# 11.5 tpi - how to do it?



## mklotz (May 9, 2009)

One of our club members solicited my help in cutting an 11.5 tpi thread on his lathe. (Don't you just wish you could get your hands on the throat of the cretin who comes up with these oddball pitches?)

The lathe has a QC (quick change) box and he has some change gears for it.

My thinking went like this... If the QC is set to cut 10 tpi, then, if I introduce a gear ratio of 10/11.5 into the train leading up to the input to the QC box, the lathe will now cut an 11.5 tpi thread on the 10 tpi setting. I picked the 10 tpi setting because

10/11.5 = 40/46

and he has 40 and 46 tooth change gears.

I don't see any problem with this approach since it entails doing something almost identical to installing the change gears used to shift an Imperial lathe to cutting metric threads.

Nevertheless, I've never done this personally so I'm raising the question here for comments, criticisms, etc.

Will it work?
What are the potential gotchas?
Has anyone done something similar?


----------



## Mainer (May 9, 2009)

Seems plausible. It will need to be a compound gear setup, of course.

What kind of lathe is it? The QC gearbox on my South Bend has 11 1/2 as one of the positions.


----------



## Tin Falcon (May 9, 2009)

Use the correct division on the thread dial IE 1&3 or 2 & 4 or to be safe just use the same main division for each pass. 
Tin


----------



## steamer (May 9, 2009)

Marv,

That is correct....I would further comment to just leave the half nuts engaged Tin if it's possible.

I did the same thing to cut a 1.6667 mm pitch buttress thread for my Waltham lathe on my Logan with QC

12 pitch with a 37/47 metric transposition set......gets you close enough.  Yes Marv it was an American company in 1899 who created that pitch thread to assure you bought your collets from them.......... 

Now Shaublin calls it a W20 and gets a $100 each for them....go figure.




Dave


----------



## DavesWimshurst (May 9, 2009)

Marv,
Live Steam magazine ran an article on this subject in December 1978 titled "You Can Cut Special Threads on Your Quick Change Lathe" which uses your suggested method. It covers odd pitch threads and approximations for metric threads and also cutting worms to mesh with gears. I've made approximate metric threads on my SB heavy ten using this method and it worked okay.

11.5 tpi sounds like a pipe thread? On my SB several really odd threads arise as fraction of other used threads such as 6.75 tpi being one fourth of 27 tpi (1/8 inch pipe).

Since the ratio is near one I'd probably cut a test on scrap first to be sure I didn't get the ratio inverted.
Dave


----------



## mklotz (May 9, 2009)

Yes, my lathe has 6.75 tpi as well. I just know that I'm going to be mightily annoyed if I ever have to make replacement parts for anything that uses such an oddball. It'll probably be something like the bolts for attaching the termite trap to a Morgan.

Getting the ratio right side up is pretty easy. To cut 11.5 tpi threads (0.087 ipt), the carriage is going to have to move more slowly than it would when cutting 10 tpi (0.1 ipt), therefore the gears must be installed in such a way as to slow the drive to the QCB. Nevertheless, my friend would be an idiot to not cut a test article first. And, yes, the threading dial becomes an entertaining but utterly useless accessory in this operation.


----------



## Tin Falcon (May 9, 2009)

> And, yes, the threading dial becomes an entertaining but utterly useless accessory in this operation.


Marv : All references I have read suggest that the threading dial is quite useful for odd pitch threads. But Adding the transposing gears may throw things off a bit. 
Tin


----------



## RonGinger (May 9, 2009)

In one of the very early issues of MODEL ENGINEERRS WORKSHOP there was an article for a very clever device for strange thread pitches. It is a thin dovetail slide made to sit under the lathe compound, and slide along the axis of the lathe. It is operated by an arm and bell-crank. The end of the arm has a guide pin that rides in a groove in an adjustable guide.

To use it the lathe is set to cut the closest standard thread pitch. For 11.5 you would probably use a 12 tpi setting. The bell crank is adjusted to cause the tool to slowly shift as the carriage slides, to make up the difference in the desired cut. The dovetail only has to be long enough to be less than one thread pitch.

My friend Rollie (of Rollies fathers fame) made one of them and has cut many very odd threads, including metric and odd tpi values. He often shows the device at shows.

The article was written by Don Unwin and listed as just 'Florida' Does anyone know Don? Id love to put his device on the web somewhere, but wouldnt want to copy his work without permission.


----------



## steamer (May 9, 2009)

Hi Ron,

I remember Rollie showing me that....thanks for reminding me.

