The Economy of Workshop Manipulation / A logical method of learning constructive mechanics. Arranged with questions for the use of apprentice engineers and students.

by John Richards

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 ¾ 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, 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. 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.