CHAPTER VI
VERTICAL BORING MILL PRACTICE
All the different types of turning machines now in use originated from the lathe. Many of these tools, however, do not resemble the lathe because, in the process of evolution, there have been many changes made in order to develop turning machines for handling certain classes of work to the best advantage. The machine illustrated in Fig. 1 belongs to the lathe family and is known as a vertical boring and turning mill. This type, as the name implies, is used for boring and turning operations, and it is very efficient for work within its range. The part to be machined is held to the table _B_ either by clamps or in chuck jaws attached to the table. When the machine is in operation, the table revolves and the turning or boring tools (which are held in tool-blocks _T_) remain stationary, except for the feeding movement. Very often more than one tool is used at a time, as will be shown later by examples of vertical boring mill work. The tool-blocks _T_ are inserted in tool-bars _T_{1}_ carried by saddles _S_ which are mounted on cross-rail _C_. Each tool-head (consisting of a saddle and tool-bar) can be moved horizontally along cross-rail _C_, and the tool-bars _T_{1}_ have a vertical movement. These movements can be effected either by hand or power.
When a surface is being turned parallel to the work table, the entire tool-head moves horizontally along the cross-rail, but when a cylindrical surface is being turned, the tool-bar moves vertically. The tool-heads are moved horizontally by the screws _H_ and _H_{1}_, and the vertical feed for the tool-bars is obtained from the splined shafts _V_ and _V_{1}_, there being a separate screw and shaft for each head so that the feeding movements are independent. These feed shafts are rotated for the power feed by vertical shafts _A_ and _A_{1}_ on each side of the machine.
These vertical shafts connect with the feed shafts through bevel and spur gears located at the ends of the cross-rail. On most boring mills, connection is made with one of the splined shafts _V_ or screw _H_, by a movable gear, which is placed on whichever shaft will give the desired direction of feed. The particular machine illustrated is so arranged that either the right or left screw or feed shaft can be engaged by simply shifting levers _D_{1}_ or _D_.
The amount of feed per revolution of the table is varied for each tool-head by feed-changing mechanisms _F_ on each side of the machine. These feed boxes contain gears of different sizes, and by changing the combinations of these gears, the amount of feed is varied. Five feed changes are obtained on this machine by shifting lever _E_, and this number is doubled by shifting lever _G_. By having two feed boxes, the feeding movement of each head can be varied independently. The direction of either the horizontal or vertical feed can be reversed by lever _R_, which is also used for engaging or disengaging the feeds. This machine is equipped with the dials _I_ and _I_{1}_ which can be set to automatically disengage the feed at any predetermined point. There are also micrometer dials graduated to thousandths of an inch and used for adjusting the tools without the use of measuring instruments.
The work table _B_ is driven indirectly from a belt pulley at the rear, which transmits the power through gearing. The speed of the table can be varied for turning large or small parts, by levers _J_ and _K_ and the table can be started, stopped or rotated part of a revolution by lever _L_ which connects with a friction clutch. There are corresponding feed and speed levers on the opposite side, so that the machine can be controlled from either position.
The heads can be adjusted along the cross-rail for setting the tools by hand-cranks _N_, and the tool slides can be moved vertically by turning shafts _V_ with the same cranks. With this machine, however, these adjustments do not have to be made by hand, ordinarily, as there are rapid power movements controlled by levers _M_. These levers automatically disengage the feeds and enable the tool-heads to be rapidly shifted to the required position, the direction of the movement depending upon the position of the feed reverse lever _R_ and lever _D_. This rapid traverse, which is a feature applied to modern boring mills of medium and large size, saves time and the labor connected with hand adjustments. The cross-rail _C_ has a vertical adjustment on the faces of the right and left housings which support it, in order to locate the tool-heads at the right height for the work. This adjustment is effected by power and is controlled by levers at the sides of the housings. Normally, the cross-rail is bolted to the housings, and these bolts must be loosened before making the adjustment, and must always be tightened afterwards.
The function of these different levers has been explained to show, in a general way, how a vertical boring machine is operated. It should be understood, however, that the arrangement differs considerably on machines of other makes. The construction also varies considerably on machines of the same make but of different size.
All modern vertical boring mills of medium and large sizes are equipped with two tool-heads, as shown in Fig. 1, because a great deal of work done on a machine of this type can have two surfaces machined simultaneously. On the other hand, small mills of the type illustrated in Fig. 2 have a single head. The toolslide of this machine, instead of having a single tool-block, carries a five-sided turret _T_ in which different tools can be mounted. These tools are shifted to the working position as they are needed, by loosening binder lever _L_ and turning or "indexing" the turret. The turret is located and locked in any of its five positions by lever _I_, which controls a plunger that engages notches at the rear. Frequently, all the tools for machining a part can be held in the turret, so that little time is required for changing from one tool to the next. Some large machines having two tool-heads are also equipped with a turret on one head.
