Fig. 1103Fig. 1103.
Fig. 1103.
An adjustable reamer designed and used by the author for holes not less than 11⁄2inches in diameter, is shown inFig. 1103, in whicharepresents the body of the reamer containing dovetail grooves tapered in depth with the least depth at the entering end. The grooves receive cuttersb, having gib heads.cis a ring or washer interposed between the gib heads of the cutter and the face or shoulder ofa, the cutters being locked against that face by a nut and a washere. By varying the thickness ofc, the cutters are locked in a different position in the length of the grooves, whose taper depth therefore causes the cutters to vary in diameter. Suppose, for example, that with a given thickness of washerc, the cutters are adjusted in diameter so as to produce a hole a tight working fit to a plug turned to a 2-inch standard gauge: a slightly thinner washer may be used, setting the cutters so as to bore a hole an easy working fit to the plug; or a slightly thicker washer may be employed so as to produce a hole a driving fit to the same plug. Three or more washers may thus be used for every standard size, their thickness varying to suit the nature of the fit required.
It will be noted that it is mentioned that threeor morewashers may be used, and this occurs because a diameter of fit that would be a driving fit for a hole of one length would be too tight for a driving fit of a much longer hole, the friction of course increasing with the length of hole, because of the increase of bearing area.
For large sizes, a reamer of this description is an excellent tool, because if it be required to guide the reamer by means of a plain cylindrical shank, a washer, or sleeve, having a bore to fit the shank at the termination of the thread, may be used, but such areamer is not suitable for small diameters, because of the reduction of shank necessary to provide for the nut and thread.
Fig. 1104Fig. 1104.
Fig. 1104.
Reamers for roughing out taper holes may be made with steps, as inFig. 1104, which is taken fromThe American Machinist, there being a cutting edge where each step meets a flute. Such a reamer may be used to enlarge parallel holes, or to rough out taper ones, and the flutes (if not to be used for brass work) may be spiral, as in the figure. The end step being guided by the hole serves as a guide to the first cutting edge; the second step serves as a guide for the cutting edge that follows it, and so on.
Fig. 1105Fig. 1105.
Fig. 1105.
The steps are best turned a trifle larger, say1⁄1000inch larger, at the cutting end. Half-round taper reamers, such as shown inFig. 1105, are used for finishing holes. The flat face is cut down, leaving rather more than a half circle; the clearance being filed or ground on the cutting side so as to enable the reamer to cut, and extending from the cutting edge to nearly half-way to the bottom of the reamer.
For holes, however, that are large enough to admit a tool of sufficient strength, the single-pointed boring tool produces the most true work.
Brass finishers use square taper reamers, which produce upon brass more true work than the half-round reamer.
Fig. 1106Fig. 1106.
Fig. 1106.
For reaming the bores of rifles, a square reamer, such as shown inFig. 1106, is employed; the edgesa bare the cutting ones, the edgesc dbeing rounded off;eis a piece of wood, beneath which slips of paper are placed to restore the size as the wear proceeds. The entering end of the reamer is slightly tapered. On account of the extreme length of this reamer in proportion to its diameter, it is fed to its cut by being pulled instead of pushed as is usually the case, the pull placing the rod of the reamer under tension and thus stiffening it; the line of pull is of course true with the axis of the rifle bore. The reamer is revolved at high speed and freely supplied with oil.
By means of the slips of paper successive cuts and minute increases of diameter may be taken with the same reamer.
Fig. 1107Fig. 1107.
Fig. 1107.
Fig. 1107represents a class of rose bit employed to reduce pins to a uniform diameter, and face off the shoulder under the head, or it may be used to cut a recess round a pin, or to cut a recess and leave a pin.
Fig. 1108Fig. 1108.
Fig. 1108.
Fig. 1109Fig. 1109.
Fig. 1109.
For making a recess round a hole, or, in other words, for cutting a flat-bottom countersink, a facing countersink,Fig. 1108, may be used, its cutting edges being ata,b,c, &c. The clearance is given at the ends of the teeth only, being shown frombtod. The pinpsteadies the tool, and is made a working fit to the hole in the work. Or if too small, a ferrule may be placed upon it, thus increasing the capacity of the tool. When a tool of this kind is to be used on iron, steel, or copper, and not upon brass, the front face of the teeth may be given rake by cutting the grooves at an angle, as inFig. 1109.
