Carbide turning inserts

[Rkweld]

Playing about with some crappy 40mm black bar on the lathe to see what sort of surface finish I can get. Usually I just accept what I get, but best learn this stuff now before I get a job it matters on, eh?

Same tool, same setup. 1000rpm, 2mm DOC, 5 TPR, and the tool rotated so swarf can't catch and mar the finish. Centre height set for each.

The best one is the top one. Chips break properly, and gives a mirror finish. No chatter marks. Works great on just about everything as long as you feed it in hard enough.

The middle one works OK. Chatters a bit towards the free end of the bar, but still gives a mirror finish. Fine, it is a roughing insert after all. But I can't get chips to break, it just gives long hot strings that are strong enough to choke a man. Maybe it's the soft material.

The bottom one is crap. It should be the sharpest of the bunch and work with shallow cuts, but no matter what I do, it chatters and gives a crap finish all over. Faster, slower, deeper, shallower, hot, cold, whatever.

Now I understand the designations and nose radius etc, but going on the limited knowledge i have, the bottom insert should be better for shallow cuts and give a better finish... But it doesn't.

So... What am I missing? Good people of the forum, please tell me what my random ebay inserts are doing

View attachment 301437

The bigger nose rads may be giving you a smoother cut because of the increased cutting area. Some times a fine point will be giving you a grainy ring appearance. The bottom of the three also has rake on so may be you are not knowingly cutting behind the point and it is giving you a rough edge (A problem i get at work if i do not put 18 degrees on my inserts and sharpen the top edge in the process). As long as the rad is sharp edged the bigger rads actually cut smooth and do not over heat as there is more material to defuse heat to. Black bar scale may be hard work for the finer point and blunt it also.

 

[Rkweld]

Are you turning or boring? Maybe a picture of the set up could help??

I reread op and it seems probably turning, my bad.

 

[Rkweld]

Also my experience of mild and tool steels seems to be that a sharp tip will give you a dull finish, that will need polishing or grinding to get a mirror finish, a dull tip will give you a mirror finish where it is rubbing/polishing rather than cutting. But that may just be because of the limits of the work I do when turning and boring.

 

[Munkul]

Well I got those nice finishing grade CNMG inserts, which aren't bad if you take a 1mm+ DOC. The chip breaker works great at 4 or 5 TPR. Unfortunately by the time I took pics, the metal was hot (CBA with coolant going everywhere) so the finish wasn't amazing. Just OK. It also got a bit of swarf trapped and marred the finish as you can see. But, to be fair the pictures make it look worse than it is.


I got a new toy tonight though... SVHCR and some VCMT16 inserts from ebay


So, bear in mind this is gummy black bar, maybe EN1, maybe ungraded. This was a 0.5mm DOC, 25mm dia, 1000rpm, 5 TPR feed.
It's almost flawless! Again, pictures pick up more than the naked eye does, it's almost mirror finish. If it was a better steel, I think it would be flawless.. but then again, better steels give better finishes anyway.



I like this tool and these inserts very, very much. I only wish I'd got it with a 25mm or 30mm shank.

Downsides: It doesn't like deep cuts. I pushed it with a 3mm DOC and was rewarded with a hot stringy mess. It also doesn't like higher than 5 TPR feed. So no good for roughing. Although it's probably obvious to some of you.

This is the tool:

Pardon our interruption...

These are the inserts:

Pardon our interruption...

 

[Hood]

Get a tool that takes DCMT, no probs with swarf getting trapped and you can take as much DOC as you can with a CNMG.

Edit.

Man I am tired looking through insurance quotes Should have said DNMG

 

[ukracer]

Get a tool that takes DCMT, no probs with swarf getting trapped and you can take as much DOC as you can with a CNMG.

Edit.

Man I am tired looking through insurance quotes Should have said DNMG

So we need Kram to find us some cheap DNMG tools and use the VCMT inserts and we can go mad on the DOC (whatever that is)

 

[Munkul]

Get a tool that takes DCMT, no probs with swarf getting trapped and you can take as much DOC as you can with a CNMG.

I've got enough tools! Between this and the MCLNR tool, and the vast array of roughing CNMG inserts I have, it covers 90% of my OD turning needs.
I thought it was you who reccomended a VC** insert? Maybe I got confused. I know it was you who said an insert with more clearance makes a nicer finish, so I have you to blame for this purchase in the first place...

 

[Hood]

I use a VCMT insert for finishing on the CNC but my roughing insert is almost always a DNMG on the CNC and on the manual I normally use it for both roughing and finishing. Only time I will use the VCMT on the manual is if I need to tickle something.

 

[Munkul]

Get a tool that takes DCMT, no probs with swarf getting trapped and you can take as much DOC as you can with a CNMG.

Edit.

Man I am tired looking through insurance quotes Should have said DNMG

I did wonder... a tight scotchman wouldn't be happy with only two cutting edges

I thought about going with a VN**16 but the tools aren't very common or cheap. And I really wanted a V shape insert for the extra reach you get with them.

 

[Brad93]

I bought a 20mm 1506 DNMG toolOn robins recommendation.

DNMG gets a great finish.

if I have to I use a CCGT to tickle things. But it doesn't like taking heavy cuts

 

[dan.taylor.1]

Is it the extra 25° angle on the d insert over the c shape that's the benefit?

 

[Munkul]

Is it the extra 25° angle on the d insert over the c shape that's the benefit?

Yes, it lets all the swarf exit without being trapped between the tool and the work while you can still turn to a shoulder and face with it.

Looking back i kinda wish I'd went DNMG but I'm fine with swapping between CNMG and VCMT on the QC toolholders. Just wish I had a few more holders... 5 isn't really enough!
- 1 for CNMG roughing
- 1 for VCMT finishing
- 1 for parting blade block
- 1 for SNMG chamfering
- 1 for boring bars and anything else that needs swapped in/out

I could use another 2 for threading tools, another 3-4 for boring bars, and a few more... straight TNMG tool, Left hand CNMG tool, grooving tools.... The thing that stops me is the average price of a T3 toolholder... £50-£60 each S/H!

 

[Hood]

You can never have enough lathe tools