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Read Ebook: Screw-Thread Cutting by the Master-Screw Method since 1480 by Battison Edwin A
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Ebook has 41 lines and 6136 words, and 1 pages
considering to bring the thread to a gentle tapering end as seen in figure 8 by gradually releasing the pressure necessary to keep the tool cutting as the end of the thread was approached.
The development of engineering theory, coupled with the rising needs of industry, particularly with the advent of the Industrial Revolution, brought about accelerated development of screw-cutting lathes through the combination of screw-cutting machines with simple lathes as seen in figure 9 and in detail in figure 11. One important advance shown here is driving the machine by means of a cord or band so that any means of rotary power could be applied, not just hand or foot power. Of greater interest and technical importance to this study is the provision, seen to better advantage in figure 11, for readily changing from one master lead screw to another. This had already been achieved in the Manuel Wetschgi machine, as far as versatility is concerned, although not in quite such a convenient way.
Figure 12, the headstock of another and more advanced lathe than shown in figures 9 and 11 but of the same type, shows "keys" , each of which is a partial nut of different pitch to engage with a thread of mating pitch. The dotted lines in figure 13 show the engaged and disengaged positions of one of these keys, and figure 14 shows the spindle with the various leads, C. At D is a grooved collar to be engaged by the narrow key shown in operating position at the left in figure 12 for the purpose of controlling the endwise movement of the spindle when used for ordinary turning instead of thread-cutting. In return for greater convenience and freedom from the expense of the many separate spindles, as typified by the Wetschgi machine, a sacrifice has been made in the length of the thread which can be cut without interruption.
This reduction in the length that could conveniently be threaded was no great drawback on many classes of work. This can be realized from figure 16 which shows a traverse-spindle lathe headstock typical of the mid-19th century. During the years intervening between the machines of figures 12 and 16, the general design was greatly improved by removing the lead screws from the center of the spindle. This made possible a shorter, much stiffer spindle and supported both ends of the spindle in one frame or headstock rather than in separate pieces attached to the bed. The screws were now mounted outside of the spindle-bearings, one at a time, while the mating nuts were cut partially into the circumference of a disk which could be turned to bring any particular nut into working position as required. With this arrangement, a wide variety of leads either right or left hand could be provided and additional leads could be fitted at any future time. Screw-cutting lathes of this design were popular for a very long time with instrument makers and opticians who had little need to cut screws of great length.
The demands of expanding industry for greater versatility in the production of engineering elements late in the 18th century set the stage for the evolution of more complex machines tending to place the threaded spindle lathes in eclipse. Maudslay's lathe of 1797-1800 appeared at this time when industry was receptive to rapid innovation. Unfortunately, the gearing which once existed to connect the headstock spindle with the lead screw has long been lost. At this time it is quite difficult to say with certainty whether the original gear set offered a variety of ratios, as was true of slightly later Maudslay lathes, or a fixed ratio. The plausibility of the fixed ratio theory is supported by the very convenient means, seen in figure 15, for removing the lead screw in preparation for substitution of one of another pitch. All that is required is to back off its supporting center at the tailstock end and withdraw the screw from its split nut and from the driving clutch near the headstock. This split nut also would have to be changed to one of a pitch corresponding to that of the screw. While more expensive than a solid nut, it neatly circumvents the need to reverse the screw in order to get the tool back to the point of beginning preliminary to taking another cut. David Wilkinson's lathe of 1798 which was developed in Rhode Island at the same time shows the same method of mounting and driving the master screw. At least in the United States, this method of changing the lead screw instead of using change gears remained popular for many years. Examples of this changeable screw feature are to be found in the lathes constructed for the pump factory of W. & B. Douglas Company, Middletown, Connecticut, in the 1830's. Middletown, at that time one of the leading metal-working centers in one of the chief industrial States, had been for many years the site of the Simeon North arms factory which rivaled Whitney's. In this atmosphere, it is reasonable to expect that machinery constructed by local mechanics, as was the custom in those days, would reflect the most accepted refinements in machine design.
Roughly twenty years later, Joseph Nason of New York patented the commercially very important "Fox" brassworker's lathe . While this does have a ratio in the pair of gears connecting the work spindle and master screw, it is clear from the patent that various pitches are to be obtained by changing screws, not by changing gears. The patent sums it up as follows:
A nut upon the end of the stud ... is unscrewed when the guide screw is to be removed or changed. The two wheels ... should have in their number of teeth a common multiple. They are seldom or never removed and their diameters are made dissimilar only for the purpose of giving to the guide screw a slower rate of motion than that of the mandrel whereby it may be made of coarser pitch than that of the screw to be cut and its wear materially lessened.
The introduction of gearing between the spindle and the lead screw, for whatever purpose, could not help but introduce variable factors caused by inaccuracies in the gears themselves and in their mounting. These were of little consequence for common work, particularly when coupled to a screw which, itself, was of questionable accuracy. The increasing refinements demanded in scientific instruments and in machine tools themselves after they had reached a relatively stable form dictated that attention be dedicated to improved accuracy of the threaded components.
