Tapping drill sizes?

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

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

BSP threads are nominated in pipe sizes, which is/was akin to pipe bore. These sizes therefore do not refer to actual thread OD.

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

....Am I the only one who reads the tapping tables in Zeus and then tests it out on a piece of scrap before touching the workpiece...always!......and not afraid to change the drill size until I feel happy.

No Bob you are not the only one.

No Bob, I too test tap on a piece of scrap first time I use a new tap. As for tapping drill sizes I use a slightly oversize tapping drill in tough materials. Here is a table of OD and tapping drill sizes for British pipe threads, and a downloadable table.

Thor.

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

Nope! My tapping drill, if it is a fresh one (I try to keep them separate from ‘the pile&rsquo, would always be checked out before adoption. I don’t have a drill sharpening set-up, per se.

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 always thought it was because one of the standard Unit Material Quantities in Industrial Engineering departments was Components Per Foot, and the extra inch was summat to hold on for the last one off.

But we also used to calculate 4" bar end of a standard 10 ft. bar amortised over the components you could get out of 9' 8" (using top drwg. limit + partoff tool width + 3%, rounded down to next whole number) into the UMQ.

And what's the LPG? I used to run a campervan on that... or have I been skip-reading again?

Last time I did a 3/8" BSP male thread for the railway, I had to screwcut it on my Warco because, astonishingly, none of the lathes in their shed appeared able to do 19 TPI. I think they had 1 die in that calibre, but it'd been thoroughly nadgered. Mercifully, they bought a tap when I had to do the female thread. IMO you can generally trust Zeus tapping drill sizes for stuff like CZ121 or LG2 - it's when you get into tougher stuffs that you might want to take advantage of the 60% depth = 90% strength calculations; especially if you ain't got every single metric, letter and number size drill...

Edited By Mick B1 on 11/02/2020 13:44:49

My Dad said I bet you'll have a decent answer within 10 minutes and you gents came through...again. Many thanks for explaining this to me. There's a lot to learn here.

Mark

A proud Yorkshireman weaned on Warrington Gauge would have no truck with that Lancashire rubbish!

Dave, was Warrington in Lancashire back in those days? Ya learn something every day.

Never been easy to get trades or workmen to standardise or change.

One only needs to read the thread on metric v imperial measurements.🙂

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

Sackcloth and ashes AGAIN!

I thought Warrington was in Cheshire last time I looked

John

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

According to Google "the French royal foot is exactly 9,00027,706 metres" or just over half a million miles!

They've cribbed wikipedia and miss-formatted 9,000 divided by 276,706!

Neil

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... ?

Now you can't leave it at that, enquiring minds will want to know the dimensions of the thread

You say it was metric so could simply have been a constant pitch one where diameter can be whatever metric size you like so things like Md x 1, 1.5, 2.0 etc can be used

Edited By JasonB on 19/02/2020 07:17:19

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