Part 16 (1/2)
As far as possible, chucks should be used for holding cylindrical parts, owing to their convenience. The jaws should be set against an interior cylindrical surface whenever this is feasible. To ill.u.s.trate, the flywheel in Fig. 3 is gripped by the inside of the rim which permits the outside to be turned at this setting of the work. It is also advisable to set a flywheel casting in the chuck so that a spoke rests against one of the jaws as at _d_, if this is possible. This jaw will then act as a driver and prevent the casting from slipping or turning in the chuck jaws, owing to the tangential pressure of the turning tool. When a cut is being taken, the table and work rotate as shown by arrow _a_, and the thrust of the cut (taken by tool _t_) tends to move the wheel backward against the direction of rotation, as shown by arrow _b_. If one of the chuck jaws bears against one of the spokes, this movement is prevented.
It is not always feasible to use a chuck jaw as a driver and then a special driver having the form of a small angle-plate or block is sometimes bolted directly to the table. Another method of driving is to set a brace between a spoke or projection on the work and a chuck jaw or strip attached to the table. Drivers are not only used when turning flywheels, but in connection with any large casting, especially when heavy cuts have to be taken. Of course, some castings are so shaped that drivers cannot be employed.
=Turning in a Boring Mill.=--The vertical type of boring mill is used more for turning cylindrical surfaces than for actual boring, although a large part of the work requires both turning and boring. We shall first consider, in a general way, how surfaces are turned and then refer to some boring operations. The diagram _A_, Fig. 4, ill.u.s.trates how a horizontal surface would be turned. The tool _t_ is clamped in tool-block _t_{1}_, in a vertical position, and it is fed horizontally as the table and work rotate. The tool is first adjusted by hand for the proper depth of cut and the automatic horizontal feed is then engaged.
When a cylindrical surface is to be turned, the tool (provided a straight tool is used) is clamped in a horizontal position and is fed downward as indicated at _B_. The amount that the tool should feed per revolution of the work, depends upon the kind of material being turned, the diameter of the turned part and the depth of the cut.
[Ill.u.s.tration: Fig. 4. (A) Turning a Flat Surface. (B) Turning a Cylindrical Surface]
Most of the parts machined in a vertical boring mill are made of cast iron and, ordinarily, at least one roughing and one finis.h.i.+ng cut is taken. The number of roughing cuts required in any case depends, of course, upon the amount of metal to be removed. An ordinary roughing cut in soft cast iron might vary in depth from 1/8 or 3/16 inch to 3/8 or 1/2 inch and the tool would probably have a feed per revolution of from 1/16 to 1/8 inch, although deeper cuts and coa.r.s.er feeds are sometimes taken. These figures are merely given to show, in a general way, what cuts and feeds are practicable. The tool used for roughing usually has a rounded end which leaves a ridged or rough surface. To obtain a smooth finish, broad flat tools are used. The flat cutting edge is set parallel to the tool's travel and a coa.r.s.e feed is used in order to reduce the time required for taking the cut. The finis.h.i.+ng feeds for cast iron vary from 1/4 to 3/4 inch on ordinary work. The different tools used on the vertical mill will be referred to more in detail later.
All medium and large sized vertical boring mills are equipped with two tool-heads and two tools are frequently used at the same time, especially on large work. Fig. 9 ill.u.s.trates the use of two tools simultaneously. The casting shown is a flywheel, and the tool on the right side turns the upper side of the rim, while the tool on the left side turns the outside or cylindrical surface. As a boring mill table rotates in a counter-clockwise direction, the left-hand tool is reversed to bring the cutting edge at the rear. By turning two surfaces at once, the total time for machining the casting is, of course, greatly reduced.
The turning of flywheels is a common vertical boring mill operation, and this work will be referred to in detail later on.
[Ill.u.s.tration: Fig. 5. Tools for Boring and Reaming Holes]
=Boring Operations.=--There are several methods of machining holes when using a vertical boring mill. Ordinarily, small holes are cored in castings and it is simply necessary to finish the rough surface to the required diameter. Some of the tools used for boring and finis.h.i.+ng comparatively small holes are shown in Fig. 5. Sketch _A_ shows a boring tool consisting of a cutter _c_ inserted in a shank, which, in turn, is held in the tool slide, or in a turret attached to the tool slide. With a tool of this type, a hole is bored by taking one or more cuts down through it. The tool shown at _B_ is a four-lipped drill which is used for drilling cored holes preparatory to finis.h.i.+ng by a cutter or reamer.
