Railroad Gazette/Volume 38/Number 5/High-Speed Tool Steel
Development and Use of High-Speed Tool Steel.[1]
In preparing high-speed steel ready for use the process may be divided principally into three stages: forging, hardening and grinding. It is, of course, very desirable that high-speed steel should be capable of attaining its maximum efficiency and yet only require treatment of the simplest kind, so that an ordinarily skilled workman may easily deal with it, otherwise the preparation of tools becomes an expensive and costly matter, and materially reduces the advantages resulting from its use. Fortunately, the treatment of the rapid steel produced by the author’s firm is of the simplest; simpler, in fact, than ordinary carbon steels or the old self-hardening steels, as great care had to be exercised in the heating of the latter steels, for if either were heated above a blood-red heat, say 1,600 deg. F., the danger of impairing their efficiency by burning was considerable; whereas with the high-speed steel, heating may be carried to a much higher temperature, even up to melting point, it being practically impossible to injure it by burning. The steel may be raised to a yellow heat for forging, say 1,850 deg. F., at which temperature it is soft and easily worked into any desired form, the forging proceeding until the temperature lowers to a good red heat, say 1,500 deg. F., when work on it should cease and the steel be reheated.
In heating a bar of high-speed steel preparatory to forging (which heating is best done in a clear coke fire) it is essential that the bar be heated thoroughly and uniformly, so as to insure that the heat has penetrated to the center of the bar, for if the bar be not uniformly heated, leaving the center comparatively cold and stiff, while the outside is hot, the steel will not draw or spread out equally, and cracking will probably result. A wise rule in heating is to “hasten slowly.”
It is not advisable to break pieces from the bar while cold, the effect of so doing tending to induce fine end cracks to develop which ultimately may extend and give trouble, but the pieces should be cut off while the bar is hot, then be reheated as before and forged to the shape required, after which the tool should be laid in a dry place until cold.
The temperature for hardening high-speed steel varies somewhat according to the class of tool being dealt with.
When hardening, turning, planing or slotting tools, and others of similar class, the point or nose of tool only should be gradually raised to a white melting heat, though not necessarily melted, but even should the point of the tool become to a more or less extent fused or melted no harm is done. The tool should then be immediately placed in an air blast and cooled down, after which it only requires grinding and is then ready for use.
Another method of preparing the tools is as follows:
Forge the tools as before, and when quite cold grind to shape on a dry stone or dry emery wheel, an operation which may be done with the tool fixed in a rest and fed against the stone or emery wheel by a screw, no harm resulting from any heat developed at this stage. The tool then requires heating to a white heat, but just short of melting, and afterwards completely cooling in the air blast. This method of first roughly grinding to shape also lends itself to cooling the tools in oil, which is specially efficient where the retention of a sharp edge is a desideratum, as in finishing tools, capstan and automatic lathe tools, brass-workers’ tools, etc.
In hardening where oil cooling is used, the tools should be first raised to a white heat, but without melting, and then cooled down either by air blast or in the open to a bright red heat, say 1,700 deg. F., when they should be instantly plunged into a bath of rape or whale oil, or a mixture of both.
Referring to the question of grinding tools, nothing has yet been found so good for high-speed steels as the wet sandstone, and the tools ground thereon by hand pressure, but where it is desired to use emery wheels it is better to roughly grind the tools to shape on a dry emery wheel or dry stone before hardening. By so doing the tools require but little grinding after hardening, and only slight frictional heating occurs, but not sufficient to draw the temper in any way, and thus their cutting efficiency is not impaired. When the tools are ground on a wet emery wheel and undue pressure is applied, the heat generated by the great friction between the tool and the emery wheel causes the steel to become hot, and water playing on the steel while in this heated condition tends to produce cracking.
With regard to the hardening and tempering of specially formed tools of high-speed steel, such as milling and gear cutters, twist drills, taps, screwing dies, reamers, and other tools that do not permit of being ground to shape after hardening, and where any melting or fusing of the cutting edges must be prevented, the method of hardening is as follows:
A specially arranged muffle furnace heated either by gas or oil is employed, and consists of two chambers lined with fireclay, the gas and air entering through a series of burners at the back of the furnace, and so under control that a temperature up to 2,200 deg. F. may be steadily maintained in the lower chamber, while the upper chamber is kept at a much lower temperature.
