Posted by Mark Gould 1 on 11/02/2020 08:40:34:
Gents,
Can anyone tell me why the tapping drill size for 3/8 BSP is over 15mm? I thought I had a basic grasp of tapping drill sizes but a hole over half an inch for a tap under half an inch doesn’t add up in my (newbie) head.
Many thanks,
Mark
As others have explained, this example is because pipes are being threaded rather than nuts and bolts. Different authority, different logic. The roots of these peculiarities generally date back to the 19th Century. Before 1860 there was very little standardisation. Individual trades and companies did whatever they thought best, maybe using local measure rather than national measure, and never anything understood internationally! Although Whitworth clearly explained the benefits of standardisation, swathes of British industry ignored him.
Wire and Sheet metal Gauges are a good example of early muddle. As late as 1950 British engineers had to pick their way through a maze of different systems. British Imperial Wire Gauge, Birmingham Wire Gauge (which is different from Birmingham Gauge), Stub's Iron Wire Gauge (which is different from Stub's Steel Wire Gauge), Whitworth Wire Gauge, Warrington Wire Gauge, Instrument Wire-Gauge, Standard Wire Gauge, English Music Wire Gauge, Lancashire Pinion Wire Gauge, American Screw and Wire Music Gauge, Wright Music Gauge, Brown and Sharpe Wire Gauge, Alhoff & Meuller, Washburn & Moen Gauge, American Steel & Wire Co Gauge, Brunton Music Wire Gauge, Poehlmann Gauge, National Wire Co Gauge, Roebling Gauge, and American Wire Gauge. Sometime the same gauge name was used for different sized wire depending on purpose or material, Soft Wire, Hard Wire, Aluminium, Silver, Gold or Resistance Wire etc.
Silver Steel in LPG was sold in 13" lengths, because that's a French Foot. Drill Rod Steel used yet another gauge system.
What's amusing about this mess is how long it persisted! Never been easy to get trades or workmen to standardise or change. A chap brought up on Whitworth would die rather than adapt to foreign muck like AWG. A proud Yorkshireman weaned on Warrington Gauge would have no truck with that Lancashire rubbish! And of course no-one accepted the beastly metric system, which is far too simple – it just measures actual diameters and sheet thicknesses in millimetres.
So don't expect too much in terms of consistency from engineering. Likely it is as it is for long forgotten historical reasons, a good idea at the time, not rational now. Unless you know for sure, don't assume anything! Well worth investing in a Zeus, Machinery's Handbook, or similar reference, especially if your workshop is going to tackle anything out of the ordinary.
Dave
Edit: typos galore
Edited By SillyOldDuffer on 11/02/2020 13:12:45