Mystery Thread Size

[sjaffe]

BSW 5/32" 32 TPI is very close, so I will go with that. I used a 8-32 tap on them and only the smallest of shavings resulted. An 8-32 fit fine after that. So thanks for the help, I learned something. I thought there was only Imperial, Metric and BA threads, but now I will need to also consider BSW. I restore antique clocks so have run into several of these various standards.

Stan

[old mart]

With antique clocks, some will be from before even Whitworth came up with the first standards, so you will have to be prepared to make custom threads.

[BCRider]

#7-32 (with a nominal OD of 0.154") is listed as a standard screw size in my 1914 Machinery's handbook.

I've always wondered what happened to the missing sizes in the number size screws. Looks like now we know. Are the others like Albums and #11 in that table as well?

BC,
As I understand Stan's original post, he's planning to make new 'upgraded' thumbscrews, so the issue of a sloppy fit wouldn't arise.

Georgineer, That's what I took it as too. But he's saying that an 8-32 would result in about .01 of slop. So that means the narrower crests of both sides would be doing most of the work. And a good pinch worth of torque might just cause the "chezzium" metal often used on these low cost third hand gizmos to bend over and smear. Depends on how much torque is needed I guess.

Some of the similar priced items I've seen are not only loose fits on the threads but the threads themselves tend to look like a rabid gerbil gnawed them from the stock. So often the fittings don't thread back and forth all that smoothly. If he's making his own or upgrading the existing to 8-32 I can't help thinking that likely as not upgrading the studs or screws would result in nicer moving thumb nuts overall.

[andywander]

Yeah, the number sizes actually went up to 30.

And to make it even more confusing, there is an "old" number series, and the "new" or present number series. ASME had a committee working the new sizes from 1908 thru at least 1921

[J Tiers]

Looks like they just decided that they were going with sizes that did not claim accuracy to a hundredth of a thou..... Didn't they think anyone could measure that difference to better than a tenth of a thou?*

Oddly, not all the new sizes are bigger. They are almost identical from 8 to 13, and then the new sizes are smaller.

Unless you have a very old set of taps/dies, the whole thing may not matter at all. And in any case the difference is such that only closer thread classes would really be affected.

* They were probably right, the hundredth of a thou was a mathematical construct and had no practicality at all.