Before placing the cutters in the furnace it is advisable to fill up the hole and keyways with common fireclay to protect them.
The mode of procedure is now as follows:
The cutters are first placed upon the top of the furnace until they are warmed through, after which they are placed in the upper chamber and thoroughly and uniformly heated to a temperature of about 1,500 deg. F., or, say, a medium red heat, when they are transferred into the lower chamber and allowed to remain therein until the cutter attains the same heat as the furnace itself, viz., about 2,200 deg. F., and the cutting edges become a bright yellow heat, having an appearance of a glazed or greasy surface. The cutter should then be withdrawn while the edges are sharp and uninjured, and revolved before an air blast until the red heat has passed away, and then while the cutter is still warm—that is, just permitting of its being handled—it should be plunged into a bath of tallow at about 200 deg. F., and the temperature of the tallow bath then raised to about 520 deg. F., on the attainment of which the cutter should be immediately withdrawn and plunged in cold oil.
Of course there are various other ways of tempering, a good method being by means of a specially arranged gas-and-air stove into which the articles to be tempered are placed, and the stove then heated up to a temperature of from 500 deg. F. to 600 deg. F., when the gas is shut off and the furnace with its contents allowed to slowly cool down.
Another method of heating tools is by electrical means, and by which very regular and rapid heating is obtained; and where electric current is available, the system of electric heating is quick, reliable and economical.
Fig. 1.
One method adopted of electrically heating the points of tools and the arrangement of apparatus is shown in Fig. 1. It consists of a cast-iron tank, of suitable dimensions, containing a strong solution of potassium carbonate together with a dynamo, the positive cable from which is connected to the metal clip holding the tool to be heated, whilst the negative cable is connected direct on the tank. The tool to be hardened is held in a suitable clip to insure good contact. Proceeding to harden the tool the action is as follows:
The current is first switched on, and then the tool is gently lowered into the solution to such a depth as is required to harden it. The act of dipping the tool into the alkaline solution completes the electric circuit and at once sets up intense heat on the immersed part. When it is seen that the tool is sufficiently heated the current is instantly switched off, and the solution then serves to rapidly chill and harden the point of the tool, so that no air blast is necessary.
Fig. 2—Apparatus for Hardening High-Speed Tools by Means of an Electric Arc.
Another method of heating the point of tools is by means of the electric arc, the heating effect of which is also very rapid in its action. The general arrangement and form of the apparatus here employed being as illustrated in Fig. 2.
The tool under treatment and the positive electrode are placed on a bed of non-conducting and non-combustible material and the arc started gradually at a low voltage and steadily increased as required, by controlling the shunt rheostat, care being taken not to obtain too great a heat and so fuse the end of the tool. The source of power in this case is a motor generator consisting of a continuous-current shunt-wound motor at 220 volts, coupled to a continuous-current shunt-wound dynamo at from 50 to 150 volts. Arcs from 10 to 1,000 amperes are then easily produced and simply and safely controlled by means of the shunt rheostat.
Electricity is also a very efficient and accurate means of tempering such forms of tools as milling, gear, hobbing and other similar cutters, also large hollow taps, hollow reamers, and all other hollow tools made of high-speed steel, where it is required to have the outside or cutting portion hard, and the interior soft and tenacious, so as to be in the best condition to resist the great stresses put upon the tool by the resistance of the metal being cut, and which stresses tend to cause disruption of the cutter if the hardening extends too deep.
Fig. 3—Apparatus for Tempering Milling Cutters, etc., Electrically.
By means of the apparatus illustrated in Fig. 3, this tempering or softening of the interior can be perfectly and quickly effected, thus bringing the cutter into the best possible condition to perform rapid and heavy work.
Tempering of hollow cutters, etc., is sometimes carried out by the insertion of a heated rod within the cutter and so drawing the temper, but this is not entirely satisfactory or scientific, and is liable to induce cracking by too sudden heat application, and further because of the difficulty of maintaining the necessary heat and temperature required, and afterwards gradually lowering the heat until the proper degree of temper has been obtained. In electrical tempering these difficulties are overcome, as the rod is placed inside the cutter quite cold, and the electric current gradually and steadily heats up the rod to the correct temperature as long as is necessary, and the current can be gradually reduced until the articles operated on are cold again, and consequently the risk of cracking by too sudden expansion and contraction is reduced very greatly. The apparatus used is very simple, as will be seen by reference to the sketch. It consist of a continuous-current shunt-wound motor directly coupled to a single-phase alternating-current dynamo of the revolving field type, giving 100 amperes at 350 volts, 50 cycles per second, the exciting current being taken from the works supply main.
