gray iron, as shown in tables II, III and IV. Of course too much cannot be judged from a test on a few samples, but from the figures shown we notice that:

First. The malleable bars machine approximately as easily as the gray iron.

Second. The malleable has almost double the strength of gray iron and an elongation of about 8 per cent as compared to no elongation in the gray iron.

Third.-Malleable castings can easily be supplied to equal this grade of cast steel in all physical tests, and to surpass it so far as smooth surface is concerned. And at the same time the malleable castings will show a superior machinability according to the operation, ranging up to that shown in the drilling test where repeated tests on cast steel showed that it required about four times as long to machine as malleable of similar physical properties.

Discussion

H. A. SCHWARTZ.-Malleable iron consisting of ferrite and free carbon forms an intermediate link in a series of alloys beginning with ingot and wrought irons and ending with very soft gray iron. Since very soft gray iron machines, in most operations, more easily than wrought iron, it may be expected that under any given conditions malleable should be intermediate in machining quality between ingot iron and gray iron. Further the higher carbon malleables should machine more nearly like gray iron than the stronger and more ductile lower carbon malleables. So much can be taken for granted.

The question of primary concern is the magnitude of the difference in machining quality between the poorest malleable which still possesses utility and the best malleable for which there is a demand and which can be commercially supplied. A second question is how much strength can be sacrificed for a given increase in machine shop production and vice versa.

The author's quotations, apparently from Touceda, indi

cating that malleable iron which is made purely from machining quality is usually weak, may well be understood as applying to extreme cases, primarily, and conclusions as to the differences in machineability of two very similar metals cannot be safely drawn from these statements which are made in general terms only.

A significant experience has come under the writer's observation within the past two years. Two foundries were furnishing castings from identical patterns in very large quantities to a given machine shop. Only one form of casting was bought by the manufacturers. Both foundries sold and furnished iron of standard quality (45,000 pounds tensile 72 per cent elongation), and the product was carefully inspected in both plants by disinterested parties.

The product of each foundry was quite uniform and safely above the specifications. That of one foundry averaged about 5000 pounds per square inch, and perhaps 3 to 5 per cent elongation higher than the other. The machine shop found that the better product machined without any difficulty, while the poorer gave a good deal of trouble. Results of this nature based on the machining of many thousands of castings would seem to indicate that a sweeping statement that the stronger of two nearly similar irons will machine less readily cannot be accepted without question.

The author's statement as to the quality of product made by manufacturers of light castings is not necessarily to be ascribed only to a desire to produce greater machining ease. Small castings are usually higher in carbon and silicon in order to secure greater fluidity of metal than is permissible in large castings, hence the physical properties are not so good. It is conceivable that a casting may be used for a purpose, say a barrel bung, in which no strength is required, and accordingly it may be good practice to sacrifice everything to machining.

The striving after production at the expense of quality has, however, great disadvantages. The experience of some automobile concerns who bought details such as wheel hubs on the basis of cost and machineability has in the past been almost disastrous.

The author's threading test measures more or less accurately the pressure of a chip of definite cross section upon the point of the cutting tool. Taylor has shown (Transactions of A. S. M. E., 1907) this pressure, on lathe tools, to be independent of the form or material of the tool or the cutting speed and to be equivalent to the value PDF where P is a constant for any given material, D is the depth of cut and F the feed per revolution. He has shown further that P bears no predictable relation to any chemical or physical property of the material or to its most economical cutting speed.

In view of these observations it is not surprising that the authors were unable to quantitatively correlate tensile properties with the load on their dies more especially in view of the fact that the tests are further complicated by the friction of the dies on the material threaded and by the clogging effect of chips to which the authors refer in their paper.

The tensile strengths in Table I run with two exceptions between 46,620 and 50,650 pounds per square inch; the two exceptions are at the extreme ends of the table as should be expected. That the intervening material is arranged in a somewhat haphazard order is not surprising considering the small range of variation in physical properties. On cast specimens it is very doubtful whether duplicate tensile tests from the same metal could be made alike much closer than 1500 or 2000 pounds per square inch. Certainly a foundry running on a specification of 46.600, the lowest of the series, would frequently overrun 50,600, the highest of the series. Similarly in Table II of the eight malleable samples, six are between 40,300 and 44,200 and should give results closely alike; the higher values are found midway in the machining range.

In Table III there is a somewhat better selection of material available. Numbering the malleable bars from 1 to 10 in the order of their tensile strengths the machining quality runs in the following order: 6, 3, 1, 8, 2, 4, 5, 9, 7, 10. The average of the first three is 3 1/3, of the middle four, 434, and of the upper three, 8 2/3, showing a progressive increase in the load on the tool with increasing tensile strength.

The only direct application of the value of P in the arts would be in the design of machine tools and cutters to prevent their failure by breaking off.

The value of P may, however, be a useful constant for checking up materials since it is easily determined. In the writer's judgment a rough relationship is shown to exist by Taylor's data to the extent that P is high in products machined with difficulty and low in soft ones.

Cutting Speed is Important Consideration

There is, as Taylor states, no predictable connection between the cutting speeds and values of P. Herbert in the Journal Iron and Steel Institute, 1910, has published the results of extended tests showing that the durability of the tool is fixed by its temperature. He has shown that for a given form and material of cutter and a given quality of material to be machined the durability of the tool is constant for constant values of atS where a is the area of the chip, t its thickness and S the cutting speed. He has shown further that his assumptions agree with Taylor's published data. The temperature of greatest durability, i. e., the values of atS3 for greatest durability could not be determined except by direct experiment, in general the durability first increases and then decreases as S increases. In many cases there is a second increase and decrease dependent on the tool steel conditions.

Under these circumstances it is not safe to attempt any prediction of permissible speeds of cut based on the authors' data. The observed differences in load may cause either enormous or negligible differences of cutting speed.

It is extremely unfortunate that the authors dealt so briefly with the surface conditions of their specimens. The column headed "Decarbonized" depth offers the only clue. Strictly speaking there is no such thing as an entirely decarburized depth of any great thickness, the carbon shading off gradually and sometimes more and sometimes less uniformly for the area of constant carbon at the center to the low carbon area at the circumference.

How did the authors determine the boundary between

decarburized and undecarburized metal? More important still what was the condition of any remaining carbon in the machined areas? The presence of amounts of pearlite, of cementite and of temper carbon, all equal to the same amount of carbon would be widely different in effect on machineability. Summary

The writer would express himself as in accord with Smith and Barr's conclusions except in the case of the second statement in group one.

Table II, which indicates the most concordant results, shows that the load in the die for the three weakest irons whose average tensile strength is 41,937 pounds is 1329 pounds, while that for the cther seven, averaging 46,164 pounds, is 1404 pounds. The writer submits that this difference does not raise a sufficient presumption as to machineability in favor of the weaker iron to justify the author's conclusion that 42,000 pounds iron should be adopted where speed of machining is of more importance than great strength.

The tests indicate that the stronger iron will require about 6 per cent more power in machining at equal cutting speeds, but show nothing conclusive as to what the effect, if any, on cutting speeds will be.

The writer would rather draw the conclusion that for the range of properties from 42,000 to 52,000 pounds per square inch, the machineability of malleable is more largely affected by other variables than by the tensile properties.

However, before either this conclusion or that of Smith and Barr can be regarded as definitely established a great Ideal of work will have to be done in developing methods of tests, studying the principles involved in the interpretation of data and investigating a much greater number of materials.

The authors have done pioneer work in a very difficult and complex problem and it is to be hoped that all interested parties will thereby be encouraged to make such investigations and publish such data as may be expected to further our knowledge.