Modern Machine-Shop Practice Part 76

A boring tool holder suitable for holes of from 2 to 4 or 5 inches is shown in Fig. 1118, in which A represents a round bar shaped at the end B to fit into the tool post of the slide rest, and having a groove across the diameter of the end C D to receive a short tool. The slot and tool may be either square or [V]-shaped, the tool being locked by a wedge. It is obvious that instead of shaping the end B as 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.

For holes of a greater diameter a holder such as shown in Fig. 1119 should be used, the body being a square bar, and the tool being held in the box A A by two set screws B. For holes of small diameter, as, say, less than 1-1/2 inches, 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 in Fig. 1120 (which are extracted from _The 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 pa.s.sed 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.

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 in Fig.

1121, in which A is the tool secured by the set-screw B into the holder C. 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, as 1/8, 1/4, 1/2 inch, &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 pa.s.ses entirely through the work, which occurs because of the presence of the set-screw B.

It is obvious that for a tool-holding bar such as this, a clamping device such as shown in Fig. 1120 is 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.

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 in Fig. 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 the latter end of each tool traverse, leaving the end of the hole last cut of smallest diameter.

If, on the other hand, the tool be below the horizontal centre, as in Fig. 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 circ.u.mference affected by the cut or bending of the cut is so small, that the height of the tool is of less consequence.

[Ill.u.s.tration: Fig. 1122.]

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 in Fig. 1123.

[Ill.u.s.tration: Fig. 1123.]

An excellent attachment for boring parallel holes is shown in Figs. 1124 and 1125, in which there is fixed to the cross slide A the bracket B, which is bored to receive a number of bushes C, whose bores are made to suit varying diameters of boring-bars or reamers D. The hub of the bracket is split on one side to enable it to be closed (by the bolt _e_) upon the bush C and grip it firmly, the bush also being split at _f_.

The bracket B is provided with a taper pin G, which brings it in position upon the slide so that the bushes C are true with the line of lathe centres. It is also provided with the screws H, 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 screw _e_), the lathe-carriage feeds the boring tool to the cut in the usual manner. Now suppose that, as shown in our ill.u.s.trations, a pulley P requires 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 bracket B, all these brackets being bored to receive the same bushes, and therefore the same boring-bars or reamers.

[Ill.u.s.tration: Fig. 1124.]

[Ill.u.s.tration: Fig. 1125.]

A bracket or stand of this kind may obviously be used to carry a bar, having a head such as is shown in Fig. 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.

[Ill.u.s.tration: Fig. 1126.]

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 former are 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.

[Ill.u.s.tration: Fig. 1127.]

"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."

[Ill.u.s.tration: Fig. 1128.]

The common form of the smaller sizes of boring bar is that shown in Fig.

1127. A A being the bar, D D the lathe centres, B the cutter pa.s.sing through a slot or keyway in the bar, and C a 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 in Fig. 1128, the recess being shown at A, and the edges B B forming the diameter of the cutters. The backing off is shown at C, from which it will be observed that the cutting duty is performed by the edge C, and not along the edge B, 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 recess A, 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.

Messrs. Wm. Sellers and Co. form the cutters for their celebrated car wheel boring bar machine as in Fig. 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.

[Ill.u.s.tration: Fig. 1129.]

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 in Fig. 1127 may 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 princ.i.p.al 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.

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 in Fig. 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 at A is 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.

[Ill.u.s.tration: Fig. 1130.]

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. 1131 represents a bar with a fixed head in operation in a cylinder, and having three cutting tools, and it will be observed that if tool A meets a low spot and loses its cut, the pressure on tools B and C, both being on the opposite side of the head, would cause the bar to spring over towards A, producing a hole or bore out of round, and it follows that four tools are preferable.

Fig. 1132 is a side view of a bar with four cutters, and Fig. 1133 an 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.

[Ill.u.s.tration: Fig. 1131.]

[Ill.u.s.tration: Fig. 1132.]

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 in Fig. 1134, so that they will be pulled rather than pushed to their cut.

[Ill.u.s.tration: Fig. 1133.]

[Ill.u.s.tration: Fig. 1134.]

An excellent form of boring bar fixed head, employed by Messrs. Wm.

Sellers and Co. on their horizontal cylinder boring machine, is shown in Fig. 1135. The boring head is split at A, so that by means of the bolt B it may be gripped firmly to the bar D, or readily loosened and slid along it. The head is provided with cutters C (of which there are four in the latest design of bar), fitting into the radial slots E. These cutters are secured to the head by the clamps and nuts at G.

Fig. 1136 represents 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.

[Ill.u.s.tration: Fig. 1135.]

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 at A in Fig. 1137.

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 end B, 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.

[Ill.u.s.tration: Fig. 1136.]

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 coa.r.s.e 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 in Fig. 1139, having no lip, and but a small amount of cutting surface; and the corner A should 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 in Fig. 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 in Fig. 1139, tends greatly to counteract a disposition to either jarring or chattering, especially as applied to bra.s.s work.

[Ill.u.s.tration: Fig. 1137.]

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.

[Ill.u.s.tration: Fig. 1138.]

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.