Pipe threads bear no relation to the size of the bore of the pipe that the fitting is for

Edited By Ian Parkin on 11/02/2020 08:49:50

A 3/8 BSP male thread is actually 0.656" diameter. The tapping size given in The Model Engineers Handbook is 15,25mm or 39/64 " (0.6094"

Brian

Edited By Brian H on 11/02/2020 08:59:35

No Bob you are not the only one.

The dimension specified for British Standard Pipe (BSP) is not the outside diameter of the pipe but refers to a nominal size of the bore. 3/8" BSP pipe has an OD of 0. 5886" – 0.6560" (14.95mm – 16.66mm).

Pipe is available with different wall thicknesses specified by a schedule. For calculating the volume of fluid that the pipe can transfer, the inside diameter is important and a nominal bore size is specified. A standard OD is important so that the pipe can mate with elbow, tees, etc.

Tube on the otherhand is specified by its OD and a wall thickness and not by its bore.

(one of the arguments that I had with a popular supplier of boiler injectors who advertised them as having 1/4" pipe fittings. When it was delivered it had a 1/4" OD tube fittings and was too small for the application)

Paul.

Edited By Paul Lousick on 11/02/2020 11:55:10

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

I really need to stop trusting my memory! I would have bet money Warrington was in Yorkshire…

Sackcloth and ashes AGAIN!

And there was me thinking they sold it in 13" lengths as that works out nicely when the longer standard 39" Nominal lengths are cut into three for tight fisted model engineers who don't want to pay out for a full length.

Same reason metric silver steel is available in 333mm lengths as that is 1/3rd of a standard 1m length.

French foot is also a bit less than 13"

Edited By JasonB on 11/02/2020 16:34:17

Just to make it more fun is if you need cut a thread unknown to any reputable publisher of thread-tables…

As when making a spare pub-trade standard, CO2 cartridge connector for the heat-exchanger at the heart of a breathing-air heater used for warding off hypothermia in the casualty, in a cave rescue.

Nothing published matched what was clearly a Metric thread, and calculating the wheels for a lathe having an 8TPI lead-screw, only one intermediate stud and change-wheels of 25 to 65 X 5, was a challenge….

As for Foots Royal, French, Stubbs or otherwise, some years ago I chanced upon a souvenir programme from a recital celebrating the complete overhaul of Oslo Cathedral organ. I do not know when Norway adopted these new-fangled Metres, nor what the country had used previously, but the instrument's voices were all given in "fus" , as in Diapason 16 fus or Viola 4-2/3 fus. Well, naturally! How can Widor, Bach or Ligeti possibly sound right on Diapason 4.877… ?

When I occasionally cut some odd thread I use the depth quoted in Zeus charts, or wherever; for a thread of that pitch and apply it to the size of blank being used. Example is 0.0534"depth (1/2 BSW ) for a 1.125" x 12 tpi thread for a Myford fitting item.

Using this technique, all sorts of hybrid threads can be produced.

In fact, the very first thread that I screwcut (after an interval of 40+, years ) was an internal 1 mm pitch for the lens panel on a 5 x 4 camera, so it was in the region of 50mm diameter.

Howard

For BA threads use 600/tpi to get the thread depth in thou.

Russell

Edited By Russell Eberhardt on 19/02/2020 15:55:46

Yeah, I've noticed that.

'S a bu99er, innit?