Boring Tools for Lathe Work.—The principal object in forming a boring tool to be held in a slide rest is to have the body of the tool as large as can be conveniently got into the size of the hole to be bored; hence the cutting edge should not stand above the level of the top of the steel. By this means the tool will be as stiff as possible, and less liable to spring away from its cut, as boring tools are apt to do, especially when the cut or hole is a long one.
It is so difficult a matter to bore a long hole parallel with a long boring tool that cutters of various forms are usually preferred, and these will be describedhereafter.
Fig. 1110Fig. 1110.
Fig. 1110.
The boring tool is, upon cast iron and brass, exceedingly liable to chatter, but this may always be avoided by making the angles forming the cutting edge less acute: thus, inFig. 1110are three boring tools,a,b,c, operating in a piece of workd. Now the lateral pressure of a cut is exerted upon the tool at a right angle to the length of the cutting edge; hence (in addition to the vertical pressure) the lateral pressure of the toolawill be in the direction of the dotted line and arrowa, that onbin the direction of dotted line and arrowb, and that oncin the direction of dotted line and arrowc; hence the pressure of the cut would tend to forceatowards the centre of the hole and off or away from its cut,bback from its cut, andcdeeper into its cut. Now as the cut proceeds, the tool edge dulls, hence it would appear that a compromise betweencandbwould be the most desirable, as giving to the tool enough of the tendency to deepen its cut to compensate for the tendency to spring away from its cut, as the cutting edge dulls (which it does from the moment the cut begins).This is quite practicable in tools to be used on wrought iron, as shown inFig. 1111, which represents the most desirable form.
Fig. 1111Fig. 1111.
Fig. 1111.
In this form the part of the cutting edge performing duty under a deep cut will be mainly in front of the tool, but in light cuts the cutting edge would be farther back, where it is more nearly parallel to the line of the work bore, and will hence cut smoother.
Fig. 1112Fig. 1112.
Fig. 1112.
Where a boring tool is intended for light cuts only on wrought iron it may have all, or nearly all, its rake at the top, as shown inFig. 1112, fromatobrepresenting the cut, andcthe tool.
Fig. 1113Fig. 1113.
Fig. 1113.
Fig. 1114Fig. 1114.
Fig. 1114.
Under ordinary conditions that in the form of tool shown inFig. 1113[15]is best for brass work, the faceabeing horizontal or slightly depressed towards the point. Boring tools require very little bottom rake, and the cutting points should be as rounded as they can be made without chattering. On wrought iron the top rake may be as much as is consistent with strength, and water should be freely applied to the cut. For cast iron the best form of tool is that shown inFig. 1114, the edgeabeing parallel with the bore of the hole, and the feed being a coarse one, taking a very light cut when finishing.
[15]From “The Complete Practical Machinist.”
[15]From “The Complete Practical Machinist.”
Fig. 1115Fig. 1115.
Fig. 1115.
In cases, however, where the tool point requires to cut up to a sharp corner, the form of tool shown inFig. 1115(which represents a top and end view) may be used. Its end facecis at an obtuse angle to the length of the tool, so that on passing up a bore and meeting a radial face the point only will meet that face. This angle, however, gives to the tool a keenness that will cause chattering on brass work unless the top face be bevelled to the tool body, as isatobin the figure.
Fig. 1116Fig. 1116.
Fig. 1116.
It frequently happens in boring cast iron that the skin or the surface of the metal is very hard, rapidly dulling the tool and forcing it away from its cut, unless the cut is deep enough to allow the point of the tool to cut beneath it, as shown inFig. 1116, in which the hardness is supposed to extend from the bore to the dotted line.
In this case a tool formed as atcis employed, the point cutting in advance of the rest of the tool, and entering the soft metal beneath the hard metal; the hard metal will then break away in lumps or pieces, without requiring to be absolutely cut into chips or turnings, because of being undercut, as shown atb.
Fig. 1117Fig. 1117.
Fig. 1117.
