what size drill bit for 1/8" NPT tap

[ralphiejantz]

just wondering what size drill bit I should be using tapping a hole with an 1/8" NPT tap,
I only have a very small set of bits like 14 or so and none seem fit all that well,
so I want to buy an individual drill bit that will work
would like to know what size I should get

[lugweld]

Most good taps have that written on them. However since it may be small to read or nonexistent, my guess, without looking, would be 3/32

[duaneb55]

'R' drill size actually (.3390"). 11/32" is closest fraction (.3437").

http://www.physics.ncsu.edu/pearl/Tap_Drill_Chart.html

[Oldiron2]

Assuming you mean American National Standard Taper Pipe Thread, also known as Briggs Standard Pipe thread, the recommended drill size is 11/32". A tapered reamer used after will help give better threads.

[arvidj]

I just happen to get the 28th edition of the Machinery's Handbook today. This is the first thing I've researched that actually might be useful - everything else today has been browsing this tome ...

1/8-27 tapered thread without a reamer - "Q" (0.332), with a reamer - 21/64, straight pipe thread - "R" (0.339).

From smallest to biggest it is 21/64, then "Q", then 8.5mm, then "R", then 11/32.

I think the common hardware store variety of pipe falls into the "tapered thread" category.

But that is probably more than you wanted to know and the 11/32 that Duane suggested will probably do the trick.

[lugweld]

ohh,

Sorry, bleary eyed. I skipped the NPT.
Just thinking NC

[farmersamm]

REAMER

This is good!!! I never think about using a reamer. I do a lot of grease fitting tapping, and I just tap straight through with a non bottoming something or other tap. I forget what they call 'em. I'll have to go lool in the AM.

Thanks Guys!!!!!!!!!!!!

[farmersamm]

It's a tapering tap, just looked it up in an old Enco catalog. I guess I'm sorta safe

[farmersamm]

Ok, here it is.

Apparently I've been messing up. I shouldn't tap thru. It destroys the taper, and doesn't give the NPT a good bite. Idiot

I'm only grabbin' good thread near the top of the hole. No taper

[Oldiron2]

This is good!!! I never think about using a reamer. I do a lot of grease fitting tapping, and I just tap straight through with a non bottoming something or other tap. I forget what they call 'em. I'll have to go look in the AM.

Is that the size you use, Samm? I've seen grease fittings in a variety of sizes, from #12-24 and/or #12-28 to 1/4-28 to 1/8" pipe to huge stuff on dozers and such, IIRC. Those first few are straight thread.

[farmersamm]

Oh no, not the size. The way I've been using the tap wrong. Everything I have is 1/4-28. And that's getting hard to find here. They're all going to metric at the store.

[farmersamm]

I think the drill size is 7/32

[farmersamm]

I need to quit running the tap all the way thru to the straight part of the tap.

Well, that added another 3 posts

[farmersamm]

Hell, the damn tap says 1/4-28, that's all I know So does the package of nipples

[MoonRise]

Samm,

1/4-28 is a 'regular' thread, a UNF thread (Unified National Fine thread). The hole and tapped threads for that are straight, not tapered.

Then what/where is that "taper chamfer tap" thing doing, you ask?

To make it easier to tap a hole and turn it into an internal (female) thread, the tap often has a taper at the tip in order to help the person line up the tap with the hole and to 'ease into' the actual cutting of the threads.

That taper chamfer tap is just fine (and the generally preferred tap to use) when you are tapping a through hole (a hole that goes all the way through the material, as opposed to a "blind hole" where the hole stops in the material and has a closed bottom to it). The taper of the tap helps you line it all up (hopefully square with the hole, or you end up breaking the tap!), and then you start turning the tap and it cuts the threads and you run the tap all the way into the hole until you are past the tapered section of the tap and the threads are then nicely cut with the straight part of the tap. Done.

