MODELING AS AN AID TO INSTRUCTION IN MA

CHINE DESIGN.

BY GEORGE W. BISSELL,

Professor of Mechanical Engineering, Iowa Agricultural College, Ames,

Iowa.

THE use of modeling in the teaching of machine design has been resorted to by the writer as a means of conveying to the student a clearer conception of form. The consideration of the form or shape of machine parts is of great importance in the production of a successful design.

In many, perhaps most, problems which the teacher of machine design presents to his students, the form or contour of each of the several pieces which go to make up the complete machine, can be depicted by a sketch, freehand or mechanical, or shown by photographs, although, of course, the most certain and convincing way of representing said form, even when very simple, is by a model or by an actual construction.

In many cases, however, the part in question may be a casting of such complex form, that the instructor, be he ever so clever, cannot exhibit it with a freehand sketch; and working drawings and models or actual examples are not accessible. For such cases a quickly produced model, however crude, is of great assistance, and an eye and a hand capable of producing it readily and quickly, become valuable adjuncts to the instructor's work. A piece of pattern pine and a jack-knife, or a piece of paper and scissors, can, in the ( 273 )

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skilful hands, be the material and tool for exhibiting the desired form. The writer is not possessed of the skill for either of the above, but has had some practice in the handling of the plastic materials used by sculptors in their work, and has turned such practice to advantage.

Modeling clay is very plastic, and, when mixed with oil, is an ever ready material.

The tools for the roughing out of the work and for the finishing touches upon the curved surfaces, are the fingers, together with two or three tools used by sculptors and called "hooks." The latter are easily made by bending lengths of wire into small isosceles triangles, wooden handles being attached at the vertices and the bases being of any desired width. The wide hooks are used for producing flat surfaces, and the narrow ones for digging out recessed portions. Special hooks with the bases curved, can be used for curved surfaces, but the writer finds that to use the finger is quicker than to be select from a multitude of tools. Having the form in mind, a lump of clay is shaped by the hands to a rough semblance of the finished form, perhaps no nearer than that of a rectangular block. Hooks are used to remove excess of material where necessary, and the surfaces are finished with a few strokes of the finger.

On models to a small scale, bosses and other projections are added after the principal outlines have been obtained.

Usually the scale is not important, provided relative proportions are maintained correct.

Where cylindrical portions are a prominent feature,

time may sometimes be saved by building the clay around or attaching it to a piece of wood turned to the right size. Similarly, flat surfaces, such as guides or tension members, can be made of other material than the clay, and incorporated into the model, it being understood that rapidity is important, especially when students are waiting for the result.

Another way in which the clay can be used instructively is in studying the effect of changes in existing constructions. Projections, such as bosses, brackets or fillets, can be rapidly created upon the existing part and the resulting form compared with the original, or with any other form. The writer has had a limited use of this method, and believes that the value to his students of his instruction in machine design is thereby enhanced.

DISCUSSION.

PROFESSOR J. J. FLATHER said that he had been much interested in the paper just read. There were many excellent suggestions in it. He would object to the statement that the scale is not important. He believed that in modeling, one is not entirely certain how a thing is going to look; it is a matter of training the eye and getting something to shape for a part that is wanted. It is a development under the hand of the modeler to suit one's preconceived ideas of shape. If the model is not made to a large scale, the relative proportions will vary considerably from those of the piece in its actual size, and moreover any scale less than full size will develop this feature. This is noticeable in drawings. A drawing of a part is made

to quarter scale or eighth scale, and it gives an entirely different idea so far as proportion is concerned, from the one that is drawn full size. The same thing must hold true to a certain extent in a model made to scale. It does not make any difference what the scale is; while there may be a resemblance, and while the model may have good clear lines, yet the proportion will be apparently modified, and the smaller the scale the greater will this be shown. In the speaker's own work in teaching design, where there is a part that is very much too large to put on the drawing board full size, as for instance, the frame of a drill press, the student has been encouraged to draw it out full size on the blackboard, so as to get the correct lines, and an entirely different proportion often results when it is thus drawn, from what would be obtained if it were drawn on the drawing board to a scale of say, one-eighth. This refers more particularly to machine forms, where the size of a boss or the curvature of a corner or location of bracket, as well as the general contour itself, will all affect the form of the piece that is to be made. It is found that a student obtains a very much better idea of correct proportions and grace of outline if it is sketched full size than he does if it is a scale drawing. In order to extend this practice, the speaker proposes to have a large slate black-board, say four feet by six, suitably mounted in a swing frame, and divided over its entire surface by fine lines one inch apart. Then after it has been determined what shape and proportions are desired, the design can readily be scaled off and reduced to any convenient size of working drawing. That system is in

use in the shops of Pedrick & Ayer, Philadelphia and possibly in other places.

PROFESSOR W. F. M. Goss did not feel sure that he understood perfectly the purpose of the modeling. Is it that the lines of the machine part may be graceful, and the prominent features of the design well joined?

PROFESSOR BISSELL answered that the object of the model is to clear up hazy impressions in a student's mind as to the shape of a relatively complicated piece. PROFESSOR GOSs suggested that it was then a means of interpreting machine forms.

PROFESSOR BISSELL said that "means of interpreting" expressed the idea very well; it was giving to the student the correct impression of what the form should be to connect certain of the important parts. For instance, in a side crank engine it is desired to connect the shaft and the cylinder and foundation in a certain way, and to provide for the motion of the connecting rods and the reciprocating parts; and it had happened that some students had come to him who had never seen an engine of the Porter type and did not have a very clear conception of what it is; there were no actual models at hand or actual examples, and that was one case in which he applied the method described.