Lathe design for high and low speed steels

This treatise on the kinematical and dynamical principles governing the construction of metal-turning lathes is based upon the following schemes of work :

(a) Experiments upon the durability of tool-steels, carried out at the Manchester Municipal School of Technology, under the auspices of a joint committee, and published as a Report by the Manchester Association of Engineers in 1903.

(b) Researches upon the cutting forces acting upon lathe-tools, made with a dynamometer constructed in the Manchester Municipal School of Technology, the results of which, partially published as a paper read by one of the Authors at the Chicago meeting of the Institution of Mechanical Engineers, are now given more generally in the form of curves,

(c) Data and particulars collected from practice, by the kind response of many of the best machine-tool makers to the requests of the Authors for information.

These data are all equally necessary and fundamental to the preparation of a treatise on the rational design of lathes. Such rational design has been hitherto non-existent; the proportioning of parts and their kinematic arrangements having proceeded upon purely empirical bases.

Until the advent of high-speed steel the necessity for a theoretical treatment was unfelt. But the new conditions imposed by the general adoption of the high-heat steel were found to have rendered obsolete the long-treasured experience and accumulated data of the tool-maker. A reasoned statement of the problems involved in lathe design, and an attempt to solve them upon a basis of experimentally ascertained fact, had consequently become imperative.

The accident of the Authors having been largely responsible for the obtaining and publishing of these experimental results transformed the task of preparing a treatise upon their application to practical designing into a duty which devolved upon them, in justice, not merely to themselves, but to the Manchester School of Technology, which had borne a large share of the expense incurred, as well as to the engineering profession at large; amongst whom, although many certainly more competent for the work might have been found, none were equally conversant with the general scope and the many details of the great field of research which had been covered.

Thus the present volume forms a natural sequence to the work which has for some years been the staple product of the machine-testing laboratory of the School of Technology. It is offered as a modest example of the way in which the Technical High School of the future may strive to advance professional knowledge and lend its aid to industrial pursuits.

The substance of the book has already appeared in large part in the columns of The Engineer. To the Proprietors and Editor of that journal the Authors wish here to tender their thanks for the many courtesies they have received from them.

They beg further to express their sense of indebtedness to the numerous firms of machine tool makers who have aided them with much-needed information.

In order to be able to design a machine upon scientific principles it is necessary to have an acquaintance with the forces which will be brought to bear upon each of its parts when it is doing its maximum and its average amount of work. The former condition governs the strength and stiffness of its various elements, whilst the latter determines their form and nature, and the materials of which they are made, from the point of view of their durability.

Strength, stiffness, and durability are, however, not the only factors which enter into the question. The conditions of manufacture which fix the cost price, the competition by makers, and the idiosyncrasies of purchasers which settle the sale price, are also very important - frequently paramount - elements in deciding what the design shall be.

It is not sufficient, therefore, to consider the design of a machine tool in its technical and scientific aspect only; the economic or commercial point of view must also be taken into account.

It is known, for instance, that a narrow belt running at a high speed is a more efficient transmitter of power than a broad one at a low speed. This high speed may be obtained by using either large pulleys at a low speed of revolution, or small pulleys rotating rapidly. The former condition is known to give a more efficient drive than the latter; but it does not follow that it ought to be adopted in every case. The extra cost involved in its use may turn the scale of economic advantage against the weight of technical considerations which are in its favour.

In the design of a fast headstock, again, it might at first appear, in these days of high speeds and heavy cuts, to be the whole duty of the designer to obtain not only the greatest possible range of speeds of the lathe spindle, but a variation of speed within those limits as nearly as may be continuous. When, however, the question of first cost, as well as that of economical cutting, is taken into account, it will be found that there is a best ratio of successive spindle speeds for each size of lathe, depending on the complex conditions which obtain in practice ; and the problem is again of the nature of a commercial compromise.

To confine our attention to the technical aspect only of such problems would be, therefore, to condemn in advance the solutions we should obtain; and it is to the constant recurrence of such incomplete treatment of engineering questions, indeed, that we must impute the disrespect with which practical men frequently regard conclusions deduced from "theory." In the last sentence of his preliminary dissertation "On the Harmony of Theory and Practice in Mechanics, ”Rankine says,” The engineer or the mechanic who plans and works with understanding of the natural laws that regulate the result of his operations rises to the dignity of a sage." This is undoubtedly true; but it requires to be emphasized that the phrase "natural laws" has reference quite as much to economic conditions as to considerations merely physical.

In the sequel, wherein we propose to discuss questions of the design and proportions of an article so commonly manufactured as a machine tool, it will, therefore, be the merest axiom of common sense to submit all technically obtained results to the crucial test of their fitness to survive under the normal and well-ascertained conditions of successful commercial production.

Until quite recently machine tools have been produced in a very stereotyped way, the designs having evolved themselves by the processes of trial and error, rule of thumb, and the survival of the fittest. If long and costly, such a process of unintelligent experimenting very frequently leads to a result which it would be difficult to excel. That man would indeed be worthy of the name of sage who should, from the recesses of his brain, excogitate a machine of higher efficiency, greater suitability, and smaller first cost, than any of those in actual use which have been the object of the constant thought and experience of practical men for centuries.

It is only when new conditions arise, such as the passing of the horse, the advent of high-speed steel, or the discarding of reciprocating in favour of rotary prime movers, that an appeal is made to theory or experiment for some assistance in bridging the chasm between past experience and present necessity.