For an NPT (National Pipe Thread) item, the actual threaded portion is tapered. And because they are based on pipes and pie sizes, the NPT numbers don't directly correspond with the size in inches. So, a 1/8 NPT thread is not 0.125 inch in diameter but is based on the thread that would be on the end of a piece of 1/8 inch pipe and is thus about 0.405 inch in 'big' diameter. The threads (and pipe end and the hole as well) are tapered to make them leak less when you put them together and if you put some thread tape/dope on the NPT items and tighten them up enough, they will usually not leak (much ).

The OP's question has already been answered. Q or R or 11/32. Tapered reamer optional, as the tapered pipe tap pretty much has the taper built in.

[farmersamm]

Hey, thanks guys. I've been doing it wrong all this time Time to get with the program.

No wonder the damn things don't snug up until the nipple is clear into the hole

[Magnetic Mechanic]

arvidj,
Did you get the book or the disk?

If disk, what format is it and can you search the whole voluum?

[arvidj]

Originally Posted by Magnetic Mechanic

arvidj,
Did you get the book or the disk?

If disk, what format is it and can you search the whole voluum?

I just got the book - the large type edition

I have an old 27th edtion CD around someplace but didn't use it very much because "the need" and "the computer" were never in the same place at the same time. I will try reinstalling it this weekend and evaluate the search capabilities.

Arvid

[arvidj]

Originally Posted by Magnetic Mechanic

arvidj,
Did you get the book or the disk?

If disk, what format is it and can you search the whole voluum?

Ok, I managed to find the CD and install it and then spent a few days working with Industrial Press trying to get the access code to work. They were great to work with and all if working now.

In the end, the book is simply a secure Adobe .pdf file so everything that you can and can not do with an Adobe reader is available - table of contents, searching, printing a page, etc.

Plus there is a 'Additional only on the CD' section with ...

Mathematics
Cement, concrete, lutes, adhesives and sealants
Surface treatments for metals
Manufacturing
Symbols for drafting
Forge shop equipment
Silent or inverted tooth chain
Gears and gearing

Note that this is the 27th edition CD so some of the extra stuff may have made it into the 28th edition hard bound.

And I see that I can upgrade to the 28th edition of the CD for about two thirds the cost of a new CD - something you can't do with the book - i.e. send the old on back and get a new one for two thirds the price. But then again lugging the laptop around the workshop doesn't seem to be to apealing either so I guess I need both.

Arvid

[Magnetic Mechanic]

Thanks.
I think I need to get that disk.
Have the book but cant hardly see the small print.

[arvidj]

Ok, hold the phone - stop the presses ...

I just noticed a nuance in the electronic version. Though it is a .pdf it is MULTIPLE pdf's. Each chapter is a .pdf file. And what this means is that you need to enable the "full reader search" capabilities of Adobe and point it at the Machinery's Handbook directory to search the entire book. The default Adobe setting is to just search the "current document", and given that each chapter is a disparate document it will only search one chapter. But be forewarned that searching it all means that you are declaring it either Miller Time or coffee break time - what ever makes you boat float - because it is going to take some time.

Note that the multiple pdf's is also why getting the 28th edition to work was reported to be such a hassle. Each of the pdf's - i.e. each of the chapters - had to be unlocked separately. Note that they may have fixed that issue. I guess I will find out when I get the upgrade.

Arvid

[Magnetic Mechanic]

Thanks - tis as i thought.

[farmersamm]

Originally Posted by Magnetic Mechanic

Thanks - tis as i thought.

Are ya sure it tis, maybe it twasn't, could be it tweren't, or maybe it ain't

[Magnetic Mechanic]

Twas!

[dakook]

Originally Posted by duaneb55

'R' drill size actually (.3390"). 11/32" is closest fraction (.3437").

http://www.physics.ncsu.edu/pearl/Tap_Drill_Chart.html

every shop I have ever been in has a big poster chart like this some where usually next to the bins/drawers were they keep the drills and taps. Would be nice to have this as a sticky some where around here maybe in the same thread as the sheet metal thickness chart?