a right and left-hand screw. It is obvious that the axial line of the screw or screws must stand parallel with the plane of the facef. It will be observed that the back of each jaw is cut away atb: this serves two purposes, first it permits of a piece of work having a small flange, head or projection being held in theVs of the jaws; and secondly, it equalizes the wear on the jaws of the chuck, because in jaw chucks generally there is more wear at the outer than at the inner end of the jaws, because work shorter than the length of the jaws, or requiring to be held as far out from the jaws as possible, does not have contact at the back end of the work holding jaw faces, hence the jaws are apt to wear, in course of time, taper. By cutting away the jaws at the back, the tendency to unequal wear is greatly reduced, hence this plan is adopted to a more or lessdegree in the dogs or jaws of all chucks, being in many cases merely a small recess from1⁄16to1⁄8inch deep only.
When the jaws have aV-groove as in the cut, the facefof the chuck does not form a guide in setting the work, the truth of theV-grooves being solely relied upon for that purpose.
Fig. 830Fig. 830.
Fig. 830.
The form of two-jawed chuck shown inFig. 830is intended for square or rectangular work, and is mainly used by wood workers. It may be operated by a right and left-hand screw, but is generally preferred with independent screws. The facefof the chuck may be employed to serve as a guide in setting the work as shown in the cut, in whichwrepresents a piece of work held between the jawsa,a, and resting against the facef, which therefore serves as a guide against which to set the work to insure that its axial line shall stand parallel with the facef, or in other words at a right angle to the line of centres of the lathe.
Fig. 831Fig. 831.
Fig. 831.
Fig. 832Fig. 832.
Fig. 832.
Fig. 833Fig. 833.
Fig. 833.
InFig. 831is an example of a machinist’s two-jawed chuck. The jaws are operated simultaneously by a right and left-hand screw. The jaws are provided with slides to receive the two separate pieces shown in figure, which may be made to suit the form of special work. The two screws shown on each side of the chuck face are to support a piece of work that is too large to be otherwise held firmly by the chuck. These screws may be operated by screw-driver wrench, to enable the face of the work to rest on them, and therefore be supported parallel or true with the chuck face. The jaws may be turned end for end in their slide ways as shown inFig. 833, to enable them to grip work of small diameter, the separate pieces shown inFig. 832, being placed on the jaws for such small pieces as drills, &c.
In the larger sizes, lathe chucks are provided with either three or four jaws, which are caused to operate either independently or simultaneously, and in some cases the construction is such that the same chuck may be used as an independent or as a universal one at will, in which case they are termed combination chucks. Concerning the number of jaws it may be observed that a three-jawed chuck will hold the work with an equal pressure on all three jaws, whether it be cylindrical or not, but in a four-jawed chuck the jaws will not have an equal grip upon the work, unless thesame be either cylindrically true or square, hence it is obvious that a three-jawed chuck is less liable to wear out of true, and is also preferable for holding unturned cylindrical work, while it is equal to a four-jawed one for true, but unsuitable for square work.
Fig. 834Fig. 834.
Fig. 834.
Fig. 834represents the construction of the Horton chuck. Upon the screws that operate the jaws are placed pinions that gear into a circular rack, so that by operating one jaw with a wrench the rack is revolved and the remaining jaws are operated simultaneously. The chuck being constructed in two halves, the rack may be removed and the jaws operated separately, or independently as it is termed.
Fig. 835Fig. 835.
Fig. 835.
Fig. 835represents one of the jaws with its operating screw and pinion removed from the chuck. The gripping surfaces of the steps in the jaws are serrated to increase their grip upon the work, and the nutsa,a, against which the works rests, are ground true with the face of the chuck. The corner between the facesaand the bite or gripping surfaces of the jaws are recessed so that the work cannot bind in them, but will bed fairly against the facesa,a, which serve to set the work against and hold it true instead of the face of the chuck.
Fig. 836Fig. 836.
Fig. 836.
Fig. 836represents a Horton chuck for work up to four inches diameter.
Fig. 837Fig. 837.
Fig. 837.
Fig. 837represents a similar chuck for all sizes between 4 and 15 inches, the designated sizes of the chuck being 6, 9, and 12 inches, these diameters being the largest the chucks will take in.
Fig. 838Fig. 838.
Fig. 838.
Fig. 838represents a Horton chuck with outside bites for opening out to grip the bores of rings or other hollow work.
The term scroll chuck is applied to universal chucks in which the jaws are operated throughout their full range by means of a scroll thread such as was shown inFig. 817. The objection to this form is that the threads on the jaws cannot be made to have a full bearing in the scroll thread.
