Modern Machine-Shop Practice Part 91

The reason for running the lathe at a comparatively fast speed is that the tool is then less likely to be checked in its movement by a seam or hard place in the metal of the bolt, and that, even if the metal is soft and uniform in its texture, it is easier to move the tool at a regular speed than it would be if the lathe ran comparatively slowly.

[Ill.u.s.tration: Fig. 1336.]

If the tool is moved irregularly or becomes checked in its forward movement, the thread will become waved or "drunken"--that is, it will not move forward at a uniform speed;[20] and if the thread is drunken when it is started, the chaser will not only fail to rectify it, but, if the drunken part occurs in a part of the iron either harder or softer than the rest of the metal, the thread will become more drunken as the chaser proceeds. It is preferable, therefore, if the thread is not started truly, to try again, and, if there is not sufficient metal to permit of the starting groove first struck being turned out, to make another farther along the bolt. It takes much time and patience to learn to strike the requisite pitch at the first trial; and it is therefore requisite for a beginner to leave the end of the work larger in diameter than the required finished size, as shown in Fig. 1336, so as to have sufficient metal to turn out the groove cut by the [V]-tool at the first trial cut, and try again.

[20] See Fig. 253, Plate II., Vol. I.

If the thread is to be cut on bra.s.s the [V]-tool must not have any top rake. Some turners start threads upon bra.s.s by placing the chaser itself against the end of the work and sweeping it rapidly from left to right (for a right-hand thread), thus obviating the use of the [V]-tool.

In all cases the work should be rounded off at the end to prevent the chaser-teeth from catching.

In applying the chaser to the groove cut by the [V]-tool the leading tooth should be held just clear of the work at first, and only be brought to touch the work after the rear teeth have found and are traversing in the groove. By this means the chaser will carry the thread forward more readily and true. The thread must be carried forward but a short distance at each pa.s.sage of the chaser, gradually deepening the thread while carrying it forward.

To start an inside thread the corner of the hole at its entrance should be rounded off and the back teeth of the chaser placed to touch the bore while the front teeth are clear. The lathe is to be run at a quick speed, and the chaser moved forward at as near the proper speed as can be judged. When the chaser is moved at the proper speed, the rear teeth will fall into the fine grooves cut by the advance ones, and start a thread, while otherwise promiscuous grooves only will be cut. It is an easy matter, however, to start a double thread with an inside chaser; hence, when the thread is started the lathe should be stopped and the thread examined.

The chaser should be placed with its top face straight above the horizontal level of the work and held quite horizontal, and the handle end then elevated just sufficient to give the teeth clearance enough to enable them to cut; otherwise, with a chaser having top rake, the thread cut will be too deep, and its sides will be of improper angle one to the other.

[Ill.u.s.tration: Fig. 1337.]

Thus, in Fig. 1337, W represents a piece of work, R the lathe rest, and T the chaser. The depth of the thread cut in this case will be from the circle A to the circle B; whereas the depth of the chaser teeth, and therefore the proper depth for the thread, is from C to D. Thus tilting the handle end of the chaser too much has caused the chaser teeth to cut a thread too deep. If on bra.s.s work the chaser has its top face ground off as in figure, tilting the handle too much will cause the thread cut to be too shallow, and in both cases the error in thread depth induces a corresponding error in the angles of the sides of the thread one to the other and relative to the axial line of the bolt or work.

If the chaser teeth are held at an angle to the work surface, the thread cut will be of finer pitch than the chaser, and the angles of the sides of the thread on the work will not be the same as those of the teeth. It is permissible, however, during the early cuts taken with a hand chaser to give the chaser a slight degree of such angle, because it diminishes the length of cutting edge, and causes the chaser to cut more freely, especially when the pitch of the thread is coa.r.s.e and the chaser is becoming dull.

In the case of a taper thread the same rule, that the thread may be roughed out with the chaser teeth at an angle to the surface lengthways of the work, but must be finished with the teeth parallel to the surface, holds good.

[Ill.u.s.tration: Fig. 1338.]

Thus, in Fig. 1338 is a taper plug fitting in a ring having a threaded taper bore, the threads matching, and having the thread sides in both cases at an equal angle to the surface, lengthways of the work, though the tops and bottoms of the thread are not parallel with the axial line of the work.

WOOD TURNING TOOLS.--Wood turning in the ordinary lathe is generally performed by hand tools, and of these the princ.i.p.al is the gouge, which in skillful hands may be used to finish as well as to rough out the work (although there are other more useful finis.h.i.+ng tools to be hereafter described).

