Just picked up this 1980 OTA coupled with a Nexstar SE. The visual back has the thread engagement issue with my new 2019 9.25 Evo visual back unable to engage past two threads. Antaries thread ring for 2" diagonal does not engage.
This is sent me on the hunt for an explanation and old CN posts shed a duplicate light on the issue between the thread spec changing fit class versus the thread type changing between ISO and Imperial / ANSI.
An on line check for UN thread designations shows the nominal 2" size to not exist under this regime, 2-20 UN_ being the last in the series. I decided to measure.
Four caliber measurements around the thread averaged to 1.995". I always ignore the 4th decimal as no vernier can measure to 10ths accurately, that's what micrometers are for.
Next I checked the pitch via gage set. 24 tpi versus 1.0mm as the closest possible. Clearly, this is a 2-24 thread by my measurements and observations.
The 1.0 ISO gauge passes back light. Also, note that 24 tpi pitch is .0417" which translates to 1.058mm.
I also measured th ID of the VB thread ring at dia 1.964" and checked the pitch via thread gauge. It's a 2-24 thread.
But it does not fit! This points to a change in the thread fit class. No doubt the old thread was a class 3A and the newer (post 1980) are more generous 2B which I believe allows for a deeper minor diameter. Another plausible explanation is the old threads may have been rolled which results in a filleted trough - better / stronger in diameter and reduced stress concentration K factor.
So now what? Well, if I can get my hands on a thread die a can chase the thread to reduce the troughs and open it up, or I can use a newer thread ring to drive lapping compound into the thread to open it up. The latter will most likely win out for practical and cost reasons.
Happy to add my views to this old dilemma.
Edit: upload glitch. Working on reducing pic sizes
Edited by B l a k S t a r, 05 December 2019 - 03:54 PM.
