4" NPT questions

[Shane123]

not much experience doing npt, but I have 2 parts i need to bore tomorrow for 4" npt internal thread on the romi lathe. I am going to use a g71 roughing cycle to bore, and a insertable thread bar to thread it with a g76 thread canned cycle. I know the standard is .75 inch taper over 12 inches (or 1/16" per inch) for npt, and standard testing is 3 full turns by hand is where you want your depth.

What i dont know is where to start/finish my bore, and then high point for my thread command, as well as the r taper command, and how deep should i thread?
also, since its internal thread, would my r be a negative value or does the canned g76 cycle know since my start point is lower on x than the finish point, like boring g71 roughing cycle?

been super busy at work, so i haven't had time to open the machinist handbook to see if its listed in there, sorry for that.

doing some google searching, i get this..... size-4" tpi-8 approx. length of threads -1 5/8 approx # of threads to be cut -13 tap drill size -4-1/4

closest i found for major diameter was in MM, @ 113,973, which is about 4.487". any help?

[djr76]

here is an npt chart - Basic Dimensions for NPT American National Taper Pipe Threads

Id try and draw it in cad to get your start and end values from the information provided in the link. Go buy a 4"- 8 npt nipple from a hardware to double check your data.

[ad64075]

Assuming a Fanuc control based on the cycles you quoted, your taper value (R) in the G76 line works in reverse from the obvious and is calculated per side. For example, when turning external NPT with the large end at the chuck, your R value will be negative. Conversely, when turning internal NPT with the small end at the chuck, your R value will be positive.

As far as bore dimensioning and the taper thereof, that can depend on the customer's requirements. We have customers that require straight bores for the length of ID NPT, which produces a very nice, clean, and practically burr-free thread; others spec a tapered bore.

[Shane123]

http://i488.photobucket.com/albums/r...CAfan/p1-2.jpg
http://i488.photobucket.com/albums/r...CAfan/p2-2.jpg
http://i488.photobucket.com/albums/r...CAfan/p3-2.jpg
http://i488.photobucket.com/albums/r...CAfan/p4-2.jpg
http://i488.photobucket.com/albums/r...CAfan/p5-2.jpg
http://i488.photobucket.com/albums/r...CAfan/p9-2.jpg
http://i488.photobucket.com/albums/r...Afan/p10-2.jpg

[djr76]

what the heck is that thing for? looks like some kind of manifold. good job!

[Shane123]

its a distribution hub, clensing water solution coming into the 4" npt inside a big tank, and then the 8 ports have 36" long tubes with sprayers 1.5" apart down the length, on both sides of the pipe (180 degrees from each other). its for terra, a fertilizer manufacturer.

[Shane123]

and thanks for the compliment djr!

[Shane123]

i will post the g76 canned cycle from work tommorrow so for future reference someone searching the forum can find the answer here.

max bore size was 4.333, tapered back an inch to 4.25. in theory your supposed to have like 1.4375 depth of threads, even tho they are not useable, but since this part has the ports closer to the face, we settled for the threads ending where the port wall was, and just adjusted the threading slowly up to ID size till the nipple fit properly. Looks cleaner with the threads ending at the edge of the hole than being deeper & interrupted by the ports.....

on a side note....... (save this info for later boys & girls)
ALWAYS make sure you check with the customer on their npt call-out & application, because if its for iso purposes to hold high pressure, the npt is different than standard, and making a standard npt for an iso application could end up with people getting seriously hurt or killed due to not holding up to the pressure..... (learned that lesson thankfully before we started the part)

[Shane123]

nc code for 4" NPT.......
g0 x4.1 z.25
g76 p040060 q0030 r.001
g76 x4.333 z-1.075 p1000 q0030 r.0625 f.125
g0 x4.1 z.25