So far as I know, there is no formula for determining this practical clearance diameter for the different metals, though one might readily be deduced, while, for the sizes mentioned, the arbitrary range of differences seems to cover the ground for either cast iron, steel or wrought iron.

Mr. Oberlin Smith.-We all know that, in order to get good results, we drill the hole in cast iron rather larger than the diameter of the bottom of the tap. Now cast iron is weaker than wrought iron or steel. We usually put in cast iron a longer tapped hole than we do in wrought iron, and yet we put less depth of thread there, for this incidental reason of "crumbling." We are making the thread weaker by construction, although the material is weaker to start with. It seems to me that we ought to make these tapping holes in cast iron just as small as we possibly can, so as to get the proper clearance. The allowance Mr. Bond speaks of is undoubtedly a good enough one, unless we should, for greater convenience and ease of remembering, take a constant for that difference, for instance a constant of three or four thousandths, or perhaps even two thousandths would be sufficient. We should adopt some such constant, or some series of figures, as a regular standard, and work to it, just as much as we should have standards regarding the threads themselves But I do not think that we ought to make a difference in this constant or series for wrought iron and steel and such stronger metals, and thus have a confusion of tapping-drill sizes. We should not have one tapping size for brass, and one for steel, and one for wrought iron, and one for cast iron, thus making great trouble and confusion, but we should have one standard for all materials. Steel and wrought iron, naturally, have stronger threads than does cast iron, and this enables us to use shorter nuts without needing the little additional advantage gained by being able to make the thread slightly deeper, because it will not crumble. In regard to having a standard with different tapping-drill sizes for different materials, I should certainly deprecate such action very strongly, on account of the great number of different drills necessary, and the number of standards to be recorded.

Mr. W. M. Barr.-I have just been experimenting with this very thing, and these experiments extended over several months. I do not find that the trouble is nearly so much with the drill as it is with the tap. There is no difficulty whatever in drilling and tapping holes in wrought iron or steel which will be all right, so

that the studs and bolts will interchange; but in regard to cast iron, I have not yet been able with standard taps to make studs or standing bolts that will interchange between cast iron and wrought iron, or between cast iron and steel. The difficulty seems to be that when we tap a hole in cast iron the finished hole is perceptibly larger than the tap. What I wanted to secure was one size of screw thread for everything, that is, the same bolt to interchange indifferently in any part of the machine and in any material; that I find is not practicable, and we have given it up. Now the trouble with an ordinary drill is, that it always drills a hole larger than itself, and the reason for that is mainly in the grinding. I have tested quite a number of twist-drill grinding machines, and I only know of one that will grind a drill which will drill a hole through a piece of metal without any perceptible enlargement of the size of the hole over that of the drill, and that is because the machine has a center grinding attachment by the use of which we are sure that the center of the drill is so ground that it is equidistant from all parts of the cir cumference. Apart from the drilling, we do not find it practicable to use the same size for threads, but are obliged to have two sizes for the same nominal size of standing bolt or stud, that is to say, the standing bolts or studs, that are made to fit into cast iron, have to be slightly larger in diameter of thread than those that fit into wrought iron tapped holes. This is a subject which interests all machine builders, it is a very important one, and one which ought to receive consideration in our society, because, if it is necessary to have two sizes, I think that we might perhaps, by throwing our experiences together, fix upon what sizes are necessary for cast iron, and see what can be done by suggesting a standard covering ordinary service.

Mr. Oberlin Smith.-It seems to me that Mr. Barr has adopted two standards, two diameters for his studs; one for those which go into cast iron and the other for those which go into wrought iron. To do that he has to have two sets of finishing dies which give the final size and compass of studs. Why wouldn't it be just as easy to have lots of studs all alike and have two kinds of taps, and then he would only have one standard? If a tap makes cast iron larger,-if that is the case that a tap will cut a larger hole in cast iron than in wrought iron, why not have a slightly smaller tap for the cast iron, so that the final result would be the same in the two materials? I doubt not the tap makers like Mr. Bond

will make those two standards when we find out how much difference there ought to be in them, or perhaps by the taps that are made, and which differ slightly on account of the little differences in tempering, etc.—perhaps there are some given number of standard taps some of which could be selected slightly smaller, which could be laid aside for cast iron taps, and those which had been used might be kept for cast iron, and those new only for wrought iron.

Prof. Sweet.-I wish to ask Mr. Barr whether the tapped holes go through or bottom in the iron.

Mr. Barr.-They do not go through.

Prof. Sweet.-Have you ever tried the plan of chambering out the hole? While I am not able to say that we get all holes of a size, we find chambering out the holes to be a very good arrangement indeed. We call the tool used in that way the "wobble drill." I do not know that any one uses them but ourselves, but they would if they knew their value and they could be found for sale.

Mr. Whitehead.-I would like to ask Mr. Barr what difference he would expect there would be in tapping steel and in tapping cast iron.

