US1863512A - Worm hob - Google Patents

Description

June 14, 1932,

N. TRBOJEVICH 1 Filed-July 21, 1930 2 Sheets-Sheet l ATTORNEYS w M V m June 14, 1932.

NI TRBOJEVICH wow aoB Filed July 21. 1930- 2 Sheets-Sheet 2 Az/YaJcz nby g ATTORNEYS Patented June 14, 1932 NIIOLJ. 133303317103, 01 DETROIT, MICHIGAN WORM HOB .lppltcation filed July 21, 1930. flerlal No. 469,577.

The invention relates to a worm hob of the tangential feed type and resides in a novel construction of flutes and a novel method of relieving the cutting teeth of the said hob.

The principle on which the new hob operates is somewhat similar to the principle exhibited in my prior application for patent ta 5, Serial N 0. 204,951 filed July 4, 1927.

he object of this invention is to provide In a hob of the specified type which although relieved in all its active cutting edges will not lose either its pitch diameter or the thickness of teeth after repeated sharpenings.

Another object is to construct a hob that 13 will ream out or burnish the teeth of the gear to be cut in the same operation in which they are generated.

Another object is to. provide a hob ground in the thread in which the grindin operation extends only to certain burnisfiing areas found in this hob and not to the roughing teeth, thereby reducing the cost of grindin Still another object is to roduce in a b5) a series of roughing teet having an increased chip room and a series of finishing teeth having a reduced chip room but an increased strength and ri idity. Another object is to construct a hob liaving roughing and finishing portions of thread in which the roughing teeth merge into the finishing teeth imperceptibly, that is, without causing the finishing teeth to bite oil a chip thicker than a predetermined small dimension, usually less than one thousandth of an inch thick and further to preserve this novel quality of the hob after sharpenings.

In the drawings:

Fig. 1 is a side-elevation of myimproved hob.

Fig. 2 is the same elevation diagrammatically represented.

Fig. 3 is a cross section of Fig. 2 in the plane 3-3.

Fig. 4 is another section in the plane 4-4.

Fig. 5 is still another section in the plane 5-5.

Fig. 6 is a diagram explanatory of the relieved hob tooth.

Fig. 7 is the plane development of the pitch cylinder of the hob shown in Fig. 2.

Fig. 8 is an enlarged and exaggerated detail of Fig. 7.

Fig. 9 is a hob tooth perspective explaining the meaning of theterms normal pressure angle and side clearance.

The'new hob (see Figs. 1 and 2) is formed from a steel worm, the threads of which are longitudinally intersected by means of a pluw rality of flutes 11, thereby forming a plurality of cutting teeth 16. The flutes 11 are of a novel construction in that they consist of two superposed systems of flutes, one of a shorter and one of a longer helical lead thus producing compound flutes which are wider at the roughing end 12 of the hob and narrower at the finishing end 13. The hob is provided with a plurality of irregularly bounded true helicoidal surfaces 14 prefer- 79 ably accurately ground over after hardening, said surfaces being distributed adjacent to the front or cutting faces 15 of the teeth 16 in generally increasing areas as approaching towards the rear or large end 13 of the ho The presence of these non-relieved helicoidal surfaces is essential to the proper functioning of the new hob and serves two purposes, first the said areas catch and removed the minute particles and surpluses of metal 39 left over b the preceding roughing and finishing teet thus producing smooth and accurate tooth surfaces and second, they form the reserve or the life of the hob so that it ma withstand a considerable number (about fi teen to twenty-five) resharpenings without losing the size or form. Thus, when this hob is resharpened, the face 15 of the lines 11 is ground over and the areas 14 will thereby be diminished. When all such areas are eventually ground oil the hob will be spent and will no longer produce the exact work for which it was intended; however, as long as the said areas remain in at least one tooth, the hob will cut true to size. i

The front end of the hob (Fig. 2) is a truncated cone 18 approximately comprising in length onehalf of the total length of the hob. The compound flutes 11 are denoted with-the numeral 19 for the flute of the longer lead ning from the said helix 21 will be re ieved. The

flutes of the above described divergent orcompound type.

The method of backing off the new hob will now be described. As shown in the pitch plane development, Fig. 7, we first mill the uired number of flutes along the helix 19 (t e helix of the longer lead) thus cutting s bounded by the helixes 19 and 20. The ho teeth 16 are now backed oif using the parallel helix 21 as the beginof the back off in such a manner that the portions of the teeth lying between helixes 20 and 21 will remain cylindrical or non-relieved whilethe portions lyin outwardly distance b of the helix 21 from the helix 20 is proportional to the distances b, Fig. 5.