This drill would probably finish a hole to within about 1/32 inch of the finish diameter, thus leaving a small amount of metal for the reamer to remove. The tool ill.u.s.trated at _C_ has a double-ended flat cutter _c_, which cuts on both sides. These cutters are often made in sets for boring duplicate parts. Ordinarily, there are two cutters in a set, one being used for roughing and the other for finis.h.i.+ng. The cutter pa.s.ses through a rectangular slot in the bar and this particular style is centrally located by shoulders _s_, and is held by a taper pin _p_. Some cutter bars have an extension end, or ”pilot” as it is called, which pa.s.ses through a close-fitting bus.h.i.+ng in the table to steady the bar.
Sketch _D_ shows a finis.h.i.+ng reamer. This tool takes a very light cut and is intended to finish holes that have been previously bored close to the required size. Sometimes a flat cutter _C_ is used for roughing and a reamer for finis.h.i.+ng. The reamer is especially desirable for interchangeable work, when all holes must have a smooth finish and be of the same diameter. When a reamer is held rigidly to a turret or toolslide, it is liable to produce a hole that is either tapering or larger than the reamer diameter. To prevent this, the reamer should be held in a ”floating” holder which, by means of a slight adjustment, allows the reamer to align itself with the hole. There are several methods of securing this ”floating” movement. (See ”Floating Reamer Holders.”)
[Ill.u.s.tration: Fig. 6. Boring with Regular Turning Tools]
Large holes or interior cylindrical surfaces are bored by tools held in the regular tool-head. The tool is sometimes clamped in a horizontal position as shown at _A_, Fig. 6, or a bent type is used as at _B_. Cast iron is usually finished by a broad flat tool as at _C_, the same as when turning exterior surfaces. Obviously a hole that is bored in this way must be large enough to admit the tool-block.
[Ill.u.s.tration: Fig. 7. Set of Boring Mill Tools]
=Turning Tools for the Vertical Boring Mill.=--A set of turning tools for the vertical boring mill is shown in Fig. 7. These tools can be used for a wide variety of ordinary turning operations. When a great many duplicate parts are to be machined, special tool equipment can often be used to advantage, but as the form of this equipment depends upon the character of the work, only standard tools have been shown in this ill.u.s.tration. The tool shown at _A_ is a right-hand, roughing tool, and a left-hand tool of the same type is shown at _B_. Tool _C_ is an offset or bent, left-hand round nose for roughing, and _D_ is a right-hand offset roughing tool. A straight round nose is shown at _E_. Tool _F_ has a flat, broad cutting edge and is used for finis.h.i.+ng. Left-and right-hand finis.h.i.+ng tools of the offset type are shown at _G_ and _H_, respectively. Tool _I_ has a square end and is used for cutting grooves.
Right-and left-hand parting tools are shown at _J_ and _K_, and tool _L_ is a form frequently used for rounding corners.
[Ill.u.s.tration: Fig. 8. Diagrams Ill.u.s.trating Use of Different Forms of Tools]
The diagrams in Fig. 8 show, in a general way, how each of the tools ill.u.s.trated in Fig. 7 are used, and corresponding tools are marked by the same reference letters in both of these ill.u.s.trations. The right-and left-hand roughing tools _A_ and _B_ are especially adapted for taking deep roughing cuts. One feeds away from the center of the table, or to the right (when held in the right-hand tool-block) and the other tool is ground to feed in the opposite direction. Ordinarily, when turning plain flat surfaces, the cut is started at the outside and the tool feeds toward the center, as at _B_, although it is sometimes more convenient to feed in the opposite direction, as at _A_, especially when there is a rim or other projecting part at the outside edge. The tool shown at _A_ could also be used for turning cylindrical surfaces, by clamping it in a horizontal position across the bottom of the tool-block. The feeding movement would then be downward or at right-angles to the work table.