The power from the alternator is by means of a stepdown transformer, reduced to current at a pressure of two volts, the secondary coil of the transformer consisting of a single turn of copper of heavy cross-section, the extremities of which are attached to heavy copper bars carrying the connecting vices holding the mandrel upon which the cutter to be tempered is placed. The secondary induced current, therefore, passes through a single turn coil, through the copper bars and vices and mandrel.
Although the resistance of the complete circuit is very low, still, owing to the comparatively high specific resistance of the iron mandrel, the thermal effect of the current is used up in heating the mandrel, which gradually attains the required temperature, slowly imparting its heat to the tool under treatment until the shade of the oxide on the tool satisfies the operator.
The method adopted to regulate the heat of the mandrel is by varying the excitation current of the alternator by means of the rheostat. An extremely fine variation and perfect heat control is easily possible by, this arrangement.
Having touched upon the development and thermal treatment of high-speed steel, it will now be opportune to refer to its practical use and to some of the most recent work done with it. It is sometimes contended that on the whole not much advantage or economy results from using high-speed steel, but it is easy to prove very greatly to the contrary, and the author proposes to give some figures and facts as to its use and advantage, not only by knowledge gained from results of his own firm, but also from information supplied by many important engineering establishments as to their present workshop practice, and for which he is indebted.
That great economy is effected is beyond all doubt, from whichever point of view the question is looked at; for it is not only rapidity of cutting that counts, but the output of machines is correspondingly increased, so that a greater production is obtained from a given installation than was possible when cutting at low speeds with the old tool steel, and the work is naturally produced at a correspondingly lower cost, and of course it follows from this that in laying down new plant and machines the introduction and use of high-speed steel would have considerable influence in reducing expenditure on capital account.
It has also been proved that high-speed cutting is economical from a mechanical standpoint, and that a given horse-power will remove a greater quantity of metal at a high speed than at a low speed, for although more power is naturally required to take off metal at a high than at a low speed (by reason of the increased work done) the increase of that power is by no means in proportion to the large extra amount of work done by the high-speed cutting, for the frictional and other losses do not increase in anything like the same ratio as a high-cutting speed is to a low-cutting speed. A brief example of this may be given in which the power absorbed in the lathe was accurately measured electrically.
Cutting on hard steel, with three-sixteenths inch depth of cut, one-sixteenth inch feed and speed of cutting 17 ft. per min., a power of 5.16 h.p. was absorbed, and increasing the cutting speed to 42 ft. per min., the depth of cut and feed being the same, there was a saving in power of 19 per cent. for the work being done.
Another experiment with depth of cut three-eighths inch and traverse one-sixteenth inch compared with one-sixteenth inch traverse and three-sixteenths inch depth of cut, showed a saving in power of as much as 28 per cent., and still proceeding with a view of increasing the weight of metal removed in a given time the feed was doubled (other conditions being the same), and a still further saving of power resulted. In a word, as in the majority of things, so it is with rapid cutting, the more quickly work can be produced the cheaper the cost of production.
Again, as regards economy there is not only a saving effected on the actual machine work, but since the advent of high-speed cutting it is now possible, in many instances, to produce finished articles from plain rolled bars, instead of following the old practice of first making expensive forgings and afterwards finishing them in the machine. By this practice not only is the entire cost of forging abolished, but the machining on the rolled bar can be carried out much quicker and cheaper in suitably arranged machines, quicker even than the machining of a forging can be done.
Many wonderful examples in proof of this can be given. Taking the two articles illustrated below: These were machined from plain rolled bars with high-speed steel in 45 min. and 13 min. respectively, as against 3¾ hrs. and 1¾ hrs. when made from forgings and using ordinary tool steel.
Fig. 4.
Fig. 5.