Mr. Barr.-I have not measured that. The question of taps and dies resolves itself into this: the Pratt & Whitney Co. have brought out a standard thread and a gauge, and that is the thing we work by; if two sizes of taps are used the one size may be too large and the other may be too small. There is no certainty that they will do what is expected of them at all; but a die is a variable thing. You can get almost any diameter out of a die. Now if you have a standard tap it is a very easy matter to so adjust the dies that you can get a proper diameter for screwing into cast iron. Therefore I am of an opinion that it is not a good plan to have two sizes of taps. Now in regard to tapping through cast iron, most of our work does not go through; most of it goes in one and one-half diameters, and there we stop. We do not do the chambering that Professor Sweet speaks of, but we do run the drill down a little further, so that when we screw in the bottoming tap it does not crowd on the taper that is left in the bottom of the hole by the taper tap. Now, what was your question, Mr. Whitehead?

Mr. Whitehead.-My question was what difference would you expect it to be in a screw tap of cast iron and one of steel tapping the same metal?

Mr. Barr.-That I cannot say; all I know is that using the same standard, the Pratt & Whitney standard, we will say, one that will fit in a hole tapped in steel, will be too loose to passinspection in a hole tapped in cast iron. What the difference is I do not know. The only thing I do know is that the work will not pass inspection.

Mr. Whitehead.-I should think that would apply more particularly to gauge work.

Mr. Barr.-No, sir, it would not; it applies to our regular, every-day work.

Mr. Bond.-It seems to me that the trouble due to the variation of size of tapped holes drilled in cast and wrought iron depends largely on the character of the tap used, because in the use of a new form of tap which has been made by us for some years for locomotive work, in which a "division of labor" is secured, the use of three taps, the first having only about one-third the depth of the U. S. Standard thread, the second about two-thirds, and the third tap the finishing size, thus dividing the work of the complete tapping equally between the three, greater uniformity has resulted in the size of the finished holes, and the loss in breakage of taps reduced to a minimum.

This form of tap is well liked by locomotive builders, and it seems to more nearly fulfill the requirements for cast iron, steel and wrought iron than the old system of using the taper, plug and bottoming taps, so familiar to all iron-workers and machinists. With these taps there is less of the crowding which tends to change the "lead" of the thread or of wearing out the thread in cast iron tapped holes.

I think taps made this way-and we are making many of them —would work as nearly uniform in different metals as it is possible to have them, and with the bolts and taps made carefully to standard gauges practical interchangeability will be readily secured.

Mr. O. C. Woolson.—I would ask Mr. Barr if the method of tapping in cast iron was the same as used in tapping wrought iron and steel? The practice in some shops of using a guide in starting a tap, in fact keeping a guide there until the tapping is finished, in a large majority of work, is a very practicable one. You suggest that to a common mechanic and he does not like it; but I will suggest this, that in tapping cast iron the quicker and smoother and truer you can get your tap in and get it out the

tighter your bolts are going to fit. In tapping with power there should be a certain amount of flexibility between spindle and tap. The wobbling of the tap without a guide will cut the cast iron hole out a little more than you want very quickly, although you may hardly appreciate that you are enlarging that hole. It is possible to make a fit with the same tap tighter in cast iron than in wrought iron, under certain conditions of workmanship..

Mr. Oberlin Smith.-I want to say that I agree entirely with Professor Sweet in thinking that a good many tapping holes in cast iron and other metals should be chambered out at the bottom. Then we have a clear hole, just the same as if it went entirely through a thin plate. Instead of using a "wobble-drill," as described, I used to take an ordinary flat drill and grind out a little notch on the corner of the grindstone. By grinding one lip away more than the other, thus throwing the center over, you get in effect what Professor Sweet speaks of as a "wobble-drill." This eccentric point striking the center of the hole as already drilled will throw the drill over gradually, and the projecting point will cut the chambering of the hole. This worked very well in the oldfashioned and aged drill-presses, because their spindles were generally an eighth of inch loose, but in some of our modern drill-presses we cannot obtain the necessary wobble. Here is a device that I got up two or three years ago (Fig. 72); a steel bar, perfectly plain, the diameter of the tapping-drill, with a little slot that went nearly through from one side, in which was a small steel cutter hung on a pivot. As soon as this is forced down in the bottom of a hole, this point strikes the conical surface of the hole and creeps over to the position shown in the second sketch, so that it cuts out a chamber of the kind desired. The whole thing runs true and does just what you want it to do. [Applause.]

Fig. 72.

Prof. Sweet.-I can hardly admit what friend Smith would imply that we use the old-fashioned drill. We do not make the drill as he has indicated, neither do we finish the hole as he has indicated. We depend upon the elasticity of the shank to spring what little is necessary. Fig. 90 shows the form of the tool. With such a tool, the hole has the appearance at the bottom shown