Minimum depth of cut possible with carbide tooling on S7

A fellow club member who has a Myford S7 wants know what is the minimum depth of cut he can use to get an excellent finish when using inserts on steel.

He was aware of tables published, but I have not seen them. Can anyone enlighten us?

Dennis

0.001" should easily be possible with **GT inserts the more common **MT wont work so well at shallow cuts.

This is the Warco 280 doing it so unless it's a really clapped out Myford that should be able to do the same or better. Best watched on Youtube at full screen so you can see the thin wisps of swarf, thats EN8 too

Edited By JasonB on 12/05/2022 20:44:32

Plus one for the "GT" inserts.

I've used GT inserts on HSS and stainless as well as basic steels and got good finishes. The rigidity of the set-up is what is most important. If you can do away with the compound slide for a job it helps. If you can use a solid rear tool post with an upside down tool for example it is more rigid than the normal set-up on a compound slide. If you have to use the tool at the front minimise overhang of any sort, that includes the tool post not being over the cross slide.

Martin C

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

How is an "excellent" finish defined? I'm generally happy with an Ra value in the range 1 to 3 um.

I rarely take cuts of less than a few thou, and only use CCGT inserts for aluminium and plastic. The finish one gets with CCMT inserts at low DOC depends on the material, insert nose radius and feedrate. I normally use 0.4mm radius inserts, with 0.8mm radius for roughing, and 0.2mm radius if I need a sharper internal corner. I almost never drop below 4 thou/rev feedrate for finishing.

Some low carbon steels, like EN1A, brass and austenitic stainless steels (303 and 316) seem fine with low DOC. But EN3, for instance, goes to pot and tends to tear with low DOC.

There is no definitive answer and never will be; experimentation is needed. Just because a particular combination works at one point doesn't mean it will work again in the future; there are too many ill defined parameters.

Andrew

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

Tounge in cheek, but true:-

Minimum DOC for a decent finish with carbide inserts on steel is about 100 thou...

Minimum DOC for a decent finish with CBN inserts is about .5 thou.

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

Andrew

Posted by Andrew Johnston 13/05/2022 14:07:26

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

Andrew

Cheapest price on Ali******s iv'e seen is £4.59 for a VNMG160408 and free shipping.

John

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

We find that the aluminium grade polished sharp inserts are very good for finishing cuts on steel. You can get away with spring cuts where a common blunt edged insert would just rub and leave a poor finish. The only way to be sure of a good finish with the harder material grades is to make your last cut deep enough to work properly. Most people would not want to chance a final deep cut and would be happier creeping up on the size, this is where the polished "GT" types come into their own. Try a box of the Chinese ones, their quality has improved greatly in recent years. The aluminium grades will put up with some interrupted cutting, I often bore soft jaws with them. WNGT06 is a trigonic insert with 6 cutting edges and may be small enough to use in Myford size lathes in 12mm holders.

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

Thank you all for taking the time to provide insightful feedback.

It is clear that on a S7 the rigidity of the cutting tool mounting is critical. Then the selection of the appropriate insert. Then the selection of d.o.c., feed rate etc.

In the past looking for good finish on my S7, I have followed the suggestions made. I found that it often does not work first time around in producing a good finish. So tweaking is required.

Note that I have never tried to use a minimum d.o.c.. I found the comments on small d.o.c. interesting.

Dennis