No, you can also get what Mercans call "agnostic" inserts – we call them "partial profile". Fine for what we need and doesn't require such a large number of insert sizes.

Good guide here if you have time…..

Murray

The thread cutting question has a specific answer, so here goes: There are two ways in which this operation is usually carried out. In both the tool is held symmetrically to the work, with the same angle both sides (ie 30 degrees for Unified and Metric, 27.5 for Whitworth and its relatives, etc). The thread is cut in two or three passes.

The difference is that in one method the tool feed (ie the inwards adjustment for the next cut) is at right angles to the workpiece, using the lower of the two handwheels on the saddle assembly. This means that both sides of the tool cut (almost exactly) the same thickness of metal with each successive cut. In the other method, the adjustment is done with the top slide, set at the angle of half the thread angle (30, 27.5 etc) and the adjustment moves the tool along one face of the new thread, with all the material removed from the other face.

The second method is described as the better way by many experts.

And no you do not need a different insert for each pitch, as long as you are not too fussy about the tip radius (or tip flat). There are those who say that for ordinary low stress stuff, you don't even need separate 30, and 27.5, tools. After all, I bet your local nuts and bolts supplier will sell you UNC studding when you ask for Whitworth, as the pitch is the same and there is enough clearance to permit them to be interchanged. (Not such a clever idea for bolts and nuts, though, as the head sizes and spanner fitment is going to be wrong).

Regarding the broader Q: What to buy, I suggest that you start with a cheap and cheerful set, and a box of free-cutting mild steel offcuts, and play. This will give you the feel of the work. Then when you actually need to cut real parts at exact angles and positions, you might need to look more closely at the better end of the market. It is cheaper that way.

Regards, Tim

Welcome Gary, as for the tool size best check that the shank top surface can be adjusted to lathe centre height with the available adjuster in the toolholder, I say top surface because that is normally the top height of any insert fitted to toolholders. You may find you have to fit smaller tooling.

What you end up buying will depend on the type and size of work you intend to do but if the toolholder will take 20mm tooling it suggests a fair size lathe, more info may help such as make and model.

Check on the advertisers sites for more info on toolholders shape and insert sizes/types but you will certainly need a Right Hand cutting tool possibly a LH and also a parting tool.
You mention threading so you will find upteen holders and inserts for various imperial threads although many can be cut with a single 55 deg tool,, most metric can be cut with a single 60 deg tool but inserts are available to provide correct radius to the crests but these are size dedicated inserts and named as Part or Full form inserts.

Buying a full set of different tools may not be the best way to go.

Emgee

If you look in the "thread designations" pages starting at pp5 in the linked document, you can see how tolerancing works for the various thread systems. Few MEs come across thread tolerancing, hence the experiences with rattle fits and stripping threads that arise from crappy rolled studding etc. However, for critical stuff like automotive and aerospace it matters, so fasteners are specified with tolerancing like a G (nut) and g (stud).

Murray

I can never be bothered with the faff of angling the topslide, plus it means you have to do mental juggling to keep track of the cut depth.

Moving the top slide across 1 unit for every 2 units of infeed will move the tool at 26.6 degrees, which is close enough to give a just shaving cut at the back with metric or imperial threads. (Not my idea, I got it from a Tubal Cain article).

Neil