Plus one for the "GT" inserts.

I also used G type (CCGT060204 for aluminium, 0.4 mm nose radius) insert on steel to get a good finish when doing shallow (0.01 mm) cuts. These polished G type inserts are very sharp.

I get the best finish on steel using a M type insert (DCMT11) that has a 0.8 mm nose radius running at 150 mm/min (Vc). When using a new insert, I can cut 0.02 mm. I have ordered some CCMT060208 inserts last week to see if the nose radius or the geometry makes the difference.

You should try to do deeper cuts at high RPM. To turn the correct diameter, do the last 2 cuts at the same depth and measure the diameter before each cut.

Cryspin has made a video on how to accurately turn a diameter. Accurately turn a diameter

Edited By Huub on 12/05/2022 23:00:00

HSS turned with a GT insert at the bottom of this picture. This is what I would consider good. I would also say good enough or acceptable but excellent is, as Andrew suggest, not well defined.

Martin C

Best to sit down and put on the seat belt before looking at prices for CBN inserts.

Andrew

Forum members are naughty to use words like 'decent' and 'excellent' because they have no scale and can't be measured. Sadly I don't suppose many of us could measure Ra, even when it's specified.

I take a jaundiced view of what goes on in my workshop and bet the farm many other amateurs are just like me – if it looks good, it must be good, even when it's wrong. The other day I turned a beautifully shiny rod by accidentally setting a carbide insert too high and rubbing the mild-steel. Less pleasing when viewed with a loupe – the polish was smeared metal, shiny scales rather than properly smooth. The apparently excellent finish would have seriously upset an inspector. Didn't matter for what I needed the rod for, so I was pleased with a dud result!

Has to be said that lathes and milling machines are both technically incapable of producing an excellent finish. Reason is that single point tools gouge and smear. Metal is ductile, and moves away from the cutting edges. Magnified lathe-cut surfaces are a mass of imperfections; waves, spiral grooves, hills, valleys, outcrops and caverns!

Grinding outperforms single-point cutting by ten times or more, and it's often worth improving on that by lapping. Making a small engine, I guess we've all noticed lathe bored cylinders and lathe turned pistons aren't good enough. Another process is needed to improve the flawed surface.

Lapping isn't excellent either! Lapped surfaces can be improved by honing, and going to the next level by super-finishing with an ultra-fine abrasive under an oscillating hone is even better. These are all grinding processes in which the crudely coarse edge of a carbide insert or HSS knife is replaced by microscopic crystalline cutting edges. Super-finishing may look perfect to the human eye, but an electron microscope can still see scratches…

My experience with carbide is it produces the best finish from relatively deep fast cuts, but sharp inserts do well taking shallow cuts. HSS seems less fussy, in that it's easier to shave off tiny whiskers of metal with slow shallow cutting. But there's not much in it compared with a sharp carbide insert.

According to my book correctly set-up carbide cuts faster, more efficiently, and produces a better finish than HSS, which would be great if my Hobby machines were man enough to run carbide optimally. They're not. Still prefer carbide to HSS though it does depends on what you're doing. As Andrew says, experiment!

Dave

Worth keeping an eye out on fleabay if one is in need of them. Don't think I've ever paid more than a fiver. although the cheapest at the moment is £9.

Fun aside, rather than creeping up on a final dimension, it's usually far better to choose a decent depth of cut that gives a good finish and then make sure that the last two cuts will be to that sort of depth. On the penultimate cut, measure before and after, then adjust the final cut to finish dead on size.

Back to fun, Taking off the embedded diamond grit from a lapping plate made from an Aldi 10kg weight with a CBN insert. Horrible stuff to turn with carbide:-

Edited By Mark Rand on 13/05/2022 20:06:51