it will scarcely be expected that any part of the present work, intended mainly for apprentice engineers, should relate to designing machines, yet there is no reason why the subject should not to some extent be treated of; it is one sure to engage more or less attention from learners, and the study of designing machines, if properly directed, cannot fail to be of advantage.
there is, perhaps, no one who has achieved a successful experience as an engineer but will acknowledge the advantages derived from early efforts to generate original designs, and none who will not admit that if their first efforts had been more carefully directed, the advantages gained would have been greater.
it is exceedingly difficult for an apprentice engineer, without experimental knowledge, to choose plans for his own education, or to determine the best way of pursuing such plans when they have been chosen; and there is nothing that consumes so much time, or is more useless than attempting to make original designs, if there is not some systematic method followed.
there is but little object in preparing designs, when their counterparts may already exist, so that in making original plans, there should be a careful research as to what has been already done in the same line. it is not only discouraging, but annoying, after studying a design with great care, to find that it has been anticipated, and that the scheme studied out has been one of reproduction only. for this reason, attempts to design should at first be confined to familiar subjects, instead of venturing upon unexplored ground.
designing is in many respects the same thing as invention, except that it deals more with mechanism than principles, although it may, and often does include both. like invention, designing should always be attempted for the attainment of some definite object laid down at the beginning, and followed persistently throughout.
it is not always an easy matter to hit upon an object to which designs may be directed; and although at first thought it may seem that any machine, or part of a machine, is capable of improvement, it will be found no easy matter to detect existing [153] faults or to conceive plans for their remedy.
a new design should be based upon one of two suppositions—either that existing mechanism is imperfect in its construction, or that it lacks functions which a new design may supply; and if those who spend their time in making plans for novel machinery would stop to consider this from the beginning, it would save no little of the time wasted in what may be called scheming without a purpose.
after determining the ultimate objects of an improvement, and laying down the general principles which should be followed in the preparation of a design, there is nothing connected with constructive engineering that can be more nearly brought within general rules than arranging details. i am well aware of how far this statement is at variance with popular opinion among mechanics, and of the very thorough knowledge of machine application and machine operation required in making designs, and mean that there are certain principles and rules which may determine the arrangement and distribution of material, the position and relation of moving parts, bearings, and so on, and that a machine may be built up with no more risk of mistakes than in erecting a permanent structure.
designing machines must have reference to adaptation, endurance, and the expense of construction. adaptation includes the performance of machinery, its commercial value, or what the machinery may earn in operating; endurance, the time that machines may operate without being repaired, and the constancy of their performance; expense, the investment represented in machinery.
the adaptation, endurance, and cost of machines in designing become resolved into problems of movements, the arrangement of parts, and proportions.
movements and strains may be called two of the leading conditions upon which designs for machines are based: movements determine general dimensions, and strains determine the proportions and sizes of particular parts. movement and strain together determine the nature and area of bearings or bearing surfaces.
the range and speed of movement of the parts of machines are elements in designing that admit of a definite determination from the work to be accomplished, but arrangement cannot be so determined, and is the most difficult to find data for. to sum [154] up these propositions we have:—
1. a conception of certain functions in a machine, and some definite object which it is to accomplish.
2. plans of adaptation and arrangement of the component parts of the machinery, or organisation as it may be called.
3. a knowledge of specific conditions, such as strains, the range and rate of movements, and so on.
4. proportions of the various parts, including the framing, bearing surfaces, shafts, belts, gearing, and other details.
5. symmetry of appearance, which is often more the result of obvious adaptation than ornamentation.
to illustrate the practical application of what has preceded, let it be supposed, for example, that a machine is to be made for cutting teeth in iron racks ? in. pitch and 3 in. face, and that a design is to be prepared without reference to such machines as may already be in use for the purpose.
it is not assumed that an actual design can be made which by words alone will convey a comprehensive idea of an organised machine; it is intended to map out a course which will illustrate a plan of reasoning most likely to attain a successful result in such cases.
the reader, in order to better understand what is said, may keep in mind a common shaping machine with crank motion, a machine which nearly fills the requirements for cutting tooth racks.