The cross slider or tool rest of a lathe should be adjusted to closely fit the cross slide of the lathe if true and parallel work is to be bored, because any lost motion that may exist in the slide is multiplied by the length the tool stands out from the tool post. Thus the centre of motion of the rest if it has play, as atb,Fig. 1117, and the direction of motion at the tool point, will be an arc of a circle of whichbis the centre, the bend of the tool from the pressure of the cut will have its point of least motion or fulcrum ata; hence, both tend to cause the tool point to dip and spring unequally under the varying cut pressure that may arise from hard or soft places in the metal, and from inequalities in the cut depth.
The pressure of the cut increases as the tool point loses its sharpness, and this makes sufficient difference for the amount of tool spring in light boring tools or in long holes to cause the tool to bore a larger hole at the beginning than it does at the end of its feed traverse; or, in other words, to bore a taper hole, whose largest end is that at which the cut was started. If, therefore, the cut is traversed from the front to the back of the hole the latter will be of the smallest diameter at the back, and conversely if the cut proceeds from the back to the front of the hole the front will be of smallest diameter. The amount of the taper so caused (or in other words the error from parallelism) will obviously increase with the length of the hole.
To obviate this taper, the slide of the rest should for the finishing cut be set up firmly, and the tool after being sharpened should take a finishing cut through the hole, and then let traverse back, which can be done providing that care be taken not to bore the hole too large.
A boring tool will take a smoother cut and chatter less if the final cut be from the back to the front of the hole, and for the following reasons: When the tool is fed in, the strain or pressure of the cut is in a direction to partly compress and partly bend the steel which is being pushed to its cut, but when it is fed in the opposite direction it is pulled to its cut and the strain is in a direction to stretch the steel, and this the tool is more capable of resisting, hence it does not so readily vibrate to cause chattering.
In consequence, however, of the liability of a boring tool to spring away from its cut, it is far preferable to finish holes with standard cutters, reamers, or bits, in which case the boring tool may beemployed to rough out and true up the hole, leaving afinecut for the finishing cutter or bit, so as to wear its cutting edge as little as possible. To further attain this latter object, the cutter or bit should be used at a slow cutting speed and with a coarse feed.
If cutters or bits are not at hand, tool holders are desirable, and the forms of these depend upon the nature, or rather the diameter, of the hole to be bored. In all cases, however, the best results will be obtained when the diameter of the tool holder is as near that of the hole to be bored as will give it clearance. This occurs on account of the rigidity of the holder being greater than that of the tool.
For large work tool holders are desirable, in that the tools, being short, are easier to forge, to handle, and to grind.
For example, a tool holder of a cross section of two inches square may contain a tool whose cross section is 1 by3⁄4inch, in which case it is necessary to forge, grind, &c., the small tool only, whereas in the absence of the holder the tool would require to be of a cross section equal to that of the holder to obtain an equal degree of rigidity.
Fig. 1118Fig. 1118.
Fig. 1118.
A boring tool holder suitable for holes of from 2 to 4 or 5 inches is shown inFig. 1118, in whicharepresents a round bar shaped at the endbto fit into the tool post of the slide rest, and having a groove across the diameter of the endc dto receive a short tool. The slot and tool may be either square orV-shaped, the tool being locked by a wedge. It is obvious that instead of shaping the endbas shown, the bar may be held (if the slide-rest head is provided with a clamp instead of a tool post) by two diametrically opposite flat faces.
Fig. 1119Fig. 1119.
Fig. 1119.
Fig. 1120Fig. 1120.
Fig. 1120.
For holes of a greater diameter a holder such as shown inFig. 1119should be used, the body being a square bar, and the tool being held in the boxa aby two set screwsb. For holes of small diameter, as, say, less than 11⁄2inches, a tool holder is especially desirable, because when a boring tool is forged out of a piece of tool steel, its length is determined, and in order to have tools suitable for various depths of hole a number of tools of varying lengths are requisite. Suppose, for example, that a piece of steel be forged into a boring tool suitable for a hole of an inch diameter, and 4 inches deep, then the steel must be forged round for a distance of at least 4 inches from the cutting end, and if such a tool were applied to a hole, say, two inches deep, the cutting edge would stand out from the tool post at least two inches more than is necessary, which would cause the employment of a tool weaker than necessary for the work. To enable the use of one tool for various depths of work, and yet hold it in each case as close to the tool post as the work depth admits, tool-clamping devices, such as inFig. 1120(which are extracted fromThe American Machinist), are employed. 1 and 2 are pieces of steel fitting in the tool post and clamping the tool, which for very small holes is made of octagon or round forged steel. The tool may be passed to any required distance through the clamp, so as to project only to the amount necessary for the particular depth of hole requiring to be bored. These clamping pieces 1 and 2 should bed upon the tool fairly along their full length; or, what is better, they may bed the firmest at their extremities, which will insure that the tool is gripped firmly as near to the cutting edge as possible.
