Turning a 4mm diameter by 25mm long should not be that hard to do. It's not a particularly long, thin job. Can you post some pics of the set up and tooling you are trying to use? A very sharp HSS toolbit might be your best bet. Less cutting force than inserts.

Othewise, you might try a moving steady that clamps to the carriage and moves just ahead of the cutting tool. I;m sure there must be one available for your model lathe.

Alternatively you could drill a small centre hole in the end of the job and support it with a tailstock centre, probably the the "half centre" type that has been ground away to allow tool access to small diameters. You could then cut the end off and face it afterwards, holding the job in a small sleeve in the three jaw chuck (or collet).

If you want to pursue the single cut idea, you might start with smaller diameter stock to reduce the depth of cut needed. You need to do a few test runs to determine the depth of cut your lathe is comfortable with. Stop looking at Youtube and get out there and just play with your lathe to see what it can do. Try different tools, speeds, set ups etc. (Remember, small diameters like this need high rpms to achieve the required 100 ft/min surface speed. Slow rpm will encourage dig ins etc. For 12mm diameter steel, your rpm should be about 800rpm with HSS toolbit. Double that for carbide insert tooling. Once you get down to 4mm diameter, rpm should be double that again, or more. Again, play with it and see what works best on your machine.

Another way of turning long slender jobs (which this is not really one of) is to do it in stages. IE, stick the job just a little out of the chuck and with progressive cuts turn it to size for say 8mm length, then slide the stock out of the chuck another 8mm and turn down the next 8mm length to size and so on.

Edited By Hopper on 30/12/2018 01:11:25

Edited By Hopper on 30/12/2018 01:13:15

I've turned up various types of mild steel pins around that size in multiple passes with no problem. Start off with a 10mm bar and work down from there. You may need to run the last pass twice without adjusting the cross-slide to make sure the piece is the correct diameter for the full length. I assume it's not some expensive rare metal you are using, so if it bends then cut it off, drop it in the bin and start again. You'll soon get a feel for what you can achieve.

I think the one shot cut is to avoid sneaking up on the final size which tends to push slender jobs away from the cutter rather than take a cut. What you need to know is how small you can go before the job loses the rigidity to cope with the cutting force. Getting the required surface finish from a cut that is going to be heavier than your normal finishing cut may need some testing to find the best speed and feed for your machine. It is probably a fair bet that getting it right first time requires experience of the tool, the machine and the material. I would think even a professional turner would need to explore a new machine to get the best from it.

Mike