Modern Machine-Shop Practice Part 1

Modern Machine-Shop Practice.

by Joshua Rose.

PREFACE.

MODERN MACHINE-SHOP PRACTICE is presented to American mechanics as a complete guide to the operations of the best equipped and best managed workshops, and to the care and management of engines and boilers.

The materials have been gathered in part from the author's experience of thirty-one years as a practical mechanic; and in part from the many skilled workmen and eminent mechanics and engineers who have generously aided in its preparation. Grateful acknowledgment is here made to all who have contributed information about improved machines and details of new methods.

The object of the work is practical instruction, and it has been written throughout from the point of view, not of theory, but of approved practice. The language is that of the workshop. The mathematical problems and tables are in simple arithmetical terms, and involve no algebra or higher mathematics. The method of treatment is strictly progressive, following the successive steps necessary to becoming an intelligent and skilled mechanic.

The work is designed to form a complete manual of reference for all who handle tools or operate machinery of any kind, and treats exhaustively of the following general topics: I. The construction and use of machinery for making machines and tools; II. The construction and use of work-holding appliances and tools used in machines for working metal or wood; III. The construction and use of hand tools for working metal or wood; IV. The construction and management of steam engines and boilers.

The reader is referred to the TABLE OF CONTENTS for a view of the mult.i.tude of special topics considered.

The work will also be found to give numerous details of practice never before in print, and known hitherto only to their originators, and aims to be useful as well to master-workmen as to apprentices, and to owners and managers of manufacturing establishments equally with their employees, whether machinists, draughtsmen, wood-workers, engineers, or operators of special machines.

The ill.u.s.trations, over three thousand in number, are taken from modern practice; they represent the machines, tools, appliances and methods now used in the leading manufactories of the world, and the typical steam engines and boilers of American manufacture.

The new p.r.o.nOUNCING AND DEFINING DICTIONARY at the end of the work, aims to include all the technical words and phrases of the machine shop, both those of recent origin and many old terms that have never before appeared in a vocabulary of this kind.

The wide range of subjects treated, their convenient arrangement and thorough ill.u.s.tration, with the exhaustive TABLE OF CONTENTS of each volume and the full a.n.a.lYTICAL INDEX to both, will, the author hopes, make the work serve as a fairly complete ready reference library and manual of self-instruction for all practical mechanics, and will lighten, while making more profitable, the labor of his fellow-workmen.

CHAPTER I.--THE TEETH OF GEAR-WHEELS.

A wheel that is provided with teeth to mesh, engage, or gear with similar teeth upon another wheel, so that the motion of one may be imparted to the other, is called, in general terms, a gear-wheel.

[Ill.u.s.tration: Fig. 1.]

When the teeth are arranged to be parallel to the wheel-axis, as in Fig.

1, the wheel is termed a spur-wheel. In the figure, A represents the axial line or axis of the wheel or of its shaft, to which the teeth are parallel while s.p.a.ced equidistant around the rim, or face, as it is termed, of the wheel.

[Ill.u.s.tration: Fig. 2.]

[Ill.u.s.tration: Fig. 3.]

When the wheel has its teeth arranged at an angle to the shaft, as in Fig. 2, it is termed a bevel-wheel, or bevel gear; but when this angle is one of 45, as in Fig. 3, as it must be if the pair of wheels are of the same diameter, so as to make the revolutions of their shafts equal, then the wheel is called a mitre-wheel. When the teeth are arranged upon the radial or side face of the wheel, as in Fig. 4, it is termed a crown-wheel. The smallest wheel of a pair, or of a train or set of gear-wheels, is termed the pinion; and when the teeth are composed of rungs, as in Fig. 5, it is termed a lantern, trundle, or wallower; and each cylindrical piece serving as a tooth is termed a _stave_, _spindle_, or _round_, and by some a _leaf_.

[Ill.u.s.tration: Fig. 4.]

An annular or internal gear-wheel is one in which the faces of the teeth are within and the flanks without, or outside the pitch-circle, as in Fig. 6; hence the pinion P operates within the wheel.

[Ill.u.s.tration: Fig. 5.]

[Ill.u.s.tration: Fig. 6.]

When the teeth of a wheel are inserted in mortises or slots provided in the wheel-rim, it is termed a mortised-wheel, or a cogged-wheel, and the teeth are termed cogs.

When the teeth are arranged along a plane surface or straight line, as in Fig. 7, the toothed plane is termed a _rack_, and the wheel is termed a pinion.

A wheel that is driven by a revolving screw, or worm as it is termed, is called a worm-wheel, the arrangement of a worm and worm-wheel being shown in Fig. 8. The screw or worm is sometimes also called an endless screw, because its action upon the wheel does not come to an end as it does when it is revolved in one continuous direction and actuates a nut.

