Thread cutting without leadscrew

look for a thread chaser

I had a look, but couldnt find much, and nothing really described what it is or how it works.

Are the threads you intend to cut pipe threads, or are you simply trying to cut a random thread on a pipe?
The typical engine lathe, without a taper attachment, can't chase a pipe thread regardless of having, or not having, a lead screw. The taper involved is beyond the ability of the machine. The sole exception to that is if the piece is short enough that you can offset the tailstock the proper amount. It wouldn't work for long pieces.
That being said, if you intend to use your proposed lathe for future projects, do not buy a lathe than doesn't have a lead screw, or one without change gears or a quick change.
There are other methods of chasing threads, but the machine must be so built, and you must have masters from which you generate the lead. Threads are precision items that can't, and should not be, generated randomly, not if you want them to work properly.
No sense discussing how they're cut at this point, but it's something you need to discuss and learn if and when you acquire a screw cutting lathe.
Good luck, (unless the pipe and lid are for a pipe bomb, that is)
Harold

According to : Hmm ... does it have a turret?
Possible, with a turret (or some other way of holding and feeding round-shanked tooling on the lathe centerline), and some Geometric (or similar) die heads with the appropriate chasers.
However -- cheap doesn't match that. Even used Geometric die heads on eBay are not cheap.
I've not seen a turret for a 3-in-1 tool (lathe/drill/mill), so you would first have to make that. Also -- I haven't seen any 3-in-1 machines without a leadscrew and at least a basket of gears threading setup, (certainly not a quick-change gearbox), unless you are talking about tiny machines like the old Unimat SL-1000, which marginally qualifies as a 3-in-1.
You mean that you not only want to make threads, but you want to make female threads in a blind hole? (I presume that you are talking about a cap for a pipe -- and those tend to have tapered threads, so you would not only need a collapsing tap but a set of tapered chasers to fit. I've gotten several sizes of Geometric die heads over the years from eBay (since I have a bed turret for my lathe), but only two collapsing taps, and very few chasers for them.
Single-point threading (instead of using a die) for a pipe thread means that the machine would need a taper attachment -- which also tends to not be available for a 3-in-1 machine. At least, not for those which I have seen.
And very few 3-in-1 machines have a large enough spindle through hole to allow feeding pipe through it for threading the end of the pipe. (Which, I will admit, you did not ask to do.
It sounds as though you really need to consider buying a more versatile lathe, not one with an afterthought of a milling machine grafted onto it.
Beyond that, since you have given very little information on what kind of machine you have (or intend to purchase), and not much on the workpiece material as well, there is not much that I can add here.
From your 50mm pipe size and your domain, I see that you are not in the US -- you are in Australia. You really should specify things like this, as advice as to which machine is best is region specific. What we have here in the US for not too much money could be quite expensive in Australia -- and vice versa.
For example, the Geometric die heads are US made. I remember that there is a similar die head in the UK, which I would expect to be easier to find where you are.
Oh yes -- and a die head to thread a 50mm pipe is going to be a bit too large for most 3-in-1 machines.
Premade pipe caps should be available from your neighborhood hardware store. Buying a lathe to make your own is hardly cost effective, unless you need something special about this.
Good Luck, DoN.

Hmm. Lots of info. Well, to start - here is the lathe I am looking at - a HQ-400 Multi Purpose Machine
One of the first things I want to make is a dive torch. Which is essentially a 50mm (or so) tube with a thread on it, and a slightly bigger tube with an internal thread on it to act as a 'lid'. The thread depth would only have to be 10mm or so. Do I need die heads for this? Or this only if it doesnt have a leadscrew? Is there any good reason not to just put a steppermotor on the control to make an electricaly controlled powerfeed?