having assumed a certain work to do, the cutting of tooth racks ? in. pitch, and 3 in. face, the first thing to be considered will be, is the machine to be a special one, or one of general adaptation? this question has to do, first, with the functions of the machine in the way of adapting it to the cutting of racks of various sizes, or to performing other kinds of work, and secondly, as to the completeness of the machine; for if it were to be a standard one, instead of being adapted only to a special purpose, there are many expensive additions to be supplied which can be omitted in a special machine. it will be assumed in the present case that a special machine is to be constructed for a particular duty only.
the work to be performed consists in cutting away the metal between the teeth of a rack, leaving a perfect outline for the teeth; and as the shape of teeth cannot well be obtained by an adjustment of tools, it must be accomplished by the shape of the tools. the shape of the tools must, therefore, be constantly maintained, [155] and as the cross section of the displaced metal is not too great, it may be assumed that the shape of the tools should be a profile of the whole space between two teeth, and such a space be cut away at one setting or one operation. by the application of certain rules laid down in a former place in reference to cutting various kinds of material, reciprocating or planing tools may be chosen instead of rotary or milling tools.
movements come next in order, and consist of a reciprocating cutting movement of the tools or material, a feed movement to regulate the cutting action, and a longitudinal movement of the rack, graduated to pitch or space, the distance between the teeth.
the reciprocating cutting movement being but four inches or less, a crank is obviously the best means to produce this motion, and as the movement is transverse to the rack, which may be long and unwieldy, it is equally obvious that the cutting motion should be performed by the tools instead of the rack.
the feed adjustment of the tool being intermittent and the amount of cutting continually varying, this movement should be performed by hand, so as to be controlled at will by the sense of feeling. the same rule applies to the adjustment of the rack for spacing; being intermittent and irregular as to time, this movement should also be performed by hand. the speed of the cutting movement is known from ordinary practice to be from sixteen feet to twenty feet a minute, and a belt two and a half inches wide must move two hundred feet a minute to propel an ordinary metal cutting tool, so that the crank movement or cutter movement must be increased by gearing until a proper speed of the belt is reached; from this the speed of intermediate movers will be found.
arrangement comes next; in this the first matter to be considered is convenience of manipulation. the cutting position should be so arranged as to admit of an easy inspection of the work. an operator having to keep his hand on the adjusting or feed mechanism, which is about twelve inches above the work, it follows that if the cutting level is four feet from the floor, and the feed handle five feet from the floor, the arrangement will be convenient for a standing position. as the work requires continual inspection and hand adjustments, it will for this reason be a proper arrangement to overhang both the supports for the rack and the cutting tools, placing them, as we may say, outside the machine, to secure convenience of access and to allow of inspection. [156] the position of the cutting bar, crank, connections, gearing, pulleys, and shafts, will assume their respective places from obvious conditions, mainly from the position of the operator and the work.
next in order are strains. as the cutting action is the source of strains, and as the resistance offered by the cutting tools is as the length or width of the edges, it will be found in the present case that while other conditions thus far have pointed to small proportions, there is now a new one which calls for large proportions. in displacing the metal between teeth of three-quarters of an inch pitch, the cutting edge or the amount of surface acted upon is equal to a width of one inch and a half. it is true, the displacement may be small at each cut, but the strain is rather to be based upon the breadth of the acting edge than the actual displacement of metal, and we find here strains equal to the average duty of a large planing machine. this strain radiates from the cutting point as from a centre, falling on the supports of the work with a tendency to force it from the framing. between the rack and the crank-shaft bearing, through the medium of the tool, cutter bar, connection, and crank pin, and in various directions and degrees, this strain may be followed by means of a simple diagram. besides this cutting strain, there are none of importance; the tension of the belt, the side thrust in bearings, the strain from the angular thrust of the crank, and the end thrust of the tool, although not to be lost sight of, need not have much to do with problems of strength, proportion, and arrangement.
strains suggest special arrangement, which is quite a distinct matter from general arrangement, the latter being governed mainly by the convenience of manipulation. special arrangement deals with and determines the shape of framing, following the strains throughout a machine. in the present case we have a cutting strain which may be assumed as equal to one ton, exerted between the bracket or jaws which support the work, and the crank-shaft. it follows that between these two points the metal in the framing should be disposed in as direct a line as possible, and provision be made to resist flexion by deep sections parallel with the cutting motion.
