Parting, HSS vs Carbide insert

Razzle

Razzle

Jack of All, Master of None
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Is there any benefit for hobby work to switching from hss to carbide insert? I have a cheap hss parting blade and have successfully parted with it. But I learned that it will not part 4130. I have some tooling I want to build from 4130 and 4140. But most of what I work is 12L14, 6160 aluminum, 360 brass. Stainless is very rare. I need to get a better parting blade but am not sure if I should get an hss p type blade or the shars retrofit carbide insert blade that can fit my existing parting tool holder. Thoughts and opinions are welcome.

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What are the best GTN2 inserts for "harder" material?
 
HSS is easy - there is HSS and Colbalt. All you need to learn how to grind 'em.

With Carbide, You've got geometry, hardness, chip breaker, and a host of other variables.

My experience is that Carbide needs more power and more rigidity in the lathe. If you have power and rigidity, carbide lasts longer, cuts faster, and is less trouble. On my 750 lb lathe (12X36) I only use HSS. On my bigger lathes (2000LB, 3800lb) I don't even have a HSS holder for them - exclusively carbide for parting.
 
Carbide needs rigidity and power to cut well, and the harder the material being parted the more rigidity and power you need. You have a Logan 820 so not fast, not that rigid and not that powerful so I would stick with HSS if I were you. A sharp P2 blade set up properly will give you the best chance for success. I have parted 4140 on a little Sherline lathe with a rear mounted P1-N parting tool and had zero issues doing it.

You also have to remember that a parting tool is a form tool. The bigger it is and the less sharp it is, which is what carbide is, the higher the cutting forces are. When parting harder material, this matters a great deal. I have carbide parting tools from Aloris and Manchester; both are very good tools and both work well on my Emco Super 11 BUT I use them only when I have to. I much prefer a sharp HSS blade because it cuts freely as long as I set it up correctly; if I can turn it with HSS then I can part it with HSS.
 
Understood. Yes my Logan 820 is definitely not very rigid or fast. So it sounds like a P type cutoff blade is my best bet. I have experimented with small carbide turning tools but they were cheap. Hss definitely fits the budget and I am getting better at grinding. I just have to stop drooling over carbide and get it through my head that hss is the best option for my setup.

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Parting 4140 HT is not a big problem with HSS; I have a fairly heavy 19" swing lathe and still do most parting with HSS with the P type blades. I also have carbide insert parting tools and am competent with them, but it is all to easy to have them break, potentially damaging the blade in the process.
 
Razzle said:
Understood. Yes my Logan 820 is definitely not very rigid or fast. So it sounds like a P type cutoff blade is my best bet. I have experimented with small carbide turning tools but they were cheap. Hss definitely fits the budget and I am getting better at grinding. I just have to stop drooling over carbide and get it through my head that hss is the best option for my setup.

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I suggest a P2, T-15 blade from AR Warner; LMS carries them. Grind a 7 degree bevel in front and hone the top with an extra-fine diamond stone and it will cut almost anything you can turn on your lathe.

Edit: if you use a parting tool holder that slants the blade upwards at a 5 deg angle, grind the front bevel on the tool at 10 degrees.
 
My lathe weighs 300 pounds. I use a carbide parting tool mounted to the rear of the cross slide. This works best for me. I'm not sure if you have the ability to do something similar. But if so, it turns all conventional wisdom on its head. A light lathe can run a carbide parting tool with absolutely no trouble, from a rear mounted position on the cross slide. I had a lot of trouble with hss parting blades. I tried and tried. Even on a 1000 pound machine, I never managed to make hss parting tools work with any lathe. But carbide solves it on the larger lathes. Rear mount tool posts solved it for the small lathe. Just something to keep in mind.
Here's an example: https://www.hobby-machinist.com/threads/rear-mounting-a-parting-tool-if-you-can.92947/
 
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