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Threading and Drill size

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LuckyKnives

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#1
Here's a question for youz guyz who know what you are talking about. I want to tap 1" x 11.5 NPT threads in a piece of brass. Looking at a man charts online for the proper drill size to prep the hole, I am given 1 5/32 - how can the hole be larger than the threads I want to cut? Everything else I've ever done has the drill being smaller than the tap - is there some magic to NPT? Thanks!
 

dulltool17

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#2
1" NPT refers to nominal inside pipe size. I don't have my tap chart handy, so I'll use an example,
A fitting of ~1" ID and 1/8" wall would be 1-1/4" OD.
If this were a straight thread, minor dia. would be 1.156" and Major dia would be a hair over 1-1/4.
Then consider that this is a tapered pipe thread..

Yeah-pipe nomenclature is a weird area..try explaining to folks that 1/2" nominal Copper pipe is 5/8 OD, while 1/2 refrigeration tubing is actually 1/2" OD
 

RJSakowski

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#3
1" black or galvanized pipe has an o.d. of 1.315", the i.d will depend on what schedule it is. Sch. 40 is 1.049" and Sch. 80 is .957". Normally, if you subtract the pitch (1/tpi) from the major diameter, you would get the tap drill diameter for a 75% thread but since pipe threads are tapered and must seal to the fittings, requiring a 100% thread, the tap drill is smaller. If you were cutting straight pipe threads, the tap drill diameter would be 1.278" for a 75% thread.
 

Mitch Alsup

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#4
I want to tap 1" x 11.5 NPT threads in a piece of brass. Looking at a man charts online for the proper drill size to prep the hole, I am given 1 5/32
1" pipe is not 1" OD nor is it 1" ID it all depends on the schedule.
 

Bob Korves

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#5
There are plenty of online charts showing the various ID and OD sizes of nominal pipe sizes. What you are going to use the pipe threads for greatly influences how you proceed and the size of the hole for tapping. There are also tapered reamers that open up the hole on a taper so the tapered tap will not need to work so hard. A lot of mechanical and lighting work is also done with straight pipe threads.
 

LuckyKnives

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#6
hey thanks for all the great replies. Not having any of the material purchased yet I didn'the have it on hand to measure, so that's a big help! I make knives and want to make one of those tubular handled survival knives. I'll post a pic if I ever get it done.
 

LuckyKnives

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#7
While i'very got you on the phone, let me ask this as well: why are pipe threads tapered? That is messing with my knife handle plans as well.
 

gwade

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#8
Pipe threads are tapered so they will seal when tightened into a female fitting. If straight threads are used, then sealing with an o-ring or something similar is required.
 

RWanke

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#10
If you're making a survival knife, why do you need tapered pipe threads? I would use a straight thread of my choosing with an O ring seal. I would think tapered threads would be harder to unscrew (I'm assuming this is for the handle cap).
 

P. Waller

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#11
NPT is an outdated standard that is for some reason still used to this day. There are many other methods of connecting fluid couplings that are far easier to produce and use. I can only surmise that a resistance to change of any kind is the underlying reason that it still exists at all.
In 1864 the origins of NPT began.
 

BaronJ

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#12
Hi Guys,

Doesn't code require that if you are working on pipe that uses tapered threads, you are required to replace fittings like for like.
 

mcostello

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#13
Pipe threads are Imperial. :grin:
 

Janderso

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#14
NPT is an outdated standard that is for some reason still used to this day. There are many other methods of connecting fluid couplings that are far easier to produce and use. I can only surmise that a resistance to change of any kind is the underlying reason that it still exists at all.
In 1864 the origins of NPT began.
Interesting, in the middle of a Civil War, we come up with pipe threads. I find that interesting.
The wheels of progress never stop.
 

eugene13

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#15
NPT is still in use because it works. Most piping systems are built to not be taken apart and will never have a problem. It's when you have to make a repair that you often run into problems. It only takes a small nudge or a slight twist in the wrong direction to loosen a fitting and cause a leak. (and 50% of the time you'll be turning in the wrong direction)
 

P. Waller

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#16
Horse drawn carriages will deliver a product to the retailer.
Few use this method in 2018 (-:
 

Eddyde

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#17
One other thing to consider, To properly tap a female thread for pipe, the hole must be tapered, not straight. Tapered reamers are available to accomplish this.
 

Bob Korves

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#18
Horse drawn carriages will deliver a product to the retailer.
Few use this method in 2018 (-:
Just about 100% of the USA is plumbed with NPT plumbing. Agricultural, domestic, industrial, and utilities. It does an excellent job of delivering products by using pipes. Replacing all that infrastructure will be an immense undertaking, and will not likely add more reliability. Yet, it is being replaced in some applications already as improved systems emerge, and will continue.
 

Tozguy

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#19
Pipe threads are definitely not suited for a survival knife. The torque required to seal the threads is too high for bare hands. Ask me how I know.
 
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