So also, since the worm is tangent to the wheel, the arrangement is sometimes called a wheel and tangent screw.

The diameter of a gear-wheel is always taken at the pitch circle, unless otherwise specially stated as "diameter over all," "diameter of addendum," or "diameter at root of teeth," &c., &c.

[Ill.u.s.tration: Fig. 7.]

When the teeth of wheels engage to the proper distance, which is when the pitch circles meet, they are said to be in gear, or geared together.

It is obvious that if two wheels are to be geared together their teeth must be the same distance apart, or the same _pitch_, as it is called.

The designations of the various parts or surfaces of a tooth of a gear-wheel are represented in Fig. 9, in which the surface A is the face of the tooth, while the dimension F is the width of face of the wheel, when its size is referred to. B is the flank or distance from the pitch line to the root of the tooth, and C the point. H is the _s.p.a.ce_, or the distance from the side of one tooth to the nearest side of the next tooth, the width of s.p.a.ce being measured on the pitch circle P P. E is the depth of the tooth, and G its thickness, the latter also being measured on the pitch circle P P. When spoken of with reference to a tooth, P P is called the pitch line, but when the whole wheel is referred to it becomes the pitch circle.

[Ill.u.s.tration: Fig. 8.]

The points C and the surface H are true to the wheel axis.

The teeth are designated for measurement by the pitch; the height or depth above and below pitch line; and the thickness.

The pitch, however, may be measured in two ways, to wit, around the pitch circle A, in Fig. 10, which is called the arc or circular pitch, and across B, which is termed the chord pitch.

[Ill.u.s.tration: Fig. 9.]

In proportion as the diameter of a wheel (having a given pitch) is increased, or as the pitch of the teeth is made finer (on a wheel of a given diameter) the arc and chord pitches more nearly coincide in length. In the practical operations of marking out the teeth, however, the arc pitch is not necessarily referred to, for if the diameter of the pitch circle be made correct for the required number of teeth having the necessary arc pitch, and the wheel be accurately divided off into the requisite number of divisions with compa.s.ses set to the chord pitch, or by means of an index plate, then the arc pitch must necessarily be correct, although not referred to, save in determining the diameter of the wheel at the pitch circle.

The difference between the width of a s.p.a.ce and the thickness of the tooth (both being measured on the pitch circle or pitch line) is termed the clearance or side clearance, which is necessary to prevent the teeth of one wheel from becoming locked in the s.p.a.ces of the other. The amount of clearance is, when the teeth are cut to shape in a machine, made just sufficient to prevent contact on one side of the teeth when they are in proper gear (the pitch circles meeting in the line of centres). But when the teeth are cast upon the wheel the clearance is increased to allow for the slight inequalities of tooth shape that is incidental to casting them. The amount of clearance given is varied to suit the method employed to mould the wheels, as will be explained hereafter.

The line of centres is an imaginary line from the centre or axis of one wheel to the axis of the other when the two are in gear; hence each tooth is most deeply engaged, in the s.p.a.ce of the other wheel, when it is on the line of centres.

There are three methods of designating the sizes of gear-wheels. First, by their diameters at the pitch circle or pitch diameter and the number of teeth they contain; second, by the number of teeth in the wheel and the pitch of the teeth; and third, by a system known as diametral pitch.

[Ill.u.s.tration: Fig. 10.]

The first is objectionable because it involves a calculation to find the pitch of the teeth; furthermore, if this calculation be made by dividing the circ.u.mference of the pitch circle by the number of teeth in the wheel, the result gives the arc pitch, which cannot be measured correctly by a lineal measuring rule, especially if the wheel be a small one having but few teeth, or of coa.r.s.e pitch, as, in that case, the arc pitch very sensibly differs from the chord pitch, and a second calculation may become necessary to find the chord pitch from the arc pitch.

The second method (the number and pitch of the teeth) possesses the disadvantage that it is necessary to state whether the pitch is the arc or the chord pitch.

If the arc pitch is given it is difficult to measure as before, while if the chord pitch is given it possesses the disadvantage that the diameters of the wheels will not be exactly proportional to the numbers of teeth in the respective wheels. For instance, a wheel with 20 teeth of 2 inch chord pitch is not exactly half the diameter of one of 40 teeth and 2 inch chord pitch.

To find the chord pitch of a wheel take 180 (= half the degrees in a circle) and divide it by the number of teeth in the wheel. In a table of natural sines find the sine for the number so found, which multiply by 2, and then by the radius of the wheel in inches.

Example.--What is the chord pitch of a wheel having 12 teeth and a diameter (at pitch circle) of 8 inches? Here 180 12 = 15; (sine of 15 is .25881). Then .25881 2 = .51762 4 (= radius of wheel) = 2.07048 inches = chord pitch.