Set Screw dilemma

[chucketn]

I purchased an assortment of socket head set screws while wandering through Horrible Freight last week. Imagine my surprise, when inspecting the contents, that the smallest size in the assortment is 1/8 x 40tpi! What’s that? Never heard of such a size. Is that the same as 5-40?
Googling the 1/8 x 40 determines that there are taps available at reasonable cost.
Chuck

 

[MachineTom]

Yes a 5-40 is the same size as the 1/8. Machine screws are sized this way, the #, 5 in this example * .013 = .065 + a constant of .060 is the the OD of the screw .125. a size "0" screw is .060 D, a size 00 is -.013 plus the constant .060= .047 D

 

[eightball]

thanks Tom i never knew thats how numbered bolts are sized

 

[modelman1838]

Strange as it may seem 1/8-40 is 1/8 B.S.Whit. but you would expect it to be B.S.Fine as it is such a fine thread, but for some unknown reason it has always been hidden away at the start of the British Standard Whitworth series.
Hugh

 

[mu38&Bg#]

BSW uses a different thread form. They are not the same.

 

[chucketn]

It ended up being a Chicom mislabel.
I dug out my trusty American made thread gage and the 1/8-40 fit the 40tpi gage. Then, I drilled and tapped a 5-40 hole in a piece of aluminum and the HF labled 1/8-40 set screw fit fine. The other fractional sizes listed also fit standard USF threaded holes.

Chuck