milling with carbide vs hss

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1949DC
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milling with carbide vs hss

Post by 1949DC »

I have been using HSS for lathe work and now have a Millrite milling machine that it's lowest speed is 250 rpm. If I'm using a boring tool or large end mill with HSS on a diameter of 2" with HSS I need to be at 133 rpm at spindle. That is using 1080 mild steel @ 70 SFM x 3.82 / 2" diameter = 133 rpm.

What is the formula for carbide tooling? I keep seeing the formula for HSS of SFM X 4 / diameter. But with so much high speed machining in modern shops what is the correct running speed formula for Carbide.
My machine is tight enough to run higher speeds and I believe it was built about the time higher machining speeds were the norm. What am I missing?
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GlennW
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Re: milling with carbide vs hss

Post by GlennW »

Easily double with decent carbide.
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Harold_V
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Re: milling with carbide vs hss

Post by Harold_V »

For clarity, 1080 is not mild steel. Mild steel ends at about 1030, and is considered low cabon. 1080 is considered high carbon steel. The material that is considered mild steel that is commonly found tends to be 1018-1020 in both hot and cold rolled.

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1949DC
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Re: milling with carbide vs hss

Post by 1949DC »

70 sfm then is probably more accurate in my general equation. And it makes large diameter work even more challenging with HSS. Still looking for a reference chart for carbide cutting. Funny so much of modern machining is done at fast speeds but so much is spoken about on hss. If rule of thumb is multiply SFM for material used by factor of 2 for carbide tooling I can work with that.
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BadDog
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Re: milling with carbide vs hss

Post by BadDog »

For low carbon steel, I use 200-300 SFPM for carbide without coolant. The 300 SFPM is the recommendation I found "on the web" long ago. And I've used it a lot. However, I noted a lot of hotter (bluer) chips that concerned me a bit. So, since I'm cheap and want my tooling to last as long as possible, and I'm not running at production rates, but I'm also impatient for getting the task done, so I now tend to run around to 200 SFPM. I can't say how much difference it's made in tooling lifespan, but I feel better about the results.

Russ
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whateg0
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Re: milling with carbide vs hss

Post by whateg0 »

I don't think carbide suffers as much from getting hot. But, there are different grades of carbide, and there are different coatings. When I first started using carbide, the general rule of thumb I found on the web was 2x HSS, but IIRC, that was for uncoated carbide. I have watched videos on youtube on CNC lathes where the work was nearly glowing or at least had enough sparks running around it to appear that way. I know ceramic lets you do that, but I think I recall seeing some carbide tooling used that way, too. I worry more about chipping carbide than dulling it or burning it up.

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BadDog
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Re: milling with carbide vs hss

Post by BadDog »

Yeah, but CNC pretty much universally uses coolant. You can't really do that on most home shop manual mills, and for those that may not know, you also can't apply lube/coolant to a hot running carbide cutter without ruining it. So, other than maybe a pre-lube with cutting oil, it's flood or dry with carbide.

And I agree regarding allowable to run much hotter. And most of my carbide EMs are TIN coated. But I still see references to reduced life span due to running carbide hotter in the HS (non-production) environment. I've suspect that that 300 SFPM often mentioned is more a compromise for light production without coolant, and thus ok with sacrificing some life span. So, I've backed off on SFPM to keep chips just barely transitioning to blue after dropping from the cut in order to extend the life. I also seem to tend to feed heavier than other home shop guys I've been around, so that likely makes my chips hotter than others may find as well (though likely with tool actually being less hot). I like fat substantial chips both from the mill, and the lathe. <shrug/> Lots of variables to consider, but I've become pretty satisfied with a bit over 200 SFPM.
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