Help with Imperial / English Thread size

GreatOldOne

GreatOldOne

R'lyeh Engineering Works
H-M Supporter Gold Member
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Mar 20, 2014
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I wonder if anyone can help here. I’m refurbishing a brown and sharpe indexing / dividing head:

Post in thread 'Brown and Sharpe Indexing Head (not BS0-3)'
https://www.hobby-machinist.com/threads/brown-and-sharpe-indexing-head-not-bs0-3.108910/post-1083091

One of the screws is badly damaged, and I would like to replace it - but I’m not sure what size it is. I’ve measured it and seems to be 28 tpi and it’s what I think is 7/32” (the old screw diameter is ~5.5mm. It’s difficult to measure as it’s truly trashed and very short). This seems to be as common as unicorn poo. I can’t find anything that size. Can anyone suggest what it might actually be?

Oh and if any one else has one of these, can you tell me what parts go under that screw (it’s the one above the direct indexing pin) and wether the pin needs a spring at opposite end as It looks as if it might?
 
Thanks for the reply. It seems as if this is pretty much impossible to get here. My usual go-to faster suppliers don’t stock anything in that size. We don’t have a McMaster-Carr equivalent, Googling for 12-28 sae or unf gets me nowhere - and eBay and Amazon aren’t giving me anything.

Looks like I may need to retap the hole, which was hoping to avoid.
 
You could probably drill and tap 6mm 1.0p and get away with it. Do the math and double check me on that.
 
Brown & Sharpe had a number of their own standards for threads; bear in mind that they were in business before there were standardized threads, so they devised their own standards for threads that were different than standards that were developed in later years. Some of their thread combinations were used for many years after SAE and USS threads systems were devised, especially for the small screws that you mentioned.
 
benmychree said:
Brown & Sharpe had a number of their own standards for threads; bear in mind that they were in business before there were standardized threads, so they devised their own standards for threads that were different than standards that were developed in later years. Some of their thread combinations were used for many years after SAE and USS threads systems were devised, especially for the small screws that you mentioned.
Click to expand...
I can't find my email exchange with Starrett (although it may have been a phone conversation): I purchased a "vintage" (1955) 196A1Z indicator kit and wanted to replace one of the holder rods, but couldn't figure out the correct thread. My Starrett contact looked up the details (The ones not available online) and told me the thread was called out as 0.225" -28, so a fat 12-28. He explained that many times heritage instruments had strange threads because that's just the way they were made (the guy that made all of the rods back in the day took a stock 1/4" rod, chucked it up cleaned up the end to have a shoulder and threaded it.

That's why I have a couple of adjustable 12-28 dies.
 
And IMHO, the bottom line answer? It costs money to change tooling and then have to also manage old stock. So why bother? Some vintage Singer threads are the same. Been too long since I worked on one to recall exactly which ones . . .
 
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