Part 3 (1/2)
[Ill.u.s.tration: Fig. 46. Boring a Cylinder Lining in an Ordinary Engine Lathe]
Cylindrical parts attached to the carriage can also be bored by using a plain solid bar mounted between the centers. The bar must be provided with a cutter for small holes or a tool-head for larger diameters (preferably holding two or more tools) and the boring is done by feeding the carriage along the bed by using the regular power feed of the lathe. A symmetrically shaped casting like a bus.h.i.+ng or lining is often held upon wooden blocks bolted across the carriage. These are first cut away to form a circular seat of the required radius, by using the boring-bar and a special tool having a thin curved edge. The casting is then clamped upon these blocks by the use of straps and bolts, and if the curved seats were cut to the correct radius, the work will be located concentric with the boring-bar. When using a boring-bar of this type, the bar must be long enough to allow the part being bored to feed from one side of the cutter-head to the other, the cutter-head being approximately in a central location.
[Ill.u.s.tration: Fig. 47. Method of Setting Circle on Work Concentric with Lathe Spindle]
=Boring Holes to a Given Center Distance.=--In connection with faceplate work, it is often necessary to bore two or more holes at a given distance apart. The best method of doing this may depend upon the accuracy required. For ordinary work sometimes two or more circles _A_ and _B_ (Fig. 47) are drawn upon the part to be bored, in the position for the holes; the piece is then clamped to the faceplate and one of the circles is centered with the lathe spindle by testing it with a pointer C held in the toolpost; that is, when the pointer follows the circle as the work is turned, evidently the circle is concentric with the spindle.
The hole is then drilled and bored. The other circle is then centered in the same way for boring the second hole. As will be seen, the accuracy of this method depends first, upon the accuracy with which the circles were laid out, and second; upon the care taken in setting them concentric. For a more accurate way of locating parts for boring, see ”Use of Center Indicator” and ”Locating Work by the b.u.t.ton Method.”
=Turning Bra.s.s, Bronze and Copper.=--When turning soft yellow bra.s.s, a tool should be used having very little or no slope or rake on the top surface against which the chip bears, and for plain cylindrical turning, the point of the tool is drawn out quite thin and rounded, by grinding, to a radius of about 1/8 or 3/16 inch. If a tool having very much top slope is used for bra.s.s, there is danger of its gouging into the metal, especially if the part being turned is at all flexible. The clearance angle of a bra.s.s tool is usually about 12 or 14 degrees, which is 3 or 4 degrees greater than the clearance for steel turning tools. Most bra.s.s is easily turned, as compared with steel, and for that reason this increase in clearance is desirable, because it facilitates feeding the tool into the metal, especially when the carriage and cross-slide movements are being controlled by hand as when turning irregular shapes.
The speed for turning soft bra.s.s is much higher than for steel, being ordinarily between 150 and 200 feet per minute. When turning phosphor, tobin or other tough bronze compositions, the tool should be ground with rake the same as for turning steel, and lard oil is sometimes used as a lubricant. The cutting speed for bronzes varies from 35 or 40 to 80 feet per minute, owing to the difference in the composition of bronze alloys.
Turning tools for copper are ground with a little more top rake than is given steel turning tools, and the point should be slightly rounded. It is important to have a keen edge, and a grindstone is recommended for sharpening copper turning tools. Milk is generally considered the best lubricant to use when turning copper. The speed can be nearly as fast as for bra.s.s.
=Machining Aluminum.=--Tools for turning aluminum should have acute cutting angles. After rough-grinding the tool, it is advisable to finish sharpening the cutting edge on a grindstone or with an oilstone for fine work, as a keen edge is very essential. High speeds and comparatively light cuts are recommended. The princ.i.p.al difficulty in the machining of aluminum and aluminum alloys is caused by the clogging of the chips, especially when using such tools as counterbores and milling cutters.
This difficulty can be avoided largely by using the right kind of cutting lubricant. Soap-water and kerosene are commonly employed. The latter enables a fine finish to be obtained, provided the cutting tool is properly ground.
The following information on this subject represents the experience of the Brown-Lipe Gear Co., where aluminum parts are machined in large quant.i.ties: For finis.h.i.+ng bored holes, a bar equipped with cutters has been found more practicable than reamers. The cutters used for machining 4-inch holes have a clearance of from 20 to 22 degrees and no rake or slope on the front faces against which the chips bear. The roughing cutters for this work have a rather sharp nose, being ground on the point to a radius of about 3/32 inch, but for securing a smooth surface, the finis.h.i.+ng tools are rounded to a radius of about 3/4 inch. The cutting speed, as well as the feed, for machining aluminum is from 50 to 60 per cent faster than the speeds and feeds for cast iron. The lubricant used by this company is composed of one part ”aqualine” and 20 parts water. This lubricant not only gives a smooth finish but preserves a keen cutting edge and enables tools to be used much longer without grinding. Formerly, a lubricant composed of one part of high-grade lard oil and one part of kerosene was used. This mixture costs approximately 30 cents per gallon, whereas the aqualine and water mixture now being used costs less than 4 cents per gallon, and has proved more effective than the lubricant formerly employed.
