DNMG110408 & CNMG090308 Inserts

Have any of you tried these inserts on a hobby Lathe?

I’ve just tidied up some tools and found a box of 5 of the DNMG and 4 of the CNMG. Given how cheap the holders are on the auction site I’m wondering if I can use these inserts on my little 8 1/2” x 14 lathe. I have been using TNMG1604’s and they work fine. TIA.

Those insert sizes work just fine on small lathes. As for the actual inserts I cant advise without more info on them.

Neil

I’m less concerned about size, than the cutting tip of the insert itself. Although brand new they appear quite blunt.

Bluntness is the nature of carbide inserts. Don't try taking off very small amounts on diameters with them. They may rub all the way along or they may get under the surface randomly resulting in a rough finish. They need to be taking a reasonable depth of cut such as 0.2mm or more for best rrsults.

Martin C

They are - deliberately so, as Martin says. Used 'correctly' carbide cuts metal differently to HSS. HSS works like a knife and it's important that it be kept sharp and hard, which is quite difficult given what it has to do. Carbide takes a different approach - it ploughs into the metal at high-speed like a wedge.

The metal peels away just in front of the insert's edge and - ideally - the blunt edge doesn't touch metal at all. Not what I'd have expected, but provided you have the necessary power, carbide brutality is not only much faster than HSS, it's also more efficient. The heat generated by wedging softens the metal making it easier to cut. HSS soon loses its hardness and edge in these conditions, but the insert is protected from the worst effects because much of the heat is carried away by violently ejected chips. This means longer tool life than HSS as well.

The powerful attack needed for carbide to perform at its best isn't easily achieved on small machines, especially if they also have a low top-speed. I've had good results using sharp inserts like those normally used on non-ferrous metals. I don't normally bother though because ordinary blunt inserts do pretty well. The trick seems to be to take deeper, faster cuts than would be reasonable with HSS. Using carbide at HSS rates is mostly disappointing, carbide does better when when worked hard.

If I can't get carbide to produce a good finish, I have no loyalty to it. When it does a better job I finish off with HSS.

Dave

There are a couple of other factors that may make your tips seem blunter than others, the "N" signifies zero front clearance so the edge does not meet the top at such a sharp angle.

A bigger pointer to the"bluntness" is to 0.8mm corner radius, I hardly ever use this size on my 280 unless I particularly want a large internal corner fillet. For general turning I use 0.4mm and on thinner parts and for fine detail go for the 0.2mm radius. The big radius makes the tool more prone to rubbing and will just deflect slender parts away from the tool.

You don't give any other details about the specific coating or chip breaker so again you may have a tip that is designed to endure heavy cuts in hard and abrasive metals where as one designed for light finishing cuts in mild steel would be better suited to use on a lighter and smaller machine.

Edited By JasonB on 10/12/2018 18:39:41

Interesting but I have a question. Starting out with my mini-lathe I bought a set of brazed carbide tip tools and not knowing any different I tried using them and they were hopeless. I then learnt that they are supplied cut to shape but not sharpened though they appeared to have an edge. Reading above on how carbide inserts work and how they appear blunt why is it that the inserts work but the carbide tips need to be sharpened..? The are both carbide but it would seem they work in different ways..? I am probably missing the obvious, I often do.

Its true that most carbide tips are blunter than a freshly ground (or even a well worn) HSS one but its not true of all carbide tips.

The ones made for machining aluminium are razor sharp! (Damhikt). They work beautifully on stainless too but are easily damaged.

Ian P

Yes Ian, the polished Carbide are sharp. I like using them on alloy as well. The round ones are popular in hollowing tools for wood turning.

Ron, good question but I don’t know the answer!

Ron, the inserts that appear blunt work by generating a lot of heat, and plasticise the material. This plasterised material then shears off the stock, with the majority of the heat going into the chip itself. For this to work, it needs surface speed, and rigidity , and a reasonable depth of cut, anywhere from 50% of the tool radius and higher, or the nose radius of the tool in diameter as a minimum cutting depth/pass. Also the edge of the insert has clearance of around 5deg to 7 deg from the radius edge looking sideways.

Your brazed blanks came most likely with 0 to very little side relief angle on them. If you try and grind them on a green wheel, under a microscope there will be micro cracking of the edge and top surface. This micro cracking does not lend itself to very good cutting and shear and surface finish. If you diamond lap the edges, by hand very slow, but a diamond lap wheel is quite fast, the surface is very good and so becomes the surface finish.

For a lot of home / hobby lathes, the Aluminium grade carbides work really well on steel as a finishing tool and can take quite small cuts. The Al inserts do not do well when facing to the very centre, so where possible, put a dimple on the very centre to eliminate that part.

I have given up on negative inserts with my myford and now use ground positive geometry inserts , like the wonder inserts etc

Thanks Neil

I only have one lathe insert tool at the moment and the insert is CCMT, what does the CCMT signify in the insert type.

C = 80/100 degree rhomboid shape

C = 7 degree relief on the sides

M = Tolerance

T = Retained by it's CSK hole

Thanks Jason, do any of the numbers relate to the letters or are they just a part number, mine is CCMT060204

The numbers relate to size, in your case Ron from left to right it’s cutting length, thickness then tip radius.

You may find this chart, which I downloaded from the internet - I can't remember from where - useful to de-code the letters/numbers used to designate various inserts.

Just like in another thread, I can't reduce the size of the image. However, if you can't read it/want a copy PM me your email address and I'll send it to you.

John

Thanks Vic

Thanks John, if I click on it I can enlarge it so yes I can read it.

Ron

You can get an app for your phone or ipad from kyocera and sandvik that explaind all of the codes .

Here’s a PDF you can download Ron.

**LINK**

I seem to have had various carbide insert charts over the years but I always seem to lose them.

Beware any ANSI insert data, these are American. I think most of the planet use ISO. Anyone confirm?

You can use sharp, polished CCGT tools in the same holder.

Neil