Odd tap sizes

[ckucia]

Recently inherited a bunch of taps from my late father and grandfather. I've gotten some waterproof tackle trays and am trying to get them all sorted. Since they're old, they're almost all inch sizes.

Have come across a few oddballs. I've got a copy of the Machinery Handbook from 1953 and managed to find most of them, and then found a PDF online that almost covers the rest.

Two oddballs I did apparently find drill sizes for are 1/2-12 and 1/2-18. Both these taps are marked "USF" after the size. I haven't been able to figure out what USF means - just about every other tap has been marked NC, NF or NS. Handbook lists the 1/2-12 as needing a 27/64 and a PDF I found also lists a Z drill. For the 1/2-18, the PDF lists "special" as the drill size but gives a range of .4400-.4530.

Anyone know what USF means on the tap size?

I also have one weird tap. It's pretty darned close to a 1/2-12 but it's marked 1/2 1/32. Have no idea what that size means. Any clues?

 

[Sumboodie]

1/2-12 is whitworth. 1/2"-18 is a standard pipe thread I believe.

 

[ybnormal]

USF- Unified Special Fine?

Tap Guide, Styles , Chamfers, Markings, Symbols, Technical, Thread Forms, Tap Thread (inch) Limits and Dimensions - Newman Tools

Lots of helpful information about taps

www.newmantools.com

Everything you wanted to know about taps but were afraid to ask, styles of taps, class of thread , gh numbers , basic point in thread measurement , constants for finding pitch diameter and minor diameter of screw threads , thread constants for vario

Everything you wanted to know about taps but were afraid to ask

www.newmantools.com

 

[RTM]

Keep those vintage taps. When resurrecting old tools, they can be a life saver. They were often oddball formats back in the early days.

The fraction 1/32 might mean undersized for a coating (guessing). Seems there should stip be a pitch after.

Or possibly oversized, but that is more likely on a die it seems (in my head).

 

[ckucia]

Appears the USF is an old standard - United States Form.

Found a thread on Practical Machinist https://www.practicalmachinist.com/forum/threads/can-someone-explain-what-a-usf-thread-is.380615/ that sent me to a thread on grabcad https://grabcad.com/questions/what-is-the-16-u-s-f-threads-meaning that sent me to page 62 of the 1919 Machinery Handbook https://www.google.com/books/editio....+thread+standard&pg=PA62&printsec=frontcover

Was apparently considered obsolete by the time my 1953 copy was produced.

 

[ckucia]

I found this tap/drill chart. Has a lot of the oddball sizes listed.

Tap Drill Chart | PDF | Drill | Cutting

Scribd is the world's largest social reading and publishing site.

www.scribd.com

 

[jayemm]

You never know where these oddball sizes will pop up. My Delta 6" bench grinder has 1/2-12 threads on the wheel arbors. Naturally one is left hand to boot.

 

[robert6715]

Oddball taps are nice to have on hand. Winchester model 70 action screws are 1/4-32 UUF.

 

[neophyte]

1/2-12 Whitworth routinely shows up on Asian machinery.
I have an older Makita grinder that uses it, and I heard it’s common on other Asian machinery since “13” is considered an unlucky number.

 

[MushCreek]

As a machinist and tool maker, I've run across some oddball threads over the years. Brown & Sharpe screw machines used 1/4-27 threads on their oldest machines. I recently restored an old park bench circa 1870, and the stretcher underneath had threads that were about .515" in diameter and 13 threads per inch. Way too big to be a true 1/2-13. I don't know if the threads hadn't yet been standardized or what. To use the original decorative nuts, I custom made a new stretcher rod to fit.

If you ever come across a size that's not on your tap drill chart, it's easy to figure it out. Just subtract the pitch from the diameter. 16 threads per inch would mean the tap drill would be 1/16" under the thread size.

 

[bwringer]

One thing I've found with old tools in general is that the survivors are the ones that never got used. The common sizes are worn out, broken, or just plain missing, leaving only the stuff that never got used.

As noted above, many are nonstandard sizes, or sizes from obsolete standards. They could end up being useful to someone somewhere who's dealing with vintage machinery.

 

[Grant Gunderson]

If you ever come across a size that's not on your tap drill chart, it's easy to figure it out. Just subtract the pitch from the diameter. 16 threads per inch would mean the tap drill would be 1/16" under the thread size.

Just learned something new. Thanks!

 

[toolmiser]

I think Stanley used some odd sizes on the planes, maybe other tools.

 

[RTM]

12-20 was Stanley's PITA thread size. Paid a ton for a dedicated tap and die to rehab them.

 

[Milton Shaw]

MSCDirect has pages of odd ball thread pitches of tap and die's. Seems like there were a dozen or more just for 1/4 inch bolts. For instance a gun silencer is something like a 1/2 28 thread, not in any tap sizes assortments but is available from MSC. Lots of different thread pitches on older industrial equipment. It helped when they standardized them 40 or 50 years ago, but before then they were all over the charts.

 

[BarrelRoll]

I've run into 1/2"-12 thread on a sheave bushing mounted on a gear box mounted on cement batch plant mixer made in Portugal.

I run into a lot of BSPT and BSPP pipe threads on foreign stuff at work. If you try hard enough you can jam a NPT fitting in and many people have. I now have a collection of BSP taps I never thought I'd need or have now.

 

[isb cornbinder]

I have restored a few camel-back drill presses from the 1800s. I had to single point turn the threads for a bolt for a WF and John Barnes drillpress. The bolt turned out perfectly, EXCEPT, I had made a left hand thread. I told myself, "That was good practice." and I got the next thread correct.

 

[2oolhound]

Funny this would come up. I was just restoring a few old woodworking hand tools. One was a small slick about 1 3/4" wide but it was an old hand made one. It had a bolt partially forge welded into the end of the blade so a handle could be screwed onto the end. The last 3/8" of the threads were badly damaged and I soon discovered the bolt was a whitworth thread, 1/2x12. I had a tap that size but no die so I ended up hand filing the threads back with a thread file.

Now it's safe to assume this old hand made tool was made in England.

This spring we had some stantion collars at work that were unique due to their slim aluminum profile. There were a number of them and they were to be shipped around the world over the next 10 years as part of a traveling exhibit. Since they would be assembled and dis-assembled over and over we wanted to include a tap with the exhibit. These were special order from a current supplier in the UK and guess what? They were tapped 1/2x12 whitworth also. The manufacturer had a special statement about these parts as to why this form factor was being used but I don't remember the details. So this is one instance where the thread is still being used today.

 

[neophyte]

Whitworth was designed for cast grey “flake” iron, by a rather brilliant engineer, so the thread profile is good for brittle materials.

 

[dogdog]

MSCDirect has pages of odd ball thread pitches of tap and die's. Seems like there were a dozen or more just for 1/4 inch bolts. For instance a gun silencer is something like a 1/2 28 thread, not in any tap sizes assortments but is available from MSC. Lots of different thread pitches on older industrial equipment. It helped when they standardized them 40 or 50 years ago, but before then they were all over the charts.

1/2-28 is also used on fuel nozzles for oil burners.

 

[dutchgray]

Whitworth was designed for cast grey “flake” iron, by a rather brilliant engineer, so the thread profile is good for brittle materials.

Its also rounded root and crest and course for the dia, excellent for any application where something will be assembled and disassembled often.