Nine Were Appointed and Ten Signed the Report

“The difficulty of obtaining the exact pitch of a thread not a multiple or sub-multiple of the inch measure is sometimes a matter of extreme embarrassment.” That is the American committee, in December 1864. Twenty-three years earlier Whitworth had written that the difficulty of ascertaining the exact pitch of a particular thread, especially when it is not a multiple or submultiple of the common inch measure, occasions extreme embarrassment. Both documents have now been read in full for this site, and the parallel runs through several sentences.

The sources are four issues of the Journal of the Franklin Institute: May 1864 for the meeting that created the committee, November and the following January for the monthly minutes, and January 1865 for the report itself, which runs from page 53 to page 56. All are out of copyright and scanned on the Internet Archive. The optical character recognition corrupts digits, so every numeral quoted below was checked against the page images.

April: nine names

The resolution was moved by Algernon Roberts and seconded by C. T. Parry, and it asked for a committee to investigate the question of a proper system of screw threads, bolt-heads, and nuts to be recommended by the Institute for general adoption by American engineers. The minutes then list the committee:

“W. B. Bement, C. T. Parry, J. V. Merrick, J. H. Towne, C. Sellers, B. H. Bartol, Edw. Longstreth, Jas. Moore, and J. W. Murphy.”

Nine men. Keep the list; it matters in December. Note also who is not on it: William Sellers, whose paper had just been read, and Algernon Roberts, who moved the resolution creating the committee.

October and December: reported progress

At the stated monthly meeting of 20 October 1864, the minutes give the committee one line, sharing it with another: the special committees on weights, measures and coins of the United States, and on a uniform system of screw-threads and bolt-heads, reported progress. That is the whole entry. Six months after the paper was read.

The meeting of 15 December 1864 opens with William Sellers, President, in the chair. Two other special committees reported progress. Then: the special committee on a uniform system of screw threads presented the following report.

Why the thread has a flat on it

The report is mostly an argument about tools, and its most substantial passage is about the shape of the crest. A sharp thread is the simplest form and needs no special tools, but its liability to accident, always great, becomes a serious matter upon large bolts. So something has to come off the point. The question is what shape to leave.

A rounded crest and root, the committee writes, presents much greater difficulties. All taps and screws chased or cut in a lathe would require a second process to finish or round them. And since the radius of the curve must vary for every thread, it would be impossible to make one gauge to answer for all sizes, and very difficult without special tools to shape it correctly even for one.

Then the principle, stated outright:

“Your Committee are of opinion that the introduction of a uniform system would be greatly facilitated by the adoption of such a form of thread as would enable any intelligent mechanic to construct it without any special tools, or if any are necessary, that they shall be as few and as simple as possible.”

So the flat crest is there because a curve could not be checked with one gauge. The amount is set the same way: as small as possible, only enough to protect the thread, for which one-eighth of the pitch would seem to be ample, leaving three-fourths of the pitch for bearing surface. Take an eighth off the top and an eighth off the bottom and three quarters remain, which is the arithmetic the sentence is doing.

An asymmetric thread was considered and rejected. One side perpendicular to the axis would give the greatest strength with the least friction, but it would only carry load in one direction and the nut would have to be cut to match, so the committee classed this form as exceptional.

Sixty degrees, and why not finer

On the angle the committee gives a mechanical reason and then, immediately, the real one. Sixty degrees seems to fulfil the conditions of least frictional resistance combined with the greatest strength, but because it is an angle more readily obtained than any other, and it is also in more general use. That is Sellers’ own argument from the paper eight months earlier, repeated by the committee reviewing it.

On pitch, the committee starts from an observation worth keeping: in every instance the threads now in use are stronger than their bolts. That made a finer scale look attractive. Three things argued against it.

Cast iron. If the thread were confined to wrought iron or brass a finer scale might have been reached, but as cast iron enters so largely into all engineering work, finer threads might not be an improvement
The flat had already done part of the job. As far as the vertical height of the thread and the strength of the bolt go, adopting a flat top and bottom was equivalent to decreasing the pitch of a sharp thread by 25 per cent, or increasing the threads per inch by 33 per cent. Those two figures are exact reciprocals, and the arithmetic checks
The shops could not hold it. Finer threads would require greater exactitude than then existed in the machinery of construction, to avoid the liability of overriding, and the wearing surface would be diminished

Cast iron making the scale coarser is the same reason Whitworth gives in 1841, in his own paper: the threads he averaged were used in cast iron as well as wrought, and that circumstance rendered them coarser than they would have been if restricted to wrought iron. Two committees, two countries, twenty-three years apart, and the same material sets the floor.

