Posted by Del Greco on 15/08/2019 14:23:07:
Hi Gents,
So, This is a total newbie follow-up question I guess…
I am turning around 10mm Brass. (I'm trying to make a small whistle, spinning top, etc etc.) all with some small stock to get in some practice.
From the posts above, and from what I can find online, I should be turning at >2000+ rpm?
Is that correct? I'm still a little nervous about spinning my lathe that fast, and have mostly been using it at around 200-300 rpm for similar work, with horrible results. I get a crappy finish, often the thinner the brass stock can bend quite visibly instead of being cut, and it can make a nasty grinding noise.
I have carbide tools, and have the extra sharp mirror looking inserts, which work great on alu at around 300-400 rpm. (Although that was thicker ~30mm alu stock!)
All this cutting speed stuff is very new to me, and I'm starting to think that my previous efforts were a lot more luck than skill. ha!
Thanks in advance.
Del 
LittleToolShop's calculator suggests 3000rpm for HSS on 3/8" brass and 10500 rpm with carbide. Although those speeds are for economic industrial production and needn't be applied religiously in a home-workshop, they're a strong hint that 200-300rpm is too slow!
Assuming a suitable tool-tip, the three factors that have to be balanced to get good finish are rpm, depth of cut and feed-rate. They vary with material and diameter of the work but there's usually a fairly broad band of conditions that will produce acceptable results.
HSS is more forgiving than carbide, but make sure it's sharp, especially if the depth of cut is low. Took me a long time to realise feed rate is important too. Feeding too slowly risks leaving a fine screw-thread effect, whilst feeding too fast can cause tearing. A shaky hand can produce both effects on the same pass.
Try taking the speed up to at least 1000 rpm and taking a few moderately deep cuts, say 0.3mm or 10thou whilst experimenting with the feed-rate. Best results from power feeding, but good results aren't hard to get by steady hand feeding, it takes a bit of practice though.
Carbide works best at speeds and cuts well beyond anything HSS can do. To get full benefits from carbide the cutters are distinctly blunt, and it pays to attack the job with vigour, provided the lathe can cope that is! One trick is to use sharp carbide cutters intended for Aluminium on other metals. The sharpness makes them behave more like HSS.
Don't be disheartened, it takes a while to learn how to get the best out of a lathe. I've been turning for several years now and still get it wrong now and again. Two common beginner mistakes, mine was pussy-footing, the other is angry gorilla with a headache. I use sound quite a lot to judge; seems to me cutting in the right zone has the lathe audibly working for a living without being flogged. Practice, practice and success will follow…
Dave
PS I failed to spot 'nasty grinding noise' = see Mick and Jason's identification of tool-height as the cause!!!
Edited By SillyOldDuffer on 15/08/2019 15:11:08