How to call out a NPT Thread

[LONDONDERRY]

How do you call out a NPT thread length on a drawing?

I have a part that is 17.75 mm thick and I called out a 1-11.5 internal NPT pipe thread. The tap has a tapper on it, s if I don't tell where to the machinist to stop in depth with the tap, the hole will be over sized, so the NTP fitting will not thread in tight. Do you understand?
So how do I call out on a drawing, how much of the tep in length to us?

Frank

 

[ewh]

I would dimension the max dia of the hole allowable.

 

[MintJulep]

Tapered pipe thread is an abomination, and should not be used in the first place.

17.75 mm is only slightly longer than the minimum recommended length of engagement for 1 inch NPT, so it is very likely that you will never get a good seal on this part.

 

[LONDONDERRY]

So in order to get a a good seal I should increase the thickness of the parts?

 

[CorBlimeyLimey]

Can you change the NPT fitting to one which uses a parallel thread?

 

[ctopher]

Don't know if this helps.

Chris
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 10-27-06)

 

[MintJulep]

What CBL said.

Metric: ISO/DIN 228/1 (BSP, JIS B0202) parallel threads with appropriate O-ring or gasket.

Inch: SAE or MS 16142

 

[LONDONDERRY]

I rather not change to parallel fittings, because I've all ready speced out all the fittings

Frank

 

[MechNorth]

You can use low-profile (short?) NPT plugs, which reduces the thickness requirements, or are you going to live witht he plugs sitting above the workpiece surface?

To specify a shallower tap, just dimension it on the drawing, but make sure you talk to the machinist if you can to confirm that he notices it. I know, the drawing should speak for itself, but this is reality.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[CorBlimeyLimey]

A less than perfect but usually workable alternative is to use a NPSF female thread.

Whichever way you go, with NPT fittings it is advisable to use teflon tape or other sealing substance to help achieve a seal.

 

[11echo]

...Let me guess most you guys are LIMEYS ...Huh!!? *L* When you use tapered pipe thread in the varying sizes they all have a normal thread engagement dept (GOOD machine shops "should" know this) ...and if I did the math correctly 17.75mm is 3/8" pipe ...normal thread engagement is 19/64" by my chart. Also it is not called out as "internal pipe thread", its called FNPT (female national pipe thread) and MNPT (male national pipe thread)...Do I need to explain the difference between female & male part? *G*
AND as Mr. CorBlimeyLimey pointed out they do make a straight nation pipe thread (NPS)...this is the stuff you'll find on electrical conduit and conduit fitting. This is NOT the normal threading for pipe that you don't want to leak.
...AND Mr. MintJulep my friend, you would not do well here in U.S.A. oil field environment! *L* …Good Luck all! …Mark

 

[powerhound]

In the Machinists Handbook there is a table for minimum handtight thread engagement for NPT. Whenever NPT is specified, a depth of tap callout is not required because there are standards specified according to the tap size. The engagement is given in inches in the Handbook but if you convert the distance to number of threads per inch you will always get a number that's somewhere around 3-6 threads. I seem to remember 1/4" NPT having a minimum handtight engagement of about 3 1/2 threads. I don't have my handbook here but when I get in to work tomorrow I'll give you the engagement on a 1" NPT if someone hasn't beat me to it by then.
MintJulep,
NPT definitely has it's applications. I'm interested to know what could be used in it's place in a plumbing environment.


Powerhound
Production Supervisor
Inventor 11
Mastercam X
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[CorBlimeyLimey]

11echo ... I have assumed that your response was intended as being somewhat playful considering the *L* & *G* usage, so I have scaled back my initial knee-jerk response;

First Mistake; (Making guesses)
"...Let me guess most you guys are LIMEYS ...Huh!!?"
Was there a point behind this comment?

Second Mistake; (Not reading the OP correctly)
The 17.75mm dimension was referring to the thickness of the plate to be tapped, not the nominal of the pipe thread.

Third Mistake; (Math)
Well actually you did the math (conversion) correctly, unfortunately you converted the wrong thing.