Fig. 839Fig. 839.
Fig. 839.
InFig. 839, for example, leta aandb brepresent grooves between the scroll threads, and if the thread on the jaws be made to the curve and width ofa a, it would not pass in that ofb b, andvice-versâ, and it would take but five revolutions of the thread to pass a nut thread fromatob. To overcome this difficulty the jaw threads are not made correct to either curvature but so formed as to fit at pointsc,d,e, when in the grooveaand at pointsf,g,h, when in grooveb. This obviously reduces their bearing area and therefore their durability. To avoid this defect the jaws of many universal chucks are operated by screws in the same way as independent jaw chucks, but provision is made whereby the operation of any one of the jaw screws will simultaneously operate allthe others, so that all the jaws are moved by the operation of one screw.
Thus in the following figures is shown the Sweetland chuck.
Fig. 840Fig. 840.
Fig. 840.
Fig. 840represents the chuck partly cut away to show the mechanism, which consists of a pinion on each jaw screw, and a circular rack beneath. The rack is shown in gear with a pinion ato, and out of gear with a pinion atc, which is effected asfollows:—
The rack is stepped, being thicker at its outer diameter, and the thin part forms a recess and the shoulder between the thick and thin part forms a bevel or cone. Between this circular rack and the face of the plate at the back of the chuck is placed, beneath each jaw, a cam block bevelled to correspond with the bevelled edge of the recess in the ring. The cam block stem passes through radial slots in the face of the chuck, so that it can be moved to and from the centre of the chuck. When it is moved in, its cam head passes into the recess in the ring rack, which then falls out of gear with the jaw screw pinion; but when it is moved outward the cam head slides (on account of the bevelled edges) under the ring rack and puts it in gear with the jaw screw pinion. Thus, to change the chuck from an independent one to a universal one all that is necessary is to push out the bolt heads on the cam block stems, the said heads being outside the chuck. The washers beneath these heads are dished to give them elasticity and enable them to steady the cams without undue friction.
Fig. 841Fig. 841.
Fig. 841.
To enable the setting of the jaws true for using the chuck as a universal one, after it has been used as an independent one, a ring is marked on the face, and to this ring the edges of all the jaws must be set before operating the cams radially to put the rack ring in gear. InFig. 841a three-jawed chuck on this principle is shown acting as an independent one to hold an eccentric. On account of the spring of the parts, which occurs when the strain is transmitted from one part to another, it is desirable when using the chuck as a universal one to first operate one screw to grip the work and then pass to the others and operate them so that they may receive the pressure direct from the screw head and not entirely through the medium of the rack, and there will be found enough movement of the screws when thus operated to effect the object of relieving the rack to some extent from strain.
Fig. 842Fig. 842.
Fig. 842.
Fig. 843Fig. 843.
Fig. 843.
Fig. 844Fig. 844.
Fig. 844.
Fig. 845Fig. 845.
Fig. 845.
Figs. 842,843,844, and845represent Cushman’s patent combination chuck, in which each jaw may be operated independently by means of its screw thread, or a circular rack may be made to engage with the respective pinions, as shown inFig. 844, in whichcase operating any one of the screws operates simultaneously all the jaws. The method of engaging and disengaging is shown inFig. 845.crepresents the circular rack andda circular ring beneath it. This ring is threaded on its circumference, screwing into the body of the chuck, so that revolving it in one direction moves the circular rack forward and into mesh with the pinions, while revolving it backward causes the rack to recede from the pinions. To operate this ring the lug shown near the top of the chuck in figure is simply pushed in the required direction, while to lock the ring when out of gear with the pinions the spring catch shown on the left of that figure is moved radially. When the rack is in gear, the chuck is a universal one, all the jaws moving simultaneously and equally, whether they be set in such position in their slots as may be necessary to grip an oval or round piece of work; when the rack is out of gear the jaws may be moved by their respective screws so as to run true as for round work, or to hold the work to any degree of eccentricity required.
The jaws may be reversed in their slots and operated simultaneously as a universal chuck, or independently as a simple jaw chuck.
It is obvious that the truth of the jaws for concentricity may be adjusted within the degree of accuracy due to the number of teeth in one pinion divided into the pitch of the jaw operating screw, because each screw may be revolved separately to bring each successive tooth into mesh until the greatest obtainable jaw truth is secured.
Fig. 846Fig. 846.
Fig. 846.
Fig. 847Fig. 847.
