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 Post subject: Anyone Have a 3-3/4" dia. x 11-1/2 TPI Taper Tap?
PostPosted: Fri Mar 15, 2019 1:48 pm 

Joined: Tue Sep 14, 2004 7:52 am
Posts: 1553
Location: Strasburg, PA
Checking out in the community:

Does anyone have a tap for tapered threads that is 3.75" major diameter on the small end, and 11-1/2 threads per inch? If so, may we borrow or rent it?

Thanks.

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"It was not easy to convince Allnutt. All his shop training had given him a profound prejudice against inexact work, experimental work, hit-or-miss work."
C. S. Forester

Strasburg Rail Road Mechanical Department


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 Post subject: Re: Anyone Have a 3-3/4" dia. x 11-1/2 TPI Taper Tap?
PostPosted: Fri Mar 15, 2019 5:16 pm 

Joined: Wed Oct 25, 2006 12:12 pm
Posts: 116
Location: Bremerton, WA
If nobody has one, you could use a thread mill. To my knowledge, our Trak mills don't have the right package to do tapered threads with a thread mill. Our Moris frequently do tapered threads. I would find a friendly job shop with a capable mill and just thread mill it using the mating part as a plug gage, if available. Buying one of those taps new would be WAY more expensive than a couple of hours of machine time and a new thread mill. Good luck.

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 Post subject: Re: Anyone Have a 3-3/4" dia. x 11-1/2 TPI Taper Tap?
PostPosted: Fri Mar 15, 2019 7:02 pm 

Joined: Tue Sep 14, 2004 7:52 am
Posts: 1553
Location: Strasburg, PA
Adam Phillips wrote:
I would find a friendly job shop with a capable mill and just thread mill it
Good idea except the part in question is the boiler, not too portable.

If we must, we can cut the rivets holding the part in question to the rest of the boiler, thread mill it and rivet it back on, but would really rather not.

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"It was not easy to convince Allnutt. All his shop training had given him a profound prejudice against inexact work, experimental work, hit-or-miss work."
C. S. Forester

Strasburg Rail Road Mechanical Department


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 Post subject: Re: Anyone Have a 3-3/4" dia. x 11-1/2 TPI Taper Tap?
PostPosted: Fri Mar 15, 2019 7:39 pm 

Joined: Mon Feb 26, 2018 12:47 pm
Posts: 3
Have you seen this?

https://www.ebay.com/itm/steam-locomoti ... Swwo1XctSg

Nyle


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 Post subject: Re: Anyone Have a 3-3/4" dia. x 11-1/2 TPI Taper Tap?
PostPosted: Fri Mar 15, 2019 8:14 pm 

Joined: Tue Sep 14, 2004 7:52 am
Posts: 1553
Location: Strasburg, PA
That's very cool, and so close, but oh so far away. That tap has a 12 pitch thread, and we need 11-1/2 threads per inch. I know it doesn't sound like much, but it's enough to make a hash of the existing threads.

Thank you for the tip just the same.

_________________
"It was not easy to convince Allnutt. All his shop training had given him a profound prejudice against inexact work, experimental work, hit-or-miss work."
C. S. Forester

Strasburg Rail Road Mechanical Department


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 Post subject: Re: Anyone Have a 3-3/4" dia. x 11-1/2 TPI Taper Tap?
PostPosted: Fri Mar 15, 2019 10:29 pm 

Joined: Fri Aug 20, 2010 8:25 pm
Posts: 295
Maybe you can super cool the material to be tapped with nitrogen, then thread the ID as a 12 TPI thread, once it warms back up you will end up with a 11-1/2 TPI thread.....

Or heat up a 12 TPI tap until it is a 11-1/2 TPI tap....

Just kidding, if nobody has one of those taps, you might get by making one out of drill rod and skipping all the heat treating steps. For a few threaded holes a sacrificial "tap" might be cost effective. Maybe taper thread a chunk of drill rod, mill in three/five/seven flutes, sharpen it with a stone and cut your thread ?


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 Post subject: Re: Anyone Have a 3-3/4" dia. x 11-1/2 TPI Taper Tap?
PostPosted: Sat Mar 16, 2019 12:45 am 

Joined: Thu Mar 15, 2007 12:47 pm
Posts: 97
Location: Arizona
One has to wonder how many innocent washout plug holes were buggered by some guy chasing the threads with a 12 TPI tap.....

I know of at least a few.....


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 Post subject: Re: Anyone Have a 3-3/4" dia. x 11-1/2 TPI Taper Tap?
PostPosted: Sat Mar 16, 2019 3:14 am 

Joined: Wed Oct 25, 2006 12:12 pm
Posts: 116
Location: Bremerton, WA
"If we must, we can cut the rivets holding the part in question to the rest of the boiler, thread mill it and rivet it back on, but would really rather not."

I know what you mean. I don't have enough mass to handle a 90-gun. The price of a new tap seems justified and it can stay well preserved on the shelf for the next job.
Good Luck.

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