Another remarkable sample of the gain resulting from the use of high-speed cutting from rolled bars is illustrated in the case of securing bolts, made by the author’s firm, for armor plates. Formerly where forgings were first made and then machined with ordinary self-hardening steel, a production of eight bolts per day of ten hours was usual. With the introduction of rapid-cutting steel, 40 similar bolts from the rolled bar are now produced in the same time, thus giving an advantage of five to one in favor of quick cutting, and also in addition abolishing the cost of first rough forging the bolt to form; in fact, the cost of forging one bolt alone amounted to more than the present cost of producing to required form 12 such bolts by high-speed machining. The cutting speed at which these bolts are turned is 160 ft. per min., the depth of cut and feed being respectively three-quarters inch and one thirty-second inch, the weight of metal removed from each bolt being 62 lbs., or 2,480 lbs. in a day of 10 hrs., the tool being only ground once during such period of work, and from such an example as this it will be at once apparent what an enormous saving in plant and costs results. On the same principle the sleeves for these bolts are produced from bars, 60 being made in one day of 10 hrs., this being even a greater saving on the old system than the bolt example shows.
The lathe on which this work is done is a 12-in. lathe of special design and strength for rapid and heavy cutting, and has a link driving belt 7½ in. wide, running at a very high velocity and driven by its own motor, so that the power absorbed can always be observed whether the lathe is running idle or cutting.
Equally remarkable results are obtained by operating on stock bars with high-speed milling cutters, and one example, among many, may be cited: Hexagon nuts for 3⅜ in. diameter bolts are made from rolled bars, the cutting speed of milling being 150 ft. per min., giving a production of 90 nuts per day, against 30 formerly. More than 90 nuts could have been produced had the machine been more powerful.
Rapid cutting with planing tools has also developed extensively, the old cutting speeds of 15 to 25 ft. per min. being now replaced by those of 50 to 60 ft. per min., and in some cases even as high as 80 ft. per min., and for the same reasons, as already described in lathe turning, the power absorbed does not increase in anything like the same proportion to the extra amount of work done, so that the wear and tear on the machine is not materially increased.
It was for some time not thought possible to plane at such high speeds on account of the tools coming into contact suddenly with the job and running risks of snapping off through shock, but where high-speed steel of proper quality is used this difficulty is overcome, and a good example or two of rapid planing may be quoted. Using a 7-ft. planing machine with two tools operating on forged steel of medium quality, the cutting speed, depth of cut and feed of each tool is respectively 54 ft., one-fourth inch, and one eighth inch, the speed of reverse being 160 ft. per min.
Another striking example of high-speed planing on a large cast-iron turbine body was: Cutting speed 36 ft. per min., depth of cut 1.25 in., and feed 7 cuts per in., the tool cutting for 10 hrs. without necessitating grinding. Two tools were cutting, each taking a cut as described, the size of the planer being 14 ft. × 14 ft. × 30 ft.
The question of cutting angles for tools is an important one, and the author would advise all interested to peruse the paper written by Professor Nicolson, of Manchester, and read before the Institution of Mechanical Engineers at Chicago this year, and in which he states that the best cutting angle as deduced from the results of experiments is 75 deg. for steel and 80 deg. for cast iron. Of course these angles may with advantage be modified according to circumstances and the nature of any particular class of work.
Objections have been made against high speed steel on the ground of its being brittle; but this is not the case where the steel has been properly annealed and the hardening confined to the cutting area, and sufficient support given to the tools when fixed in the machine.
An example of the great pressure-resisting powers of high-speed steel may be given.
When cutting forged steel of about 30 tons per sq. in. tensile strength and offering a resistance to cutting of about 100 tons per sq. in., a tool of 1¼ sq. in. section was used, taking a cut of seven-eighths inch in depth by one-fourth inch feed per revolution, equivalent to an area of metal under cut of 0.21875 sq. in., the cutting speed being 90 ft. per min., and removing 60¾ lbs. of metal per min., or the enormous weight of 4,010 lbs. per hr. The tool in this instance was projecting a distance of 1⅛ in. beyond the rest (see Fig. 4), and a calculation shows the stress on the tool to be as high as 78.5 tons per sq. in. In another case, cutting forged steel of 35 tons tensile strength and offering a resistance to cutting of 115 tons per sq. in., a 1¼-in. square tool being used, the diameter of forging was reduced by 1 in., equal to one-half inch depth of cut, while the tool advanced three-eighths inch every revolution, the cutting speed being 25 ft. per min. and removing 14¼ lbs. of metal per min. With the point of the tool projecting three-fourths inch beyond the rest, the tool successfully withstood a stress of 51.6 tons per sq. in. (See Fig. 5.)