CHAPTER II
LATHE TURNING TOOLS AND CUTTING SPEEDS
Notwithstanding the fact that a great variety of work can be done in the lathe, the number of turning tools required is comparatively small. Fig.
1 shows the forms of tools that are used princ.i.p.ally, and typical examples of the application of these various tools are indicated in Fig.
2. The reference letters used in these two ill.u.s.trations correspond for tools of the same type, and both views should be referred to in connection with the following description.
=Turning Tools for General Work.=--The tool shown at _A_ is the form generally used for rough turning, that is for taking deep cuts when considerable metal has to be removed. At _B_ a tool of the same type is shown, having a bent end which enables it to be used close up to a shoulder or surface _s_ that might come in contact with the tool-rest if the straight form were employed. Tool _C_, which has a straight cutting end, is used on certain cla.s.ses of work for taking light finis.h.i.+ng cuts, with a coa.r.s.e feed. This type of tool has a flat or straight cutting edge at the end, and will leave a smooth finish even though the feed is coa.r.s.e, provided the cutting edge is set parallel with the tool's travel so as to avoid ridges. Broad-nosed tools and wide feeds are better adapted for finis.h.i.+ng cast iron than steel. When turning steel, if the work is at all flexible, a broad tool tends to gouge into it and for this reason round-nosed tools and finer feeds are generally necessary. A little experience in turning will teach more on this point than a whole chapter on the subject.
[Ill.u.s.tration: Fig. 1. Set of Lathe Turning Tools for General Work]
[Ill.u.s.tration: Fig. 2. Views ill.u.s.trating Use of Various Types of Lathe Tools]
The side-tools shown at _D_ and _E_ are for facing the ends of shafts, collars, etc. The first tool is known as a right side-tool because it operates on the right end or side of a shaft or collar, whereas the left side-tool _E_ is used on the opposite side, as shown in Fig. 2.
Side-tools are also bent to the right or left because the cutting edge of a straight tool cannot always be located properly for facing certain surfaces. A bent right side-tool is shown at _F_. A form of tool that is frequently used is shown at _G_; this is known as a parting tool and is used for severing pieces and for cutting grooves, squaring corners, etc.
The same type of tool having a bent end is shown at _H_ (Fig. 2) severing a piece held in the chuck. Work that is held between centers should not be entirely severed with a parting tool unless a steadyrest is placed between the tool and faceplate, as otherwise the tool may be broken by the springing of the work just before the piece is cut in two.
It should be noted that the sides of this tool slope inward back of the cutting edge to provide clearance when cutting in a narrow groove.
At _I_ a thread tool is shown for cutting a U. S. standard thread. This thread is the form most commonly used in this country at the present time. A tool for cutting a square thread is shown at _J_. This is shaped very much like a parting tool except that the cutting end is inclined slightly to correspond with the helix angle of the thread, as explained in Chapter IV, which contains descriptions of different thread forms and methods of cutting them. Internal thread tools are shown at _K_ and _L_ for cutting U. S. standard and square threads in holes. It will be seen that these tools are somewhat like boring tools excepting the ends which are shaped to correspond with the thread which they are intended to cut.
[Ill.u.s.tration: Fig. 3. Turning Tool with Inserted Cutter]
A tool for turning bra.s.s is shown at _M_. Bra.s.s tools intended for general work are drawn out quite thin and they are given a narrow rounded point. The top of the bra.s.s tool is usually ground flat or without slope as otherwise it tends to gouge into the work, especially if the latter is at all flexible. The end of a bra.s.s tool is sometimes ground with a straight cutting edge for turning large rigid work, such as bra.s.s pump linings, etc., so that a coa.r.s.e feed can be used without leaving a rough surface. The tools at _N_ and _O_ are for boring or finis.h.i.+ng drilled or cored holes. Two sizes are shown, which are intended for small and large holes, respectively.
The different tools referred to in the foregoing might be called the standard types because they are the ones generally used, and as Fig. 2 indicates, they make it possible to turn an almost endless variety of forms. Occasionally some special form of tool is needed for doing odd jobs, having, perhaps, an end bent differently or a cutting edge shaped to some particular form. Tools of the latter type, which are known as ”form tools,” are sometimes used for finis.h.i.+ng surfaces that are either convex, concave, or irregular in shape. The cutting edges of these tools are carefully filed or ground to the required shape, and the form given the tool is reproduced in the part turned. Ornamental or other irregular surfaces can be finished very neatly by the use of such tools. It is very difficult, of course, to turn convex or concave surfaces with a regular tool; in fact, it would not be possible to form a true spherical surface, for instance, without special equipment, because the tool could not be moved along a true curve by simply using the longitudinal and cross feeds. Form tools should be sharpened by grinding entirely on the top surface, as any grinding on the end or flank would alter the shape of the tool.
[Ill.u.s.tration: Fig. 4. Heavy Inserted-cutter Turning Tool]