After havin the teeth 16, thus, partially completed in t at each tooth will now be of a hexa nal contour B B C C D D? (Fig. 7), comprising a non-relieved portion B B E E, in the form of a parallelogram in front of it and a trapezium E E D in the rear of it, we intersect the entire series of cuttin teeth by means of the helical flute 15 of a s orter lead, said flute forming an angle G with the flute 21. The effect 'of this last operation will be to make each tooth in the hob different from any other tooth; howevenall teeth takon together will form an organic series exhibiting gradual variations, which is a matter of importance in this theory.

The introduction of the second helix 15 produces two important results; first, all teeth to the left of the point A, which is the intersection of the hclixes 15 and 21 respec- .time the point A reaches, thus, p p of the hob, the hob will be spent, but not'unt1l""' tively, are of an ever decreasing thickness C C as counted from the said point A, while the teeth to the right of the lpoint. A are all of a uniform thickness but a possess cylindrical lands H H of an ever increasing width as counted from the point A. Thus, in one single operation we ave produced first, a serles of roughin of an ever increasing width so that each toot will take its proportional share of the work, second, a finishing tooth at the point A and third, a series of burnishing teeth having burnishing surfaces arranged according to ever increasing areas.

Still further, these conditions will not change after resharpening. Let us sharpen the hob along the helix '22 parallel to the helix 15, Fig. 8. The point of intersection A will now travel to the right of the hob in its new location A but the hob will still have the roughing series, the finishing tooth and the burnishing series, as formerly. By the out from between the movin the rightend roughing seri'es eading to and connecting with the said-burnishing series should do so wlthout'forming an interval or a gap. I consider the practical means of doing this as the most valuable part of this invent on.

As diagrammatically shown in Fig. 6, the non-relieved tooth 16 is forced to take a chip of a thickness e from the workpiece. Let the contacting area of the tooth be :denoted with F and the cuttin velocity with '0 then, the force P with whic the tooth has to be pressed against the work to produce the deformation 0 will be P= bFe 1 where b and c are constants, the exponent a being of a higher power than one.

The developed heat per second W .will be proportional to the delivered frictionwork,

W=dPfv 2 where d is another constant and f the coefiicient of friction.

I observed that thehighly injurious force P will be generated at an amazing rate as soon as the jump e exceeds a'certain ver small limit, maybe less than one thonsandt of an inch. However, for the values of 6 less than that the tool will pass through the work freely, in very much the same manner as if it were relieved. This action ma be explained first b the structure of the Fquation 1 and second the fact that by exceeding a certain force the oil is suddently squeezed bobbing machine-thus establishing ametal to metal contact in the said parts.

Therefore, it is imperativev to keep the force P within bounds and thatis most eflectivel done by reducing the jump e and-the area The angle G included by the first helix 21 and the second helix 15, Fig. 8, may be calculated to produce a jump e not to exceed a predetermined limit.

When a hob tooth 16 the lathe tool travelsalong an Archimedean spiral 23 having a drop it from tooth to tooth. The said tool also moves parallel tothe axis of the hob thus producing actually a resultant composed of an Archimedean spiral and a straight line, i. e. a conical helix. 7

The clearance angle M at'the top ofthe hob tooth (Fig. 9) may the equation be determined from parts of the Fig. 4, is backed ofl',

, where m is the shortest distance from flute to flute as indicated in Fig. 7

Let now K be the normal pressure angle of the hob tooth (Fig. 9) and M be the side clearance angle of the said tooth on pitch line, Figs. 7 and 8. Then it stands that tan M=tan M tan K (4) The side clearance angle M will usually be from 3 to 4 degrees in a coarse pitch high a1=E0 tan M (5) but E0==EA tan G. (6)

now, the thickness of the chip c, Fig. 6 will be equal to where N is the number of flutes in the hob.

