# Cobalt blanks? Other questions.



## Pcmaker (Oct 15, 2018)

I just ordered some 3/8" cobalt blanks for my lathe. Any experiences with them? I've been using 1/4" HSS and it the cutting edge dulls out quick, so I'm going to try out cobalt. 

Maybe the fact that it's 1/4" had something to do with it, so I ordered some 5/16" HSS blanks, too. Maybe larger blanks don't dull out as much. 

Also, any tips on prolonging carbide inserts for cutting tools? I chip them pretty quick. Besides taking smaller cuts, what other steps should I do/avoid in order to prolong these inserts?


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## mmcmdl (Oct 15, 2018)

The size of the tool has nothing to do with you losing your edge . What are you cutting ? You'll have better luck with the cobalt than regular hss though . Make sure your carbide inserts are on center or just below !


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## Pcmaker (Oct 15, 2018)

I'm cutting mild steel 99% of the time. I use carbide for hardened steel.


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## Cadillac (Oct 15, 2018)

I seem to break insert tips mostly from not pulling out when returning to start a lot. Using carbide you have to be on your speeds for them to work properly.


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## 4ssss (Oct 15, 2018)

They make good parallels


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## Cobra (Oct 15, 2018)

Pcmaker said:


> I'm cutting mild steel 99% of the time. I use carbide for hardened steel.


One question.  How many SFM are you turning at?  Too fast and you'll just burn/wear off the edge.


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## Pcmaker (Oct 15, 2018)

Cobra said:


> One question.  How many SFM are you turning at?  Too fast and you'll just burn/wear off the edge.




I still can't grasp the idea of SFM. I'm turning 1/2" mild steel at around 450 RPMs and using the leadscrew, taking out around .015


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## mikey (Oct 15, 2018)

Pcmaker said:


> I still can't grasp the idea of SFM. I'm turning 1/2" mild steel at around 450 RPMs and using the leadscrew, taking out around .015



Interesting that you're mostly turning mild steel and dulling HSS. For one thing, you're going too slow for 1/2" mild steel, although there are many kinds of mild steel and you haven't told us what you're using. On average, though, mild steel has a cutting speed of about 100 sfm and for a 1/2" work piece, that gives you an rpm of about 764. A HSS ground well at conventional tool angles will easily go 0.025 to 0.030 per pass in mild steel. A modified HSS tool will easily double that or more, and it will cut for a long time before needing sharpening. So, I suspect that your grind is the issue. If you show us your tool we might be able to help.

Size of your HSS tools does matter. 1/4" tooling will work for most things but a 3/8" tool is stiffer and will cut better if your lathe can handle it.

As for grasping SFM, it's pretty simple. Every material has a particular cutting speed that is given in surface feet per minute, or SFM. You can find a good one on LMS: https://littlemachineshop.com/reference/cuttingspeeds.php

Look up the cutting speed for the material you're cutting and plug it into this equation: RPM = SFM X 3.82 / D, where SFM is the cutting speed for your material, 3.82 is a constant and D is the diameter of your work piece. Solve for RPM and that is the speed you run your lathe at for that material and work piece. Note that cutting speed varies with the type of tooling you're using, HSS or carbide. Using your stated work piece as an example, the cutting speed for mild steel is about 100 so RPM = 100 X 3.82 / 0.5" = 764 rpm when using a HSS tool. Pretty simple.

Note that this speed is only a recommended starting point. You can adjust this speed and feed as needed. For example, if you tried turning at 750 rpm and encountered chatter, slow the speed to 600 and increase your feed a bit and the chatter should resolve. Again, the calculated rpm is not written in stone.

Cannot comment on your breaking carbide inserts without more info - material, tool holder and insert, cutting conditions, etc. More info will help us help you.


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## P. Waller (Oct 16, 2018)

"Cobalt" is HSS with cobalt added to the alloy, it is still high speed steel.
What exactly do yo mean by "chipping" when using carbide tools?
Sintered carbide lathe and mill tools are very hard therefore very brittle, a sharp lathe insert is easily broken by touching it off when the part is not rotating. The often recommended method of setting center height on a lathe using a hard steel ruler pinched between the tool and the part will easily break a sharp carbide tool, do not do this.

Chatter during turning will cause carbide tool failure when using a sharp tool. If you want to increase the life of inserts use negative rake tooling with the largest nose radius and largest edge radius that your machine will run.

You do not want the tool cutting edge to be sharp as it will not last long.


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## Larry42 (Oct 17, 2018)

Total amatur here but I've found that if I cut more aggressively with carbide inserts I get a better finish. Since I'm chicken to aggressively cut right up to finish size I will change to HSS for the last couple of passes. In a facing operation don't let a carbide insert travel past center. The force will be up on the cutting edge which usually has very little support.


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