Fig. 853Fig. 853.
Fig. 853.
An excellent example of special chuck is shown inFig. 853, representing a chuck for holding piston rings. It resembles a face plate screwing on the live spindle atb, and having 8 radial dogs or jawsa, let into the faced, and secured thereto, when adjusted by the bolts and nutse. A mandrel is fast in the centre of the chuck carrying the conec, upon which rest the cone surfaces on the ends of the dogsa, so that screwing upc, by means of the nut shown, throws the dogsaoutwards, causing them to grip the inside of the piston ring as shown in the face view of the chuck.
Fig. 854Fig. 854.
Fig. 854.
InFig. 854is shown Swazey’s expanding chuck.bis the body of the chuck driven on an arbora. The hub ofbis turned taper to receive a discc, which is split partly through in three places, and wholly through atz. By means of the nut and washerd e, the disc is forced up the taper hub and caused to expand in diameter and grip the bore of the work, or ringr, the face ofbserving to set the face of the ring against to hold it true sideways.
The chucks employed by wood workers for driving work without, the aid of the back or dead centre of the lathe are as follows:— On account of the fast speed at which the wood-workers’ lathe revolves, it would be undesirable to have their chucks of iron, because of the time it would take the lathe to start them to full speed, and also to stop them after shifting the belt from the driving to the loose pulley of the countershaft, and further becauseof the damage the tool edges would receive if they accidentally came into contact with the face of the chuck. For these reasons wood workers’ chucks are usually built up upon small iron face plates.
Fig. 855Fig. 855.
Fig. 855.
Fig. 855represents a cement chuck, consisting of a disc of hard wooda, screwed firmly to the face plateb; atcis a round steel point located at the axis of the chuck.
This chuck is employed to drive very thin work by the adhesion between the surface of the work and that of the chuck. The surface of the chuck is coated with a mixture of 8 parts of resin to one part of beeswax run into sticks. The chuck is waxed or cemented by rotating it at high velocity while holding the sticks against it. The whole surface of the chuck being thus coated, the centre of the work is forced on the steel pointc, and the lathe is kept running until the surface of the work nearly touches that of the chuck, when the belt is passed to the loose pulley overhead and the work forced against the chuck surface until it stops or else revolves the work against the hand pressure, the friction between the surfaces having melted the wax or cement, and cemented the work to the chuck. This leaves the face and the circumference of the work free to be operated upon. The work is removed from the chuck by the gradual insertion between the two of a long thin-bladed knife.
Fig. 856Fig. 856.
Fig. 856.
For work of large diameter, however, a mere disc of wood will not answer, it being too weak across the grain: and here it may be remarked that the work often supports the chuck, and therefore we should always, in fixing, make the grain of the work cross that of the chuck, because the centrifugal force due to the high velocity is so great that both the chuck and the work have before now been rent asunder by reason of the non-observance of this apparently small matter. When it is considered that the chuck has not sufficient strength across the grain, battens should be screwed on at the back; but a chuck so strengthened will require truing frequently on account of the strains to which its fibres will be subjected from the unequal expansion or contraction of its component parts.Fig. 856shows the back of a chuck strengthened by the battensa,a,a.
Fig. 857Fig. 857.
Fig. 857.
Another and superior method of making a chuck suitable for work of about the same diameter is shown inFig. 857. Its construction enables it to better resist outward strains in every direction, while the strains to which it must necessarily be subject, from variations of temperature and humidity, are less than in the former. It will also be found that it can be trued with greater facility, especially on the diameter, as the turning tool will not be exposed to the end grain of the wood.
The crossed bars at the back of the chuck are half checked, as shown ata, so that both pieces may extend clear across the chuck and not terminate at the centre. They are fastened together at the centre by glue, and also with screws. Upon these bars as a frame, the four pieces composing the body or face of the chuck are fastened by both glue and screws. These pieces need not extend clear to the centre, but may leave an open square as shown, because the centre of a large chuck rarely requires to be used.
Fig. 858Fig. 858.
Fig. 858.
