# Single Depth of Thread for 26 TPI

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#### alcostich

##### Member
I attempting to make a draw in nut for a 3C collet. The thread wants to be 26 TPI. Nowhere can I find the single depth of thread for that.
This will be an internal thread. The dimensions on the plan I'm working from call out a minimum Dia of .520 and a max of .635.
Based on the calculation: 1/26 x .866 I come up with .033.
Does that sound right?
Incidentally, my Machinerys Handbook does not have a listing for this 26 TPI. Or, at least, none that I'v been able to find.

Thanks.

Alan

#### Scott_M

##### Well-Known Member
You can also use the formula for obtaining tap drill size and then divide by 2 to get single depth.

outside dia. of thread minus .01299 x percentage of full thread divided by threads per inch = drilled hole size

if we use 100% and skip the outside diameter we get

.1299x100= 1.29 dived by 26 =.04961 ( double depth ) /2 = .0248" single thread depth

Scott

#### Tim Wescott

##### Well-Known Member
Even if I know the exact thread dimensions I'm not a good enough machinist to hit them -- I just get close, then shave & try, shave & try.

#### jkimberln

##### Well-Known Member
I may not understand this project very well but if you have a min of 0.520" and a max of 0.635" why can't you just use a 5/8"-26 tap instead of trying to cut an internal thread on the lathe?

#### Richard Hed

##### Well-Known Member
I attempting to make a draw in nut for a 3C collet. The thread wants to be 26 TPI. Nowhere can I find the single depth of thread for that.
This will be an internal thread. The dimensions on the plan I'm working from call out a minimum Dia of .520 and a max of .635.
Based on the calculation: 1/26 x .866 I come up with .033.
Does that sound right?
Incidentally, my Machinerys Handbook does not have a listing for this 26 TPI. Or, at least, none that I'v been able to find.

Thanks.

Alan
But your machine has a setting for 26tpi, right?

#### Richard Hed

##### Well-Known Member
HOld on, Alcostich is in USA, our threads are a differnt shape, the 60deg type.

#### DavidBC

##### Active Member
The thread is a very early English thread. I have attached a jpeg file with the info you are after. It was known as a bicycle thread but is now called British Standard Brass (55 deg). I have a 5/8" x 26 TPI tap that you are after but living in Australia I cannot help you but I hope the attached file does. The column on the right is the tapping drill size. The image was taken of a thread chart of an Australian company called Sutton Tools.

www.sutton.com.au

Regards
David

#### Attachments

• British Stamdard Brass (55 deg).jpg
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#### DavidBC

##### Active Member
Further to my post if you look at a chart that shows BSF Screw Threads you will find the depth of a 26 TPI thread is .0246".

Regards

David

#### MRA

##### Well-Known Member
Even if I know the exact thread dimensions I'm not a good enough machinist to hit them -- I just get close, then shave & try, shave & try.
Me too! not forgetting that when I cut threads I sometimes push a burr up at the crowns, which can make things seem tight until I take it off, at which point things are really not tight at all

#### Charles Lamont

##### Well-Known Member
HMEM Supporting Member
26 TPI is a pitch that is common in the UK. 1/4" and 9/32" BSF are 26 TPI. "Gas" or "brass gas" thread is 26 TPI in all sizes and is used extensively in UK model engineering designs as the coarsest of the fixed pitch model engineering thread series, the others being 32, and 40 (and sometimes 60).

All these threads are 55° Whitworth form, as shown above by fcheslop. The thread depth is 0.64 P, so for 26 TPI, the depth of a Whitworth form thread is 0.025".

Of course Alan's thread may not be Whitworth form, but if it is 26 TPI, I think it might well be.

(I typed all this before reading DavidBC's posts)

#### fcheslop

##### Well-Known Member
If you have any 26tpi tap ? .You could use it as a chaser to finish the thread
Like most I suck it and see for the final few cuts.
From memory British Standard Brass is 55 degrees .British Standard Cycle is 60 degrees. It often causes confusion especially if you want a good fit
The chart works well enough for my simple needs
cheers

#### ajoeiam

##### Well-Known Member
I would suggest changing that internal thread to something to is more 'common' today. Then you change its mating part and - - - bob's yer uncle.
Why beat yourself about the head and buy very very expensive tools that you are only going to use once?
If you feel you must keep the thread - - - - by all means - - - - I would 'update' the drawing - - - life is too short!

Went looking for the specs - - - -what a boondoggle!
Doesn't look like there is any way of getting around the issue!!!!!!!!!!
Isn't backward compatibility fun!!!!!!!

Looking at pgs 966-8 (25th ed Machinery's handbook) - - - - well they're ALL oddball threads!!!!

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#### danallen

##### Well-Known Member
Since the pitch is almost exactly 1mm I would use the metric way of calculating tap drill size or thread bore. Subtract the pitch, 1mm or .040, from the nominal OD and start there.

#### JLaning427

##### Active Member
I have a South Bend 9" lathe that uses the 3C collet, so this piqued my interest.

I went looking in my "old" Machinery Handbook (11th edition from 1943), since others had already said 25th Ed didn't have useful info. It lists a bunch of collet sizes, including 3SB / 3C (listed separately but the dimensions are the same). It lists the thread as 0.645" - 26.

It also has a listing for the United States Standard thread form for 26 tpi. It lists 26 this as having a 0.0250" depth of thread, and 0.0048" flat width.

I don't have a 26tpi thread gage, but checked my 3C's with 24, 27, and 28 (which I had) and they definitely don't fit.

James

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#### JLaning427

##### Active Member

-USS threads are 60° and are the predecessor of American Standard threads. I am assuming that they are USS threads, based on finding 26TPI info in the USS section. And the fact that South Bend, Hardinge, and Cataract (the original designer of the 'C' series of collets) were all US based companies, but it could be something else. It could also be British Standard Fine Thread, which shows thread depth at 0.0246 for 26 TPI (I would call that 0.025") and that is a 55° thread, as mentioned above.

-I measured the OD of a couple of my 3C collets, and they were around 0.637" to 0.638" across the threads, vs the 0.645" nominal listed in Mach Handbook.

James

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#### MRA

##### Well-Known Member
If you have any 26tpi tap ? .You could use it as a chaser to finish the thread

A bit pot of mismatched odds and ends which once would have fit a Coventry die head, came my way very cheap (no sets). I thought I might grind them up for small tools (and make a holder), but I ended up using them for chasers which seems a better use.

#### fcheslop

##### Well-Known Member
Also if youre cackhanded enough to break a split style die the wee bits left can also be made into chasers. I have some very old hand chasers that often get me out of trouble although I've only hand chased in brass for old scientific instruments.
Keep well

#### Richard Hed

##### Well-Known Member
Me too! not forgetting that when I cut threads I sometimes push a burr up at the crowns, which can make things seem tight until I take it off, at which point things are really not tight at all
Oh, yeas, that happens, keep a file close by and keep the burrs down, however, there is another thing too if you make American style or metrics with 60deg angles: valleys and peaks on the threads where the peak should be cut down a couple percent. I find that usually a file over the top is enough if my diameter is what is called out for in the drawing

#### Richard Hed

##### Well-Known Member
If you have any 26tpi tap ? .You could use it as a chaser to finish the thread
Like most I suck it and see for the final few cuts.
From memory British Standard Brass is 55 degrees .British Standard Cycle is 60 degrees. It often causes confusion especially if you want a good fit
The chart works well enough for my simple needs
cheers
This is why I don't bother with Whitworth or the other type of threads, only 60deg

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