You don't say what inserts you use. I'd suggest some **GT ones worked well for someone a month or so back I suggested it to wanting a good finish on Titanium using a Sherline lathe

check tool centre height

Check saddle / top slide gibs are nipped up so that tool height doesn't change when cutting load comes on.

Check cutting speed for tool / work material.

Use correct lubricant if using HSS tooling.

I don’t know whether such things are available as inserts [but I suspect Jason could advise] … If you could grind, or find, a ‘shear’ tool … that would better-approximate the cutting action of the graver.

MichaelG.

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Edit: __ You may like the toolholder design that I linked in this previous thread:

https://www.model-engineer.co.uk/forums/postings.asp?th=132573&p=1 

Edited By Michael Gilligan on 14/01/2023 22:12:30

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For clarity … this is the shape of insert that Dell told us he was using: **LINK**

https://littlemachineshop.com/products/product_view.php?ProductID=3574&category=

MichaelG.

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Edit: __ and here is someone using a graver properly, on blue steel:

Edited By Michael Gilligan on 15/01/2023 07:03:12

HT socket head cap screws are often a cow to machine to a nice finish. No idea why. They just often are. Plus you are using a very small, not very rigid lathe on some very tough steel. Not a good combination. Try on some mild steel of known grade and see how it compares. If the finish on that is acceptable using the same tools and methods, the problem lies in the material.

With a small lathe and difficult material, you should expect to have to hand finish the keys. You only make them once, and your normal work is very different and more suited to your machine.

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Then I think we must ask Dell to describe each of the attempts he made, in a little more more detail

… as I read it: he ended up using a hand graver [with which he is familiar] to get a decent finish.

MichaelG.

That's a smaller size insert than your previous link, so you would want something like this for a CCGT option.

From the picture I would have the tool at right angles to the lathe axis, the more you angle it the more "pointed" the tool is.

Going back to basics.

Your Pultra does not have a Leadscrew.

So it is not possible to arrange a power feed for sliding or facing. The feed rate is literally in your hands.

(Just like a training lathe such as the larger Lougborough )

At the risk of teaching granny to suck eggs.

The bad news is that if you are getting an obvious "screw thread", all other things being equal, (i e. the material is not some nasty exotic "Wont machineium" ) you are feeding too quickly. It is assumed that the tool is sharp, possibly with a small nose radius, and mounted exactly on centre height.

Bear in mind that moulded carbide tips are not absolutely sharp. A HSS tool can be ground and honed to be sharper.

Carbide tips are not ideal for shallow cuts at fine feeds, although they will do it, given the right conditions.. They were developed so that industry could remove lots of metal, as quickly a possible.

The objective is to feed at a lower rate per rev than the nose radius (Remember that a large nose radius tends to encourage chatter, so keep things as rigid as possible) .

Whatever the feed rate, you are going to generate a helix. The objective is is produce one of such fine pitch that the radiused cuts overlap without leaving any sharp peaks.

One of the first things that we were taught as Apprentices was how to produce a steady, slow, manual feed.

Spend time, practicing this, on "scrap" metal, gently feeding from one hand to the other You are trying to reproduce the steady consistent motion that a low power feed would give, with a shallow cut (Again less than 0.005" ).

For a fine finish, you are looking to feed at about 0.003 – 0,004" per rev. (As an example, to produce a 40 tpi Model Engineer thread the tool would need to be fed at 0.025 per rev ) And you are looking for something much finer than that.

So start learning with the lathe running at a low speed. Speed up as you become more proficient.

We are all learning!

Practice will make perfect. (A friend who recently bought his first, basic, lathe has learned this so well that now he is better at it than I am! )

Keep trying, and you will succeed.

Howard