For very large chucks a cross of this kind would not afford sufficient strength, hence, the form shown inFig. 858is employed. The arms are bolted to an iron face plate, as shown, their number increasing with the diameter of the chuck. To keep the chuck true, the arms should have a level and fair bed upon the face plate, the segments composing the rim being fairly bedded to the arms and well jointed at the ends. They should be both glued and screwed, care being taken that the points of the screws do not meet the face of the chuck, in which case they would damage the turning tools used to true the chuck.
As wooden chucks are liable to warp and become out of true it is requisite to test them on each occasion before use, and true them if necessary. The work is fastened to these chucks by means of screws whose heads are sunk beneath the work surface a sufficient depth so that there is no danger of their coming into contact with the turning tools. In other cases the work is glued to the chuck, a piece of paper being interposed between the work and the chuck, which, by being damped, will enable the more ready removal of the work from the chuck.
Fig. 859Fig. 859.
Fig. 859.
Another form of chuck used by wood workers is shown inFig. 859. It consists of a disc of wooda; screwed to the face plateand carrying the two piecesb,b. The piecesc,care wedges which slide endways to grip the work. This chuck is especially handy for small work of rectangular form.
From the shape of some work, it cannot be chucked in jaw chucks of any description, and this is especially the case with work of large diameter, hence, large lathes, as, say those that will swing more than three feet, are not usually provided with universal chucks, although sometimes provided with independent jaw-chucks. So likewise in small lathes there are many forms of work that cannot be chucked in jaw chucks, and yet other forms that can be more conveniently held or chucked on face or chuck plates, &c.
If, for example, the surface of the chuck requires to be used in setting the work, the jaws will often be in the way of the tools or instruments employed to set the work. Again, there may be projections on the work which will require the body of the work to be held too far from the face of the chuck to enable its jaws to grip the work.
To meet the requirements of these classes of work chucking devices, which may be classified as follows, areemployed:—
1st. Chucking by bolting work to the face plate or chuck plate with bolts and plates.
2nd. Chucking between dogs movable about the face chuck plate, and holding the work from that plate.
3rd. Chucking with the aid of the angle plate, or with the angle plate employed in conjunction with the chuck plate.
Fig. 860Fig. 860.
Fig. 860.
Fig. 861Fig. 861.
Fig. 861.
The chuck plate is simply a face as large in diameter as the lathe will swing, and is sometimes termed the large face plate. Chuck plates for smaller lathes, as 30 inches swing, or less, are sometimes provided with numerous round or square holes to receive the bolts which hold the work, but usually with slots and holes as inFig. 860. The larger sizes of chuck plates are similarly formed, but are sometimes provided with short slots that meet the circumference of the plate as inFig. 861, which represents a chuck plate of the Whitworth pattern. The face of the chuck plate must be maintained true in order that true work may be produced, and it is necessary when putting it upon the lathe to carefully clean its threads and those of the live spindle, as, on account of its large diameter, a very little dirt between it and the live spindle will throw it considerably out of truth at the circumference.
It is better if there be any error in a chuck plate or face plate that it be hollow rather than rounding when tested with a straightedge, because in that case a given amount of error in the plate will produce less error in the work.
Fig. 862Fig. 862.
Fig. 862.
Fig. 863Fig. 864Fig. 863.Fig. 864.
Fig. 863.Fig. 864.
Fig. 865Fig. 866Fig. 865.Fig. 866.
Fig. 865.Fig. 866.