The offset round-nose tools _C_ and _D_ are for turning exterior or interior cylinder surfaces. The shank of this tool is clamped in the tool-block in a vertical position and as the bent end extends below the tool-block, it can be fed down close to a shoulder. The straight type shown at _E_ is commonly used for turning steel or iron, and when the point is drawn out narrower, it is also used for bra.s.s, although the front is then ground without slope. Tool _F_ is for light finis.h.i.+ng cuts and broad feeds. The amount of feed per revolution of the work should always be less than the width of the cutting edge as otherwise ridges will be left on the turned surface. The offset tools _G_ and _H_ are for finis.h.i.+ng exterior and interior cylindrical surfaces. These tools also have both vertical and horizontal cutting edges and are sometimes used for first finis.h.i.+ng a cylindrical and then a horizontal surface, or _vice versa_. Tool _I_ is adapted to such work as cutting packing-ring grooves in engine pistons, forming square or rectangular grooves, and similar work. The parting tools _J_ and _K_ can also be used for forming narrow grooves or for cutting off rings, etc. The sketch _K_ (Fig. 8) indicates how a tool of this kind might be used for squaring a corner under a shoulder. Tool _L_ is frequently used on boring mills for rounding the corners of flywheel rims, in order to give them a more finished appearance. It has two cutting edges so that either side can be used as when rounding the inner and outer corners of a rim.
The turning tools of a vertical boring mill are similar, in many respects, to those used in a lathe, although the shanks of the former are shorter and more stocky than those of lathe tools. The cutting edges of some of the tools also differ somewhat in form, but the principles which govern the grinding of lathe and boring mill tools are identical, and those who are not familiar with tool grinding are referred to Chapter II, in which this subject is treated.
=Turning a Flywheel on a Vertical Mill.=--The turning of a flywheel is a good example of the kind of work for which a vertical boring mill is adapted. A flywheel should preferably be machined on a double-head mill so that one side and the periphery of the rim can be turned at the same time. A common method of holding a flywheel is shown in Fig. 9. The rim is gripped by four chuck jaws _D_ which, if practicable, should be on the inside where they will not interfere with the movement of the tool.
Two of the jaws, in this case, are set against the spokes on opposite sides of the wheel, to act as drivers and prevent any backward s.h.i.+fting of work when a heavy cut is being taken. The ill.u.s.tration shows the tool to the right rough turning the side of the rim, while the left-hand tool turns the periphery. Finis.h.i.+ng cuts are also taken over the rim, at this setting, and the hub is turned on the outside, faced on top, and the hole bored.
[Ill.u.s.tration: Fig. 9. Turning the Rim of a Flywheel]
The three tools _A_, _B_ and _C_, for finis.h.i.+ng the hole, are mounted in the turret. Bar _A_, which carries a cutter at its end, first rough bores the hole. The sizing cutter _B_ is then used to straighten it before inserting the finis.h.i.+ng reamer _C_. Fig. 10 shows the turret moved over to a central position and the sizing cutter _B_ set for boring. The head is centrally located (on this particular machine) by a positive center-stop. The turret is indexed for bringing the different tools into the working position, by loosening the clamping lever _L_ and pulling down lever _I_ which disengages the turret lock-pin. When all the flywheels in a lot have been machined as described, the opposite side is finished.
[Ill.u.s.tration: Fig. 10. Tool B set for Boring the Hub]
[Ill.u.s.tration: Fig. 11. Diagrams showing Method of Turning and Boring a Flywheel on a Double-head Mill having one Turret Head]
In order to show more clearly the method of handling work of this cla.s.s, the machining of a flywheel will be explained more in detail in connection with Fig. 11, which ill.u.s.trates practically the same equipment as is shown in Figs. 9 and 10. The successive order in which the various operations are performed is as follows: Tool _a_ (see sketch _A_) rough turns the side of the rim, while tool _b_, which is set with its cutting edge toward the rear, rough turns the outside. The direction of the feeding movement for each tool is indicated by the arrows. When tool _a_ has crossed the rim, it is moved over for facing the hub, as shown by the dotted lines. The side and periphery of the rim are next finished by the broad-nose finis.h.i.+ng tools _c_ and _d_ (see sketch _B_).
The feed should be increased for finis.h.i.+ng, so that each tool will have a movement of say 1/4 or 3/8 inch per revolution of the work, and the cuts should, at least, be deep enough to remove the marks made by the roughing tools. Tool _c_ is also used for finis.h.i.+ng the hub as indicated by the dotted lines. After these cuts are taken, the outside of the hub and inner surface of the rim are usually turned down as far as the spokes, by using offset tools similar to the ones shown at _C_ and _D_ in Fig. 7. The corners of the rim and hub are also rounded to give the work a more finished appearance, by using a tool _L_.