And the same method, in the same words

The sentence that closes the pitch argument is the one to keep: we are of opinion that the average practice of the mechanical world would probably be found better adapted to the general want than any proportions founded upon theory alone. And then the method: we have taken some pains to ascertain what the proportions in use are, and submit the following, as being in our judgment a fair average.

That is Whitworth’s method and very nearly Whitworth’s wording, arrived at independently or otherwise. This page reports the parallel and does not assert how it came about. What it does establish is that the three founding documents this site has now read in full all reach for the same instrument. Survey the trade, take the average, and round the awkward numbers off.

Where the across-flats rule comes from

The resolutions include the head and nut proportions, and this is where a rule that is still recognisable enters the record.

Dimension	Rough bolt	Finished bolt
Across the flats of head and nut	One and a half diameters of the bolt, plus one eighth of an inch	One sixteenth of an inch less than rough
Head thickness	Half the distance between its parallel sides	Equal to the thickness of the nut
Nut thickness	Equal to the diameter of the bolt	One sixteenth of an inch less than rough

Our page on the Sellers paper records that one of the Philadelphia firms brought drawings to the April meeting carrying a house rule: the width across the flats of finished heads and nuts was one and a half times the diameter of the bolt. The committee adopted that house rule and added an eighth of an inch. What decides the spanner size on a modern bolt is a table rather than a formula, but this is the formula it replaced, and one of the firms that used it sat on the committee.

How it was meant to spread

The second resolution is the interesting one, because it names the mechanism. A copy of the resolutions was to be forwarded to the Quarter Master General, the Chief of the Bureau of Steam Engineering of the Navy, the Chiefs of Ordnance for the Army and Navy, the Chiefs of the Engineer and Military Railroad Corps, and the superintendents and master mechanics of the railroad companies, asking them to promote adoption by requiring all builders under any new contracts to conform to the proportions recommended. A third copy went to the engineering societies and the principal machine and engine shops.

The route to a national thread ran through the people who signed purchase contracts, and in December 1864 that meant the Army, the Navy and the railroads. Fifty-four years later the same reasoning produced a federal commission with two Army members, two Navy members and a six-month deadline. The instrument did not change.

Ten signatures

The last resolution reads that this Committee be now discharged. Below it are the names, each with a firm.

Signed	Firm as printed	On the April list?
Wm. B. Bement	Firm of Bement and Dougherty	Yes
C. T. Parry	Supt. Baldwin’s Locomotive Works	Yes
J. Vaughan Merrick	Firm of Merrick and Sons	Yes
John H. Towne	Firm of I. P. Morris, Towne and Co.	Yes
Coleman Sellers	Eng. Wm. Sellers and Co.	Yes
B. H. Bartol	Supt. Southwark Foundry	Yes
Edward Longstreth	Foreman Baldwin’s Locomotive Works	Yes
James Moore	Firm of Matthews and Moore	Yes
Wm. Sellers	Firm of Wm. Sellers and Co.	No
Algernon Roberts	of the Pencoyd Iron Works	No

Nine were appointed and ten signed. J. W. Murphy, who was appointed, is not among the signatures. William Sellers and Algernon Roberts, who signed, were not on the April list. Roberts had moved the resolution that created the committee. Sellers was the author of the proposal under review and chaired the meeting that received the report, and his firm supplied two of the ten signatures.

We read every issue from June 1864 to January 1865 and found no minute recording any addition to or change in the committee’s membership. This page reports what the two documents say and stops there. Nineteenth century committee minutes are often thin, and the absence of a record is not evidence of anything in particular.