According to : Its a HQ-400 Multi Purpose Machine
O.K. Without more details of the maker and model, and just going by the photo shown, it appears to have a leadscrew (connected to the hand crank at the right hand end), and it appears to go into the gearing under the cover at the left-hand end. It looks as though the control for forward or reverse feed is on the belt and gear cover, with a yellow plate behind the knob.
That long column from the carriage to the toolpost will be a problem when dealing with heavy cutting, because it is long enough to both flex and to add leverage to flex other things. It is necessary because of the large "swing" (maximum diameter of workpiece). The actual reasonable size is probably suggested by the relatively small 3-jaw chuck on the spindle.
There *should* be a collection of gears to change under the left-hand cover to set up for various threads.
UK "torch" is the same as US "flashlight" I believe?
By "depth" do you mean the actual depth of cut? If so, that would leave you with only 30mm inside diameter (taking 10mm out of each side of the pipe). I hope that you only mean the length of the threaded section.
How deep do you need to take this? That will determine how thick the walls need to be, and how good the seals. (And no -- *I* can't give you the answers given the depth, but others probably can.)
I would suggest that the threads should be straight threads in this case, with O-rings in grooves.
And the material is probably a bronze (for long term life in the presence of seawater), or brass for infrequent immersion. Brass will be easier to machine, FWIW.
The die heads are used for quick cutting of threads -- many operations per workday. They (and a turret lathe) are production machines.
You have a leadscrew on this -- and hopefully also have the change gears (make sure before you bid, as Chinese replacement parts become very hard to get after a few years, and I think that this has seen a few years of service.
And to cut the threads, you would grip one end in the 3-jaw chuck, and support the other end with a live center in the tailstock, aided by a plug of some sort for the center to engage, as that one appears to be tiny. The chuck (with the alternate set of jaws, which are for gripping larger items) will probably handle your 50mm tube and an appropriate cap. At least, I *think* so -- assuming that your electrical plugs are a bit larger than the US standard ones, as there is nothing which says how big this is, and the pointer to the ad from which he bought it seems to not work. Reading that might give me more information on what it might be able to do -- if it is complete.
Well ... if you want it for cutting threads, you need to have information from the spindle as to how far it has rotated, and to step a few steps to be in the right position. This is one of the functions of a CNC conversion of a lathe. Until you do a full CNC conversion, you really need to stick with using the gears inside the headstock covers for your threading.
And based on experience with other such machines, I suspect that you will need to add some extra belts and pulleys to slow the spindle down, as the slowest spindle speed makes it difficult to enage the half-nuts or the dog clutch (which I suspect this one has) to start the thread at the right time for each pass. You can't cut the thread to full depth in a single pass -- you need to make each one a little deeper.
Stopping soon enough will be more of a problem for the cap which you wish to thread on the inside, as you are threading towards a solid piece of metal, so slower makes the chance to disengage it in time somewhat better.
You'll have to spend quite a while learning the machine and the operations before you start on your project, I fear. Plan to make mistakes and to learn from them. And try to learn without breaking parts of the lathe, as repair parts may be quite difficult to find.
And ideally, you need someone nearby who can help you to learn to use the machine.
Good Luck, DoN.

It's a waste of good money so I hope you didn't win it.
By the time you buy a lathe, the tooling, learn to use it etc etc you could not only have bought a torch, you coulda bought a complete dive outfit. Your dive torch threads have to be accurate and a good fit, *and* you'll need O rings etc to keep the water out. Just go & buy one. I wouldn't bother making one myself and I have a full workshop.
You can use die heads for cutting external threads on relatively small dia workpieces. 50mm is not small by die head standards. I sold a small die head some years ago for a couple hundred, IIRC. OTOH a reasonably small lathe can thread both internally & externally at 50mm dia without dramas.
Get a lathe first, then learn to cut metal, and only THEN start modifying it. That's my advice. Also don't confuse a power feed with a threadcutting feed. If you're thinking that you can take a piece of crap, add steppers, DRO etc et and end up with a good machine, you're dreaming.
Very little good used machinery, IMO, appears on the Australian Ebay site. In fact I've been thinking of selling some of my surplus tooling. You need to haunt the second hand machinery dealers. If you're in Sydney or close, I can tell you a couple places to start at. Some of the Chinese lathes aren't bad and are infinitely better than those 3 in 1 pieces of crap.
is a more reasonable machine if the price doesn't go much higher although the low speed is too high for my liking. Otherwise Hare & Forbes in Sydney, Melbourne & Brisbane import Asian lathes. The AL-32G would be worth a look if you want to spend $2K on a machine. It has a low speed of 60 rpm, good for beginners threading, and a 38mm spindle bore.
Can't afford new? Join the club, start hunting second hand machinery. Trading Post is good but you need to know what you want and be prepared to move fast. If your budget is less than $1000, look for a Hercus lathe in good shape.
PDW