Specifying female 1/16-27 NPT on drawing.

[KENAT]

I’m checking a drawing that has a long (11 in) thru hole (dia.201 in).

One end calls up “1/16-27 NPT” (which receives a plug at the next level assy).

1. Per ASME B1.20.1 do I need to specify the minimum thread depth on the drawing? Simplistically I’d expect so but I suppose this could be set in the standard.

2. If I should then what value should I give? Looking at the table in Machineries should depth of thread = L1+L3 where L1 = length of hand-tight/normal engagement and L3 is Wrench makeup Length?

I don’t have ASME B1.20.1 but have been looking in machineries (27th edition page 1860-1863) and can’t make it out.

I looked at thread1103-170383 and

http://i15.tinypic.com/2nv438j.jpg

referenced there on but am still confused.

Please note, no offence to anyone but I’m not talking about tap drill depth; I’m not interested in a debate on the merit of NPT threads; I’m not interested in what a ‘competent’ machine shop will normally do in lieu of fully detailed drawing or any of the other things that seem to take other threads on this or related subjects off topic.

Thanks.


KENAT, probably the least qualified checker you'll ever meet...

 

[drstrole]

At my current place we do not specify the thread depth. The reason for this is that the thread depth is determined by the standard and checked with the gages.

David Strole
Engineering Systems Administrator
GDTP S-0132

 

[ctopher]

I have called out the same as David, but in some circumstances and depending on the design, I have called out MIN thd depth and sometimes the drill depth. Depends on the mating part and overall design.
I have had parts not fit because min thd depth was not indicated.

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Apr 30, 2008)

 

[MintJulep]

If the plug needs to be flush or below the surface, then I would say yes, you need to spec the thread depth to ensure that the plug will end up where you need it to be.

If it's ok for the plug to stick out by some unspecified amount, then probably not.

 

[powerhound]

As David said, the depth should be per standards. For functionality, specifying a minimum depth below the standard minimum handtight engagement is not an option. If you receive parts that are not deep enough per the standard, send them back for rework at your vendors expense. They did it wrong. Tapping too deep will result in a non-functional part. Page 1849 of Machinery's Handbook #26 has the standard to use. There is definitely a lot of leeway between the minimum handtight engagement and a thread that is so deep that it doesn't seal.
Any design using NPT threading should take all this into account. If the standard thread depths are insufficient then the design may need to change to use o-rings.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[KENAT]

So just to finalize it then:

According to ASME B1.20.1 (which is invoked explcitly on the drawing) I do not need to specify a minimum depth in order to guarantee getting a working hole, this depth is given by the standard (or a calculation there in).

By working hole I mean the plug will go in and not bottom out before sealing (I know PTFE tape or compound etc is needed to create a full seal).

Thanks all.

KENAT, probably the least qualified checker you'll ever meet...

 

[fcsuper]

Mint,

Because of the nature of NPT (tapered threads), there is no way to say where the plug would end up. One certainly could no specified it to end up flush with the surface, as this would defeat the purpose of the NPT (to create a barrier).

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[CheckerRon]

I concur with what the others have written. The depth an NPT taper pipe thread is normally never specified on the drawing. Tap drill for it would be "in-process" and not specified either on the product drawing.
As Mint Julip says, if a flush plug is used (internal square socket), then assure that the thread depth is sufficient to flush mount the plug.

 

[KENAT]

Thanks, enough of you agree that I'm confident not putting a depth on the drawing, I don't believe the plug needs to be flush.

Appreciate the help.

KENAT, probably the least qualified checker you'll ever meet...