=Boring and Turning in a Vertical Boring Mill.=--The vertical boring mill is, in many respects, like a lathe placed in a vertical position, the table of the mill corresponding to the faceplate or chuck of the lathe and the tool-head to the lathe carriage. Much of the work done by a vertical mill could also be machined in a lathe, but the former is much more efficient for work within its range. To begin with, it is more convenient to clamp work to a horizontal table than to the vertical surface of a lathe faceplate, or, as someone has aptly said, "It is easier to lay a piece down than to hang it up." This is especially true of the heavy parts for which the boring mill is principally used. Very deep roughing cuts can also be taken with a vertical mill. This type of machine mill is designed for turning and boring work which, generally speaking, is quite large in diameter in proportion to the width or height. The work varies greatly, especially in regard to its diameter, so that boring mills are built in a large range of sizes. The small and medium sizes will swing work varying from about 30 inches to 6 or 7 feet in diameter, whereas large machines, such as are used for turning very large flywheels, sheaves, etc., have a swing of 16 or 20 feet, and larger sizes are used in some shops. The size of a vertical mill, like any other machine tool, should be somewhat in proportion to the size of the work for which it is intended, as a very large machine is unwieldy, and, therefore, inefficient for machining comparatively small parts.
=Holding and Setting Work on Boring Mill Table.=--There are three general methods of holding work to the table of a boring mill; namely, by the use of chucks, by ordinary bolts and clamps, or in special fixtures. Chucks which are built into the table (as illustrated in Fig. 2) and have both universal and independent adjustments for the jaws can be used to advantage for holding castings that are either round or irregular in shape. The universal adjustment is used for cylindrical parts, such as disks, flywheels, gear blanks, etc., and the independent adjustment, for castings of irregular shape. Chucks which have either an independent or universal movement for the jaws are known as a "combination" type and usually have three jaws. There is also a four-jaw type which has the independent adjustment only. This style is preferable for work that is not cylindrical and which must be held very securely. Chuck jaws that do not form a part of the machine table, but are bolted to it in the required position, are also employed extensively, especially on comparatively large machines.
Most of the work done in a vertical mill is held in a chuck. Occasionally, however, it is preferable to clamp a part directly to the table. This may be desirable because of the shape and size of the work, or because it is necessary to hold a previously machined surface directly against the table in order to secure greater accuracy. Sometimes a casting is held in the chuck for turning one side, and then the finished side is clamped against the table for turning the opposite side. Parts which are to be machined in large quantities are often held in special fixtures. This method is employed when it enables the work to be set up more quickly than would be possible if regular clamps or chuck jaws were used.
Work that is to be turned or bored should first be set so that the part to be machined is about central with the table. For example, the rim of a flywheel should be set to run true so that it can be finished by removing about the same amount of metal around the entire rim; in other words, the rim should be set concentric with the table, as shown in Fig. 3, and the sides of the rim should also be parallel to the table.
A simple tool that is very useful for testing the position of any cylindrical casting consists of a wooden shank into which is inserted a piece of wire, having one end bent. This tool is clamped in the toolpost and as the work revolves the wire is adjusted close to the cylindrical surface being tested. The movement of the work with relation to the stationary wire point will, of course, show whether or not the part runs true. The advantage of using a piece of wire for testing, instead of a rigid tool, is that the wire, owing to its flexibility, will simply be bent backward if it is moved too close to a surface which is considerably out of true. The upper surface of a casting can be tested for parallelism with the table by using this same wire gage, or by comparing the surface, as the table is revolved slowly, with a tool held in the toolpost. An ordinary surface gage is also used for this purpose. The proper surface to set true, in any case, depends upon the requirements. A plain cylindrical disk would be set so that the outside ran true and the top surface was parallel with the table. When setting a flywheel, if the inside of the rim is to remain rough, the casting should be set by this surface rather than by the outside, so that the rim, when finished, will be uniform in thickness.
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 illustrate, 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, illustrates 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.
Most of the parts machined in a vertical boring mill are made of cast iron and, ordinarily, at least one roughing and one finishing 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 coarser 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 coarse feed is used in order to reduce the time required for taking the cut. The finishing 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 illustrates 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.
=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 finishing 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 finishing 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 illustrated 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 finishing. The cutter passes 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 passes through a close-fitting bushing in the table to steady the bar. Sketch _D_ shows a finishing 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 finishing. 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.")
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.
=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 illustration. 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 finishing. Left-and right-hand finishing 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.
The diagrams in Fig. 8 show, in a general way, how each of the tools illustrated in Fig. 7 are used, and corresponding tools are marked by the same reference letters in both of these illustrations. 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 brass, although the front is then ground without slope. Tool _F_ is for light finishing 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 finishing exterior and interior cylindrical surfaces. These tools also have both vertical and horizontal cutting edges and are sometimes used for first finishing 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