What a reader can take from it

The flat on the thread is a gauging decision. A rounded crest needs a different radius for every pitch, so one gauge could not serve all sizes, and the committee wanted a form any mechanic could make without special tools
The flat is one eighth of the pitch at top and at bottom, leaving three quarters of the pitch as bearing surface, and the report does that arithmetic in the sentence
Cast iron set the floor on fineness, in Philadelphia in 1864 exactly as it had in England in 1841
Across the flats was one and a half diameters plus an eighth of an inch, taken from a member firm’s house rule with an eighth added
Adoption was to be driven by procurement, through the Army, Navy and railroad purchasing officers, half a century before Congress did the same thing

The habit worth borrowing is the one all three documents share. None of them claims to have found the right number. Each says, in its own words, that the right number cannot be derived, and then goes and measures what the trade is already doing.

This page covers step 2, the thread. The whole order is substrate, thread, head, drive, finish, documentation, and why doing it out of order is rework rather than a tweak is in specifying a screw.

Common questions

What did the Franklin Institute committee of 1864 conclude?

It reported on 15 December 1864 and resolved that the Institute recommend for general adoption by American engineers a thread with straight sides at 60 degrees, a flat at top and bottom equal to one eighth of the pitch, and a stated set of pitches, together with proportions for bolt heads and nuts. It then resolved that copies be sent to Army, Navy and railroad officials and to the engineering societies, and that the committee be discharged.

Why does the Sellers thread have a flat crest instead of a rounded one?

Because of gauging and tooling. The committee wrote that a rounded top and root would require a second process to finish taps and screws cut in a lathe, and that since the radius of the curve must vary for every thread it would be impossible to make one gauge to answer for all sizes. It stated the principle that a uniform system would be facilitated by a form any intelligent mechanic could construct without special tools.

How much of the thread is flat?

One eighth of the pitch at the top and one eighth at the bottom. The report says the amount should be as small as possible and only sufficient to protect the thread, that one eighth of the pitch would seem to be ample, and that this will leave three fourths of the pitch for bearing surface.

Why did the committee not adopt a finer pitch?

It gives three reasons. Cast iron enters so largely into all engineering work that finer threads might not be an improvement, where a scale confined to wrought iron or brass might have gone finer. Adopting a flat top and bottom had already achieved the equivalent of decreasing the pitch of a sharp thread by 25 per cent. And finer threads would need greater exactitude than existed in the machinery of construction, risking overriding, while reducing the wearing surface.

Where does the rule of one and a half diameters across the flats come from?

From this report, which resolved that the distance between the parallel sides of a bolt head and nut for a rough bolt be one and a half diameters of the bolt plus one eighth of an inch, with finished work one sixteenth of an inch less. One of the Philadelphia firms represented on the committee had brought drawings to the April meeting carrying one and a half times the diameter as a house rule.

Who signed the 1864 report?

Ten men, each with a firm printed beside the name: Wm. B. Bement, C. T. Parry, J. Vaughan Merrick, John H. Towne, Coleman Sellers, B. H. Bartol, Edward Longstreth, James Moore, Wm. Sellers and Algernon Roberts. The committee appointed in April had nine names, and J. W. Murphy, who was on that list, is not among the signatures, while Wm. Sellers and Algernon Roberts, who signed, were not on it.

How was the recommendation supposed to be adopted?

Through purchase contracts. The committee resolved that copies of the resolutions go to the Quarter Master General, the Chief of the Bureau of Steam Engineering of the Navy, the Chiefs of Ordnance for the Army and Navy, the Chiefs of the Engineer and Military Railroad Corps, and railroad superintendents and master mechanics, asking them to promote adoption by requiring all builders under any new contracts to conform.

References

All four issues used here are out of copyright and were read from the scanned text of Internet Archive copies, none of which is access restricted. The optical character recognition corrupts digits on these scans, so every numeral quoted above was checked against images of the printed pages, specifically printed pages 55 and 56 of the January 1865 issue. That check mattered: the text layer renders the equivalence as 26 per cent where the printed page says 25 per cent, which is the figure that makes it an exact reciprocal of the 33 per cent beside it. No value from the report’s pitch table is quoted here, that table being badly garbled in the text layer. The parallel between the committee’s opening sentences and Whitworth’s 1841 paper is reported as a textual observation from having read both documents in full; no claim is made about how it arose, and none about copying or influence. On the membership, this page states only what the April minutes and the December signatures record, and that the issues from June 1864 to January 1865 were read and contain no minute recording any change in the committee’s membership; no motive or impropriety is suggested or implied. No claim is made that this report caused any later adoption, no document recording that being in hand. The recommendation is not compared with any modern standard.