Fig. 1121Fig. 1121.
Fig. 1121.
In place of a steel tool, a tool holder turned cylindrically true and parallel may be used to carry a short boring tool, as shown inFig. 1121, in whichais the tool secured by the set-screwbinto the holderc. The latter may be provided with a line running true longitudinally, and may have a fine groove similar to a thread, and having a pitch measuring some part of an inch, as1⁄8,1⁄4,1⁄2inch, &c., so that the distance the tool projects from the holder may be known without measuring the same. But when a tool and holder of this description are used, the tool cannot be employed unless the hole passes entirely through the work, which occurs because of the presence of the set-screwb.
It is obvious that for a tool-holding bar such as this, a clamping device such as shown inFig. 1120is requisite, and that the position of the clamping device may be adjusted to suit the work by setting it more or less through the tool post.
Fig. 1122Fig. 1122.
Fig. 1122.
The manner in which the deflection of a boring tool will affect the bore of the work depends upon the height of the boring tool in the work. If the tool is above the horizontal centre of the work, as inFig. 1122, the spring vertically will cause it to leave the cut, and bore the hole to a corresponding amount smaller; and since the tool gets duller as the wear proceeds, it will spring more at thelatter end of each tool traverse, leaving the end of the hole last cut of smallest diameter.
Fig. 1123Fig. 1123.
Fig. 1123.
If, on the other hand, the tool be below the horizontal centre, as inFig. 1123, the vertical spring will be in a direction to increase the amount of the cut, and thus offset the tapering effect of the increased tool spring due to the wear of the tool. Furthermore, the shaving will be easier bent if the tool be below than if above the horizontal centre, because the metal will be less supported by the metal behind it. It is always desirable therefore to have the cutting edge of a boring tool used on small work below rather than above the horizontal centre of the work. On large work, however, as say, having a bore of 6 inches and over, the curve of the bore in the length of the circumference affected by the cut or bending of the cut is so small, that the height of the tool is of less consequence.
To enable the use of a stout-bodied boring tool, while keeping its cutting edge below the centre, the top face of the tool may be depressed, as shown inFig. 1123.
Fig. 1124Fig. 1124.
Fig. 1124.
Fig. 1125Fig. 1125.
Fig. 1125.
An excellent attachment for boring parallel holes is shown inFigs. 1124and1125, in which there is fixed to the cross slideathe bracketb, which is bored to receive a number of bushesc, whose bores are made to suit varying diameters of boring-bars or reamersd. The hub of the bracket is split on one side to enable it to be closed (by the bolte) upon the bushcand grip it firmly, the bush also being split atf. The bracketbis provided with a taper ping, which brings it in position upon the slide so that the bushescare true with the line of lathe centres. It is also provided with the screwsh, which lock it firmly to the cross slide and prevent any spring or movement from play or looseness.
When the bracket is adjusted and the bar fastened up (by screwe), the lathe-carriage feeds the boring tool to the cut in the usual manner. Now suppose that, as shown in our illustrations, a pulleyprequires to be bored, and the boring tool or reamer may be set to have its cutting end stand out just as far as the length of the hub requires, and no farther, so that the bar will be held and supported as close to the pulley hub as is possible from the nature of the job. There need not be a separate bush for every size of reamer, because the bodies of several size bars may fit to one size of bush, especially if the set of reamers for every size of bush be made with its smallest size equal to the bore of the bush; because in that case the whole of the set may be adjusted to bore any required depth of hole by sliding the reamer through the bush to the required distance. If there are a number of lathes in a shop, each lathe may have its own bracketb, all these brackets being bored to receive the same bushes, and therefore the same boring-bars or reamers.
Fig. 1126Fig. 1126.
Fig. 1126.
