The above table was furnished by the superintendent of a mill in Connecticut in 1894. He says: While each mill has its own proportions for various mixtures, depending upon the purposes for which the product is intended, the figures given are about the average standard. Thus, between cartridge brass with 33% per cent zinc and common high brass with 38 per cent zinc, there are any number of different mixtures known generally as "high brass," or specifically as "spinning brass," "drawing brass," etc., wherein the amount of zinc is dependent upon the amount of scrap used in the mix. ture, the degree of working to which the metal is to be subjected, etc.

15-100

16-100

17-100

BB. 18-100

BBB 19-100

20-100

No. C

SCREW-THREADS, SELLERS OR U. S. STANDAR In 1864 a committee of the Franklin Institute recommended the ado of the system of screw-threads and bolts which was devised by Mr. Wi Sellers, of Philadelphia. This same system was subsequently adopte the standard by both the Army and Navy Departments of the United St and by the Master Mechanics' and Master Car Builders' Association that it may now be regarded, and in fact is called, the United States St ard.

The rule given by Mr. Sellers for proportioning the thread is as follo Divide the pitch, or, what is the same thing, the side of the thread, eight equal parts; take off one part from the top and fill in one part in bottom of the thread; then the flat top and bottom will equal one eight the pitch, the wearing surface will be three quarters of the pitch, and diameter of screw at bottom of the thread will be expressed by the mula

Diar

eter

Screw-Threads, Whitworth (English) Standard.

15-16 9

U. S. OR SELLERS SYSTEM OF SCREW-THREADS.

LIMIT GAUGES FOR IRON FOR SCREW THREADS. In adopting the Sellers, or Franklin Institute, or United States Standard, as it is variously called, a difficulty arose from the fact that it is the habit of iron manufacturers to make iron over-size, and as there are no over-size

Pitch.

1%

214

4

31

Diam.

312

Pitch.

screws in the Sellers system, if iron is too large it is necessary to cut it away with the dies. So great is this difficulty, that the practice of making taps and dies over-size has become very general. If the Sellers system is adopted it is essential that iron should be obtained of the correct size, or very nearly 80. Of course no high degree of precision is possible in rolling iron, and when exact sizes were demanded, the question arose how much allowable variation there should be from the true size. It was proposed to make limitgauges for inspecting iron with two openings, one larger and the other smaller than the standard size, and then specify that the iron should enter the large end and not enter the small one. The following table of dimensions for the limit-gauges was commended by the Master Car-Builders' Association and adopted by letter ballot in 1883.

Caliper gauges with the above dimensions, and standard reference gauges for testing them, are made by The Pratt & Whitney Co.

THE MAXIMUM VARIATION IN SIZE OF ROUGH IRON FOR U. S. STANDARD BOLTS.

Am. Mach., May 12, 1892.

By the adoption of the Sellers or U. S. Standard thread taps and dies keep their size much longer in use when flatted in accordance with this system than when made sharp "V," though it has been found advisable in practice in most cases to make the taps of somewhat larger outside diameter than the nominal size, thus carrying the threads further towards the V-shape and giving corresponding clearance to the tops of the threads when in the nuts or tapped holes.

Makers of taps and dies often have calls for taps and dies, U. S. Standard, "for rough iron."

An examination of rough iron will show that much of it is rolled out of round to an amount exceeding the limit of variation in size allowed.

In view of this it may be desirable to know what the extreme variation in iron may be, consistent with the maintenance of U. S. Standard threads, i.e., threads which are standard when measured upon the angles, the only place where it seems advisable to have them fit closely. Mr. Chas. A. Bauer, the general manager of the Warder, Bushnell & Glessner Co., at Springfield, Ohio, in 1884 adopted a plan which may be stated as follows: All bolts, whether cut from rough or finished stock, are standard size at the bottom and at the sides or angles of the threads, the variation for fit of the nut and allowance for wear of taps being made in the machine taps. Nuts are punched with holes of such size as to give 85 per cent of a full thread, expe rience showing that the metal of wrought nuts will then crowd into the threads of the taps sufficiently to give practically a full thread, while if punched smaller some of the metal will be cut out by the tap at the bottom of the threads, which is of course undesirable. Machine taps are made enough larger than the nominal to bring the tops of the threads up sharp, plus the amount allowed for fit and wear of taps. This allows the iron to be enough above the nominal diameter to bring the threads up full (sharp) at top, while if i is small the only effect is to give a flat at top of threads; neither condition affecting the actual size of the thread at the point at which it is intended to bear. Limit gauges are furnished to the mills, by which the iron is rolled the maximum size being shown in the third column of the table. The minimum diameter is not given, the tendency in rolling being nearly always to exceed the nominal diameter.

In making the taps the threaded portion is turned to the size given in the eighth column of the table, which gives 6 to 7 thousandths of an inch allowance for fit and wear of tap. Just above the threaded portion of the tap a

place is turned to the size given in the ninth column, these sizes being the same as those of the regular U. S. Standard bolt, at the bottom of the thread, plus the amount allowed for fit and wear of tap; or, in other words, d' U. S. Standard d+ (D' — D). Gauges like the one in the cut, Fig. 72, are furnished for this sizing. In finishing the threads of the tap a tool

FIG. 72.

Tap Gauge

is used which has a removable cutter finished accurately to gauge by grinding, this tool being correct U. S. Standard as to angle, and flat at the point. It is fed in and the threads chased until the flat point just touches the portion of the tap which has been turned to size d'. Care having been taken with the form of the tool, with its grinding on the top face (a fixture being provided for this to insure its being ground properly), and also with the setting of the tool properly in the lathe, the result is that the threads of the tap are correctly sized without further attention.

It is evident that one of the points of advantage of the Sellers system is sacrificed, i.e., instead of the taps being flatted at the top of the threads they are sharp, and are consequently not so durable as they otherwise would be; but practically this disadvantage is not found to be serious, and is far overbalanced by the greater ease of getting iron within the prescribed limits; while any rough bolt when reduced in size at the top of the threads, by filing or otherwise, will fit a hole tapped with the U. S. Standard hand taps, thus affording proof that the two kinds of bolts or screws made for the two different kinds of work are practically interchangeable. By this system

iron can be .005" smaller or .0108" larger than the nominal diameter, or, in other words, it may have a total variation of .0158", while 13" iron can be .0105 smaller or .0309" larger than nominal-a total variation of .0414"~ and within these limits it is found practicable to procure the iron. STANDARD SIZES OF SCREW-THREADS FOR BOLTS AND TAPS, (CHAS. A. BAUER.)

Inches. Inches Inches. Inches. Inches. Inches. Inches. Inches.

14 20 .2608

.1855

.0379 .0062 .006

.2668 .1915

.2024

5-16 18 .3245

36 16 .8885

.006 .0078 .006

7-16 14 .4530