Fig. 847.
Fig. 846represents a front, andFig. 847a sectional view, of the Westcott combination chuck.fis the main body of the chuck screwing on to the lathe spindle.fcarries the annular ringd, which has a thread on its face, as shown.dis kept in place by the ringe, which screws in an annular recess provided in the back of the chuck.cis a box fitting in the radial slots of the chuck. The back of the boxcmeshes into the radial thread ond, hence, whendis revolved, the boxescmove radially in the slots. Now the boxescafford journal bearing to, and carry the worm or screwsbas well as the chuck jawsa, hence revolvingdoperates the jaws simultaneously and concentrically as in a scroll or universal chuck. By means of the screwsb, the jaws may be operated individually (the boxescand ringdremaining stationary) as in an independent jaw chuck.
Suppose, now, the jaws to have been used independently, and that they require to be set to work simultaneously and concentric to the centre of the chuck, then the screwsbmay be operated until the jaws at their outer edge are even with the circumference of the chuck (or, if the jaws are nearer the centre of the chuck, they may be set true with a pointer), and the ringdmay be operated. In like manner, if a number of pieces of work are eccentric, the screwsbmay be used to chuck the work to the required eccentricity, and when the next piece is to be chucked the ringdmay be operated, and the chuck will be used as a universal one, although the shape of the work be irregular, all that is necessary being to place the same part of the work to the same jaw on each occasion.
Fig. 848Fig. 848.
Fig. 848.
The faces of the jaws of jaw chucks when they are true with the face of the chuck (or what is the same thing, run true, and are at a right angle to the axial line of the lathe centres), form guides wherefrom to set the work true, but this will only be the case when they remain true, notwithstanding the pressure of the jaws upon the work. Their truth, however, is often impaired by their wear in the chuck slots which gives them play and permits them to cant over. Thus inFig. 848is shown a chuck gripping a piece of workw, and it is obvious that to whatever extent the jaws may spring, or have lost motion in the ways or slots in the chucks, the jaws will move in the direction of the dotted linesa a, the face of the jaw then standing in the direction of dotted linesb b, instead of being parallel to the chuck face. If the spring or wear of the mechanism were equal for each jaw, the work would be held true, notwithstanding that the jaws be out of line, but such is not found to be the case, and as a result the work cannot be set quite true.
Fig. 849Fig. 849.
Fig. 849.
When the jaws are applied within the work, as inFig. 849(representing the jaws of the chuck within the bore of a ring or piece of workw), the jaws spring in the opposite direction asdenoted by dotted linesc,c, and when the jaws are locked to the work the latter moves in the direction ofdand away from the chuck face. It will be observed that there is no true surface to put the face of the work against in either case.
Fig. 850Fig. 850.
Fig. 850.
This is remedied in independent dog chucks by the construction shown inFig. 850, in which each jaw has a squarea, fitting in the grooves of the chuck, and a nut and washer atbsecure the jaw to the face of the chuck so that the lost motion due to wear of the parts may be taken up.
Fig. 851Fig. 851.
Fig. 851.
Fig. 852Fig. 852.
Fig. 852.
The Judson patent chuck is designed to overcome this difficulty, and is constructed as shown inFigs. 851and852, the former being a face view and the latter a sectional edge view of the chuck.
The jawsaof the chuck are hollow, and the nut instead of being solid in the jaw is a separate piece, having two wings, the outer of which bears upon a pin in the jaw, while the inner bears upon an inclined surface as plainly shown in the cut, so that the pressure of the screw is distributed equally upon the pin and the inclined surface. The nutbbeing below the centre of the pin and inclined surface causes the pressure to throw the jaw fair against the face of the chuck, hence the faces of the jaws will serve (equally as well as the surface of the chuck) as a guide to set the work against.
From the short length of gripping surface on the jaws of jaw chucks, they are incapable of holding work of any greater length than, say, about 6 inches, without the aid of the dead centre at the other end of the work; but if the dead centre be used in this way the work will be out of true, unless the jaws of the chuck be quite true, which is not always the case, especially after the chuck has been much in use. Furthermore, it is at times a difficult if not even an impracticable job to set work quite true in this way.
For special work made in quantities the form of the chuck may be varied to conform to the special requirements of the work. The variety of chucks that may thus be formed is obviously as infinite as the variations in form of the work. Thus threaded work may be screwed into threaded chucks, or cylindrical work may be driven into bored blocks forming chucks, or a ring may be chucked and then used as a mandrel to drive the work by friction.