It is used mainly, however, to rough out the work and to round out corners and sweeps. The proper form for this tool is shown in Fig. 1339, the bevel on the end of the back or convex side being carried well round at the corners, so as to bring those corners up to a full sharp cutting edge on the convex or front side.

The proper way to hold a gouge is shown in Fig. 1340, in which the cut taken by the tool is being carried from right to left, the face plate of the lathe being on the left side, so that by holding it in the manner shown the body and arms are as much as possible out of the way of the face plate, which is a great consideration in short work. But if the cut is to be carried from left to right, the relative position of the hands may be changed.

When the work runs very much out of true, or has corners upon it, as in the case of square wood, the forefinger may be placed under the hand rest, and the thumb laid in the trough of the gouge, pressing the latter firmly against the lathe rest to prevent the tool edge from entering the work too far, or, in other words, to regulate the depth of the cut, and prevent its becoming so great as to force the tool from the hands or break it, as is sometimes the case under such circ.u.mstances. When the gouge is thus held, its point of rest upon the lathe rest may be used as a fulcrum, the tool handle being moved laterally to feed it to the cut, which is a very easy and safe plan for learners to adopt, until practice gives them confidence. The main point in the use of the gouge is the plane in which the trough shall lie. Suppose, for example, that in Fig.

1341 is shown a piece of work with three separate gouge cuts being taken along it, that on the right being carried in the direction of the arrow.

Now the gouge merely acts as a wedge, and the whole of the pressure placed by the cut on the trough side or face of the gouge is tending to force the gouge in the direction of the arrow, and therefore forward into its cut, and this it does, ripping along the work and often throwing it out of the lathe. To avoid this the gouge is canted, so that when cutting from right to left it lies as shown at B, in which case the pressure of the cut tends rather to force the gouge back from the cut, rendering a slight pressure necessary to feed it forward. The gouge trough should lie nearly horizontal lengthwise, the cutting edge being slightly elevated. The gouge should never (for turning work) be ground in the trough (as the concave side is termed), and should always be oilstoned, the trough being stoned with a slip of stone lying flat along the trough, the back being rotated upon a piece of flat stone, and held with the ground surface flat on the surface of the stone, and so pressed to it as to give most pressure at and near the cutting edge.

[Ill.u.s.tration: Fig. 1339.]

[Ill.u.s.tration: Fig. 1340.]

[Ill.u.s.tration: Fig. 1341.]

[Ill.u.s.tration: Fig. 1342.]

For finis.h.i.+ng flat surfaces, the chisel shown in Fig. 1342 is employed.

It should be short, as shown. It should be held to the work in a horizontal position, or it is apt to dig or rip into the work, especially when it is used upon soft wood. Some expert workmen hold it at an angle for finis.h.i.+ng purposes, which makes it cut very freely and clean, but increases the liability to dig into the work; hence learners should hold it as shown.

[Ill.u.s.tration: Fig. 1343.]

[Ill.u.s.tration: Fig. 1344.]

Another excellent finis.h.i.+ng tool is the skew-chisel, Fig. 1343, so called because its cutting edge is at an angle, or askew with the body of the tool. This tool will cut very clean, leaving a polish on the work. It also has the advantage that the body of the tool may be kept out of the way of f.l.a.n.g.es or radial faces when turning cylindrical work, or may, by turning it on edge, be used to finish radial faces. It is shown in Fig. 1343 by itself, and in Fig. 1344 turning up a stem. It is held so that the middle of the edge does the cutting, and this tends to keep it from digging into the work. The bevels forming the cutting edge require to be very smoothly oilstoned.

The whole secret of the skillful and successful use of this valuable tool lies in giving it the proper inclination to the work. It is shown in Fig. 1344, at E, in the proper position for taking a cut from right to left, and at F in position for taking a cut from left to right. The face of the tool lying on the work must be tilted over, for E as denoted by line A, and for F as denoted by the line B, the tilt being only sufficient to permit the edge to cut. If tilted too much it will dig into the work; if not tilted, the edge will not meet the work, and therefore cannot cut. For cutting down the ends of the work, or down a side face, it must be tilted very slightly, as denoted in figure by C D, the amount of the tilt regulating the depth of the cut, so that when the cutting edge of the tool has entered the wood to the requisite depth, the flat face of the tool will prevent the edge from entering any deeper. In cutting down a radial face the acute corner of the tool leads the cut, whereas in in plain cylindrical work the obtuse is better to lead.

For cutting down the ends, for getting into small square corners, and especially for small work, the skew chisel is more handy than the ordinary chisel, and leaves less work for the sand-paper to do.

Beginners will do well to practise upon black walnut, or any wood that is not too soft, roughly preparing it with an axe to something near a round shape.

