Don,


Another possibility is that this is a "Sellers Thread" - fractional sizes below
1/4" which are just not the same as the numbered sizes.


Do you have the years in which Wm. Johnson operated in Newark? That would be a
clue because WIlliam Sellers presented his threading system at the Franklin
Institute on   September 15, 1864.  If your wing dividers are after that date,
but before the adoption of the current threading system, then Sellers threads
are likely.


In addition to oddball diameters and threads per inch, the thread profile is
somewhat different than the current standard with regard to the peaks and
valleys, although the slopes are still 60 degrees.


"Of threads, and threading systems, there is no end!"  [  (tm) John Ruth ]


Victor Machinery is a good source for odd threading dies and taps.


John Ruth

Who collects odd taps and dies.


________________________________
From: OldTools  on behalf of Don Schwartz 
Sent: Monday, December 11, 2017 5:34:53 PM
To: old tools
Subject: [OldTools] Thumb screw wanted


I have a pr. of wing dividers made by Wm. Johnson Mfg. of Newark, New
Jersey which is in need of a thumb screw to secure the setting. I tried
all of my loose odds and found nothing that fit. The local Bolt Supply
House agreed that an 8-32 almost fits, but jams up quite quickly, and
suggested it may be an 8-28 which is not available here due to lack of
demand. ;-(

The sales person suggested that even though it is an American tool, that
it may be a British standard threading. Can anyone shed light or point
me to a supply house, or maybe spare an extra? It's a decent tool, with
a fine adjustment screw, and I'd like to put it to use.

Don

--
"You can tell a man that boozes by the company he chooses"
The Famous Pig Song, Clarke Van Ness

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