Table for Small Screw of Fine Angular Threads.*

Diameters in vulgar fractions of the inch

1/2

15/32

7/16

13/32

3/5

11/32

5/16

9/32

1/4

7/32

1/5

Diameters in hundths of the inch nearly

.50

.47

.44

.41

.37

.34

.31

.28

.25

.22

.20

Number of threads to the inch . . .

16

18

18

20

20

24

24

28

28

32

36

Diameters in hundredths of the inch .

.18

.16

.14

.12

.10

.09

.08

.07

.06

.05

.04

Number of threads to the inch . . .

36

40

40

48

48

56

56

64

72

80

100

" This table was arranged by Mr. Chidson, of Liverpool, who made, first, a set of coarse angular thread taps from 1/4 to 1 inch, agreeably to the terms of Mr. Whitworth's table, giving to the screw tool the angle of 55 degrees, and also a set of square thread taps, of the same diameters, and, as usual, of twice the pitch. This led Mr. Chidson to set out and construct a series of finer and deeper threads, from 1/2 inch to 14 hundredths diameter, agreeably to the arrangement in the second table, and with screw tools of the angle of 45 degrees.

I have great pleasure in stating my individual opinion of the suitability of the table to its intended purpose, and on comparing the screws with those of similar diameters used by Holtzapffel and Co., I found about one third to be nearly identical in pitch, one third to be slightly coarser, and the others slightly finer. As regards the workmanship of these taps, made by Mr. Chidson for his own use, and principally with his own hands, by means of the change wheels and single point tools, it gives me great pleasure to report most favourably.

The tables above given, and which have been selected and not calculated, will serve to explain the inapplicability of the mode of calculation proposed in various popular works; namely, for angular thread screws, to divide the diameter by 8 for the pitch, when, it is said, such screws will all possess the angle of 3 1/4 degrees nearly; and for square threads to divide by 4, thus giving an angle of 7 degress) nearly; therefore

Angular thread screws of would have pitches of or rates of which differ greatly from

8

6

4

2

1

1/2

1/4

inches diameter.

1

3/4

1/2

1/4

1/8

1/16

1/32

inches rise.

1

1 1/3

2

4

8

16

32

threads per inch.

2 1/2

3

4 1/2

8

12

2

Whitworth's observational numbers.

By the use of the constant divisor 8, the one-inch screw agrees with Whitworth's table, the extremes are respectively too coarse and too fine; as instead of 8 being employed, the actual divisors vary from about 5 to 16, and therefore a theoretical mods would probably require a logarithmic schema Bat were this followed out with care, the adjustment of the fractional threads so obtained, for those of whole numbers, would completely invalidate the precision of the rule; and the result would not bo in any respect better than when adjusted experimentally, as at present.

There is little doubt that if we could entirely recommence the labours of the mechanist, or if we could sweep away all the screwing tools now in use, and also all the existing engines, machines, tools, instruments, and other works, which have been in part made through their agency, these proposed scales, or others not greatly differing from them (as the choice is in great measure arbitrary), would be found of great general advantage; the former for the larger, the latter for the smaller works. But until all these myriads of objects are laid on one side, or that repairs are no longer waanted in them, the old tools must from absolute necessity be retained, in addition to those proposed in these or any other schemes. It would be of course highly judicious in new manufacturing establishments to adopt such conventional scales, as they would, to that extent, promote this desirable but almost impracticable end, namely, that of unity of system; but which, although highly fascinating and apparently tenable, is surrounded by so many interferences, that it may perhaps be considered both as needless and hopeless to attempt to carry it out to the full, or to make the system absolutely universal: and some of the circumstances which affect the proposition will be now briefly given.

First, agreement with standard measure, although convenient, is not indispensable. It may be truly observed, that as regards the general usefulness of a screw such as 615, which was supposed to measure 3/4 inch diameter, and to have 10 threads per inch, it is nearly immaterial whether the diameter be three or four hundredths of an inch larger or smaller than 3/4 of an inch; or whether it have 9, 9 1/16, 9 3/4, 10 1/2, or 11, threads per inch, or any fractional number between these; or whether the thread be a trifle more or less acute, or that it be slightly truncated or rounded; so long as the threads in the screw and nut are but truly helical and alike, in order that the threads mutually bear upon each other at every part; that is, as regards the simple purpose of the binding screw or bolt, namely, the holding of separate parts in firm contact. And as the same may be said of every screw, namely, that a small variation in diameter or pitch is commonly immaterial, it follows, that the good office of a screw does not depend on its having any assigned relation to the standard measure of this or any other country.

Secondly, The change of system would cause an inconvenient increase in the number of screwing tools used. - Great numbers of excellent and useful screws, of accidental measures, have been made by various mechanicians; and the author hopes to be excused for citing the example with which he is most familiar.

Between the years 1794 - 1800, the author's father made a few varieties of taps, dies, hobs, and screw tools, after the modes explained at pages 635 and 636; these varieties of pitch were ultimately extended to twelve kinds, of each of which was formed a deep and shallow hob, or screw tool-cutter. These, when measured many years afterwards, were found nearly to possess in each inch of their length, the threads and decimal parts that are expressed in the following table.

Approximate Valuta of I. I. Holtzapffel's Original Screw Threads.

Number . . .

1

2

3

4

5

6

7

8

9

10

11

12

Threads in 1 inch

6.58

8.25

9.45

13.09

16.5

19.89

22.12

25.71

28.88

36.10

39.83

55.11

The angle of the deep threads is about 50 degrees; of the shallow 60 degrees

This irregularity of pitch would not have occurred had the screw-lathe with change-wheels been then in use; but such was not the case. For a long series of years I. I. Holtzapftel, (in conjunction with his partner, I. G. Deyerlein, from 1804 to 1827,) made, as occasion required, a large or a small screw, a coarse or fine, a shallow or deep thread, and so forth. By which accumulative mode, their series of working taps and dies together with screw tools, gages, chucks, carriers, and a variety of subordinate apparatus, became extended to not less than one hundred varieties of all kinds.