NPT Tapping
NPT Tapping
(OP)
Hello,
I am wondering about tapping a 1/4" NPT threads. The drawing from my customer shows the hole 18mm deep and tapped this depth. I see from the handbook the tapping size is 7/16". when i drill my hole then go to tap it, the tap won't go down this depth, maybe only 10mm. Should the hole be tapered ?
Also he has given me a dwg. of a 10mm thick plate with a hole drilled & tapped 1/8" NPT. I assume/guess i just tap from one side as far as the tap will go??
There must be some standard that these thread depths should be. any help appreciated.
I am wondering about tapping a 1/4" NPT threads. The drawing from my customer shows the hole 18mm deep and tapped this depth. I see from the handbook the tapping size is 7/16". when i drill my hole then go to tap it, the tap won't go down this depth, maybe only 10mm. Should the hole be tapered ?
Also he has given me a dwg. of a 10mm thick plate with a hole drilled & tapped 1/8" NPT. I assume/guess i just tap from one side as far as the tap will go??
There must be some standard that these thread depths should be. any help appreciated.





RE: NPT Tapping
I have a Screw Data Selector that shows the tap drill diameter and depth of effective thread per size for NPT.
For 1/8"-27 it states a tap drill diameter of 0.339"(R) and an effective thread length of 0.264" (7mm approx)
For 1/4-18 it states a tap drill diameter of 0.437" and an effective thread length of 0.402" (Less than 11mm)
As for the drawing "SHOWING" full depth threads; I would contact the customer and ask if the illistration is incorrect.
If you try and tap to what the drawing shows, you had better purchase several taps!! They break rather easily.
Hope this helps.
RE: NPT Tapping
Thank you very much, this makes sense.where would i get a screw data selector like yours?
RE: NPT Tapping
This is not the exact scale I have but this is a link to site that contains the information http://www.engineersedge.com/pipe_internal_thrd.htm
If you go to http://www.engineersedge.com there is information on all types threads ect...
The exact scale is made by Alvin & Co in Dallas Texas. I really don't remember where I got it. You may be able to puchase one at your local CAD supply store.
RE: NPT Tapping
J&L Industrial, www.jlindustrial.com, and www.mscdirect.com have them in their catalogs.
RE: NPT Tapping
http://bellsouthpwp.net/r/s/rsnmar/METhreads.zip
RE: NPT Tapping
RE: NPT Tapping
I think mrainey's last point is a key issue. "length of effecive thread is not the same as required tap depth." looking at your link http://bellsouthpwp.net/r/s/rsnmar/METhreads.zip
as you have allready pointed out
it seems that the tap depth of 18mm shown on my drawing is correct. the chart says 17.445mm.
So basically i'm back to my original problem.
so with your help, we've estblished:
1)Tapping Drill 7/16"
2)Tapping Depth 17.5mm approx.
3)effective thread length of 0.402" or 10.2mm
the only way i can see of achieving the tapping depth is to increase the tapping size! or some way make a tapered hole! when the tap gets down approx.10mm then the top of the hole is smaller than the root of the tap.
Thanks guys.
RE: NPT Tapping
RE: NPT Tapping
if i had a gauge then it would tell me that i needed to tap the hole deeper. but the tap wont go down any deeper.
RE: NPT Tapping
Just a thought.
RE: NPT Tapping
A pipe tap machines a tapered thread from a cylindrical hole, thus the top of the tap WILL be cutting the minor diameter.
Look at the value for "Minor Diameter at Face of Part" for an internal thread. It equals .4471, which is larger than your 7/16 drill.
Pipe tapping requires a good bit of torque, and can be a bit brutal in some materials. What sort of machine are you using. How does it stack up in terms of power and rigidity?
If your setup is marginal, you might want to consider Ed's idea of prepping the hole with a tapered pipe reamer after drilling.
RE: NPT Tapping
thanks, mrainey.
RE: NPT Tapping
RE: NPT Tapping
http://groups.google.com/groups?dq=&hl=en&lr=&ie=UTF-8&threadm=20030723022209.23307.00001579%40mb-m29.aol.com&prev=/groups%3Fhl%3Den%26lr%3D%26ie%3DUTF-8%26group%3Dalt.machines.cnc
RE: NPT Tapping
You might ask the customer what his mating part looks
like. Normally the depth of a tapered pipe tap is not
shown. His requirement does not make any sense unless he
is using some non-standard male thread. The answer might
be to provide a counterbore at the surface if he is
certain about the depth. Ask if that is an option.
RE: NPT Tapping
Robert Setree
www.advanced-grinding.com
RE: NPT Tapping
IE. REMOVE THE LEAD-IN THREADS, TO ACHIEVE YOUR 18MM(.708")
NO, THE HOLE IS NOT TAPERED. THE TAP IS TAPERED. THATS'
HOW YOU ACHIEVE A SEAL BETWEEN YOUR FEMALE/MALE THREADS.
WHERE DOES THE CUSTOMER WANT TO SEAL, AT TOP OR BOTTOM OF HOLE? STANDARDS SAY 1/4-18 NPT IS 7/16"DIA. HOLE X 7/16" DEEP. ROUGH CALCULATIONS YOU NEED TO GRIND OFF .074" OFF
THE END OF YOUR TAP TO ACHIEVE GAGE AT 18MM.
RE: NPT Tapping