InFig. 862, for example,arepresents a chuck plate hollow across the face, andba link requiring to be bored through its double eyec, the centre line of the lathe being linee e, and the centre line of the hole in the hubdof the link being denoted byf, and aseandfare not parallel one to the other it is obvious that the holes will not be parallel. Suppose, now, that the chuck face was rounding, and the centre line ofdwould stand atg g, and the holes incanddwould be out of true in the opposite direction. In this case the error would be equal, but suppose we have a ring or disc such asbinFig. 863to chuck by bolts and platesc,dand it will be chucked true, notwithstanding that the face of the plate is hollow. But were the face of the plate rounding the disc may be chucked as inFig. 864, the facefof the work not being held at a right angle to the line of centreseas it is inFig. 863. The truth of the chucking inFig. 864depends upon whether the clampscwere screwed up with equal force upon the work. A hollow chuck plate will lose this advantage in proportion as the work covers more of one side of the chuck plate than it does of the other, but in any event it will chuck more true than a roundingone. Suppose we have, for example, a ring chucked eccentrically as inFigs. 865and866, the chuck being as much hollow in the one case as it is rounding in the other, and that shown inFig. 866will stand out of true to an amount greater than the chuck is in an equal amount of its radius. While that shown inFig. 865would be nearer true than the chuck is in an equal length of its radius, both amounts being in proportion to the length of the lineato that of lineb.
If the chuck plate is known to be either rounding or hollow, pieces of paper of sufficient thickness to remedy the error may be placed atcanddrespectively. It is better, however, to true up the faces of plates so that the surface of the work bolted against it will be true and stand at a right angle to the line of lathe centres.
In truing up a face plate, the bearings of the live spindle should be adjusted so that there is no play on them, and the screw or other device used to prevent end motion to the live spindle should be properly adjusted.
Fig. 867Fig. 867.
Fig. 867.
A bar or rod of iron should also be placed between the lathe centres to further steady the live spindle, and the square holes or radial slots should have the edges rounded or bevelled off, as shown inFig. 867, so that when the tool point strikes the sidesaof the holes or slots it will leave its cut gradually and not with a sudden jerk or jump, while, when it again takes its cut on the sideb, it will also meet it gradually and will not meet the sand or hard skin on the face of the casting, which would rapidly dull the tool.
In facing or truing up a chuck plate, the feed nut should be put in gear with the feed screw or feed spindle, and the cut should be put on by revolving the feed spindle or feed screw. This will take up any lost motion in the feeding mechanism, after which the carriage may, if there are devices for the purpose, be locked to the lathe bed so as to prevent its moving.
It is better that the thread of the chuck be not too tight a fit upon that on the lathe spindle, the radial face of the chuck hub and of the cone spindle collar being relied upon to set the chuck true, because it is somewhat difficult to produce threads so true as to hold the faces true.
To preserve the threads both upon the chuck bore and the lathe spindle from undue wear, the chuck when taken off the lathe should be stood on edge so that falling dust may not accumulate in the thread. Before putting the chuck upon the lathe spindle the threads of both and the radial faces of the chuck hub and cone spindle collar should be carefully cleaned, because the presence of any dirt or dust on those faces will throw the face of the chuck plate out of true to an amount that may be of importance at and near the chuck’s circumference.
Fig. 868Fig. 868.
Fig. 868.
As an example of simple chucking on a face plate, or chuck plate, let it be required to bore, cut a thread in the bore, and recess the piece of work shown inFig. 868, the radial faces being already true planes not requiring to be turned.
Fig. 869Fig. 869.
Fig. 869.
Fig. 870Fig. 870.
Fig. 870.
This could be held as shown inFig. 869, in whichcis the chuck plate,wthe work,sa strap plate, andb,bare bolts and nuts, a face view of the work already chucked being shown inFig. 870. The surface of the work being bolted direct against the face of the chuck plate will be held true to that face, and all that is necessary is to set it true concentrically. While performing this setting, the work should not be bolted too firmly, but just firm enough to permit of its being moved on the chuck plate by light blows, the final tightening of the clamps being effected after the work is set true. The bolts should be tightened upon the work equally, otherwise one end of the plate will grip the work firmly, while the other being comparatively slack, the work will be apt to move under the pressure of a heavy cut.
Fig. 871Fig. 871.
Fig. 871.
A form of strap not unusually employed for work chucked in this manner is shown inFig. 871, its advantage being that it is capable of more adjustment about the chuck plate, because the slots afford a greater range for the bolts to come even with the holes in the chuck plate.
Fig. 872Fig. 872.
Fig. 872.
Fig. 873Fig. 873.
