Threading and Drill size

LuckyKnives

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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!
 
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
 
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.
 
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.
 
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.
 
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.
 
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.
 
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).
 
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