Whitworth bolt's head size

[Henry Harris]

I'll see if I can find my 1961 Technical Drawing O Level notes later, but I remember the formula taught as being 1 1/4 D + 1/4 inch. 

So for instance, 1/4 inch D x 1 1/4 + 1/4 inch  =  0.25 x 1.25 + 0.25  =  0.5625"  =  9/16"

However, checking original A7 nuts, it's oversize!

[Tony Griffiths]

 As Holmes once remarked to Watson "That man owns a 2CV" "Why, how do you know that Holmes?" "I note an M7 spanner in his tool box."

Holmes always was a smart-arse, making guesses based on incomplete evidence.

He could have owned a Bugatti.

Charles

...possible - indeed likely - if his bench vice was a Dolex, Leinen or Boley...

[Colin Morgan]

If the formula for AF from the diameter in inches is 1.5D + 1/6", which seems to give about the right numbers, then this could be written as AF = (9D+1)/6 .

As a hexagon is made up of 6 equilateral triangles, the length of the flat is the AF distance divided by the square root of 3, so the formula relating the flat length to the diameter would be F = (9D+1)/(6 x Root3)  ?

[Mike Costigan]

I don't know whether this helps the conversation along, but fifty years ago the VSCC published this chart in their quarterly Bulletin ( No 104 Winter 1969 p41):

Spanner     Jaw size (inches)

3/16 AF      0.191
6mm          0.236
1/4 AF        0.254
7mm          0.276
8mm          0.315
5/16 AF      0.318
11/32 AF    0.350
9mm          0.354
3/8 AF        0.381
10mm        0.394
7/32 BSF    0.417
11mm        0.433
7/16 AF      0.443
1/4 BSF      0.449
12mm        0.472
1/2 AF        0.506
13mm        0.512
5/16 BSF    0.531
14mm        0.551
9/16 AF      0.569
15mm        0.591
19/32 AF    0.601
3/8 BSF      0.606
16mm        0.630
5/8 AF        0.633
17mm        0.669
11/16 AF    0.695
18mm        0.709
7/16 BSF    0.717
19mm        0.748
3/4 AF        0.759
20mm        0.787
25/32 AF    0.790
13/16 AF    0.821
21mm        0.827
1/2 BSF      0.827
22mm        0.866
7/8 AF        0.884
23mm        0.906
9/16 BSF    0.929
24mm        0.945
15/16 AF    0.947
25mm        0.984
1" AF         1.010
5/8 BSF      1.019
26mm        1.024

[Stuart Giles]

---

The comparable  hexagon (AF) sizes of Whitworth barstock are
.445" for 1/4"
.525" for 5/16"
.600" for 3/8"
.71" for 7/16"
.82" for 1/2"
I keep these sizes  in stock, but it seems very difficult to find them in material other than mild steel or brass, I have on occasion milled the hexagon size I need in the material I need from a chunk of round bar.

[Colin Morgan]

As BSF, and more recently BSW, are one size down (e.g. 5/16" is the old 1/4") on the original Victorian (and up until about WW2) sizes, this can add a further complication... I was looking at the old sizes.

[Charles P]

 As Holmes once remarked to Watson "That man owns a 2CV" "Why, how do you know that Holmes?" "I note an M7 spanner in his tool box."

Holmes always was a smart-arse, making guesses based on incomplete evidence.

He could have owned a Bugatti.

Charles

...possible - indeed likely - if his bench vice was a Dolex, Leinen or Boley...

I reckon my Maho probably has one if I look hard enough!


C

[Denis Sweeney]

I remember when I first purchased my 36 Ruby I had some very old Whitworth spanner’s which I noticed would never fit any new “modern” Whitworth nuts, even though they did fit original nuts.
When I investigated I found pre-war tables which gave larger dimensions than later tables.
I believe, but stand to be corrected, the dimensions were reduced as a means to save material during the Second World War.
If I can find the tables I will post these, but it is probably 30 years since I last saw them!
Denis S

[Tony Press]

Early Whitworth bolts were to BS 190 1924 for Bright Hex Bolts 

Across the Flats dimensions-

.525 to .520" for 1/4" Nominal Thread Size.
.600 to 595" for 5/16"
.710 to .705" for 3/8"
.820 to ,815" for 7/16"
.920 to .915" for 1/2"


During the war as noted to save metal this was changed to the pre-war BSF standard across the flat dimensions (which apparently had been used for Whitworth bolts by some pre-war Auto makers ) resulting in- 

BSW to B.S. 1083 1951  for Bright Hex Bolts 

Across the Flat dimensions-

.445 to .438" for 1/4" Nominal Thread Size

.525 to .518" for 5/16"
.600 to .592" for 3/8"
.710 to .702" for 7/16"
.820 to .812" for 1/2"


Later B.S 1083 1965 covered both BSW and BSF Precision Hexagon Bolts all with the same across the flat dimensions.

Not relevant to the head discussion- the famous Meccano Cheese Head 'bolt' had an uncommon 5/32" BSW thread apparently originally used to hold door knobs on their square shaft.

Tony.  

[39Jet]

This is just what I was hoping for.
Like Paul N-M I wonder where else on the interweb such informed opinions would be offered.
I believe that the diameter across the points, the length of the flats and the distance across the flats (AF) are all simply related to each other and aren't related linearly to the shank diameter in Whitworth bolts. Like Cliff Ringrose I have read that the diameter across the points is twice the shank diameter, but I am pretty sure that this isn't true.
I have tested the formulae offered and, within the limits of my calculator skills, I think Colin Morgan's 1.5 x Shank diameter + 0.16" gives good results.
I have seen it suggested that the head diameters were specified because there was hex-bar available in an approximately suitable size, so that the head dimensions are basically due to chance. Colin's formula would suggest that this isn't the case.

A detailed and amusing description of various systems of fasteners from an American perspective is on the "Progress is fine but it has gone on too long" web site.

Joseph Whitworth produced the specifications for the Whitworth thread in 1841, but I can't see where he worked out head sizes for bolts.
By the start of last century a finer thread was needed for steel (rather than cast iron) fasteners and so BSF was introduced. With a head size one size smaller than Whitworth. In the 1920s Auto-whit, Whitworth with smaller head-size, steel bolts began to be used in some cars. In the early years of WWII it was decided to reduce the head size of all Whitworth bolts to that of the BSF bolts of the same shank diameter in order to save metal - the bolts could be produced from smaller hex-bar. The new Whitworth bolts were called BSW. The same spanner will fit both BSF and BSW bolts of the same shank diameter and are often marked just BS.