Fig. 873.
If the work be light, it may be held to the face plate while the holding or clamping plates are applied as shown inFig. 872, inwhichfis the face plate or chuck plate,wthe work,pa plate of iron,da rod, andcthe back lathe centre. The latter is forced out by the hand wheel of the tailstock with sufficient force to hold the work by friction while the bolts and plates are applied. It is obvious, however, that if the work has no hole in its centre, the platepmay be dispensed with, and that if a strap plate, such as shown inFig. 871, be employed, it must first be hung on the tail spindle so that it may be passed over the roddto the work. Strap plates are suitable for work not exceeding about 6 inches in diameter. For larger work, bolts and plates are used, as shown, for example, inFig. 873, which represents a piece of workwheld to the chuck plate by platespand boltsb, there being ate epacking pieces or pieces of iron to support those ends of the clamps or clamping platesp. It is necessary that these packing piecesebe of such a height as to cause the platespto stand parallel to the face of the chuck for the followingreasons:—
Fig. 874Fig. 874.
Fig. 874.
Suppose that inFig. 874,wis a piece of work clamped to the chuck plate, and that packing pieceeis too high, and packing piecee′is too low, as shown, both pieces throwing the platespout of level, then in setting the hole in the work to run true it will be found difficult to move it in the direction of the arrow, because moving it in that direction acts to force it farther under platep′, and therefore, to tighten its nut. In the case of platep, the packing pieceewill be gripped by the plate more firmly than the work is, which will be held too loosely, receiving so little of the plate pressure as to be liable to move under the pressure of the tool cut. It is better, however, that the packing piece be slightly above, rather than below the level of the work surface. The position of the plates with relation to the work should be such as to drive rather than to pull it, which is accomplished in narrow work by placing them as inFig. 873.
The position of the bolts should be as close as possible or convenient to the work, because in that case a larger proportion of its pressure falls upon the work than upon the packing piece. For the same reason, the packing piece should be placed at the end of the plates. This explains one reason why it is preferable that the packing piece be slightly above rather than below the level of the work surface, because, the bolt being nearer to the work than to the packing piece, will offset in its increased pressure on the work the tendency of the packing piece to take the most bolt pressure on account of standing the highest.
Fig. 875Fig. 875.
Fig. 875.
Fig. 876Fig. 876.
Fig. 876.
If a packing piece of the necessary height be not at hand, two or more pieces may be used, one being placed upon the other. Another plan is to bend the end of the clamping plate around, as inFig. 875, in which case a less number of packing pieces will be required, or, in case the part bent around is of the right length or height, packing pieces may be dispensed with altogether. This is desirable because it is somewhat difficult to hold simultaneously the plate in its proper position and the packing pieces in place while the nut is screwed up, there being too many operations for the operator’s two hands. To facilitate this handling, the nuts upon the bolts should not be a tight fit, because, in that case, the bolt will turn around in the bolt holes or slot of the chuck, requiring a wrench to hold the head of the bolt while the nut is screwed up, which, with holding the plate, would be more than one operator could perform. If the holes in the chuck plate are square, as they should be, the bolt may be made square under the head, as inFig. 876ata, which will prevent it from turning in the hole. This, however, necessitates that the head of the bolt be placed at the back of the chuck, the nut end of the bolt being on the work side, which is permissible providing that the bolt is not too long, for in that case the end of the bolt projecting beyond the nut would prevent the slide rest from traversing close up to the work, which would necessitate that the cutting tools stand farther out from the slide rest, which is always undesirable. Bolts that are not square under the head should, therefore, be placed with the head in the work side of the chuck plate, because it is of little consequence if the bolt ends project beyond the nuts at the back of the chuck plate.
The heads of the bolts should be of larger diameter than thenuts, because the increased area under the head will tend to prevent the bolt from turning when the nut is screwed up.
Fig. 877Fig. 877.
Fig. 877.
