Turning and Surface finishing - fail.

[udimet]

Neo Tech,
                       Nose radius is directly linked to feedrate AND depth of cut.   Looking at the photo told me that it was a small nose radius and it was, 0.4 . It is unlikely to be micro cracking as that usually happens when the insert is suddenly cooled after becoming hot [like if you were dry cutting and then suddenly decided to cool thing down a bit ] but there was no mention of coolant in your post, so I presume you were dry cutting.   Really you should try a larger [0.8] nose radius and slow down the feedrate also flood coolant does have a big effect on the final finish  [in your case not to cool the insert but to wash away any chips that can rub on the finished surface.
 Also, DCMT inserts are not so versatile as CCMT so if you you have a SCLC R/L or a STFC R/L   then try a CCMT 06 02 08 for a small size toolholder  ie 8/10/12mm or CCMT 09 03 08 for 16mm size.                                 
                                                                                              Regards,
                                                                                               Udimet.
 

[Mikey]

Hey NeoTech,
I saw the pictures you posted and said, "Ooh, ooh, I know what that is!" I rushed to join this forum and raise my hand but it took a few days to get it done so I'm late but would like to agree with jiihoo and udimet. That pattern is one I know well, having done them my very own self!

Basically, you have too fast a feed rate, too slow a cutting speed, and too shallow a depth of cut for a chipbreaker insert. Feed rate for this insert should be, as jiihoo pointed out, about 1/2-2/3 the nose radius (0.02-0.026mm/rev). Cutting speed for aluminum with a carbide insert is faster than the top speed of your lathe so it needs to run at least that. DOC for a chipbreaker insert depends on the manufacturer's recommendation but in general should be about NR+0.020".

While I agree that HSS tools would work really well for you there is that little hump of learning to grind them. It isn't difficult but you still have to learn it, and while I think its a good idea I can certainly understand why you went with carbide. Lots of us do it for the same reasons, myself included once upon a time. If you stay with carbide, which is far more expensive than HSS in the long run, I agree with udimet that going to a 0.08mm NR would be a good idea and only because it will finish better for you. I am not familiar with the Sandvik series you are using but if they offer a flat top insert to fit your lathe tool instead of the chipbreaker one you are using then that will lower cutting forces and greatly aid in both roughing and finishing at lower speeds (reduced deflection).

On the other hand, HSS tools should not cost much on ebay. If you use 1/2" bits you can do with the imports until you learn to grind them well and then invest in some higher quality bits. In fact, I suggest using mild steel keystock to learn on and go to HSS only when you are confident that you can grind what you intend.

Sorry I was so late to the dance and I hope this helps. You've gotten some solid advice from the others - I would take it.

Mikey

[SKIPRAT]

Hi There

Long long ago when i was in the apprentice training school the turning instructor gave us a talk about tool grinding he then gave us a bit of 1/2" square bright mild steel and told us to grind it up as a lathe tool.When he was satisfied we had got it right he then gave us a bit of HSS and we had to grind that up and use it in a lathe to prove it worked i wonder if that kind of stuff is still taught to apprentices these days that was a long time ago and it is like riding a bicycle once you have learnt it you don't forget it.

Cheers Paul

[Boucher]

The carbide tools need to be sharpened also. In fact most new carbide tools are not as sharp as one would like. A quick momentary touch to a diamond wheel can work wonders.
There is an article posted by bogstandard describing grinding a HSS flycutter that produces an outstanding surface finish. It will do the same as a turning tool. The surface finish of your part can be improved by a little filing followed by poilishing with some 320 grit paper backed by the file.

[Paddy OFernichur]

Just a quick note on carbide to, There are hundreds and thousands of differant grades of carbide.

Each grade is specific to ONE machining condition on ONE type of material yet almost every single carbide tip listed on ebay states they are for General maching on steel.

 Ive seen inserts for nylon listed as stainless grade tips, cast iron as steel tips. Unless you have been tought the carbide companies grading system you are buying grades of carbide that are utter usless to you.

Actually, I'll disagree with the part in bold. There are many good inserts out there which can work in a wide range of applications. Perhaps you just haven't found any yet, but they do exist. Is there ONE insert size, shape, grade and coating that can work in 90% of all applications? Not even close, but there are some very useful, almost "universal"  inserts out there.

[Jonny]

Looks like movement or vibration to me cutter digging in.
Try dragging the cutter, if a lot better you know where to look.

These types of bull nose were the only things apart from a sharp point that worked in my ML7 20 yr ago, that site 150698711080

[Fergus OMore]

Gentlemen,

                    May I make a plea? The poster is a novice and English is certainly not his first language and he has, like many  of us, no professional training and is using a hobby lathe. Can we attempt to concentrate on the issue and not involve him in the perplexities and issues of what is production engineering?

As far as I see it, all that he needs is a set of FOUR tools in a turret ( or 4 tools in a quick change holder) to do a simple turning operation- in stainless steel.

Can someone bring himself to address the specific problem- using carbide tools which seem- who knows why, to be his choice?

At some point, he is going to discover that he is going to have to use hss if only to drill holes-- and they do get blunt. However, that is not yet the issue!

[Pete.]

I think he will have used some HSS by now