View Full Version : Maximumn Pipe Thread Hole diameters?

02-26-2009, 07:03 PM
I used the 55 percent hole sizes for my stainless threading, I was getting better at it having the larger hole helped a LOY thanx Guys for the Tip. NOW i am having a time with a 1/4-18 NPT ppe taps. I already threw one in the garbage. i wondered if there was a chart for 55 % hole size for the Pipe tap series taps?? thanx

Mark Hockett
02-26-2009, 07:16 PM
Pipe taps in stainless are a pain, I usually use a thread mill on the CNC for NPT threads. I haven't tried one of the interrupted cut NPT taps on stainless but it might be worth trying,
http://www1.mscdirect.com/CGI/NNSRIT?PMCTLG=00&PMAKA=88201033&partnerURL=http://catalogs.shoplocal.com/mscdirect/index.aspx%6Fpagename=shopmain%50circularid=13665% 50storeid=1040626%50pagenumber=375%50mode=

02-26-2009, 07:35 PM
I have been having a time on this Project. i had 2 pieces that wouldnt fit my 15 inch Lathe, SO i send it out the guiy wants 250 bucks i tell him ill give you 300 .NOW i show up a week and a yhalf later and also give him a brand new grooving tool as a present well he HASNT EVEN STARTED MY PROJECT. Im not even surprised NOW i spend all day hacking it out (lathe work) on my rotary table and now its done BUT the other guy who was going to do my stainless tube machining job (he said oh ill whip that off on my large lathe after he visits my small home shop and saw my hacky LOL setup. I go to see him also today well he isnt even there and my stuffs sitting unfinished nbeklow his bench. NOW i gotta jam again everything into a big rush delivery is monday sucks to be me LOL Mike PS i made the ss pipe hole way bigger and finally tapped it but is sure didnt feel rught. Tough as hell going in,.

02-26-2009, 07:42 PM

thanks for the prompt.

You are the first that I've seen that mentioned doing milled threads.

I am not sure if some who may have the capacity realise just what it is and how useful it is - particularly for internal threading.

It sure does make some difficult threading a lot less difficult.

Is there any chance of you posting a video of the process please?

For those that are interested see Machinery's Handbook - under "Thread Milling" (pages 1962>1964 in MHB 27).

It is quite feasible to mill threads with a non-CNC-ed lathe with a single point cutter mounted in a rotating spindle - a good "die grinder" might do it - tilted at the thread helix angle and the thread pitch diameter. It works very well in a three-axis CNC mill.

Screw-threading - using a 3-axis CNC mill as a lathe works too.

Check out the following links and videos - as well as other videos referenced/linked to those sites too.

Its quite an eye-opener.





02-26-2009, 07:47 PM
Hey guys i own a 1960 Bridgeport with a step pulley head LOL No CNC Here But I wish i had one

Just Bob Again
02-26-2009, 08:33 PM
It is quite feasible to mill threads with a non-CNC-ed lathe with a single point cutter mounted in a rotating spindle - a good "die grinder" might do it - tilted at the thread helix angle and the thread pitch diameter. It works very well in a three-axis CNC mill.

On a 3-axis mill, you're not tilting the spindle to match the helix angle. You'd need a 5-axis machine. Why bother on a lathe? You also end up with clearance issues. Angled cutter shank will hit the ID on internal threads. The thread angle won't be precisely 60 degrees that way but it won't on a mill either. Except with full-form cutters made for a specific pitch. I think they jigger the angle of the cutter so it's closer to correct at that one pitch. Those full-form cutters are way expensive. Like $300 each. I can buy a lot of taps for $300. I can see it for large threads but not a general thing.

02-26-2009, 08:38 PM
For cutting pipe threads, a tapered reamer will make life much easier. In mild steel, brass & cast iron, you can get away without them up to about 1" or so. In stainless, you might be better off using them down to 1/4". It will save you the cost of a tap.

02-26-2009, 09:52 PM
Jim is spot-on with the tapered reamer. Also keep in mind that the tap is to be run in to a specific depth and not up to the top or shank. You could be over tapping the hole and galling the tap and/or part.

02-26-2009, 09:54 PM
I vote for the taper pipe reamer too. Stainless is a real bugger to cut a pipe thread into.

02-26-2009, 10:33 PM
I have heard of guys machining tapered bores for pipe threads on big mills but never heard of a tapered pipe reamer? Sounds interesting I ended uop making the hole WAY bigger and it worked out after a couple of taps .

02-26-2009, 11:12 PM


This is just one example. They come in spiral or straight flute but the spiral is easier to use.