Such a state of things now obtains with regard to machine tools. The rapid development and universal adoption of the new high-speed steel has necessitated the abandonment of the old standard rules of lathe construction and design, which are no longer found equal to the demands of the new material. The volume of results of experience so far compiled by single firms is too small to act as a universal guide in projecting new designs. It therefore appears to be opportune to offer a new series of rules of design based upon a long series of experiments on cutting with the new steels, and checked by numerical data collected from the most recent practice of a number of eminent firms of machine tool makers.

The deductions of a theory sound both technically and commercially cannot but be of value to those engaged in the design and manufacture of a class of machines which is at the present time in a state of transition owing to the new departure in tool steel production. It is hoped also that if the purchaser can be reached, he may be to some extent educated to a higher sense of responsibility in regard to altering standard lines of design, unless for proper and sufficient reason ; and, on the other hand, he will be told what he ought to expect in a lathe of a given size in the way of cutting power, spindle speeds, gear ratios, belt or motor drive, and the numerous details of design on which it is possible to give a verdict for or against.


CHAPTER III

CUTTING FORCES - METHODS OF EXPERIMENTING

In the trials of the Manchester committee, which we described in our second chapter, there appeared an entire lack of uniformity in the shapes and angles of the tools submitted by the eight competing firms of steel makers. There was also no obvious connection between the shapes and angles of the tools and the cutting forces exerted by these tools as deduced in the report from the electrical power measurements made by the committee. Neither did the shape or angle supply a clue to the causes of success and failure in the various durability trials with different tools.

The type of lathe for heavy and rapid cutting, which was soon found to be generally necessary owing to the universal adoption in practice of the improved steel, called, on the other hand, for a reconsideration of the whole question of lathe design from the point of view of the forces exerted by the tool Approximately correct values for the vertical cutting force at the heavier cuts had been obtained by the Manchester committee; but the magnitude of the forces required for the traversing and surfacing movements of the slide rests were quite unknown.

It was also very desirable to have a more definite knowledge of the law of variation of these forces with the shape and size of the cut, and with the shape and cutting angle of the point of the tool.

A special lathe-tool dynamometer was therefore designed and constructed at the Manchester Municipal School of Technology for the more exact measurement of the quantities in question by a direct method, instead of as the relatively small difference of two measurements of electrical Korse-power.

Thanks are due to the authorities of that school for defraying the not inconsiderable expense incurred for power, light, and mechanical assistance, and to Messrs. Sir W. G. Armstrong, Whitworth and Co., for the lathe and materials used in the prosecution of a somewhat extensive research.


CUTTING FORCES - RESULTS

More than one thousand trials on different materials with various shapes and sizes of cut, and with differently formed tools, have been carried out by the authors with the dynamometers above described. A first report, dealing with trials made up to April 1904, was read by one of them at the Chicago meeting of the Institution of Mechanical Engineers. A second report, in which the mass of data accumulated by the other author as the result of about six hundred trials, made partly in the course of laboratory work with students as well as by special research, is now in preparation.

The accompanying Figures, Nos. 8 and 9, give a general abstract in pictorial form of these new results. They allow of certain rough generalizations being made which we require for the purposes here in view.

Fig. 8 refers to experiments made upon Whitworth fluid-pressed steel of medium hardness, and Fig. 9 to those made upon medium-hard cast iron. Both bars were presented to the School of Technology for the purpose of these experiments by Messrs. Sir W. G. Armstrong, Whitworth & Co. Their compositions are closely similar to those of the corresponding bars made use of by the Manchester Committee in 1902.

In both figures the vertical cutting pressures (expressed in tons per square inch) are plotted on two bases: the first being the area of the cut; the second the cutting angle of the tool. (Here the cutting angle means the "included angle" plus the front clearance.) These pressures are represented by the full line curves. The broken line curves give the traversing and surfacing pressures expressed as percentages of the vertical cutting pressures.


CHAPTER V

DURABILITY OF HIGH-HEAT STEEL TOOLS - VARIATION OF CUTTING SPEED WITH ANGLE OF TOOL AND WITH SIZE AND SHAPE OF CUT

In order to understand the nature of the phenomena occurring during the failure of a turning tool, a close study of the mode of formation of the chip was undertaken.

To facilitate matters in this formidable research it was first arranged to take a cut at a very slow speed, so that the action might be observed at leisure. The lathe was driven by taking several turns of a wire rope round one of the cone pulleys and then leading it to a hand winch. When the labourer turned the winch handle, the cutting speed was 1 ft. in from one to five hours, according to the number of back gears in use. Lines having been ruled on the uncut surfaces of the work, forming squares of inside, the distortion of these squares during and after the cut could be easily studied.

The action at high speed is in many respects quite different from what takes place at "dead slow" The successive main shears - or, rather, slides - of the shaving take place at shorter space intervals, for the slipping of the shaving over the tool now takes place continuously, and the cracks are no longer healed up by compression. The tear from each of the cracks is now continued right out to the surface of the shaving. The progression of this tear across the shaving is not instantaneous, however, and by the time it has reached the upper surface the chip has advanced some distance from the point of the tool. By reference to Fig. 13, which shows the variation of the force during chip formation at slow speed, it will be found that the vertical cutting force is greatest between the moment when the tear starts and that when it reaches the surface.


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Lathe design for high and low speed steels