A bracket or stand of this kind may obviously be used to carry a bar, having a head such as is shown inFig. 1126, each dovetail groove carrying a cutting tool, and for wrought iron or steel work these grooves may be at an angle to the bar axis, as in the figure, to give each cutter front rake, and increase its keenness.
Boring Bars for Lathe Work.—Boring bars for lathe work are of two kinds, those in which the cutters are held in a fixed position in the length of the bar, and those in which the cutters are held in a head which traverses along the work. The formerare the least desirable, because they require to be more than twice the length of the work, which must be on one side of the cutter at the commencement of the cut, and on the other at the termination of the same. But to traverse the head carrying the tools along the bar necessitates a feed screw either within the bar or outside of it. If within, the metal removed to give it place weakens the bar, while in small holes there is no room for it; hence solid bars with fixed cutting tools are used for small holes, and tools held in a traversing head for those sufficiently large to give room for a head without weakening the bar too much. A boring bar is best driven from both ends.
“The boring bar is one of the most important tools to be found in a machine shop, because the work it has to perform requires to be very accurately done; and since it is a somewhat expensive tool to make, and occupies a large amount of shop room, it is necessary to make one size of boring bar answer for as many sizes of hole as possible, which end can only be attained by making it thoroughly stiff and rigid. To this end a large amount of bearing and close fitting, using cast iron as the material, are necessary, because cast iron does not spring or deflect so easily as wrought iron; but the centres into which the lathe centres fit are, if of cast iron, very liable to cut and shift their position, thus throwing the bar out of true. It is, therefore, always preferable to bore and tap the ends of such bars, and to screw in a wrought-iron or steel plug, taking care to screw it in very tightly, so that it shall not at any time become loose. The centres should be well drilled and of a comparatively large size, so as to have surface enough to suffer little from wear, and to well sustain the weight of the bar. The end surface surrounding the centres should be turned off quite true to keep the latter from wearing away from the high side, as they would do were one side higher than the other.”[16]
[16]From “The Complete Practical Machinist.”
[16]From “The Complete Practical Machinist.”
Fig. 1127Fig. 1127.
Fig. 1127.
Fig. 1128Fig. 1128.
Fig. 1128.
The common form of the smaller sizes of boring bar is that shown inFig. 1127.a abeing the bar,d dthe lathe centres,bthe cutter passing through a slot or keyway in the bar, andca key tapered (as is also the back edge of the cutter) to wedge or fasten the cutter to the bar. It is obvious that, if the cutter is turned up in the bar, and is of the exact size of the hole to be bored, it will require to stand true in the bar, and will therefore be able to cut on both ends, in which case the work may be fed up to it twice as fast as though only one edge were performing duty. To facilitate setting the cutter quite true, a flat and slightly taper surface should be filed on the bar at each end of the keyway, and the cutter should have a recess filed in it, as shown inFig. 1128, the recess being shown ata, and the edgesb bforming the diameter of the cutters. The backing off is shown atc, from which it will be observed that the cutting duty is performed by the edgec, and not along the edgeb, further than is shown by the backing off. The recess must be made taper, and to fit closely to the flat places filed on the bar. Such a cutter, if required to be adjustable, must not be provided with the recessa, but must be left plain, so that it may be made to extend out on one side of the bar to cut any requisite size of bore; it is far preferable, however, to employ the recess and have a sufficient number of cutters to suit any size of hole, since, as already stated (there being in that case two cutting edges performing duty), the work may be fed up twice as fast as in the former case, in which only one cutting edge operates.
Fig. 1129Fig. 1129.
Fig. 1129.
Messrs. Wm. Sellers and Co. form the cutters for their celebrated car wheel boring bar machine as inFig. 1129, the bottom or plain edge performing the cutting. By this means the recess to fit the bar is not reduced in depth from sharpening the tool. The tool is sharpened by grinding the ends of the lower face as shown by the unshaded parts, and the cutter is said to work better after the cutting part has begun to be oblique from grinding.
The cutter is hardened at the ends and left soft in the middle, so that the standard size of the cutter may be restored when necessary, by pening and stretching the soft metal in the middle. These cutters will bore from 50 to 250 car wheels, without appreciable reduction of size.