[Ill.u.s.tration: Fig. 1345.]

For finis.h.i.+ng hollow curves the tool shown in Fig. 1345 is employed, the cutting edge being at B; the degree of the curve determines the width of the tool, and, for internal work the tool is usually made long and without a handle.

[Ill.u.s.tration: Fig. 1346.]

[Ill.u.s.tration: Fig. 1347.]

The tool shown in Fig. 1346 is employed in place of the gouge in cases where the broad cutting edge of the latter would cause tremulousness. It may be used upon internal or external work, being usually about two feet long. For boring purposes, the tools shown in Fig. 1347 are employed, the cutting edges being from the respective points along the edges C, D, respectively. But when the bore is too small to admit of the application of tools having their cutting edges on the side, the tool shown in Fig.

1347 at E is employed, which has its cutting edge on the end.

[Ill.u.s.tration: _VOL. I._ =THE ROGERS-BOND UNIVERSAL COMPARATOR.= _PLATE XIV._

Fig. 1348.

Fig. 1349.]

CHAPTER XIV.--MEASURING MACHINES, TOOLS, AND DEVICES.

Measurements are primarily derived in Great Britain and her colonies, and in the United States, from the English Imperial or standard yard.

This yard is marked upon a bar of "Bailey's metal" (composed of 16 parts copper, 2-1/2 parts tin, and 1 part zinc), an inch square and 38 inches long. One inch from each end is drilled a hole about three-quarters through the whole depth of the bar, into which are fitted gold plugs, whose upper end faces are level with the axis of the bar. Across each plug is marked a fine line, and the distance between these lines was finally made the standard English yard by an Act of Parliament pa.s.sed in 1855. A copy of this bar is in the possession of the United States Government at Was.h.i.+ngton, and all the standard measuring tools for feet, inches, &c., are derived from subdivisions of this bar.

The standard of measurement in France and her colonies, Italy, Germany, Portugal, British India, Mexico, Roumania, Greece, Brazil, Peru, New Granada, Uruguay, Chili, Venezuela, and the Argentine Confederation, is the French metre, which is also partially the standard in Austria, Bavaria, Wurtemberg, Baden, Hesse, Denmark, Turkey, and Switzerland. It consists of a platinum bar, called the "metre des archives," whose end faces are parallel, and the length of this bar is the standard metre.

But as measuring from the ends of this bar would (from the wear) impair its accuracy, a second bar, composed of platinum and iridium, has been made from the "metre des archives." This second bar has ruled upon it two lines whose distance apart corresponds to the length of the "metre des archives," and from the distance between these lines the subdivisions of the metre have been obtained.

As all metals expand or contract under variations of temperature, it is obvious that these standards of length can only be accurate when at some given temperature: thus the English bar gives a standard yard when it is at a temperature of 62 Fahr., while the French standard bar is standard at a temperature of 32 Fahr., which corresponds to 0 in the centigrade thermometer. But if a bar is copied from a standard, and is found to be too short, it is obvious that if its amount of expansion under an increase of temperature be accurately known, it will be an accurate standard at some higher temperature, or in other words, at a temperature sufficiently higher to cause it to expand enough to compensate for its error, and no more.

As all bars of metal deflect from their own weight, it is obvious that the bar must be supported at the same points at which it rested when the lines were marked, and it has been determined by Sir George Airy, that the best position for the points of support for any bar may be obtained as follows: Multiply the number of the points of support by itself (or, as it is commonly called, "square it"), and from the sum so obtained subtract 1. Then subtract the square root of the remainder, which gives a sum that divided into the length of the bar will represent the distance apart for the points of support. It will be obvious that the points of support must be at an equal distance from each end of the bar.

Measurement may be compared in two ways, by sight and by the sense of feeling. Measurement by sight is made by comparing the coincidence of lines, and is called "line measurement." Measurement by feeling or touch is called "end measurement," because the measurement is taken at the ends. If, for example, we measure the diameter of a cylindrical bar, it is an end measurement, because the measurement is in a line at a right angle to the axis of the bar, and the points of touch on each side of the bar are the ends of the measurement, which is supposed to have no width.

In measuring by sight we may, for rude measurements, trust to the unaided eye, as in using the common foot rule, but for such minute comparisons as are necessary in subdividing or transferring a standard, we may call in the aid of the microscope.

The standard gauges, &c., in use in the United States have been obtained from Sir Joseph Whitworth, or duplicated from those made by him with the aid of measuring and comparing machines. It has been found, however, that different sets of these gauges did not measure alike, the variations being thus given by Mr. Stetson, superintendent of the Morse Twist Drill and Machine Co.