Fourth Mistake; (Forum Etiquette)
Criticising other posters contributions without contributing a damn thing to help the OP.


The OP stated that the pipe thread he wanted to use was 1"-1.5 which is 1.327" actual O/D and has an effective thread length of .682". (Yes, I can read a chart too.) The .682" dimension by the way is 17.32mm, which is why your "friend" suggested that the 17.75mm thickness might not yield a succesful seal.

BTW, straight NP threads are also used on many regular pipe fittings (valves, elbows, connectors, etc) ... not just conduit types.
Also, the OP mentioned "NTP fitting" not pipes.

For future reference, my handle is CorBlimeyLimey (or CBL for a more user-friendly reference) not "Mr. CorBlimeyLimey". IMO, the "Mr" title (in the context you used) came across as being condescending ... but then I do have an intense dislike of titles so that could just be me.

 

[powerhound]

Hi CBL,
I too had to go back and edit my original response to 11echo's post as I figured 2 wrongs would not make a right. I decided to post something potentially helpful instead of critical of others' contributions.
As far as location goes, I live here in the USA but am, by no means, a "roughneck". I still have all my fingers and teeth.

Powerhound
Production Supervisor
Inventor 11
Mastercam X
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[11echo]

…You know as soon as I posted my first response I KNEW I would probably catch H*LL from somebody. SO let me say up front I apologize …IF I stepped on any toes in my attempt at humor here! I used the term “LIMEY” not as any derogatory comment, but only as tile as the Brits had called me “YANK” in my dealing with them…I didn’t take it as derogatory then or do I now. Also I use the tile of “MR.” as a sign of respect; if you feel that is also derogatory I can only suggest you “lighten-up”! I was feeling particularly cheerful that week with the birth of my first grandchild, and again I can only say I’m sorry if I got any extra “cheerfulness on you”.

 

[CorBlimeyLimey]

11echo ...
First of all, congratulations on the birth of your Grandchild. Is it a boy or girl? Either way I hope you will have as much fun with him/her as I am having with my grandkids.

Just to clarify things, I didn't and don't take offence at being called a Limey. How could I with my handle? I just didn't understand the relevance.

As for the "Mr" thing, well as I wrote before, that's probably just me, but given the overall content of your post, I interpreted it as being condescending ... especially when followed by your derogatory opinion of MintJulep's competence and the "my friend" reference. Unfortunately the "intonations" of the written word can be all too easily misinterpreted.

Anyway, please accept my apologies if my response detracted from the enjoyment of your happy news.

 

[11echo]

...It's a Boy 8 Lbs. 12Oz.s! *G* …Well I hope I haven't alienated the whole world of engineering/design with my "shot" at levity, but I found it funny that the guys on the left side of the pond were having problems with a connection system that is in common use here. I must confess that I have an equal problem dealing with B.S.P. (British Standard Parallel) connections myself, and have the same thoughts as "Mr." MintJulep regarding them!
AGAIN my humble apologies for a “lame” attempt at humor …AND my best wishes for a merry holiday season!

 

[JohnNabors]

Londonderry-

I don't know what sort of volume you are running, but a 1-11.5 NPT plug gage would only run you about $150 USD. That will control your depth.

 

[looslib]

While this debate has been going on far a while, let me inject my $.02.

Do you really need to call out the depth of the NPT tap?
Any machinist should know how deep to tap the hole to get a proper fit on the thread.

Your job on the drawing is to define what you want.
The machinist's job is to make the part to your design.

By only calling out the NPT thread on the hole, you have done your part. How deep the machinist taps the hole to get the proper fit, is his job.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli
Sr IS Technologist
L-3 Communications

 

[powerhound]

I agree! When I was still a machinist, I knew where to look for the spec for thread depth. As I stated in a previous post, the Machinist Handbook has the table for this spec. I've never seen a print that specified depth, only what size thread. I then referred to the Handbook for the minimum handtight engagement and went from there.

Powerhound
Production Supervisor
Inventor 11
Mastercam X
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II