The description of bar shown inFig. 1127may be provided with several slots or keyways in its length, to facilitate facing off the ends of work which requires it. Since the work is fed to the cutter, it is obvious that the bar must be at least twice the length of the work, because the work is all on one side of the cutter at the commencement, and all on the other side at the conclusion of the boring operation. The excessive length of bar, thus rendered necessary, is the principal objection to this form of boring bar, because of its liability to spring. There should always be a keyway, slot, or cutter way, near to the centre of the length of the bar, so as to enable it to bore a hole as long as possible in proportion to the length of the boring bar, and a keyway or cutter way at each end of the bar, for use in facing off the end faces of the work.
Fig. 1130Fig. 1130.
Fig. 1130.
If a boring bar is to be used only for work that does not require facing at the ends, the cutter, slot, or keyway should be placed in such position in the length of the bar as will best suit the work (keeping in mind the desirability of having the bar as short as possible), and the bar should be tapering from the middle towards each end, as shown inFig. 1130. This will make the bar stronger in proportion to its weight, and better able to resist the pressure of the cut and the tendency to deflect. The parallel part atais to receive the driving clamp, but sometimes a lug cast on at that end is used instead of a clamp.
For bores too large to be bored by the bar alone, a tool-carrying head is provided, being sometimes fixed upon the bar by means of a locking key, and at others fed along the bar by a feed screw provided on the bar.
When the head is fixed on the bar the latter must be twice as long as the bore of the work, as the work is on one side of the head at the beginning, and on the other at the end of a cut; hence it follows that the sliding or feeding head is preferable, being the shortest, and therefore the most rigid, unless the bar slides through bearings at each end of the head.
Fig. 1131Fig. 1131.
Fig. 1131.
Fig. 1131represents a bar with a fixed head in operation in a cylinder, and having three cutting tools, and it will be observed that if toolameets a low spot and loses its cut, the pressure on toolsbandc, both being on the opposite side of the head, would cause the bar to spring over towardsa, producing a hole or bore out of round, and it follows that four tools are preferable.
Fig. 1132Fig. 1132.
Fig. 1132.
Fig. 1133Fig. 1133.
Fig. 1133.
Fig. 1132is a side view of a bar with four cutters, andFig. 1133an end view of the same shown within a cylinder, and it will be seen that should one of the cutters lose its cut, the two at right angles to it will steady the bar.
Fig. 1134Fig. 1134.
Fig. 1134.
When the cutters require to stand far out from the head, the bar will work more steadily if the cutters, instead of standing radially in the head, are placed as inFig. 1134, so that they will be pulled rather than pushed to their cut.
Fig. 1135Fig. 1135.
Fig. 1135.
An excellent form of boring bar fixed head, employed by Messrs. Wm. Sellers and Co. on their horizontal cylinder boring machine, is shown inFig. 1135. The boring head is split ata, so that by means of the boltbit may be gripped firmly to the bard, or readily loosened and slid along it. The head is provided with cuttersc(of which there are four in the latest design of bar), fitting into the radial slotse. These cutters are secured to the head by the clamps and nuts atg.
Fig. 1136Fig. 1136.
Fig. 1136.
Fig. 1136represents a boring bar, with a sliding head fed by a feed screw running along the bar, and having at its end a pinion that meshes upon a gear or pinion upon the dead centre of the lathe.
Fig. 1137Fig. 1137.
Fig. 1137.
The tools employed for the roughing cuts of boring bars should, for wrought iron, cast iron, steel, or copper, have a little front rake, the cutting corner being atainFig. 1137.
Fig. 1138Fig. 1138.
Fig. 1138.
If the cutters are to be used for one diameter of bore only, they will work more steadily if but little or no clearance is given them on the endb,Fig. 1138, but it is obvious that if they are to be used on different diameters of bores they must have clearance on these ends. The same tool may be used both for roughing and finishing cuts.
Fig. 1139Fig. 1139.
Fig. 1139.
The lip or top rake must, in case the bar should tremble during the finishing cut, be ground off, leaving the face level; and if,from the bar being too slight for its duty, it should still either chatter or jar, it will pay best to reduce the revolutions per minute of the bar, keeping the feed as coarse as possible, which will give the best results in a given time. In cases where, from the excessive length and smallness of the bar, it is difficult to prevent it from springing, the cutters must be made as inFig. 1139, having no lip, and but a small amount of cutting surface; and the cornerashould be bevelled off as shown. Under these conditions, the tool is the least likely to chatter or spring into the cut.
