View Full Version : how to thread a taper

Black Forest
03-19-2011, 02:12 PM
I have a connector I made on the lathe. It has a hex head in the middle and two shafts either side if the hex head. A double sided bolt. One left thread, one right thead. This connects the ends of plastic pipe together. The plastic pipe is a circle with 150cm diameter. The "bolt" is solid as in no hole for any liquid to flow. It works but it would be better if I turned a taper and threaded the taper. That would make it much easier to get started in the plastic pipe.

So, how does one make threads on a taper? The picture is what I have made already but I would like to make one with each threaded shaft tapered.


Black Forest
03-19-2011, 02:21 PM
A picture of what I have made without the taper. I would like the threaded shafts to have a taper.

doctor demo
03-19-2011, 02:29 PM
If You are doing this in the lathe ,ya have to set up Your taper attachment....if the lathe has one.


03-19-2011, 02:59 PM
Might you be able to do it without a taper attachment if you turn between centers with the tailstock offset a touch?


03-19-2011, 03:41 PM
I dont even know weather the threads will be with the taper or with the strait part..that question could do with being answered .

youd have to have a tapperd nut as well

ps anyone having trouble with this site ..missing half the day ..and very slow now

all the best.markj

mike os
03-19-2011, 04:24 PM
only time I have ever cut taper threads was with a pipe threader... die head with tapered cutters....

03-19-2011, 04:46 PM
Cutting pipe threads is explained in South Bend's "How to Run a Lathe" book. I've got two copies and I can't find either of them, but my rather doubtful memory tells me you would set the tailstock over towards you to achieve the proper pipe taper (3/4" per foot, I think) and set your 60 degree threading tool perpendicular to the main body of the work, not to the part you are threading.

I imagine Machinery's Handbook would confirm (or refute) the thread alignment to the work.

03-19-2011, 05:13 PM
I have no idea what you call it, but the thread used on drilling pipe/drills is a very sharp taper, and is threaded. I know of a shop that makes them, but I've never had the opportunity to watch him do it. I would think a taper attachment would be needed for something this aggressive.

03-19-2011, 08:12 PM

I don't think you need to make tapered threads since you're not trying to make a tight seal for air or liquid. Just ease the ends of your connector. Kind of like a plug tap. And give a bit of bevel to the pipe also.


03-19-2011, 08:39 PM
When I used to use my plain lathe - no carriage movement, the top slide does it all - turning a tapered thread was no different from tunring a straight thread, except the top slide was at the taper angle.

All threads had to be fed in straight using the cross slide. Well, for heavy beasts I would play tricks with the gears to move the cutter along a bit, but that's by the way.

So, set up just as you did for the original thread, but with the top slide at the taper angle. Lock the carriage, and go for it.

I should think that for precision work you have to know whether you want to measure the pitch along the taper or along the axis, although by definition you can't use a long tapered thread.

03-19-2011, 08:48 PM
The difference between a tapered thread and a taper pipe thread is in the preparation of the workpiece and the position of the cutting tool. The tailstock set over method or a taper attachment can be used in either case.

When cutting a taper pipe thread, the tool is positioned at 90* to the workpiece and the thread tapers from full depth to almost nothing along its length, the workpiece remains full diameter.

A tapered thread is cut on a workpiece that is first machined to the desired taper. The tool is positioned at 90* to the taper and the thread is full profile for its entire length.

03-19-2011, 08:53 PM
one way for pipe threads ...
I do not know if they make left hand pipe threads


but the pipes you are joining usually have their threads one the outside not the inside in which case you would be wanting a female coupler ...
maybe I need to go back and reread the question

03-19-2011, 09:06 PM
I did a lot of this type of threading. All the way up to 6" dia. It is done with a taper attachment on an engine lathe. The taper is as some one else said is 3/4" per foot. We used 11 1/2 thread per inch for most of the larger sizes we did. I don't recall what the 6" required. It was 20 yrs . ago. You can do it by shifting the tailstock over and holding part between centers when you do. You single tool thread it and use the compound set at 29 1/2 degrees. Advancing the compound a few thousands at at time and backing off at the end of thread with the crosslide and return in the crosslide to "0" to start again. that way you only advance on the compound. Watch Twobalcain on the You Tube site for threading example. You actually only cut on one side of the thread form doing it this way. Good luck . I wish I could show you on you engine lathe. Also use the thread dial on engine lathe. Simple to do.

03-19-2011, 09:23 PM
Yes you have to cut taper first and then use your fishtail to set the threading tool 90 to the taper. And yes the thread will diminish as you cut to the end of thread.

03-19-2011, 10:00 PM
Like JC said the tool is set 90* to the pipe's axis.If using the tailstock offset method,set the compund to 28-1/2*IIRC and singlepoint as any other thread.Edited to clear brainfart:D

Everything you need to know about thread specs and profiles.


03-19-2011, 10:46 PM
Rohat, I've been machining for 60+ years and you totally lost me. How can you cut a thread if you lock the carrage down? Also how do you make the compound move the correct threads per inch. Peter, old fart turner from North Wales moved to the colonies:eek:

03-19-2011, 10:47 PM
Like this:


03-19-2011, 10:55 PM
The chaser still has to move one pitch per revolution. Or are those anular grooves? Peter

03-19-2011, 10:57 PM
That is a threading die. You set it to the correct angle and then proceed as if single point threading. The back end starts cutting first. By the time you have advanced the depth so that the leading tooth is cutting to full depth the full taper is developed. It is no different in operation than single point threading.

03-20-2011, 02:04 AM
I have done it that way myself but rohat said he locked the carrage, that is what I was questioning. Peter

John Stevenson
03-20-2011, 08:56 AM
Like this:


That will work but only if the length of the taper is equal to the width of the die insert.
Anything longer will leave you with a parallel trailing end which will be at the smaller diameter.

Still probably the quickest way to do short threads.

Most taper threads are usually 1.5 x D in length so this teels you what the width of the die insert needs to be, if it's possible to get one that length.

03-20-2011, 09:24 AM
Anything longer will leave you with a parallel trailing end which will be at the smaller diameter

Just start over again on the smaller diameter making sure the threads are timed.

John Stevenson
03-20-2011, 09:29 AM
Yes that would work, well worth remembering.

Errol Groff
03-20-2011, 03:56 PM
I have scanned and posted the Taper and Threading chapter from the text we use at school. It can be found at:


Scroll down to the link Tapers and Threading.

Setup for taper threading is on the last page of the file.

03-20-2011, 04:54 PM
Why don't you use a connector thats what they are made for, no threading, no screwing, just heat the ends in some hot water and push together, done.

03-21-2011, 08:58 AM
Yes, the O/P asked for a threaded taper as shown in Evans pikky, not to be confused with a tapered thread. A threaded taper must be cut with the thread at right angles to the taper, a tapered thread is first cut on a parallel bar and the taper then cut. In real terms, a threaded taper is a "Bastard" pitched thread, unless cut with a taper turning attachment.

Regards Ian.