Update

Hi, I have tried to take on board the various bits of advice offered here, regarding improving the surface finish of my cast-iron turning. Unfortunately I don't have a grinding wheel so I can't really make or use HSS cutting tools, so I have to stick with carbide inserts.
At the time of purchasing this chuck for the mill, I didn't have a boring head or a rotary table, so it was helpful to be able to have the workpiece rotating in the chuck, whilst bringing the turning or boring tools up to it; gripping the tools using the vice on the milling bed.

Anyway, previously my cast-iron facing cuts were with a generic CCMT060204 carbide insert. This has an angle of 80 degrees, with a tip radius of 0.4mm. The rotational speed and feed-rate was too high, and the depth of cut too shallow. The insert material was not ideal for cast-iron, and it is not designated as a 'finishing' insert.

The image below shows the 100mm chuck backplate. The central region shows my initial turning finish, which is quite dull and roughly scored.

I have just bought new inserts and holders. The new ones are DCMT11T308-UF KC5010, these have an angle of 55 degrees, and a tip radius of 0.8mm, combined with a "wiper"-style tip for ultra-fine surface finish.
The recommended operating conditions are that the depth of cut be no LESS than 2/3 of the cutter radius.
In this case, this means a minimum of (2/3)*0.8=0.53mm (21 thou).

Also, I selected 150rpm, and turned the feed wheel as smoothly as I could using two hands.

The results are shown in the below image, where the shiny outer turned region is the result.
This was tricky as this area is an interrupted cut, due to the 4 bolt-holes, but it still worked ok,
As you can see, it isn't quite mirror-finish, but it is a lot shinier and smoother than my original attempt at turning.
For my application, this finish quality it is probably good enough, although any suggestions for improvement are always welcome. I'm not even sure how much better than this a manual machine can do?

Edited By Nicholas Lee on 28/01/2016 22:29:12

Steve, that made me laugh.

In comparison with a Sieg lathe, I would hope their lathes would be much better out-of-the-box.

I say this because the column alignment of a mill can be trammed, but (having never owned a lathe) I don't know if a lathe's alignment can even be adjusted?

The Sieg 'hobby' machines doesn't seem to come with any published accuracy figures.
If you need to ask, then presumably they are too cheap for you.

On the X3, I think that tramming the head is generally left as an "exercise for the reader", as part of your initial machine setup. Mine was pretty much spot-on out of the box, but maybe I was just lucky. It's hard to tell with a sample size of 1.

Tramming the column itself needs to be done only once, ref: http://www.yuriystoys.com/2013/03/tramming-sieg-x3-column-and-head.html

The only component of the mill's overall accuracy that can't be adjusted-out would be the run-out of the spindle taper. If that's wonky then the whole machine is junk.

Having searched various forums, other people have measured the run-out on their X3 mill's spindle tapers to be 0.0001" (0.1 thou)

I have a feeling mine isn't quite that good, but it is probably not too shabby.

PS: The Sieg lathes can be greatly improved by changing the headstock bearings for Taper Roller Bearings.

Edited By Nicholas Lee on 29/01/2016 00:01:06

Thanks Paul.

You are correct of course; a lathe could take deeper cuts without flexing or chattering. Unfortunately I don't have a lathe (or even anywhere to put one), so I make do.

I suspect the 25mm hole drilling, and 50mm facing cut, limits of the Super X3 mill are more down to the 1KW motor power than any indication of machine accuracy, but I do take your point about machine rigidity.

This (comparative) lack of rigidity simply means I have to make slightly shallower cuts to reduce the bending forces on the column, so things will take a bit longer than using a lathe, but it still does the job.

As you can see from the image I posted, it can (with care and patience) still give a nice turned finish, even when facing a piece of cast-iron 100mm across. (Which is a pretty extreme test)

Mostly I work in brass and aluminium, and with parts less than 50 mm across, so I shouldn't have too much trouble.

This 4-jaw chuck is especially handy for holding square-stock that I want turn a round feature onto; or for turning round wheels that are too large to grip in a collet.

A dedicated lathe is always going to be better at turning, and a dedicated mill is always going to be better at milling. (For machines of the same size and price)
I only had the physical floor space (and budget) for one machine, and as most of my machining operations involve milling rather than turning, I naturally went for a mill.