The shape of the cutting corner of a cutter depends entirely upon the position of its clearance or rake. If the edge forming the diameter has no clearance upon it, the cutting being performed by the end edges, the cutter may be left with a square, slightly rounded, or bevelled corner; but if the cutter have clearance on its outside or diametrical edge, as shown on the cutters inFig. 1137, the cutting corner should be bevelled or rounded off, otherwise it will jar in taking a roughing cut, and chatter in taking a moderate cut. The principle is that bevelling off the front edge of the cutter, as shown inFig. 1139, tends greatly to counteract a disposition to either jarring or chattering, especially as applied to brass work.
The only other precaution which can be taken to prevent, in exceptional cases, the spring of a boring bar is to provide a bearing at each end of the work, as, for instance, by bolting to the end of the work four iron plates, the ends being hollowed to fit the bar, and being so adjusted as to barely touch it; so that, while the bar will not be sprung by the plates, yet, if it tends to spring out of true, it will be prevented from doing so by contact with the hollow ends of the plates, which latter should have a wide bearing, and be kept well lubricated.
It sometimes happens that, from play in the journals of the machine, or from other causes, a boring bar will jar or chatter at the commencement of a bore, and will gradually cease to do so as the cut proceeds and the cutter gets a broader bearing upon the work. Especially is this liable to occur in using cutters having no clearance on the diametrical edge; because, so soon as such a cutter has entered the bore for a short distance, the diametrical edge (fitting closely to the bore) acts as a guide to steady the cutter. If, however, the cutter has such clearance, the only perceptible reason is that the chattering ceases as soon as the cutting edge of the tool or cutter has lost its fibrous edges. The natural remedy for this would appear to be to apply the oil-stone; this, however, will either have no effect or make matters worse. It is, indeed, a far better plan to take the tool (after grinding) and rub the cutting edge into a piece of soft wood, and to apply oil to the tool during its first two or three cutting revolutions. The application of oil will often remedy a slight existing chattering of a boring bar, but it is an expedient to be avoided, if possible, since the diameter or bore cut with oil will vary from that cut dry, the latter being a trifle the larger.
The considerations, therefore, which determine the shape of a cutter to be employed are as follows: Cutters for use on a certain and unvarying size of bore should have no clearance on the diametrical edges, the cutting being performed by the end edge only. Cutters intended to be adjusted to suit bores of varying diameter should have clearance on the end and on the diametrical edges. For use on brass work the cutting corner should be rounded off, and there should be no lip given to the cutting edge. For wrought iron the cutter should be lipped, and oil or soapy water should be supplied to it during the operation. A slight lip should be given to cutters for use on cast iron, unless, from slightness in the bar or other cause, there is a tendency to jarring, in which case no lip or front rake should be given.
Fig. 1140Fig. 1140.
Fig. 1140.
“In boring work chucked and revolved in the lathe, such, for instance, as axle boxes for locomotives, the bar shown inFig. 1140is an excellent tool.arepresents a cutter head, which slides along, at a close working fit, upon the bard d, and is provided with the cuttersb,b,b, which are fastened into slots provided in the headaby the keys shown. The bard dhas a thread cut upon part of its length, the remainder being plain, to fit the sliding head. One end is squared to receive a wrench, which resting against the bed of the lathe, prevents the bar from revolving upon the lathe centresf,f, by which the bar is held in the lathe.g,g,gare plain washers, provided to make up the distance between the thread and plain part of the bar in cases where the sliding headarequires considerable lateral movement, there being more or less washers employed according to the distance along which the sliding head is required to move. The edges of these washers are chamfered off to prevent them from burring easily. To feed the cutters, the nuthis screwed up with a wrench.
“The cutter headais provided in its bore with two feathers, which slide in grooves provided in the bard d, thus preventing the head from revolving upon the bar. It is obvious that this bar will, in consequence of its rigidity, take out a much heavier cut than would be possible with any boring tool, and furthermore that, there being four cutters, they can be fed up four times as fast as would be possible with a single tool or cutter. Care must, however, be exercised to so set the cutters that they will all project true radially, so that the depth of cut taken by each will be equal, or practically so; otherwise the feeding cannot progress any faster than if one cutter only were employed.”[17]