It is seen now that we can control the thickness of the chip 6 (the maximum which the burnishing tooth will be required to take) by properly selecting the dimensions fo 1nd in the Equations 3 to 6 inclusive. In articular, the whole procedure hinges most upon the selection of the divergence angle G. I have found out in practical work that the angle G should be around 4 to 5 degrees, the angle M about 6 to 7 degrees, thusgiving a bllllllllilShlIlg chip e between .0007 and .001inch t ic Regarding the manufacture of these hobs it may be noted that by one process the flutes 19 may be milled first, then the hob is backed off and the flutes 15 are milledafterwards, and by another process the flutes 19 and 15 both may be milled first and the hob may be backed off afterwards. The hob is then hardened and the faces 15 together with the cylindrical lands 14 are ground. The grinding of the lands 14.- may be done in the same machine in which the worms (of which the hob is a counterpart) are ground, thus insuring a perfect duplication of tooth contours at a low cost.

As shown in Fig. 1 which was traced from a photograph, the cylindrical lands 14 are of a more or less irregular shape showing that even when the hob as first backed cif was not quite perfect, which is indicated by the irregularity of the lands, yet such a defect will not impair the ability of the hob to produce good work. In this system a single hob tooth may be of such a form that a part of it will be roughing, another part finishing and still another part burnishing without affecting its correct working.

In calculating the dimensions of such a hob we first determine the lead of the helical flute 15 such that it will be at right angles to the thread helix aseis customary in all hobs. The angle G is determined afterwards as was shown in detail and the lead of the auxiliary flute 19 is from that calculated.

The principal advantage of this s stem is that now both the roughing and nishing teeth in a hob may be backed off in the same and continuous operation, thereby producing a series having no jump or interruption at any intermediate point, whereas formerly the roughing teeth had to be backed off separately by means of two divergent helixes, one for each side of the roughing tooth Series. This last procedure invariably placed anundue and mostly unforeseen and incalculable excessive loading u on the first finishing tooth; thusshortening t elife of the hob.

What I claim as my invention is:

1. A rotary thread tool comprising a threaded portion interrupted by longitudinal flutes forming cuttin teeth in which the flutes are of a convergin cross section from the entering toward the nishing end and in Which the cutting teeth are relieved be inning from a line intersecting the cutting aces at an acute angle at an intermediate point, thereby reducing the entering teeth in thickness and providing the finishing teeth with seri s of non-relieved helicoidal flanks adjacent to the cutting edges and progressive ly increasing in area.

2. A worm hob of the tangential feed t e com rising a threaded portion interrupte y ap urality of longitudinal flutes forming cutting teeth in which the teeth from the entering toward the finishing end are all backed off with a uniform relief and in an uninterrupted sequence, said back ofl' beginning from a line intersecting the cutting faces at an acute angle at an intermediate point, thus automatically obtainin a differentiated series of roughing and finis ing teeth, each tooth differing from the preceding one either in regards to its thickness at one side of the intermediate point or in regard to the width of the cylindrical lands adj accnt to the cutting edge at the other side of the said point.

3. A worm hob of the tangential feed t pe comprising a threaded ortion interrupted by a plurality of helical flutes forming cutting teeth in which the flutes are compounded from two helixes, one of a shorter lead forming the cutting faces and the other of a longer lead forming the rear sides of the cuttin teeth and in whichthe teeth are backed 0 along a helix substantially parallel to the said second helix of the longer lead.

4. A worm hob having a threaded portion and a plurality of compound flutes, each flute being compounded from two divergent helixes in which one helix forming the cutting faces is substantially perpendicular to the hob thread and the second helix forming the rear ends of the cutting teeth is diverging from the first helix at an angle and is substantially parallel to the helix at which the f hob teeth are backed off.

' 5. A worm hob of the tangential feed type comprising a threaded portion interrupted by a plurality of longitudinal flutes forming cutting teeth in which the teeth from 1 the enterin toward the finishing end are backed off fieginning from a line intersecting the cutting faces at an acute angle at an intermediate oint, thus automatically obtaining a di erentiated series of roughing and finishing teeth, each tooth differing from the preceding one in regard to its thickness at one side of the intermediate point and in regard to the width of the cylindrical lands adjacent to the cutting edge at the other side 2, of the said point and in which the said lands are corrected in form by a passage of a grinder in a. helical path relative to the hob axis, thus obtaining a partly ground hob of the same degree of accuracy as if it were ground all over.

6. A worm hob having a plurality of cutting teeth backed off from a helix disposed at an angle relative to the cutting faces'of the said teeth and a ground over in a helical path concentric with and equidistant from the hob axis, thus producing a series of corrective cylindrical helical surfaces adjacent to the cutting edges and increasing the cutting accuracy of the hob.

In testimony whereof I afiix my kgnature. I I

NIKOLA TRBOJ E CH.

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