An attack on this problem, which interestingly reverts to the fundamental principle of motion derived from a master screw without the intervention of other mechanism , is covered by a patent issued to Charles Vander Woerd, one-time superintendent of the Waltham Watch Company. The problem is well stated in the patent:
This invention relates to the manufacture of leading screws to be used for purposes requiring the highest attainable degree of correctness in the cutting of the screw-threads of said screw ... as, for example, in machines for ruling lines in glass plates to produce refraction gratings for the resolution of the lines of the solar spectrum, such machines being required to rule many thousands of lines on an inch of space by a marking device which is reciprocated over the glass plate and is fed by the action of a leading screw after the formation of each line. Great difficulty has been experienced in constructing a leading screw for this and other purposes, in which the thread is so nearly correct as to produce no perceptible variation in the microscopic spaces between the ruled lines or gratings.... Various causes prevent the formation of a thread on the rod or blank, which is absolutely uniform and accurate from end to end of the rod. Among other causes are the variations of temperature from time to time, the imperfections of the operating leading screw, the springing of the leading screw and of the rod that is being threaded, and other unavoidable causes, all of which, although apparently trivial and producing only slight variations in the thread at different parts of the rod or blank, are of sufficient moment to be seriously considered when a screw of absolute accuracy is desired.
It is interesting to note in figure 19 that Vander Woerd's machine, to avoid the problems outlined in his patent, has returned to a starkly simple design. We are not told, however, how he originated this master screw which is used to produce the accurately threaded work pieces. Later generations, in the search for ever-greater accuracy, also returned to the fundamental simplicity of a master screw as we shall see when we consider the refinements in mechanism necessary to the extended development of the automobile and the airplane.
Attempts to produce gear hobs free of the imperfections and distortions introduced by heat treatment led to another return to the use of the master lead screw. Figure 20 illustrates a machine having this feature which was patented in 1932 by Carl G. Olson. In speaking of the spindle-driving mechanism disclosed in earlier patents, the patent goes on to say:
This driving mechanism includes an integral spindle 20, one extremity thereof being designed for supporting a hob 22 and the other extremity thereof being formed so as to present a lead screw 24. The spindle 20 is mounted between a bearing 26 and a bearing 28, the latter bearing providing a nut in which the lead screw 24 rotates.... From the description thus far given it will be apparent that the rotation of the lead screw 24 within the bearing or nut 28 will cause the hob to be moved axially, the lead of the screw 24 being equal to the lead of the thread in the hob.
Claim 8 which concludes the descriptive portion of the patent states in part:
In a hob grinding machine of the class described, a rotary work supporting spindle, means for effecting longitudinal movement of the spindle, a tool holder for supporting a grinding wheel in operative position with respect to the work supported by the spindle during the rotary and longitudinal movement thereof, ...
Even before this patent was applied for, another patent was pending for the purpose of modifying the pitch of the lead screw without the use of change gears in spite of the wide acceptance of such gear mechanisms for over a hundred years.
Figure 21 shows a plan view of the machine, and figure 22 a detailed view of the sine-bar mechanism actuated by the master screw, 6, to modify the effective pitch of the lead screw in accordance with the realities of practice as stated in the preamble of the patent:
This invention relates to material working machines, and particularly to machines such as hob grinders and the like, wherein the work is reciprocated through the agency of a lead screw.
In the manufacture of hobs it is common practice to employ the same machine for grinding hobs of varied diameters, and in order to employ such a machine in this manner the pitch of the lead screw, thereof, which actuates the work carrier, must conform to the axial pitch of the hob to be ground. This will be readily apparent when it is understood that the helix angles of hobs vary in accordance with their diameters and, consequently, the difference between the normal pitch and the axial pitch correspondingly varies. While the requirement for the normal pitch may be the same for hobs of different diameters, it is necessary to change the axial pitch in accordance with a change in the hob diameter, and this axial pitch of the hob is equal to the pitch of the lead screw which actuates the work carrier in grinding machines heretofore used. Hence, in order to adapt such machines to cover a wide range of leads, it is necessary to provide a large number of interchangeable lead screws and obviously this represents a large investment, and the interchanging of these screws requires the expenditure of considerable time in setting up the machine for each job.
Thread-grinding machines were being designed concurrent with the development of hob-grinding machines. Many were entirely concerned with features peculiar to the problems of wheel-dressing and to automatic characteristics. An invention to embody the use of a master screw and concerned with the precision grinding of worm threads, for use in gearing, was patented by Frederick A. Ward in this era. That part of the invention pertaining to the use of a master screw, "a rotary work holder mounted on said carriage and provided with a driving spindle, an exchangeable master screw and stationary nut detachably secured to said spindle and head,..." is shown in figure 23.
FOOTNOTES:
U.S. patent 10383 issued to Joseph Nason of New York, January 3, 1854.
U.S. patent 293930 issued to Charles Vander Woerd of Waltham, Massachusetts, February 19, 1884.
U.S. patent 1874592, filed June 8, 1929, issued to C. G. Olson of Chicago, Illinois, August 30, 1932, and assigned to the Illinois Tool Works, also of Chicago.
U.S. patent 1901926, filed February 16, 1928, issued to C. G. Olson of Chicago, Illinois, March 21, 1933, and assigned to the Illinois Tool Works, also of Chicago.
U.S. patent 1899654, filed August 31, 1931, issued to F. A. Ward of Detroit, Michigan, February 28, 1933, and assigned to the Gear Grinding Company of Detroit, Michigan.
U.S. GOVERNMENT PRINTING OFFICE: 1964
For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402--Price 20 cents
INDEX
Besson, Jacques, 107
Douglas, W. & B., Company, 113
Maudslay, Henry, 106, 113
Nason, Joseph, 114
North, Simeon, arms factory, 114
Olson, Carl G., 118
Vander Woerd, Charles, 116, 117
Ward, Frederick A., 120
Wetschgi, Emanuel, 108
Wetschgi, Manuel, 108, 111
Whitney arms factory, 114
Wilkinson, David, 113
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