Dave


----------



## Cliff (May 10, 2009)

I always thought that 11.5 tpi was a big pain when I use to cut them I always wondered why didn't they use 12 tpi. I think that water hose fittings are 11.5 tpi and some big pipe thread's. When I cut them I would engage the half nut on the same number every time just in case I was always afraid that I was going to screw up any time. Cliff.


----------



## rake60 (May 10, 2009)

All of the lathes I have ever used including my Seig made Grizzly G4000 listed
the gearing and thread dial numbers for cutting an 11½ pitch thread.












I must have just been lucky.

Rick


----------



## shred (May 10, 2009)

Interestingly my "big China" lathe (4003G 12x36) doesn't have 11.5 on the QCGB, but my 1930's Sheldon does have it on the change wheel chart, along with such fun things as 5.75 tpi (half of 11.5) .. I guess somebody once really wanted 11.5 or 23


----------



## steamer (May 10, 2009)

My 2557T Logan has 4.5 5.5  5.75 6.5 7 and of course 11.5....224 also.....


I've used the 128 once....








Dave


----------



## kvom (May 11, 2009)

My 1942 10EE has 11.5 as well.


----------



## mklotz (May 11, 2009)

Disappointingly, the quest for a way to squeeze 11.5 tpi out of a lathe that lacks it on the QCB has come to an end. My friend wimped out and got another buddy, who has a lathe that will do 11.5, to make him a tap. (Don't you just hate those friends who will solicit your help and analysis and then ignore it and go out and do it some bog simple way?)

My (Taiwanese) lathe doesn't have an 11.5 setting but, as several of you have shown, older American lathes do. Perhaps that pitch was once more common here in the USA and so American manufacturers had to cater to it.

At any rate, thanks to all of you for your time and suggestions. Hopefully this thread may be useful someday to someone who wants to coax his lathe into producing a thread not available on the QCB. I'm pleased to learn that my idea has already been presented (Dave W's comment about seeing it in "Live Steam"). That means it's not entirely implausible and, so, worth remembering for the next time a similar problem crops up.


----------



## steamer (May 12, 2009)

quote
 "That means it's not entirely implausible and, so, worth remembering for the next time a similar problem crops up."


It's no different than determining the ratios for a loose change wheel lathe. Its just a gear train Marv.


Dave


----------



## kvom (May 12, 2009)

This thread made me wonder if many of the threads a lathe can cut are "accidental". For example, the 10EE gearbox has A, B, and C settings which cause the TPI to double on each setting of the dial. So we have 5-3/4, 11-1/2, and 23 as one series. If 11-1/2 was a common thread in 1942, perhaps 5-3/4 was just there to allow the gearbox to cut the other two.


----------



## steamer (May 12, 2009)

Certainly a possiblity KVOM.  Mechanical Design is a series of compromises...sometimes you take the bad with the good.


Dave


----------



## DavesWimshurst (May 12, 2009)

Kvom,
You have certainly hit the reason for some of the oddball thread pitches, on my southbend each row
of the gearbox is half the tpi of the row below. If any one of the row is needed you get the others whether useful or not. 
However 11.5 tpi is used for pipe threads in sizes from 1'' to 2":
http://www.engineeringtoolbox.com/npt-national-pipe-taper-threads-d_750.html
These are a tapered thread so a taper attachment is probably needed to get much use out of this pitch also.
Dave


----------



## gilessim (May 12, 2009)

Why don't we just rename 11.5 TPI to 1 OTPU (odd thread pitch unit)?

Giles


----------



## steamer (May 12, 2009)

Rof} Rof} Rof}


Great!


But only if I can call 1.6667mm pitch buttress threads 1 totally wicket bizare thread unit ( 1 TWBT)


----------



## shred (May 12, 2009)

I suspect some thread pitches are driven by marketing: "The competition's new lathe can cut 96 thread pitches. We need more! Joe, throw another gear on there! But Boss, nobody will ever use those. I don't care, I want more! We'll advertise 128 threads! The North Bend Lathe Company Name will live forever!"

The rest could happen like so: "Well, old Eli never was much for sitting down figuring things out or making drawings, he just sorta cut a thread that looked about right. When the Army wanted a million of his frambulators, they took Eli's prototype to a guy to make drawings and he didn't dare change a thing. Eli always did get a chuckle out of that five-and-three-quarters tpi"


----------



## steamer (May 12, 2009)

The 128 I did was for a needle valve on a model airplane engine......which came first the lathe with 128 or the part with 128.....well some mysteries are better left untold....

Dave


----------

