Thread: Brass Taps

Originally Posted by bob colles

Idle Question: Was the 26 TPI thread chosen for brass taps an accident or is there something about that particular metal that requires that thread spacing at certain sizes???

Bob
I also have been intrigued with the origin of the 26 TPI Brass thread.
My " Machinerys Screw Thread book " of the 1950s says the origin of this thread is obscure however it was used for many years for gas fittings, brass tubing & general brass work.Perhaps its shallow thread depth was good for brass tubing work.
It was used a lot for gas fittings, but below quarter inch diameter the BA or British Association thread was used. The Brass thread was of Whitworth form ie 55 degree whereas the BA thread is 47 & a half degree.
Both are still very handy threads to use & there are lots of them still about.
regards
Bruce
ABRATOOL

This www site might help. Untitled Document


I don't have any brass taps/dies as such but have used a lot of BA threads in building my "Simplex" steam loco. These threads have been great to use where a small dia. screw/bolt is required. I have also used some American Number Series screws in the chassis as well. Both of these formats are excellent for when small screws are required.

Originally Posted by Kody

This www site might help. Untitled Document


I don't have any brass taps/dies as such but have used a lot of BA threads in building my "Simplex" steam loco. These threads have been great to use where a small dia. screw/bolt is required. I have also used some American Number Series screws in the chassis as well. Both of these formats are excellent for when small screws are required.

Kody
I agree. I used BA threads in building a No 7 Bolton Stationary steam engine, & had the need for a 40 TPI model engineers thread also.Occasionally I will make an odd size threading tap from a piece of Silver steel & then harden & temper it to get over an immediate problem, rather than start trying to purchase the required tap.
Like yourself I find the American number series screws very useful.
I always substitute 3 sixteenths Whitworth threads with a No 2 BA thread.
The 3 sixteenths Whitworth has a small core dia & is a weak thread leading to tap breakage compared to alternatives like 2 BA & others.
I am 69 years of age & notice you are of a similiar "Vintage".I think this era could share a lot of common knowledge.
regards
Bruce
ABRATOOL

FWIW BSF threads used to be very common on older British cars and machinery. 1/4 BSF is also 26 TPI 55 degree thread angle, Whitworth thread form, so 1/4" Brass and 1/4" BSF taps and dies are interchangeable.

The 1/4 x 26 TPI BSC (Cycle thread), however, is a 60 degree thread, so is not really interchangeable with the others. Like the brass threads, 26 TPI was used for BSC threads on a number of different material diameters besides 1/4".

No idea why the 26 TPI thread count was so popular though.

Frank.

Thanks guys, its always been one of those things that has tickled my curiosity bone

Brass thread in 26 tpi was developed for brass tubing which had a constant wall thickness regardless of diameter hence the "standard" thread form.

Roger

I have no specific Knowledge but my feeling is that most of these threads sort of evolved over the years rather than being specifically designed. The first threads were one off hand made with chisels and files. The first person to invent a tool to do them would have copied roughly his own handmade thread dimensions. The whitworth threads (and UNC etc) are a bit coarse and I have had breakages of 3/16" and 1/4" taps. My understanding is that the nearest metric threads are stronger in both shaft and thread, and are finer and easier to cut, with taps and dies at least.

I did not like metric threads for a long time but circumstances have changed my mind. I have a huge number of metric bolt and nuts from work. After contractors have finished a job there is often many left around and these get picked up and I usually take them home as no one else wants them. Most of them are used but still ok. I have about 100 18mm bolts 50mm to 65mm long. I used to have hundreds of 12mm bolts nuts and washers 25mm to 40mm long but have used about half.

Dean

Generally coarse threads are used in low strength (cast iron) and soft alloy materials (brass , copper and Al) Fine threads are used in high tensile strength materials and alloys or where a thicker wall is required in tubing etc.

BSF came about during WWII where the British reworked the specs to save material so that the bolt heads and nuts dropped down a size. The American system (UNF/UNC) is a hybrid of both metric and imperial (French and English) systems i.e 60deg thread form and Imperial Units. More or less the best of both worlds. 60deg form gives a large root diameter and stronger stud.

BSW and UNC pitches match for the same thread diameter apart from 1/2" (12TPI BSW and 13TPI UNC) and 2 1/4" (4. 1/2TPI BSW and 4TPI UNC). UNC studs fit BSW holes.

Where you see BSF or UNF threads in cast iron this has been done because coarse threads have a tendency to vibrate loose more easily. This is where the torque wrench comes in.

Originally Posted by Swarfmaker1

BSF came about during WWII where the British reworked the specs to save material so that the bolt heads and nuts dropped down a size.

Swarfmaker,

The British Standard Fine thread was around for many years before WW2 - I think the Standard came out about 1908. In the late forties I restored an Austin 7 of about 1925 vintage which used BSF fasteners pretty well throughout, and I remember them being used on on even older vehicles.

I think what you are referring to is the dropping of the original Whitworth standard hex sizes which were still in common use up to the beginning of WW2 in favour of the already existing British Standard hex sizes as used on BSF fasteners. The modified fasteners were known as British Standard Whitworth fasteners. These differed by one spanner size from the original Whitworth standard hex sizes. The actual threads were unchanged. I have a couple of spanner sets from the 1940s which have the Whitworth size stamped on one side, and the British Standard size stamped on the other - e.g one side will be stamped 1/4W, and the other side of the same spanner end stamped 5/16BS.

Frank.