lathe Cutting American NPT taper threads F and M

My friend has given me a iron Bung removed from a liquid holding engine and wants me to tap/cut a thread through it to fit a drain tap, that he has supplied.

The bung has a 3/4" NPT thread 14 TPI (which is very rusty and I will most likely have to make a new one).

The tap has a 1/4" NPT thread 18TPI.

So I want to drill and thread the Bung to fit the Tap.

I have searched the Web and found lots of Data but nothing to explain how to go about cutting these threads in a lathe.

Any help much appreciated.

Paul

I reckon you will need to set your tail stock over to have the plug run parallel, this will turn a thread that looks parallel but actually cutting it on the taper, you will have to run between centres to allow movement. You could always use a boring head with a running centre ( if you have one ) rather than move your tailstock over... check out 'Doubleboost ' YouTube site, he shows on one of his videos where he is turning a tapered thread...

George

I use taps for internal NPT threads; seems to work fine. For external NPT threads I'd screwcut in the normal way, but use the hydraulic copy unit to move the tool along the taper. I expect you could achieve the same thing using a taper turning attachment.

Andrew

Paul...

See this... Doubleboost cutting a tapered thread...

**LINK**

George.

Do they not sell 3/4 to 1/4NPT reducers where you live?

A 3/4 BSP plug has the same TPI as the NPT, and perhaps could be drilled and tapped to suit the 1/4 NPT draincock?

Seems like a lot of mucking about to try to turn one from scratch unless you have a taper turning attachment.

Have a look at this recent thread, although the title is about using a die it is mostly about screwcutting taper threads

**LINK**

Dear Helpers,

Thanks for the help supplied, this all helped me to understand in my own head what the options were.

I attended the Guildford model engineering society Rally at Stoke Park Guildford, on Sunday and a very nice man on the Home and Workshop Machinery stand looked through his stock of taps and found a 1/4" NPT tappered Tap, how good was that! Therefore I decided to Tap rather than turn the thread in the Lathe, thank you Andrew for your suggestion. Hopper suggested that I buy a Plug/Bung from my local Plumbers merchants, I visited two Trade suppliers and neither could supply a Iron or Steel Plug/Bung, they had Brass ones but these had very little meat in them to Tap a thread, I then sorted through all my plumbing stuff and found a brass Plug/Bung with ten mm of meat.

What drill to use? I found some data suggesting 7/16" for a 1/4" NPT tappered Tap, this I tried but got only one thread into it, I then tried 11.5mm drill and got nearly three threads, I then tried a 12mm drill and this was just right as Goldilocks said to the three bears.

So I ended-up with the Rusty Plug/Bung supplied by my friend Tapped to take the drain tap, plus a Brass one if the rusty one is beyond use.

Will I ever use the lathe to make Tappered threads? well with the suggestions you have all made I probably would!

Many thanks again,

Paul

You can hand chase male threads. Gets a bit tricky on small female threads - and larger ones too.

It's not as hard to do as some may think. The speed of the lathe needs to be fast enough to suit the pitch being cut. Too slow and it can be rather hard to get an even feed but as the chaser gets deeper it tends to mostly look after itself.

John

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