# How to bore taper for  tapping 1./2" NPT threads



## martik777 (Jul 6, 2019)

Looks like the taper angle is 1.79 degrees so for a 1/2" thread I would drill thru close to .703" and taper bore the rest out. Set the compound just under 2 degrees.

Is that about right?


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## FOMOGO (Jul 6, 2019)

Duck Duck Go is your friend ( a little Google goad ). Mike



			NPT Thread Chart Tap Drill Size Calculator


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## MontanaLon (Jul 6, 2019)

No need to bother with the taper, the tap will put in the correct taper. Same way if you were working external threads on a pipe, the die will give you the correct taper.

Unless you are talking about single point threading on the lathe. But you still wouldn't necessarily have to taper it before cutting the threads but you would lose the ability to make a scratch pass to check your thread pitch as it would cut the largest diameter before cutting any smaller diameter and cut progressively deeper with each pass you make.


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## martik777 (Jul 6, 2019)

Tried to tap with a non tapered hole but it's extremely difficult, I read there were tapered reamers available so figured I could  do the same by boring with the compound.


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## darkzero (Jul 6, 2019)

I wish I had a taper attachment so I could do things like single point tapered pipe threads.

With pipe threads I always tap with a straight hole. But I've never tapped anything larger than 3/8" NPT.


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## benmychree (Jul 6, 2019)

You should be able to tap 1/2 NPT without worrying about tapering the hole; reamers are available, but I have never had to use them.  Just do it!


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## MarkM (Jul 6, 2019)

You can single point a pipe thread.  Offset your tailstock and cut between centres.  Then cut it like a regular thread.
Pipe thread is 1/16" per inch or 3/4" per foot. 1* 47 min.
Another way is to set up normally and cut the taper with the crosslide when threading.  Cut the taper with the compound.    Thread out from the chuck with the tool upside down.  Divide a sixteenth by the number of threads will give you your cutting depth per rev. for taper.  Now this is where skill and a good craftsman comes into play.  Set up a dial and feed the amount per rev.  Threading away from the chuck allows you to be cautious and not worry about running in to the shoulder and the fact the taper gets smaller it s easier to not ruin the threads.  Practice freehand over the part for awhile until your comfortable with timing.  Normal with compound for setting thread depth and crosslide for feeding to match taper and return to zero on crosslide after each cut then again feed crosslide per rev. Amount you calculated.   Once done a time or two you can catch on.  I wouldn t send these parts to Nasa but a good craftsman can get a good seal.  I set the compound 90 degrees to the shaft (not taper) so infeeding in with the compound doesn t affect taper to tool location.


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## MrWhoopee (Jul 6, 2019)

It takes a lot of OOMPH to tap 1/2 npt in a drilled hole, but it can be done. I had to do one in a well cap last year. I did it on the mill using a 12 in. crescent wrench with the tap supported by a center in the drill chuck. Would have been easier with a large tap handle, if I had one. Yes, boring the taper (using the compound or taper attachment) will reduce the hp required.


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## martik777 (Jul 6, 2019)

Thanks MarkM, I will try your method next time


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## Illinoyance (Jul 6, 2019)

There are tapered reamers to be used ahead of the pipe tap.  It makes tapping a lot easier.  Boring the taper on the lathe works well but is not as convenient.


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## jbobb1 (Jul 6, 2019)

Like John, I've tapped many pipe threads (much larger than 1/2") without the need for a reamer or cutting a taper. Support your tap with the tailstock, use plenty of cutting oil and a fair sized crescent wrench or tap handle and feed the tailstock as you go.


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## MarkM (Jul 7, 2019)

You can do an internal pipe thread the same way I described in my earlier post.  Treat it like an external thread with the spindle running in reverse then the infeed is in the same direction as an external running in reverse. 
You do need a fairly slow spindle speed but it is more about rythym with practise. 
Running from inside out takes away all the pucker factor with the shoulder and the taper going from small to big.
I ve mostly done this with much larger pipe thread.  In tough materials i ve also drilled two different sizes for the same hole to help the tap with the taper.


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