Fishtail question ?

[vintageandclassicrepairs]

Hi All
I picked up a Whitworth 55degree fishtail among some other tools at an autojumble at the weekend
It has scales marked along either side of the "tail"
One side in 20ths of an inch and 14ths on the other side
Do any of you on here know what is the purpose of the 14ths scale is? 

John

[Pete.]

Can't think of any reason except perhaps whomever made it messed up in their marking out. I bet they thought they were making a 1/16th graduation.

[seadog]

Is it marked on the other side too? Maybe to measure thread pitch?

[awemawson]

7/16 Whit is 14 tpi - perhaps whatever environment it was used in made a lot of 14tpi threads

1/2 , 5/8, 3/4 and 7/8 BSP are all 14 tpi so there's another possibility 

[vintageandclassicrepairs]

Hi All,
I thought of a couple of possibilities like those mentioned !!
Its plain on the other side
So a bit more searching on the web and I found this,
which also shows 14 and 20 on an American version

http://www.chaski.org/homemachinist/viewtopic.php?t=60268

Lots of versions show up on a Google images search
https://www.google.ie/search?q=Fishtail+thread+gauge&client=firefox-b&dcr=0&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwiKnvC_weTWAhXSKVAKHVNVAsoQsAQIJw&biw=1198&bih=626

Since I almost always use tipped threading tools I'm not sure I need  to dig so deep
I have an upcoming project that will require probably grinding up a tool so thought the fishtail might come in useful

 John