Help me understand NPT holes.

BRIAN.T · Jan 17, 2019

What is the hole diameter at the top of an NPT tapered hole before tapping? What am I shooting for with a taper reamer? Obviously drill size is easy, but drill size plus taper has me stumped. I just can't seem to find what I'm looking for in the handbook, obviously I'm missing something.

Thanks.

litlerob1 · Jan 17, 2019

Mixed with some very basic Trig. all the answers are in Machinery's Handbook.

Taper amount is clearly stated, the minor diameter of the small end is stated clear as mud.

R

BRIAN.T · Jan 17, 2019

litlerob1 said:
Mixed with some very basic Trig. all the answers are in Machinery's Handbook.

Taper amount is clearly stated, the minor diameter of the small end is stated clear as mud.

R

Ha yeah I figured this is the answer I would get. So I am looking for the basic minor diameter at the small end of pipe, by the thread length (bottom of taper?) at 1.789 deg? See what's happening here... I'm just guessing. Thanks for your help.

Spruewell · Jan 17, 2019

Pipe threads are based on the size of schedule 40 black pipe which is measured by the inside diameter of the pipe. 1/2” schedule 40 pipe has a 1/2” hole through it. As pipe schedule changes, the outside diameter remains the same and the inside diameter shrinks for the same size pipe.

litlerob1 · Jan 17, 2019

If you just want the answer to your question then here;

The taper per foot is 3/4". Meaning the taper per Inch is .0625" 12/.75=.0625/2=.03125 Indicator rise per Inch.

So if the Minor Diameter at the small end is 8.75mm/25.4=.344"

If the hole is .5" deep/through, then you would take .5*.0625=.03125----soooo .344+.03125=.375

It's all there in MH

Is that more helpful? Come back any time.

R

BRIAN.T · Jan 17, 2019

litlerob1 said:
If you just want the answer to your question then here;

View attachment 247200

The taper per foot is 3/4". Meaning the taper per Inch is .0625" 12/.75=.0625/2=.03125 Indicator rise per Inch.

So if the Minor Diameter at the small end is 8.75mm/25.4=.344"

If the hole is .5" deep/through, then you would take .5*.0625=.03125----soooo .344+.03125=.375

It's all there in MH

Is that more helpful? Come back any time.

R

Awesome, thanks for the reply. I certainly understand the math behind it, but I'm still unsure of the starting point for my taper. Say I've got .560 hole thru 2 inches, 3/8 npt nearside, I can't start the taper at the far side of the two inches depth. The hole would be much too large at the nearside.

litlerob1 · Jan 18, 2019

Again its in the handbook. Look at dimension L4.

R

Bobw · Jan 18, 2019

Just spend the $100 on some thread pal.. Its all there. Best $100 I ever spent.
Its made by a member here...

I haven't touched a machinery handbook in about 5 years. I haven't had to dig
for the J thread spec.. I haven't had to use a calculator to calculate wires.
I haven't had to look up drill sizes for form taps...

And it saves a ton of time when turning pipe threads.. That was always a pain
in the ass trying to figure out what the hell the handbook was telling you..

Now. Diameter at pipe face, taper out from there, and thread depth..
Even prints out a pretty little report for you..

BRIAN.T · Jan 18, 2019

litlerob1 said:
Again its in the handbook. Look at dimension L4.

R

Thanks again! Now I know, and I appreciate it!

BRIAN.T · Jan 18, 2019

Bobw said:
Just spend the $100 on some thread pal.. Its all there. Best $100 I ever spent.
Its made by a member here...

I haven't touched a machinery handbook in about 5 years. I haven't had to dig
for the J thread spec.. I haven't had to use a calculator to calculate wires.
I haven't had to look up drill sizes for form taps...

And it saves a ton of time when turning pipe threads.. That was always a pain
in the ass trying to figure out what the hell the handbook was telling you..

Now. Diameter at pipe face, taper out from there, and thread depth..
Even prints out a pretty little report for you..

I like this, I'm definitely going to look into this. Anything to make life easier is worth spending money on. I appreciate the help man!

GENERALDISARRAY · Jan 18, 2019

On a comparator, measure the minor tsc of your plug gage at the notch.

Spoiler. It will be whats in Machinery's Handbook

BRIAN.T · Jan 18, 2019

GENERALDISARRAY said:
On a comparator, measure the minor tsc of your plug gage at the notch.

Spoiler. It will be whats in Machinery's Handbook

Come on man, why do you have to ruin the surprise! Thanks for the tip!

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Help me understand NPT holes.

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