surface finish

[m_c]

Also as the diameter reduces so does the surface speed, which can effect results

I was just going to mention that.
It looks like classic low surface speed issues, which is a major factor in surface finish with carbide inserts.

Inserts primarily aimed for steel, typically have negative rake, and rely on pressure/force to keep them engaged in the material. At low speed it can result in the material tearing rather than cutting, which gives a very rough surface finish. However under optimum cut conditions, can produce a mirror finish.

The three main issues with typical steel inserts, is they need a good depth of cut (DOC), a suitable surface speed, and a suitable feedrate.
On a manual lathe, especially at smaller diameters, getting a good balance between those can be challenging. In order to get a suitable surface speed, you need to run the spindle fast, which then means you need to have a high feedrate.
The last thing you want with carbide, is for it to be rubbing the surface, and not cutting.

However, if you want a good finish, aluminium inserts, which have a nice sharp positive rake, can be used for light cuts on steel.
They won't last as long, and are far more prone to damage, but you can get a brilliant finish where steel inserts don't.

The other option are diamond tipped inserts.

 

[8ob]

was getting some (seemingly) random results when playing about with an old rod from a ram -

You need to be quite brutal to get under the chrome on a ram, if you go too light with the cut the tip tends to ride up onto the surface and rub instead of cutting.

Bob

 

[smashweld]

inserts are actually just the only ones I had … CCMT so positive 7 rake

it does seem like the tool is rubbing but again the inconsistency is the annoying thing, at least got more to think about next time I’m trying something

 

[daedalusminos]

It helps if your depth of cut is greater than the cutting tool tip radius, that way the cutting force vector moves towards the workpiece axis (more rigid) and away from the workpiece radius.

 

[minimutly]

Lots of experience of using all kinds of scrap steels here (rams, shock absorber stems, driveshafts etc). The variation in hardness is amazing, usually hard outside, softening as you go in, very often varying as you go along as well.. it’s a lottery really, but free steel. I don’t think a pro is likely to be using scrap?