lastly, proportions; having estimated the cutting force required at one ton, although less than the actual strain in a machine of this kind, we proceed upon this to fix proportions, [157] beginning with the tool shank, and following back through the adjusting saddle, the cutting bar, connections, crank pins, shafts, and gear wheels to the belt. starting again at the tool, or point of cutting, following through the supports of the rack, the jaws that clamp it, the saddle for the graduating adjustment, the connections with the main frame, and so on to the crank-shaft bearing a second time, dimensions may be fixed for each piece to withstand the strains without deflection or danger of breaking. such proportions cannot, i am aware, be brought within the rules of ordinary practice by relying upon calculation alone to fix them, and no such course is suggested; calculation may aid, but cannot determine proportions in such cases; besides, symmetry, which cannot be altogether disregarded, modifies the form and sometimes the dimensions of various parts.
i have in this way imperfectly indicated a methodical plan of generating a design, as far as words alone will serve, beginning with certain premises based upon a particular work to be performed, and then proceeding to consider in consecutive order the general character of the machine, mode of operation, movements and adjustments, general arrangement, strains, special arrangement, and proportions.
with a thorough knowledge of practical machine operation, and an acquaintance with existing practice, an engineer proceeding upon such a plan, will, if he does not overlook some of the conditions, be able to generate designs which may remain without much modification or change, so long as the purpose to which the machinery is directed remains the same.
perseverance is an important trait to be cultivated in first efforts at designing; it takes a certain amount of study to understand any branch of mechanism, no matter what natural capacity may be possessed by a learner. mechanical operations are not learned intuitively, but are always surrounded by many peculiar conditions which must be learned seriatim, and it is only by an untiring perseverance at one thing that there can be any hope of improving it by new designs.
a learner who goes from gearing and shafts to steam and hydraulics, from machine tools to cranes and hoisting machinery, will not accomplish much. the best way is to select at first an easy subject, one that admits of a great range of modification, and if possible, one that has not assumed a standard form of construction. bearings and supports for shafts and spindles, [158] is a good subject to begin with.
in designing supports for shafts the strains are easily defined and followed, while the vertical and lateral adjustment, lubrication of bearings, symmetry of supports and hangers, and so on, will furnish grounds for endless modification, both as to arrangement and mechanism.
in making designs it is best to employ no references except such as are carried in the memory. the more familiar a person is with machinery of any class, the more able he may be to prepare designs, but not by measuring and referring to other people's plans. dimensions and arrangement from examples are, by such a course, unconsciously carried into a new drawing, even by the most skilled; besides, it is by no means a dignified matter to collect other people's plans, and by a little combination and modification produce new designs. it may be an easy plan to acquire a certain kind of proficiency, but will most certainly hinder an engineer from ever rising to the dignity of an original designer.
symmetry, as an element in designs for machinery, is one of those unsettled matters which may be determined only in connection with particular cases; it may, however, be said that for all engineering implements and manufacturing machinery of every kind, there should be nothing added for ornament, or anything that has no connection with the functions of the machinery.
modern engineers of the abler class are so thoroughly in accord in this matter of ornamentation, both in opinion and practice, that the subject hardly requires to be mentioned, and it will be no disadvantage for a learner to commence by cultivating a contempt for whatever has no useful purpose. of existing practice it may be said, that in what may be called industrial machinery, the amount of ornamentation is inverse as the amount of engineering skill employed in preparing designs.
a safe rule will be to assume that machinery mainly used and seen by the skilled should be devoid of ornament, and that machinery seen mainly by the unskilled, or in public, should have some ornament. steam fire engines, sewing machines, and works of a similar kind, which fall under the inspection of the unskilled, are usually arranged with more or less ornament.
as a rule, ornament should never be carried further than graceful proportions; the arrangement of framing should follow as nearly as possible the lines of strain. extraneous decoration, such as detached filagree work of iron, or painting in colours, is [159] so repulsive to the taste of the true engineer and mechanic that it is unnecessary to speak against it.
(1.) name some of the principal points to be kept in view in preparing designs?—(2.) why should attempts at designing be confined to one class of machinery?—(3.) what objection exists to examining references when preparing designs?
