Mr. Stetson-For ourselves we have been to considerable expense to get a perfect thread to start with. The only thing in this line that I have seen was in one of the technical papers, and it was something on this principle; that the error in the thread was found, and then a system of calculations made: something that would, first, gear up the lathe for cutting a screw and the gearing then applied to overcome the error of the thread, so that it produced a perfect thread. Prof. Egleston-That is not it.

Mr. Sellers-The best threads in the country are only an approximation to correctness. I know of no true thread. The thread we use was copied from one purchased from Mr. Whitworth some years ago. The pitches are not even for the whole length of it. A screw to be cut absolutely correct would have to be cut under water or in some way that temperature may not vary; but the best way, perhaps, to correct a thread is, that of causing the nut to vibrate back and forth, about the axis of the screw, following a curved line of a former-bar which corrects it. That is, you cause a division to an extent that is necessary to overcome the error that is found to exist in the thread itself, and by that means it is possible, in a measure, to correct the thread. Some lead-screws have been cut very carelessly indeed, while others are near enough right to answer all practical purposes. I think it is safe to say that the lead-screws of most of the first-class makers of lathes will compare very favorably with one another, and that screws cut in them will be sufficiently accurate for all practical purposes.

Mr. Weightman-In manufacturing dies, we have first to fall back on the gauges. When we have tested the accuracy of the gauges, and compare the bolts, they too disagree. Let two manufacturers, making bolts, start with the same gauges, and we shall find at the end of six months that the bolts produced are of different gauge.

TRANSACTIONS OF THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS]

AN ADAPTATION OF BESSEMER PLANT TO THE BASIC PROCESS.

Read at the Annual Meeting, 1880.

BY A. L. HOLLEY, MEMB. INST. C. E., ETC.

The maintenance of refractory linings in Bessemer converters, in such a way as to promote regular and maximum production, has been the subject of more experimenting than any other feature of the Bessemer system, and it is still the least perfect and satisfactory feature, excepting perhaps the casting of steel. Linings are not only eroded by the mechanical action of the charge, but they are chemically decomposed by its various slags. The silica linings usually employed have, indeed, been so improved that an average of, say, sixty charges per twenty four hours can be got out of a pair of converters, and the shifting of interchangeable converter bottoms (containing the tuyeres) is so rapid that it does not delay production; but the repairing of the fixed lining just above the tuyeres, where both mechanical and chemical action are most severe, is frequently the cause of delay, and the operation rapidly performed between heats is tedious and costly. The accumulations of slag on other parts of the lining must also be quarried out, else the converter will become too small for the charge. These are the conditions of maintaining silica linings; but the difficulties are increased, probably about threefold, when the linings are made of lime, for the basic process. The basic process consists in removing phosphorus from the iron under treatment, by retaining the phosphorus oxidized by the blast, in a basic slag formed of, say, twenty per cent. of lime added to the charge. An acid (silica) lining would vitiate the basic slag, and would also be rapidly destroyed by it. Lime containing some magnesia, and produced by burning magnesian limestone (dolomite), is at present the only basic material successfully used for converter linings. It is usually made into bricks, which are hard burned and built up with mortar of similar material to form the lining.

Basic bottoms and tuyeres stand ten to fifteen charges, nearly equaling acid bottoms, and they may be readily changed; but basic linings, near the tuyeres, and also in other parts where abrasion is severe, wear rapidly and must be frequently repaired by cooling the converter and inserting new bricks, or patching in some suitable manner. The converter is thus put out of use for at least twenty-four hours ―a very serious delay to production. From a wide observation, the

author feels safe in saying that a basic lining is rarely run above sixty charges without extensive repairs, and in some works repairs are made every time a bottom is set. With some irons there is also an accumulation of slag around the mouth of the converter; its removal sometimes also causes further delay.

The output of a pair of converters in Europe averages about half that of a pair of converters of the same size in the United States, and is often less than half. The limited endurance of basic linings in Europe is, therefore, a less conspicuous defect than it is here, where one converter must make twenty-five or thirty charges in twenty-four hours, so that the repairs of basic linings, as at present conducted, would keep an American plant idle half the time. This delay is really as important in Europe as it is here; the greater the output from a given plant, the cheaper the product.*

In order, therefore, that the basic process may come into extensive use, basic linings must be so maintained that their output will nearly equal that of acid linings.

There are two reasonable conditions of improvement: the one is to prolong the endurance of basic materials, so that their repairs can be made with little delay, while the converter is in position for use. There seems to be little or no progress, or probability of immediate progress, in this direction. The other is the rapid and complete removal of a worn lining and the replacement of a repaired one. A third system, seriously proposed, is to double or treble the entire converting plant. The only practicable way to replace a refractory lining (which cannot be handled by itself), is to replace the vessel which contains it. The worn portions of the lining may thus be repaired at leisure, in another part of the works, rather than in position for use, where repairs would retard output.