Although in actual practice tools of much greater section would be used, the results clearly show that, if proper care be taken, tools of high-speed steel are quite capable of withstanding any pressure likely to be met in ordinary workshop practice.
A most important point to observe when taking heavy cuts is that of having the tools quite flat on the bottom side and supported as near as possible up to the extreme edge, as by so doing the pressures tending to break the tool are very considerably reduced. For example, the position of the tool as placed in the rest shown in Fig. 4 would cause a stress of something like 78.5 tons per sq. in. to be thrown on it, whereas when the overhang is reduced to one-half of the original distance, equal to nine-sixteenths inch, the stress is lowered to 14.27 tons per sq. in., a reduction of 80 per cent.
Perhaps one of the most unlooked-for developments in the use of high-speed steel has been the manufacture from it of twist drills, and it would be safe to say that in no other sphere has the new steel justified itself to a greater extent than in the operations of drilling and boring, as its powers in that respect have revolutionized completely modern workshop practice. It is now possible in many cases to drill holes through stacks of thin steel plates as quickly and economically as by punching them, thus avoiding the consequent liability to distress the material due to punching action.
The plates of torpedo and other boats, which are comparatively thin and of high tensile strength, can now be drilled in stacks with such facility that it is no longer necessary to punch the holes, whilst in many articles where it was formerly the practice to core in the holes, as, for example, in cylinder and other covers, or pipe flanges, etc., it is now cheaper and quicker to use high-speed steel and drill the holes out of the solid.
A considerable amount of doubt has been thrown from time to time on the inability to take finishing cuts with high-speed steel, and in the early stages of its development this contention was to a large extent justified, but experience and practice have brought the steel into line and rendered it possible to obtain an excellent finish at high speeds with tools suitably formed and properly arranged in the machines. Some very good examples of finished bright work at high speeds have been made mostly in semi-automatic machines, high-speed steel being used and one cut only taken, the surface finish being most excellent.
This finishing quality of high-speed steel is especially advantageous for tools used in automatic and capstan lathes, because it enables the work to be produced so very much more rapidly; and also, on account of the great resistance of the steel to wearing action, greater accuracy is insured.
As regards the quality of retaining a sharp edge, high-speed steel makes excellent razors, and will long retain without sharpening an extremely keen cutting edge. The author may add that it is thus now possible to those whose time is precious to indulge even in “high-speed shaving.”
The author hopes that the few facts he has given as to the use and development of high-speed steel may indicate some of its uses and progress, but he can scarcely refrain from remarking that many are saying. and rightly so, “Yes! these results are very remarkable; but what of the machines to perform such prodigious work?” and this leads him to speak before concluding as to how one important development often leads up to another of even greater magnitude, and that is in this case the complete revolution in the design of machine tools to cope with the extraordinary increased cutting powers of the latest rapid cutting steels.
It is impossible that the design of machine tools can remain on the old lines, since the difference between them and the cutting powers of the steel is so abnormal, and a sphere of immense area for the redesigning of machine tools is opened out to the ingenuity of the world’s engineers.
That much has been already done is admitted, but the work is naturally of such a nature that only time and experience will accomplish, gradually enabling as nearly as possible the relative powers of the steel and machines to be equated.
In the machine tool department of the author’s firm, this branch of the subject of remodeling their tools has received the closest attention, and a type of their modern 18-in. center lathe for high-speed cutting may be mentioned. It is capable of exerting 65 h.p. equivalent to a belt width of 12 in., and with the aid of a variable speed motor a range of cutting speeds from 16 to 400 ft. per min. is possible, this comparing with an old-type 18-in. lathe having a belt of 4-in. width, and capable of exerting only about 12 h.p.
In a similar way the old types of planing, milling, drilling machines, etc., are all more or less obsolete, and new designs are already constructed to cope with work at speeds and feeds described in this paper.
It is indeed a pleasure to see the new type of machine tool operating with high-speed steel, and treating the work it has to turn out in such a businesslike way, throwing off shavings from steel and iron as one usually sees in turning wood, and imparting a life and energy to the whole establishment in remarkable contrast to the sleepy rate at which metals used to be turned and machined for so many years past, thus exerting an influence on everybody therein to get “a hustle” on that is positively exhilarating in its effects.
- ↑ Extract of a paper by J. M. Gledhill at the New York meeting of the Iron and Steel Institute, October, 1904.