It sometimes happens that a projection on the work prevents the surface that should go against the surface of the chuck plate from meeting the latter. In this case, what are known as parallel pieces are employed. These are pieces of metal, such as shown inFig. 877, the thicknessavarying from the widthbso as to be suitable for work requiring to stand at different distances from the chuck plate surface, it being always desirable to have the work held as near as possible to the chuck plate so that it may not overhang the live spindle bearings any more than necessary.
Fig. 878Fig. 878.
Fig. 878.
An example of chucking with bolts and plates and with parallel pieces is given inFig. 878, in which the work has projectionsa,aandb,b, which prevent it going against the face of the chuck;e,eare the parallel pieces which, being of equal thickness, hold the inside face of the work parallel to the chuck face.
Fig. 879Fig. 879.
Fig. 879.
Another example of the employment of parallel pieces is shown inFig. 879, which represents a connecting rod strap with its brasses in place, and chucked to be bored.bis a small block of iron inserted so that the key may bind the brasses in the strap andp pis one parallel piece, the other being hidden beneath the key and gib. The object in this case is to chuck the brasses true with the faceaof the strap, the platessbeing placed directly above or over the parallel pieces. This is a point requiring the strictest attention, for otherwise the pressure of the clamping plates will bend both the work and the chuck plate.
Fig. 880Fig. 880.
Fig. 880.
InFig. 880, for example, the parallel pieces being placed atp,p, and the clamping plates atp,p, the pressure of the latter will bend the work as denoted by the dotted lines, and the chuck plate in the opposite direction, and in this case the work being weaker than the chuck plate will bend the most.
As a result the face of the work will not be true when released from the pressure of the bolts and nuts holding it. Parallel pieces should therefore always be placed directly beneath the clamping plates, especially in the case of light work, because if they be but an inch away the work will be bent, or spring as it is termed, from the holding plate pressure. In very large work the want of truth thus induced would be practically discernible, even though the work be quite thick, as, say, three inches, if the parallel pieces were as much as, say, 6 inches from the holding plates.
Fig. 881Fig. 881.
Fig. 881.
Fig. 881shows an example of chucking by means of parallel strips in conjunction with parallel pieces.b,bare a pair of brasses clamped by the stripss,s, which are bolted together by the boltsa,a;p,pare the parallel pieces.
The strips being thus held parallel to the surface of the chuck plate, all that is necessary is to set the flanges of the work fair against the surface of the strips and true with the dotted circle, and the brass bore will be bored at a true right angle to the inside face of the flange. If the inside face of the brasses was true, the parallel pieces might be omitted, but this is rarely the case.
Fig. 882Fig. 882.
Fig. 882.
An excellent example of bolt and plate chucking is given in a heavy ring of, say, three feet diameter, and 5 or 6 inches cross section, requiring to be turned quite true, and of equal thickness all over. This job may be chucked in three different ways; for example, inFig. 882,a,b,c,dare four-chucking dogs, so holding the work that its two radial faces and outside diameter may be turned. This being done, four more dogs may be placed to grip the diameter of the work, and the inside ones may then be removed and the bore turned out. In this way the work would not be unchucked until finished. There is danger, however, that the dogs applied outside may spring the work out of true, in which case it would require setting by a pointer in the slide rest.
Fig. 883Fig. 883.
Fig. 883.
Fig. 884Fig. 884.
Fig. 884.
Another plan would be to hold the work by dogs applied on the outside, and turn the bore and both of the faces. To these fasten four plates on the chuck plate, and turn their ends to the size of the bore and place the work on them, as inFig. 883, in whicha,b,c,dare the four plates, and are clamping plates. This plan is often employed, but it is not a desirable one in heavy work, because the weight of the work is quite apt to move the plates during its setting. A better plan than either of these is tofirst turn off one face and then turn the work around in the lathe and hold it as inFig. 884. The bore may then be turned, and all that part of the face not covered by the plates. Four holding plates must then be applied with the bolts within the bore, and when screwed firmly down the outside plates may be removed, leaving the work free to have the remainder of its face and its circumference turned up. In this way the work may be turned more true than by either of the two previously described methods, because it has no opportunity to move or become out of true.