An obvious way to replace an entire converter lining is to replace the entire converter. This system is already under construction in Europe. The method is also obvious-lifting the converter bodily out of its pillow blocks, and conveying it to the repair shed by means of an overhead traveler; then setting a repaired converter in place by the same means. Such a plant is doubtless cheaper than a duplicate plant, and its output should be materially greater than that of fixed converters. But the operation of changing an entire converter must be slow and tedious. When the arrangement is such that pillow block caps are required, these must be loosened by unscrewing heavy nuts; then they must be made fast to the crane chain, lifted, traversed

*The statement, sometimes made in England, that the rapid production in America impairs quality of product is but a cover for inadequate plant. Steel is obviously no better because five hours instead of one are consumed in setting a vessel bottom, or because it may take twice as long in an English works to handle materials and product.

and set down. The blast pipe connection must be broken, and possibly some platforms must be removed. Then the traveler is placed. exactly centrally over the converter, ponderous chains are made fast, the mass is raised high enough to clear surrounding parts, and drawn laterally to the repair shed; then the converter is placed centrally over its seat and lowered and steadied (as it swings from a chain) into its pillow blocks. The repaired converter is raised, traversed and set in place by repeating all these operations; the blast connection is then made, and the pillow block caps are lifted, traversed, steadied into place and screwed down. If the converter is removed in sections, transferring each section and making the refractory joints will occupy much more time. The chimneys and the openings in the side of the building must be high enough to make passage not only for the traveler but for the converter when lifted out of its seat, and for the chains that sustain it. A traveler of the required power, height and length, is obviously a ponderous and costly structure, and to work with reasonable speed it must have independent steam power -the hydraulic system of the works cannot well reach it.

The method of replacing the lining proposed by the author, and shown in the engravings, is removing only the shell of the converter; lowering it out of the trunnion ring easily and rapidly, by means of a simple lift and car, and replacing a repaired shell by the same means. No pillow block caps, blast connections, nor other surrounding parts are touched; a dozen cotters are knocked out, the shell is lowered and run straight back to the repair shed, the new shell is run in, lifted and cottered on; this is all. The machinery and transference are on the general level, and not forty feet or more up in the air. The car may be moved by a small reversing engine or by a hydraulic capstan, by means of a wire rope and sheaves suitably arranged. The car runs against a stop, and the lift is perfectly vertical, so that the shell may be put in place by two rapid motions without the delay of adjustment.

The lining may be heated before the shell is put in place, and bottoms (and tuyeres) may be separately removed, as at present, or they may be taken away with the shell and repaired without removal from it. In the latter case, the shell must be placed in trunnions, in the repair shed, so that the bottom may be turned downward for repairs. But if the bottom is first removed, the shell need not be placed in trunnions in the repair shed; the shell will stand mouth downward on the car, a position most favorable for repairing both the mouth and the lining about the tuyeres, which are the two places chiefly needing repairs. This is doubtless the better plan, and it saves the cost of supplementary trunnion rings and turning gear. The engravings show the converter hung so high above the general level that

author feels safe in saying that a basic lining is rarely run above sixty charges without extensive repairs, and in some works repairs are made every time a bottom is set. With some irons there is also an accumulation of slag around the mouth of the converter; its removal sometimes also causes further delay.

The output of a pair of converters in Europe averages about half that of a pair of converters of the same size in the United States, and is often less than half. The limited endurance of basic linings in Europe is, therefore, a less conspicuous defect than it is here, where one converter must make twenty-five or thirty charges in twenty-four hours, so that the repairs of basic linings, as at present conducted, would keep an American plant idle half the time. This delay is really as important in Europe as it is here; the greater the output from a given plant, the cheaper the product.*

In order, therefore, that the basic process may come into extensive use, basic linings must be so maintained that their output will nearly equal that of acid linings.

There are two reasonable conditions of improvement: the one is to prolong the endurance of basic materials, so that their repairs can be made with little delay, while the converter is in position for use. There seems to be little or no progress, or probability of immediate progress, in this direction. The other is the rapid and complete removal of a worn lining and the replacement of a repaired one. A third system, seriously proposed, is to double or treble the entire converting plant. The only practicable way to replace a refractory lining (which cannot be handled by itself), is to replace the vessel which contains it. The worn portions of the lining may thus be repaired at leisure, in another part of the works, rather than in position for use, where repairs would retard output.

An obvious way to replace an entire converter lining is to replace the entire converter. This system is already under construction in Europe. The method is also obvious-lifting the converter bodily out of its pillow blocks, and conveying it to the repair shed by means of an overhead traveler; then setting a repaired converter in place by the same means. Such a plant is doubtless cheaper than a duplicate plant, and its output should be materially greater than that of fixed converters. But the operation of changing an entire converter must be slow and tedious. When the arrangement is such that pillow block caps are required, these must be loosened by unscrewing heavy nuts; then they must be made fast to the crane chain, lifted, traversed

* The statement, sometimes made in England, that the rapid production in America impairs quality of product is but a cover for inadequate plant. Steel is obviously no better because five hours instead of one are consumed in setting a vessel bottom, or because it may take twice as long in an English works to handle materials and product.