Posted by IanT on 02/05/2021 12:33:05:
Hi Bill,
That vertical slide looks about the same size as the one I have for my 2.5" EW. The one I was referring to is probably twice as big.
As with most things here, there are different experiences and opinions. I don't know what work you are planning to do – or how often you intend to do it. My parts are generally small and I do have a choice of machines available. But I'll try to give a bit more detail on my experience in this area.
Do you need to make vertical height adjustments whilst machining? Some operations (drilling, slot milling, edge milling/fly-cutting) often don't – and if the work can be correctly positioned and the right tool fitted – then a vertical slide might not be required, indeed a heavy angle plate might be better. If you know the 'bed to centre' and 'bed to top of cross-slide' measurements, then you can set work either on (or off) the lathe fairly easily. Packing blocks can give simple work height adjustments too by removing or inserting blocks of known thickness. Slip gauges are not required!
Apart from rigidity, there is another reason I don't feel 'swivels' work for many vertical slide operations. My large slide came with a rotary base but I soon discovered that most times I needed the actual table to overhang the cross-slide. This is because otherwise the slide's table will be mostly above the lathe centre height – and things like vices make lack of travel even more of a problem. Hanging the sliding face over the edge of the cross-slide really helps but also limits any use of a swivel. So I replaced my rotary base with a plain adaptor plate to match the vertical slide with the cross-slide, whilst giving a more secure mounting. Same for my EW slide.
So in summary, you can certainly mill in the lathe without a vertical slide and 'swivels' may not be as useful as they first seem (think of a vertical mill and it's table – generally the table doesn't swivel!) 
If you have a simple milling job to do, why not try doing it with what kit you already have? Clamp the part in a tool-holder or on an angle plate if you have one. You could hold the milling tool directly in your 3-jaw but better to make a simple round tool holder (in mild steel – just drill or bore it and fit a grub screw). The chuck will grip the mild steel much better than the hardened tool and the grub screw will hold the tool if a small flat is ground in it.
Then have a go. Small cuts and slow feeds, saddle locked and with the gibs on the tight side. It may not be perfect but it will give you some idea of what is possible with patience and practice. You may decide it's not going to work for you – or that it's perfectly good enough. Either way it will help you decide what is best for your needs.
It might even be fun trying too! 
Regards,
IanT
Ian – thanks for a very comprehensive, thought provoking reply.
TBH I have no idea what Milling Operations I'll be doing as I am only just starting out learning how to use the Lathe e.g. I don't know what half of the parts are called or which cutters are for which job. Although in my first go I did manage to make this tool, even knurled the handle 
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So based on the info so far and the fact that I have no idea what I am doing, I think the cheapest option would be the best so that I can cut my teeth and have a go.
So forget the Swivel Slide and opt for:
– Heavy Angle Plate
– Clamps – not sure what I'd need
– Some HSS Cutters
– Home made Cutter Holder
Angle plates – I need to work out which would be best and offer the largest working area. There are so many different types to choose from ????? Also found this in a search from Chronos (who I have bought some tools before – decent quality I thought)
Clamps – Again need to work out what I need and I guess this could depend on the Angle Plate?
Cutters – Depends on what type of milling – but again I need to research what I need for various tasks?
Iv'e only been on here a day and I've already learnt so much, so thanks to everyone's support.
One job I would like to try, is cutting a recess in the area of a valve seat on a cylinder head. This needs to be done with a Fly Cutter, but I don't believe I'd be able to get the cylinder head bolted to an angle plate on the lathe and even if I did I doubt I'd be able to get at all the valves, maybe if I moved the backing on the Warco back (as I have seen Ade do to stop aluminium swarf wrapping around the head stock
)