Fig. 885Fig. 885.
Fig. 885.
Cylindrical work to be chucked with its axis parallel to the face plate is chucked by wood workers as shown inFig. 885, in whichb,bare two blocks screwed to the chuckc, and havingVs in to receive the work as shown; the work is held to the blocksb, by means of the strapss,s, which are held tob,bby screws. An example of a different class of chucking by bolts and clamps may be given in the engine crank. A common method of chucking such a crank is to level the surface of the crank in a planing machine, and to hold that surface to the chuck-plate by bolts and plates, while boring both the holes, merely reversing the crank end for end for the second chucking.
This method has several inherent defects, especially in the case of large cranks. First, it is a difficult matter to maintain large chuck plates quite true, and as a result by this method of chucking any want of truth in the surface of the chuck will be doubled in the want of parallelism in the bores of the crank.
Suppose, for example, that the chuck surface is either slightly hollow or rounding as tested with a straight-edge placed across its face, then the axial line of the hole bored in the crank will not be at a true right angle with the planed surface of the crank. When the crank is turned end for end on the chuck-plate and again bolted with its plain surface against the surface of the chuck, the second hole bored will again not stand at a true right angle to the planed surface, and furthermore the error in one hole will be in a directly opposite direction to that of the other hole, so that the error in the crank will be double the amount that it is on the chuck surface. To this it may be answered that if such an error is known to exist it may be corrected by placing a piece of paper of the requisite thickness at the necessary end of the crank for both chuckings. But this necessitates testing the chuck on each occasion of using it, and the selection of a sheet of paper of the exact proper thickness, which is labor thrown away so long as an equally easy and more true way of chucking can be found. Furthermore there is a second and more important element than want of truth in the chuck to be found, which is that of the alteration of form which occurs in the crank (as each part of its surface is cut away) as explained in the remarks with which the subject of chucking is prefaced.
First, the planed surface of the crank will alter in truth so soon as the crank is released from the pressure of the holding devices on the planer or planing machine; second, that surface will again alter in form and truth from the removal of the metal around the surface of the hole first bored; and third, the planed surface will beto some extentsprung from the pressure of the plates holding the crank to the chuck plate, hence the following method is far preferable.
Fig. 886Fig. 886.
Fig. 886.
If it is intended to plane the back surface of the crank let that be done first as before, and let it be held to the face-plate by bolts and plates as before, while the hole and its radial face at the large end of the crank are turned and finished. In doing this, however, first rough out the radial face, and then rough out the hole, so that if the work alters in form a fine finishing cut on both the radial face and the bore will correct the evil. Then release the crank from the pressure of the holding plates; and itis obvious that however the planed surface may have altered in truth from removing the surface metal, the radial face just turned will be true with the bore turned at the same chucking. Now to chuck the crank to bore the second hole, turn it end for end as inFig. 886, and bolt the face already turned to the chuck plate (as atain the figure) with one or more bolts and strap plates. To steady the other end of the crank, and prevent it from moving under the pressure of the cut, take two bolts and platesb, and place a washer between them and the chuck surface as shown atc, then bolt the plates to the chuck plate, so adjusting them that their ends just have contact with the crank when it is set true. In setting it true it may be moved by striking the outer ends of the plates.
In this method of chucking, we have the followingadvantages:—
1st. If the chuck plate is not true we may place a piece of paper beneath the crank surfacea, to correct the error as in the former method, or if this is neglected, the second hole bored will be out of true to an amount answerable to the want of truth in the chuck, and not to twice as much as in the former method.
2nd. Any alteration of form that may take place during the first chucking does not affect the truth of the second chucking as in the other case.
3rd. The crank being suspended during the second chucking, any alteration of form that may accompany the boring of the second hole will be corrected by the finishing cut, hence the crank will be bored with its two holes as axially true as they can be produced in the lathe.
