Page:EB1911 - Volume 27.djvu/48

both to drive and cut the blank in an ordinary machine. When worms are not produced by these methods the envelope cannot be obtained, but each tooth space is cut by an involute milling cutter set at the angle of thread in a universal machine, or else in one of the general gear-cutting machines used for spur, bevel and worm gears, and only capable of yielding really accurate results in the case of spur-wheels.

The previous remarks relate only to the sectional forms of the teeth. But their pitch or distance from centre to centre requires dividing mechanism. This includes a main dividing or worm wheel, a worm in conjunction with change gears, and a division plate for setting and locking the mechanism. The plate may have our divisions only to receive the locking lever or it may be drilled with a large number of holes in circles for an index peg. The first is adopted in the regular gear-cutters, the second on the universal milling machines which are used also for gear-cutting. In the largest number of machines this pitching has to be done by an attendant as often as one tooth is completed. But in a good number of recent machines the itching is effected by the movements of the machine itself witiiout human intervention. With s ur-wheels the cutting proceeds until the wheel is complete, when tfie machine is often made to ring a bell to call attention to the fact. But in bevel-wheels only one side of the teeth all the way round can be done; the attendant must then effect the necessary settings for the other side, after which the pitching's are automatic. As a general rule only one tooth is being operated on at one time. But economy is studied in spur-gears by setting several similar wheels in line on a mandrel and cutting through a single tooth of the series at one traverse of the tool. In toothed racks the same device is adopted. Again, there are cases in which cutters are lrnade to operate simultaneously on two, three or more adjacent teet ..

Recently a generating machine of novel design has been manufactured, the spur-wheel hobbing machine. In appearance the hob resemblesgthat employed for cutting worm-gears, but it also generates the teeth of spur and spiral gears. The hob is a worm cut to form teeth, backed off and hardened. The section of the worm thread is that of a rack. Though it will cut worm-wheels, spiral-wheels or spur-wheels equally correctly, the method of presentation varies. When cutting worm-wheels it is fed inwards perpendicularly to the blank; when cutting spirals it is set-at a suitable angle and fed across the face of the blank. The angle of the worm thread in the hob being about 2é°, it has to be set by that amount out of parallel with the plane of the gear to be cut. It is then fed down the face of the wheel blank, which is rotated so as to synchronize with the rotation of the worm. This is effected through change gears, which are altered for wheels having different numbers of teeth. The advantage is that of the hob over single cutters; one hob serves for all wheels of the same pitch, and each wheel is cut absolutely correct. While using a set of single cutters many wheels must have their teeth only approximately correct. VI.-GRINDING MACHINES A

The practice of finishing metallic surfaces by grinding, though very old, is nevertheless with regard to its rivalry with the work of tlhe ordinary rlgiaching tools a development of tgeéast paét of ltlhe 19t century. rom eing a non- recision met o, grin ing as become the most perfect device für producing accurate results measured precisely within thousandths of an inch. It would be rather difficult to mention any class of machine-shop work which is not now donefby the grinding Thr; mostdreceng divelopi ments are grin ing out engine cy in ers an grin ing the ips twist drills by automatic movements, the drills rotating constantly. There are five very broad divisions under which grinding machines ma}i be cla;sif1ed, hbuié tlae inldlividual, véell-defined grciupsh/(fr types mig t num er a un re e main ivisions are: I achines for dealing with plane surfaces; (2) machines for plain cylindrical work, external and internal; (3) the universals, which embody movements rendering them capable of angular setting; (4) the tool grinders: and (5)' the specialized mac ines.' Most of these might be again classed under two heads, the non-precision and the precision types. The difference between these two classes is that the first does not embody provision for measuring the amount of material removed, while the second does. This distinction is a most important one.

The underlying resemblances and the differences in the main designs of the groups of machines just now noted will be better understood if the essential conditions of grinding as a corrective process are” grasped. The cardinal point is that accurate results a{e prloiluced by wlheels that are themselves being abraded constant y. at is not the case in steel cutting tools, or at least in but an infinitesimal degree. A steel tool will retain its edge for several hours (often for days) Without the need for regrinding, but the particles of abrasive in an emery or other grinding wheel are being incessantly torn out and removed. A wheel in traversing along a shaft say of 3 ft. in length is smaller in diameter at the term mat ion than at the beginning of the traverse, and therefore the shaft must be theoretically larger at one end than the other. Shafts, nevertheless, are ground parallel. The explanation is, and it lies at the basis of emery grinding, that the feed or amount removed at a single traverse is extremely minute, say a thousandth or half a thousandth of an inch. The minuteness of the feed receives compensation in the repetition and rapidity of the traverse. The wear of the wheel is reduced to a minimum and true work is produced.

From this fact of the wear of grinding wheels two important results follow. One is that a traverse or lateral movement must always take place between the wheel and the piece of work being ground. This is necessary in order to prevent a mutual grooving action between the wheel and work. The other is that it is essential to provide a large range in quality of wheels, graded according to coarseness and iineness, of hardness and softness of emery to suit all the different metals and alloys. Actually about sixty grades are manufactured, but about a dozen will generally cover average sho practice. With such a choice of Wheels the softest brass as well) as the hardest tempered steel or case-hardened glass-like surfaces that could not possibly be cut in lathe or planer, can be ground with extreme accuracy.

FIG. 50.-Universal Grinding Machine, 7 in. centres; 3 ft. 6 in. between centres. (H. W. Ward & Co., Ltd., Birmingham.) A, Base or body, with waste J, Headstock for carrying and water tray round top edge, driving work, used for and interior fitted as cup- chuck work or dead centre boards, with shelves and work; the base is graduated doors. into degrees.

B, Sliding table. a, Dogs, which regulate auto-C, Swivel table. matic reversals. An internal D, Grinding wheel. grinding fixture, not shown, E, Wheel guard. is fitted to wheel head. F, Wheel headstock swivelling L, Countershaft pulley driving to in a horizontal plane, and wheel pulley. having the base graduated M Pulley driving to cones. into degrees for angular N, Pulley driving to work head setting. stock pulley.

G, Slide carrying headstock. O, Belt from line shaft. H, Hand-wheel for traversing P, Water pipe from pump. table. Q, Water guards above table.

Plane surfacing machines in many cases resemble in general outlines the well-known planing machine and the vertical boring mill. The wheels traverse across the work, and they are fed vertically to precise fractional dimensions. They fill a large place in finishing plane surfaces, broad and narrow alike, and have become rivals to the planing and milling machines doing a similar class of work. For hardened surfaces they have no rival. Cylindrical grinders include many subdivisions to embrace external and internal surfaces, either parallel or tapered, small or