It now remains to explain the uses of the pieceswinFig. 886, simply weights termed counterbalances bolted to the chuck plate to balance it against the overhanging weight of the crank on one side of the chuck plate. If these weights are omitted the holes in the work will be bored oval, because the centrifugal force generated by the revolution of the work will take up any lost motion there may be between the cone spindle journal and its bearings, or if there be no such lost motion the centrifugal force will in many cases be sufficient to spring the cone spindle.
In selecting these weights it is well to have them as nearly as possible heavy enough to counterbalance the work when placed at the same distance from the lathe centre as the outer end of the work. The proper adjustment of the weight is ascertained by revolving the lathe and letting it slowly come to rest, when, if the outer end, or overhanging end as it termed, of the work comes to rest at the bottom of the circle of revolution on two or three successive trials the weight of the counterbalance must be increased by the addition of another weight, or the weight may be moved farther from the lathe centre.
Fig. 887Fig. 887.
Fig. 887.
To enable a piece of work, such as a crank for example, to have two or more holes bored at one chucking, a class of chuck such as shown inFig. 887is sometimes employed.sis a slide in one piece with the hub that screws on the live spindle and standing at a true right angle with the axial line of the cone spindle and made as long as will swing over the lathe bed. It contains a dovetail groove (as shown in the edge view) into which a bart, running across the back of the face platep, passes. To cause the bartto accurately fit the dovetail, notwithstanding any wear of the surfaces, a slipgis introduced, being set up totby set-screws passing through that side of the dovetailed piece. The work, as the crankc, is bolted to the face plate, and the set-screws ongare eased so that the plate can be moved to set the work true; when true, the set-screws are tightened, and the first hole may be bored. To bore the second hole all that is necessary is to slacken the set-screws ong, move the plate, which will slide in the dovetail groove, and set the work; when the set-screws are again set up tight, the boring may again be proceeded with. In this way both holes may be bored without unclamping the work. The whole truth of the job,before being unclamped from the chuck plate, depends in this case upon the dovetail groove being at a true right angle to the axial line of the lathe cone spindle, it being of no consequence whether the face plate stands true or not. But suppose the removal of the metal to have released strains in the casting or forging, then the clamping plates will have prevented the crank from quite assuming its normal shape after the release of those strains, and the crank, when finished, though true while clamped, will change its form the instant the clamping plates are removed, and the holes bored will in all probability not have their axial lines true one with the other. Another objection is that throwing the chuck plate out of balance on the lathe spindle as well as the crank induces the evils due to the centrifugal motion. This may be offset by increased counterbalancing, of course, but the counterbalancing becomes cumbersome, and is not so easy a matter. For these reasons, chucks of this class are not desirable unless it may be for comparatively small and light work. It is obvious that the dovetail groove may be provided with a screw, and the back of the plate with a nut, so as to move the plate along the groove by revolving the screw. This will assist in adjusting or setting the work, but it will increase the amount of weight requiring to be counterbalanced.
Fig. 888Fig. 888.
Fig. 888.
Fig. 889Fig. 889.
Fig. 889.
When a number of pieces are to be bored with their holes of equal diameters and of the same distance apart, the chucking should be performed as inFigs. 888and889; one and the same end of each link should be bored and faced, the links being held by the stem, placed on parallel pieces with plates. A pin such as shown inFig. 889should then be provided, its diameter acrossabeing a close sliding fit into the bores of the links; while the length ofashould be slightly less than the length of the hole in the link, the partdshould be made to accurately fit the hole bored by any suitably sized reamer; a washerbshould be provided, and each end should be threaded to receive nuts. There should then be provided in the chuck plate a hole whose distance from the centre of the chuck must exactly equal the distance apart the holes in the links are required to be, and into whose bore the enddof the pin shown inFig. 889must drive easily. The pin should be locked in this hole by a nut as shown inFig. 889. The bored ends of the links may then be placed on the pin andfastened by a nut as inFig. 888, which will regulate the distance apart of the holes.
It is obvious that the pin may be passed through one of the radial slots in the chuck, and set the required distance from the centre, but in this case the pin would be liable to become moved in its position in the slot.
Side plates to prevent the link from moving should of course be applied as atd,din the figure.